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{{Short description|Order of reptiles characterized by a shell}}
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{{other uses|Turtle (disambiguation)}}
{{redirect|Chelonian|||Chelonian (disambiguation)}}
{{Featured article}}
{{Use mdy dates|date=February 2018}}
{{Automatic taxobox
| name = Turtles
| image = Florida Box Turtle Digon3.jpg
| image_caption = [[Common box turtle]]
| fossil_range = {{fossilrange|Late Jurassic | Present|earliest=Middle Triassic}} Stem-group turtles are from the [[Middle Triassic]]
| taxon = Testudines
| authority = [[Batsch]], 1788<ref name="TTWG 2017">{{Cite book |author=Turtle Taxonomy Working Group |author-link=Turtle Taxonomy Working Group |year=2017 |title=Turtles of the World: Annotated Checklist and Atlas of Taxonomy, Synonymy, Distribution, and Conservation Status |publisher=Chelonian Research Foundation: Turtle Conservancy |edition=8th |url=http://images.turtleconservancy.org/documents/2017/crm-7-checklist-atlas-v8-2017.pdf |series=Chelonian Research Monographs |volume=7 |issue=1 |pages=10, 24 |doi=10.3854/crm.7.checklist.atlas.v8.2017 |isbn=978-1-5323-5026-9 |access-date=January 20, 2018 |archive-date=February 25, 2021 |archive-url=https://web.archive.org/web/20210225035801/http://images.turtleconservancy.org/documents/2017/crm-7-checklist-atlas-v8-2017.pdf |url-status=live |oclc=1124067380 }}</ref>
| range_map = World.distribution.testudines.1.png
| range_map_caption = Blue: sea turtles, black: land turtles
| diversity_link = List of Testudines families
| diversity = 14 living families
| subdivision_ranks = Subgroups
| subdivision = *[[Cryptodira]]
*[[Pleurodira]]
*†[[Paracryptodira]]?
| synonyms = * Chelonii <small>Latreille, 1800</small>
* Chelonia <small>Ross & Macartney, 1802</small>
| synonyms_ref = <ref name="Dubois2010"/>
}}

'''Turtles''' are [[reptiles]] of the [[order (biology)|order]] '''Testudines''', characterized by a special [[turtle shell|shell]] developed mainly from their ribs. Modern turtles are divided into two major groups, the [[Pleurodira]] (side necked turtles) and [[Cryptodira]] (hidden necked turtles), which differ in the way the head retracts. There are 360 living and recently extinct [[species]] of turtles, including land-dwelling [[tortoise]]s and freshwater [[terrapin]]s. They are found on most continents, some islands and, in the case of [[sea turtle]]s, much of the ocean. Like other [[Amniote|amniotes]] (reptiles, [[bird]]s, and [[mammal]]s) they breathe air and do not lay eggs underwater, although many species live in or around water. 

Turtle shells are made mostly of [[bone]]; the upper part is the domed [[Turtle shell#Carapace|carapace]], while the underside is the flatter [[plastron]] or belly-plate. Its outer surface is covered in [[scale (anatomy)|scales]] made of [[keratin]], the material of hair, horns, and claws. The carapace bones develop from ribs that grow sideways and develop into broad flat plates that join up to cover the body. Turtles are [[ectotherm]]s or "cold-blooded", meaning that their internal temperature varies with their direct environment. They are generally opportunistic [[omnivore]]s and mainly feed on plants and animals with limited movement. Many turtles [[animal migration|migrate]] short distances seasonally. Sea turtles are the only reptiles that migrate long distances to lay [[Egg#Amniote eggs and embryos|their eggs]] on a favored beach.

Turtles have [[Cultural depictions of turtles|appeared in myths and folktales]] around the world. Some terrestrial and freshwater species are widely kept as pets. Turtles have been [[turtling (hunting)|hunted]] for their meat, for use in traditional medicine, and for their shells. Sea turtles are often killed accidentally as [[bycatch]] in fishing nets. Turtle habitats around the world are being destroyed. As a result of these pressures, many species are extinct or threatened with extinction.

==Naming and etymology==

The word ''turtle'' is borrowed from the [[French language|French]] word {{Lang|fr|tortue}} or {{Lang|fr|tortre}} 'turtle, [[tortoise]]'.<ref>{{OEtymD|turtle|access-date=October 28, 2021}}</ref> It is a [[common name]] and may be used without knowledge of taxonomic distinctions. In North America, it may denote the order as a whole. In Britain, the name is used for [[sea turtle]]s as opposed to freshwater [[terrapin]]s and land-dwelling tortoises. In Australia, which lacks true tortoises (family Testudinidae), non-marine turtles were traditionally called tortoises, but more recently turtle has been used for the entire group.{{sfn|Orenstein|2012|p=9}}

The name of the order, ''Testudines'' ({{IPAc-en|t|E|'|s|tj|uː|d|ᵻ|n|iː|z|audio=En-us-Testudines.oga}} {{Respell|teh|STEW|din|eez}}), is based on the [[Latin]] word {{lang|la|testudo}} 'tortoise';<ref>{{L&S|testudo|testudo|ref}}</ref> and was coined by German naturalist [[August Batsch]] in 1788.<ref name="TTWG 2017" /> The order has also been historically known as ''Chelonii'' ([[Pierre André Latreille|Latreille]] 1800) and ''Chelonia'' (Ross and [[James Macartney (anatomist)|Macartney]] 1802),<ref name="Dubois2010">{{cite journal |last1=Dubois |first1=Alan |last2=Bour |first2=Roger |year=2010 |title=The Distinction Between Family-Series and Class-Series Nominain Zoological Nomenclature, With Emphasis on the Nomina Created by Batsch (1788, 1789) and on the Higher Nomenclature of Turtles|journal=Bonn Zoological Bulletin |volume=57 |issue=2 |pages=149–171 |url=https://www.zoologicalbulletin.de/BzB_Volumes/Volume_57_2/149_171_BzB57_2_Dubois_Alain_and_Bour_Roger.PDF |archive-url=https://ghostarchive.org/archive/20221009/https://www.zoologicalbulletin.de/BzB_Volumes/Volume_57_2/149_171_BzB57_2_Dubois_Alain_and_Bour_Roger.PDF |archive-date=2022-10-09 |url-status=live}}</ref> which are based on the [[Ancient Greek]] word {{lang|grc|χελώνη}} ({{transliteration|grc|chelone}}) 'tortoise'.<ref>{{LSJ|xelw/nh|χελώνη|ref}}</ref><ref>{{cite web |title=Chelonia |website=Merriam-Webster |url=https://www.merriam-webster.com/dictionary/Chelonia |access-date=August 29, 2021}}</ref> Testudines is the official order name due to the [[principle of priority]].<ref name="Dubois2010"/> The term '''chelonian''' is used as a formal name for members of the group.<ref name="TTWG 2017"/>{{sfn|Franklin|2011|p=14}}

== Anatomy and physiology==
<!--"Form and function" arguably more comprehensive-->

===Size===

The largest [[Extant taxon|living]] species of turtle (and fourth-largest [[reptile]]) is the [[leatherback turtle]], which can reach over {{convert|2.7|m|abbr=on}} in length and weigh over {{convert|500|kg|abbr=on}}.<ref>{{cite journal |last1=Chen |first1=Irene H. |last2=Yang |first2=Wen |last3=Meyers |first3=Marc A. |year=2015 |title=Leatherback Sea Turtle Shell: a Tough and Flexible Biological Design |journal=Acta Biomaterialia |volume=28 |pages=2–12 |doi=10.1016/j.actbio.2015.09.023 |pmid=26391496|doi-access=free  |issn = 1742-7061 }}</ref> The largest known turtle was ''[[Archelon|Archelon ischyros]]'', a [[Cretaceous|Late Cretaceous]] sea turtle up to {{convert|4.5|m|ft|0|abbr=on}} long, {{convert|5.25|m|ft|0|abbr=on}} wide between the tips of the front flippers, and estimated to have weighed over {{cvt|2200|kg}}.<ref>{{cite web |url=http://www.bhigr.com/pages/info/info_arch.htm |title=The ''Archelon'' |publisher=Black Hills Institute of Geological Research |access-date=23 December 2018 |archive-date=March 12, 2016 |archive-url=https://web.archive.org/web/20160312042116/http://www.bhigr.com/pages/info/info_arch.htm|url-status=live}}</ref> The smallest living turtle is ''[[Chersobius signatus]]'' of South Africa, measuring no more than {{convert|10|cm|in|abbr=on}} in length<ref>{{cite book |last1=Bonin |first1=Franck |last2=Devaux |first2=Bernard |last3=Dupré |first3=Alain|year=2006 |title=Turtles of the World |publisher=Johns Hopkins University Press |page=230 |isbn=978-0-8018-8496-2}}</ref> and weighing {{convert|172|g|abbr=on}}.<ref>{{cite book |last=Frazier |first=Jack |year=2020 |contribution=Galapagos tortoises: Protagonists in the spectacle of life on Earth |page=26 |title=Galapagos Giant Tortoises |editor-last=Gibbs |editor-first=James |editor-last2=Cayot |editor-first2=Linda |editor-last3=Aguilera |editor-first3=Washington Tapia |publisher=Academic Press |isbn=978-0-12-817554-5 }}</ref>

===Shell===
{{main article|Turtle shell}}
[[File:Turtle skeleton cross-section, labelled as infographic.svg|thumb|left|upright=1.3|[[Sagittal section]] of a tortoise skeleton|alt=Photograph of one half of a tortoise skeleton, cut in half vertically showing the vertebrae following curving along the carapace]]

The shell of a turtle is unique among [[vertebrate]]s and serves to protect the animal and provide shelter from the elements.<ref name=Firefly>{{cite book |last1=Iverson|first1=John |last2=Moll|first2=Edward O|year=2002 |contribution=Turtles and tortoises |title=The Firefly Encyclopedia of Reptiles and Amphibians |editor=Halliday, Tim |editor2=Adler, Kraig |publisher=Firefly Books |pages=118–129 |isbn=978-1-55297-613-5 }}</ref>{{sfn|Orenstein|2012|p=22}}<ref>{{Cite web |last=Hutchinson |first=J. Howard |year=1996 |title=Introduction to Testudines: The Turtles |url=http://www.ucmp.berkeley.edu/anapsids/testudines/testudines.html |publisher=University of California Museum of Paleontology |access-date=June 4, 2003 |archive-date=June 29, 2016 |archive-url=https://web.archive.org/web/20160629134148/http://www.ucmp.berkeley.edu/anapsids/testudines/testudines.html |url-status=live }}</ref> It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back) [[Turtle shell#Carapace|carapace]] and the flatter, ventral (belly) [[Turtle shell#Plastron|plastron]]. They are connected by lateral (side) extensions of the plastron.<ref name=Firefly/>{{sfn|Orenstein|2012|p=16}}

The [[carapace]] is fused with the vertebrae and ribs while the plastron is formed from bones of the [[shoulder girdle]], [[sternum]], and [[gastralia]] (abdominal ribs).<ref name=Firefly/> During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the [[dermis]] (inner skin) of the back to support the carapace. The development is signaled locally by proteins known as [[fibroblast growth factor]]s that include [[FGF10]].<ref name="Cebra-Thomas Tan 2005">{{cite journal |last1=Cebra-Thomas |first1=Judith |last2=Tan |first2=Fraser |last3=Sistla |first3=Seeta |last4=Estes |first4=Eileen |last5=Bender |first5=Gunes |last6=Kim |first6=Christine |last7=Riccio |first7=Paul |last8=Gilbert |first8=Scott F. |title=How the Turtle Forms its Shell: a Paracrine Hypothesis of Carapace Formation |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=304B |issue=6 |year=2005 |pages=558–569 |issn=1552-5007 |doi=10.1002/jez.b.21059 |pmid=15968684 |bibcode=2005JEZB..304..558C |s2cid=2484583 |url=http://doc.rero.ch/record/15491/files/PAL_E2884.pdf }}</ref> The shoulder girdle in turtles is made up of two bones, the scapula and the [[coracoid]].<ref name=gaffney>{{cite journal |last=Gaffney |first=Eugene S. |year=1990 |title=The Comparative Osteology of the Triassic Turtle ''Proganochelys'' |journal=Bulletin of the American Museum of Natural History |issue=194 |pages=1–263 |hdl=2246/884 |url=http://digitallibrary.amnh.org/handle/2246/884 |access-date=May 16, 2021 |archive-date=May 16, 2021 |archive-url=https://web.archive.org/web/20210516221736/http://digitallibrary.amnh.org/handle/2246/884 |url-status=live |oclc=263164288 }}</ref> Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.<ref name="Schoch Sues 2019"/>
[[File:How the Turtle Gets its Shell.svg|thumb|upright=1.6|right|[[Embryogenesis|Development]] of the shell. The ribs are growing sideways into the carapacial ridge, seen here as a bud, to support the carapace.<ref name="Cebra-Thomas Tan 2005"/>|alt=Drawing of a section through a turtle embryo showing formation of the shell, with the ribs growing sideways]]
The shell is covered in [[epidermal]] (outer skin) scales known as [[scute]]s that are made of [[keratin]], the same substance that makes up hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron, giving them 54 in total. Carapace scutes are divided into "marginals" around the margin and "vertebrals" over the vertebral column, though the scute that overlays the neck is called the "cervical". "Pleurals" are present between the marginals and vertebrals.{{sfn|Orenstein|2012|pp=16–17}} Plastron scutes include gulars (throat), humerals, pectorals, abdominals, and anals. [[Pleurodira|Side-necked turtles]] additionally have "intergular" scutes between the gulars.{{sfn|Orenstein|2012|p=16}}<ref>{{cite book |last=Pritchard |first=Peter C. H. |chapter=Evolution and Structure of the Turtle Shell |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2 |oclc=144570900 |page=56}}</ref> Turtle scutes are usually structured like [[mosaic]] tiles, but some species, like the [[hawksbill sea turtle]], have overlapping scutes on the carapace.{{sfn|Orenstein|2012|p=16}}

The shapes of turtle shells vary with the adaptations of the individual species, and [[sexual dimorphism|sometimes with sex]]. Land-dwelling turtles are more dome-shaped, which appears to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells that allow them to cut through the water. Sea turtles in particular have streamlined shells that reduce [[drag (physics)|drag]] and increase stability in the open ocean. Some turtle species have pointy or spiked shells that provide extra [[Anti-predator adaptation|protection from predators]] and [[camouflage]] against the leafy ground. The lumps of a tortoise shell can tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the tip of the plastron is thickened and used for butting and ramming during combat.{{sfn|Orenstein|2012|pp=22–26}}

Shells vary in flexibility. Some species, such as [[box turtle]]s, lack the lateral extensions and instead have the carapace bones fully fused or [[ankylosis|ankylosed]] together. Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract. [[Softshell turtle]]s have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, but has thick [[connective tissue]] and an outer layer of leathery skin.{{sfn|Orenstein|2012|pp=22–23, 26–27}}

===Head and neck===
[[File:Mocsári teknős Kiserdő 2016.jpg|thumb|Head and neck of a [[European pond turtle]]|alt=Closeup of the head and neck of turtle|left]]
The turtle's skull is unique among living [[amniote]]s (which includes reptiles, birds and mammals); it is solid and rigid with no openings for muscle attachment ([[temporal fenestrae]]).{{sfn|Franklin|2011|p=18}}{{sfn|Orenstein|2012|p=33}} Muscles instead attach to recesses in the back of the skull. Turtle skulls vary in shape, from the long and narrow skulls of softshells to the broad and flattened skull of the [[mata mata]].{{sfn|Orenstein|2012|p=33}} Some turtle species have developed large and thick heads<!-- for their size-->, allowing for greater muscle mass and stronger bites.{{sfn|Franklin|2011|p=28}} In several species, some individuals can develop larger heads and thicker muscles than others, a phenomenon known as 'megacephaly'. This is likely due to differences in diet.<ref>{{Cite journal |last=Iverson |first=John B. |date=2020-06-16 |title=Potential Environmental Basis for Megacephaly in the Eastern Musk Turtle (Sternotherus odoratus) |journal=Chelonian Conservation and Biology |volume=19 |issue=1 |page=137 |doi=10.2744/CCB-1407.1 |issn=1071-8443|doi-access=free }}</ref><ref name=":2">{{Cite journal |last1=Iverson |first1=John B. |last2=Ernst |first2=Carl H. |last3=Gotte |first3=Steve |last4=Lovich |first4=Jeffrey E. |date=1989 |title=The Validity of Chinemys megalocephala (Testudines: Batagurinae) |journal=Copeia |volume=1989 |issue=2 |pages=494–498 |doi=10.2307/1445452 |jstor=1445452 |bibcode=1989Copei1989..494I |issn=0045-8511}}</ref> 

Turtles that are carnivorous or [[durophagy|durophagous]] (eating hard-shelled animals) have the most powerful bites. For example, the durophagous ''[[Mesoclemmys nasuta]]'' has a bite force of  {{convert|432|lbf|N|abbr=on}}. Species that are [[insectivorous]], [[piscivorous]] (fish-eating), or [[omnivorous]] have lower bite forces.<ref>{{cite journal |title=Evolution of Bite Performance in Turtles |last1=Herrel |first1=Anthony |last2=O'Reilly |first2=James C. |last3=Richmond |first3=Alan M. |s2cid=54067445 |journal=Journal of Evolutionary Biology |volume=15 |issue=6 |pages=1083–1094 |year=2002 |doi=10.1046/j.1420-9101.2002.00459.x|citeseerx=10.1.1.484.5540 }}</ref> Living turtles lack teeth but have beaks made of [[rhamphotheca|keratin sheaths]] along the edges of the jaws.{{sfn|Orenstein|2012|pp=33–34}}<ref name=Firefly/> These sheaths may have sharp edges for cutting meat, serrations for clipping plants, or broad plates for breaking [[mollusks]].{{sfn|Orenstein|2012|p=34}} Sea turtles, and several extinct forms, have evolved a bony [[secondary palate]] which completely separates the oral and nasal cavities.<ref>{{cite journal|last1=Abramyan|first1=J|last2=Richman|first2=J. M.|year=2015|title=Recent insights into the morphological diversity in the amniote primary and secondary palates|journal=Developmental Dynamics|volume=244|issue=12|pages=1457–1468|doi=10.1002/dvdy.24338|pmid=26293818|pmc=4715671}}</ref>

The necks of turtles are highly flexible, possibly to compensate for their rigid shells. Some species, like sea turtles, have short necks while others, such as [[snake-necked turtle]]s, have long ones. Despite this, all turtle species have eight [[cervical vertebrae|neck vertebrae]], a consistency not found in other reptiles but similar to mammals.{{sfn|Orenstein|2012|p=36}} Some snake-necked turtles have both long necks and large heads, limiting their ability to lift them when not in water.{{sfn|Franklin|2011|p=28}} Some turtles have folded structures in the [[larynx]] or [[glottis]] that vibrate to produce sound. Other species have [[elastin]]-rich [[vocal cord]]s.<ref>{{cite journal|last1=P. Russell|first1=Anthony|last2=M. Bauer|first2=Aaron|year=2020|title=Vocalization by extant nonavian reptiles: A synthetic overview of phonation and the vocal apparatus|journal=The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology|volume=304|issue=7|pages=1478–1528|doi=10.1002/ar.24553|pmid=33099849|s2cid=225069598|doi-access=free}}</ref><ref>{{cite book|author1=Capshaw, Grace|author2=Willis, Katie L.|author3=Han, Dawei|author4=Bierman, Hilary S.|year=2020|title=Neuroendocrine Regulation of Animal Vocalization|contribution=Reptile sound production and perception|pages=101–118|editor1=Rosenfeld, Cheryl S.|editor2=Hoffmann, Frauke|publisher=Academic Press|isbn=978-0-12-815160-0}}</ref>

===Limbs and locomotion===

Due to their heavy shells, turtles are slow-moving on land. A [[desert tortoise]] moves at only {{cvt|0.22|–|0.48|km/h|abbr=on}}. By contrast, sea turtles can swim at {{cvt|30|km/h|abbr=on}}.<ref name=Firefly/> The limbs of turtles are adapted for various means of locomotion and habits and most have five toes. Tortoises are specialized for terrestrial environments and have column-like legs with elephant-like feet and short toes. The [[gopher tortoise]] has flattened front limbs for digging in the substrate. Freshwater turtles have more flexible legs and longer toes with [[Webbed foot|webbing]], giving them thrust in the water. Some of these species, such as [[snapping turtles]] and [[mud turtle]]s, mainly walk along the water bottom, as they would on land. Others, such as terrapins, swim by paddling with all four limbs, switching between the opposing front and hind limbs, which keeps their direction stable.<ref name=Firefly/>{{sfn|Orenstein|2012|p=38}}

[[File:Green Sea Turtle swimming.jpg|thumb|[[Sea turtle]]s have [[streamlined]] shells and limbs adapted for fast and efficient swimming.<ref name="Davenport 1984"/>|alt=Marine turtle swimming]]

Sea turtles and the [[pig-nosed turtle]] are the most specialized for swimming. Their front limbs have evolved into flippers while the shorter hind limbs are shaped more like rudders. The front limbs provide most of the thrust for swimming, while the hind limbs serve as stabilizers.<ref name=Firefly/>{{sfn|Orenstein|2012|pp=38–40}} Sea turtles such as the [[green sea turtle]] rotate the front limb flippers like a bird's wings to generate a propulsive force on both the upstroke and on the downstroke. This is in contrast to similar-sized freshwater turtles (measurements having been made on young animals in each case) such as the [[Caspian turtle]], which uses the front limbs like the oars of a rowing boat, creating substantial negative thrust on the recovery stroke in each cycle. In addition, the streamlining of the marine turtles reduces drag. As a result, marine turtles produce a propulsive force twice as large, and swim six times as fast, as freshwater turtles. The swimming efficiency of young marine turtles is similar to that of fast-swimming fish of open water, like [[mackerel]].<ref name="Davenport 1984">{{cite journal |last1=Davenport |first1=John |last2=Munks |first2=Sarah A. |last3=Oxford |first3=P. J. |title=A Comparison of the Swimming of Marine and Freshwater Turtles |journal=Proceedings of the Royal Society of London. Series B. Biological Sciences |volume=220 |issue=1221 |date=February 22, 1984 |issn=0080-4649 |doi=10.1098/rspb.1984.0013 |jstor=35758 |pages=447–475 |bibcode=1984RSPSB.220..447D |s2cid=84615412 }}</ref>

Compared to other reptiles, turtles tend to have reduced tails, but these vary in both length and thickness among species and between sexes. [[Snapping turtles]] and the [[big-headed turtle]] have longer tails; the latter uses it for balance while climbing. The [[cloaca]] is found underneath and at the base, and the tail itself houses the reproductive organs. Hence, males have longer tails to contain the penis. In sea turtles, the tail is longer and more [[prehensile]] in males, who use it to grasp mates. Several turtle species have spines on their tails.{{sfn|Orenstein|2012|p=40}}{{sfn|Franklin|2011|p=18}}

=== Senses ===
[[File:Red Eared Slider's Eye.jpg|thumb|The [[red-eared slider]] has an exceptional seven types of color-detecting cells in its eyes.<ref name="Fritsches et al 2013"/>|alt=head of a red-eared slider turtle|left]]

Turtles make use of [[Visual perception|vision]] to find food and mates, avoid predators, and orient themselves. The [[retina]]'s light-sensitive cells include both [[Rod cell|rods]] for vision in low light, and [[Cone cell|cones]] with three different [[photopigment]]s for bright light, where they have full-color vision. There is possibly a fourth type of cone that detects [[ultraviolet]], as hatchling sea turtles respond experimentally to ultraviolet light, but it is unknown if they can distinguish this from longer wavelengths. A freshwater turtle, the [[red-eared slider]], has an exceptional seven types of cone cell.<ref name="Fritsches et al 2013"/><ref name="Granda Dvorak 1977 pp. 451–495">{{cite book |last1=Granda |first1=Alan M. |last2=Dvorak |first2=Charles A. |title=The Visual System in Vertebrates |chapter=Vision in Turtles |series=Handbook of Sensory Physiology |publisher=Springer Berlin Heidelberg |publication-place=Berlin, Heidelberg |year=1977 |volume=7 / 5 |issn=0072-9906 |doi=10.1007/978-3-642-66468-7_8 |pages=451–495|isbn=978-3-642-66470-0 }}</ref><ref name="Jacobs 1981">{{cite book |last=Jacobs |first=Gerald |chapter=Reptiles: The Turtle |title=Comparative Color Vision |publisher=Academic Press |publication-place=New York |year=1981 |isbn=978-0-12-378520-6 |pages=102–105}}</ref>

Sea turtles orient themselves on land by night, using visual features detected in dim light. They can use their eyes in clear surface water, muddy coasts, the darkness of the deep ocean, and also above water. Unlike in terrestrial turtles, the [[cornea]] (the curved surface that lets light into the eye) does not help to focus light on the retina, so focusing underwater is handled entirely by the lens, behind the cornea. The cone cells contain oil droplets placed to shift perception toward the red part of the spectrum, improving color discrimination. Visual acuity, studied in hatchlings, is highest in a horizontal band with retinal cells packed about twice as densely as elsewhere. This gives the best vision along the visual horizon. Sea turtles do not appear to use [[polarized light]] for orientation as many other animals do. The deep-diving leatherback turtle lacks specific adaptations to low light, such as large eyes, large lenses, or a reflective [[Tapetum lucidum|tapetum]]. It may rely on seeing the [[bioluminescence]] of prey when hunting in deep water.<ref name="Fritsches et al 2013">{{cite book |last1=Fritsches |first1=Kerstin A. |last2=Warrant |first2=Eric J. |chapter=Vision |editor-last=Wyneken |editor-first=Jeanette |title=The Biology of Sea Turtles |publisher=CRC Press |year=2013 |isbn=978-1-4398-7308-3 |oclc=828509848 |pages=31–58}}</ref>

Turtles have no ear openings; the [[eardrum]] is covered with scales and encircled by a bony [[otic capsule]], which is absent in other reptiles.{{sfn|Orenstein|2012|p=36}} Their hearing thresholds are high in comparison to other reptiles, reaching up to 500 [[Hz]] in air, but underwater they are more attuned to lower frequencies.<ref>{{cite book|last=Willis|first=Katie L. |title=The Effects of Noise on Aquatic Life II |chapter=Underwater Hearing in Turtles |series=Advances in Experimental Medicine and Biology |year=2016 |volume=875 |pages=1229–1235 |doi=10.1007/978-1-4939-2981-8_154 |pmid=26611091|isbn=978-1-4939-2980-1 }}</ref> The [[loggerhead sea turtle]] has been shown experimentally to respond to low sounds, with maximal sensitivity between 100 and 400&nbsp;Hz.<ref name="MartinAlessi2012">{{cite journal |last1=Martin |first1=Kelly J. |last2=Alessi |first2=Sarah C. |last3=Gaspard |first3=Joseph C. |last4=Tucker |first4=Anton D. |last5=Bauer |first5=Gordon B. |last6=Mann |first6=David A. |title=Underwater Hearing in the Loggerhead Turtle (''Caretta caretta''): a Comparison of Behavioral and Auditory Evoked Potential Audiograms |journal=Journal of Experimental Biology |volume=215 |issue=17 |year=2012 |pages=3001–3009 |issn=1477-9145 |doi=10.1242/jeb.066324 |pmid=22875768 |bibcode=2012JExpB.215.3001M |s2cid=459652 |url=https://scholarcommons.usf.edu/etd/3233 |doi-access=free |url-access=subscription }}</ref>

Turtles have [[Olfactory system|olfactory]] (smell) and [[vomeronasal organ|vomeronasal]] receptors along the nasal cavity, the latter of which are used to detect chemical signals.{{sfn|Orenstein|2012|pp=35–36}} Experiments on green sea turtles showed they could learn to respond to a selection of different odorant chemicals such as [[triethylamine]] and [[cinnamaldehyde]], which were detected by olfaction in the nose. Such signals could be used in navigation.<ref name="MantonKarr1972">{{cite journal |last1=Manton |first1=Marion |last2=Karr |first2=Andrew |last3=Ehrenfeld |first3=David W. |title=Chemoreception in the Migratory Sea Turtle, ''Chelonia mydas'' |journal=The Biological Bulletin |volume=143 |issue=1 |year=1972 |pages=184–195 |issn=0006-3185 |doi=10.2307/1540338 |jstor=1540338 |url=https://www.biodiversitylibrary.org/part/24653 |access-date=June 6, 2021 |archive-date=June 6, 2021 |archive-url=https://web.archive.org/web/20210606225146/https://www.biodiversitylibrary.org/part/24653 |url-status=live }}</ref>

=== Breathing===
[[File:Indian softshell turtle (Nilssonia gangetica) Babai River nose-breathing at surface.jpg|thumb|A submerged [[Indian softshell turtle]] nose-breathing at river surface|alt=photo of a river turtle with only its nose above water]]
The rigid shell of turtles is not capable of expanding and making room for the lungs, as in other amniotes, so they have had to evolve special adaptations for respiration.<ref name="Cordeiro 2016">{{Cite journal |last1=Cordeiro |first1=Tábata E. F. |last2=Abe |first2=Augusto S. |last3=Klein |first3=Wilfried |date=April 2016 |title=Ventilation and Gas Exchange in Two Turtles: ''Podocnemis unifilis'' and ''Phrynops geoffroanus'' (Testudines: Pleurodira) |journal=Respiratory Physiology & Neurobiology |volume=224 |pages=125–131 |doi=10.1016/j.resp.2014.12.010 |pmid=25534144 |issn=1569-9048 |hdl=11449/158795 |s2cid=37446604 |url=https://repositorio.unesp.br/bitstream/11449/158795/1/WOS000373540800015.pdf |hdl-access=free |access-date=September 25, 2019 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105252/https://repositorio.unesp.br/bitstream/handle/11449/158795/WOS000373540800015.pdf;jsessionid=7500C1BBBBCF7CD5F9A0DE4D94B0AC03?sequence=1 |url-status=live }}</ref><ref name="Lyson 2014">{{Cite journal |last1=Lyson |first1=Tyler R. |last2=Schachner |first2=Emma R. |last3=Botha-Brink |first3=Jennifer |last4=Scheyer |first4=Torsten M. |last5=Lambertz |first5=Markus |last6=Bever |first6=G. S. |last7=Rubidge |first7=Bruce S. |last8=de Queiroz |first8=Kevin |date=7 November 2014 |title=Origin of the Unique Ventilatory Apparatus of Turtles |journal=Nature Communications |volume=5 |article-number=5211 |doi=10.1038/ncomms6211 |pmid=25376734 |issn=2041-1723 |bibcode=2014NatCo...5.5211L|doi-access=free }}</ref><ref>{{Cite journal |last1=Lee |first1=Stella Y. |last2=Milsom |first2=William K. |year=2016 |title=The Metabolic Cost of Breathing in Red-eared Sliders: An Attempt to Resolve an Old Controversy |journal=Respiratory Physiology & Neurobiology |volume=224 |pages=114–124 |doi=10.1016/j.resp.2015.10.011 |pmid=26524718 |s2cid=5194890 |issn=1569-9048}}</ref> The lungs of turtles are attached directly to the carapace above while below, connective tissue attaches them to the organs.{{sfn|Orenstein|2012|p=41}} They have multiple lateral (side) and medial (middle) chambers (the numbers of which vary between species) and one terminal (end) chamber.<ref>{{Cite journal |last1=Lambertz |first1=Markus |last2=Böhme |first2=Wolfgang |last3=Perry |first3=Steven F. |date=July 2010 |title=The Anatomy of the Respiratory System in ''Platysternon megacephalum'' Gray, 1831 (Testudines: Cryptodira) and Related Species, and its Phylogenetic Implications |journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology |volume=156 |issue=3 |pages=330–336 |doi=10.1016/j.cbpa.2009.12.016 |pmid=20044019 |issn=1095-6433}}</ref>

The lungs are ventilated using specific groups of abdominal muscles attached to the organs that pull and push on them.<ref name="Cordeiro 2016"/> Specifically, it is the turtle's large liver that compresses the lungs. Underneath the lungs, in the [[coelomic cavity]], the liver is connected to the right lung by the [[Root of the lung|root]], and the stomach is directly attached to the left lung, and to the liver by a [[mesentery]]. When the liver is pulled down, inhalation begins.<ref name="Lyson 2014"/> Supporting the lungs is a wall or [[septum]], which is thought to prevent them from collapsing.<ref>{{Cite journal |last1=Klein |first1=Wilfried |last2=Codd |first2=Jonathan R. |year=2010 |title=Breathing and Locomotion: Comparative Anatomy, Morphology and Function |journal=Respiratory Physiology & Neurobiology |volume=173 |pages=S26–S32 |doi=10.1016/j.resp.2010.04.019|pmid=20417316 |s2cid=28044326 |issn=1569-9048}}</ref> During exhalation, the contraction of the [[transversus abdominis muscle]] propels the organs into the lungs and expels air. Conversely, during inhalation, the relaxing and flattening of the [[Abdominal external oblique muscle|oblique abdominis muscle]] pulls the transversus back down, allowing air back into the lungs.<ref name="Lyson 2014"/>

Although many turtles spend large amounts of their lives underwater, all turtles breathe air and must surface at regular intervals to refill their lungs. Depending on the species, immersion periods vary between a minute and an hour.<ref>{{Cite journal |last1=Morera-Brenes |first1=Bernal |last2=Monge-Nájera |first2=Julián |year=2011 |title=Immersion Periods in Four Neotropical Turtles |url=https://revistas.uned.ac.cr/index.php/cuadernos/article/view/212 |journal=UNED Research Journal |volume=3 |issue=1 |page=97 |doi=10.22458/urj.v3i1.212 |doi-access=free |access-date=June 11, 2020 |archive-date=August 9, 2020 |archive-url=https://web.archive.org/web/20200809164614/https://revistas.uned.ac.cr/index.php/cuadernos/article/view/212 |url-status=live }}</ref> Some species can [[Enteral respiration|respire through the cloaca]], which contains large sacs that are lined with many finger-like projections that take up dissolved [[oxygen]] from the water.<ref>{{Cite journal |last1=Priest |first1=Toni E. |date=December 2002 |last2=Franklin |first2=Craig E. |title=Effect of Water Temperature and Oxygen Levels on the Diving Behavior of Two Freshwater Turtles: ''Rheodytes leukops'' and ''Emydura macquarii'' |journal=Journal of Herpetology |volume=36 |issue=4 |pages=555–561 |jstor=1565924 |issn=0022-1511 |doi=10.1670/0022-1511(2002)036[0555:EOWTAO]2.0.CO;2 |s2cid=85279910 }}</ref>

=== Circulation ===

[[File:Snapping turtle in Mud.JPG|thumb|[[Chelydra serpentina|Snapping turtle]] emerging from period of [[brumation]], in which it buried itself in mud. Turtles have multiple circulatory and physiological adaptations to enable them to go long periods without breathing.<ref name="Wyneken 2008 Circulatory"/>|alt=photo of a turtle climbing out of mud|left]]

Turtles share the linked [[Circulatory system|circulatory]] and pulmonary (lung) systems of vertebrates, where the three-chambered heart pumps deoxygenated blood through the lungs and then pumps the returned [[oxygenated blood]] through the body's tissues. The cardiopulmonary system has both structural and physiological adaptations that distinguish it from other vertebrates. Turtles have a large lung volume and can move blood through non-pulmonary blood vessels, including some within the heart, to avoid the lungs while they are not breathing. They can hold their breath for much longer periods than other reptiles and they can tolerate the resulting low oxygen levels. They can moderate the increase in acidity during [[anaerobic respiration|anaerobic (non-oxygen-based) respiration]] by [[buffer solution|chemical buffering]] and they can lie dormant for months, in [[aestivation]] or [[brumation]].<ref name="Wyneken 2008 Circulatory"/>

The heart has two [[Atrium (heart)|atria]] but only one [[Ventricle (heart)|ventricle]]. The ventricle is subdivided into three chambers. A muscular ridge enables a complex pattern of blood flow so that the blood can be directed either to the lungs via the [[pulmonary artery]], or to the body via the [[aorta]]. The ability to separate the two outflows varies between species. The leatherback has a powerful muscular ridge enabling almost complete separation of the outflows, supporting its actively swimming lifestyle. The ridge is less well developed in freshwater turtles like the sliders (''[[Trachemys]]'').<ref name="Wyneken 2008 Circulatory">{{cite book |last=Wyneken |first=Jeanette |chapter=The Structure of Cardiopulmonary Systems of Turtles: Implications for Behavior and Function |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2|oclc=144570900 |pages=213–224}}</ref>

Turtles are capable of enduring periods of anaerobic respiration longer than many other vertebrates. This process breaks down sugars incompletely to [[lactic acid]], rather than all the way to [[carbon dioxide]] and water as in [[aerobic respiration|aerobic (oxygen-based) respiration]].<ref name="Wyneken 2008 Circulatory"/> They make use of the shell as a source of additional buffering agents for combating increased acidity, and as a sink for lactic acid.<ref>{{Cite journal |year=2002 |last=Jackson |first=Donald C. |title=Hibernating Without Oxygen: Physiological Adaptations of the Painted turtle |journal=Journal of Physiology |volume=543 |issue=Pt 3 |pages=731–737 |doi=10.1113/jphysiol.2002.024729 |pmid=12231634 |pmc=2290531}}</ref>

===Osmoregulation ===
In sea turtles, the bladder is one unit and in most freshwater turtles, it is double-lobed.<ref>{{cite book |last1=Miller |first1=Jeffrey D. |last2=Dinkelacker|first2=Stephen A.|chapter=Reproductive Structures and Strategies of Turtles |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2 |oclc=144570900 |page=234}}</ref> Sea turtle bladders are connected to two small accessory bladders, located at the sides to the neck of the urinary bladder and above the [[Pubis (bone)|pubis]].<ref>{{cite journal |last1=Wyneken |first1=Jeanette |last2=Witherington |first2=Dawn |date=February 2015 |title=Urogenital System |url=http://www.ivis.org/advances/wyneken/16.pdf?LA |journal=Anatomy of Sea Turtles |volume=1 |pages=153–165 |access-date=May 18, 2021 |archive-date=June 8, 2019 |archive-url=https://web.archive.org/web/20190608150657/http://www.ivis.org/advances/wyneken/16.pdf?LA |url-status=live }}</ref> Arid-living tortoises have bladders that serve as reserves of water, storing up to 20% of their body weight in fluids. The fluids are normally low in [[solute]]s, but higher during droughts when the reptile gains [[potassium]] salts from its plant diet. The bladder stores these salts until the tortoise finds fresh drinking water.<ref name="Bentley 2013">{{cite book |last=Bentley |first=Peter J. |url=https://books.google.com/books?id=U0D3BwAAQBAJ&pg=PA143 |title=Endocrines and Osmoregulation: A Comparative Account in Vertebrates |date=2013 |publisher=Springer |page=143 |isbn=978-3-662-05014-9 |access-date=May 18, 2021 |archive-date=May 31, 2021 |archive-url=https://web.archive.org/web/20210531050224/https://books.google.com/books?id=U0D3BwAAQBAJ&pg=PA143 |url-status=live }}</ref> To regulate the amount of salt in their bodies, sea turtles and the [[brackish water|brackish]]-living [[diamondback terrapin]] secrete excess salt in a thick sticky substance from their [[lacrimal gland|tear gland]]s. Because of this, sea turtles may appear to be "crying" when on land.{{sfn|Franklin|2011|p=31}}

=== Thermoregulation ===
[[File:Two basking cooter turtles (5861462496).jpg|thumb|Smaller pond turtles, like these [[northern red-bellied cooter]]s, regulate their temperature by basking in the sun.|alt=cooter turtles basking in sunshine near their pond]]

Turtles, like other reptiles, have a limited ability to [[Homoiotherm|regulate their body temperature]]. This ability varies between species, and with body size. Small pond turtles regulate their temperature by crawling out of the water and basking in the sun, while small terrestrial turtles move between sunny and shady places to adjust their temperature. Large species, both terrestrial and marine, have sufficient mass to give them substantial [[thermal inertia]], meaning that they heat up or cool down over many hours. The [[Aldabra giant tortoise]] weighs up to some {{convert|60|kg|lb}} and is able to allow its temperature to rise to some {{convert|33|C|F}} on a hot day, and to fall naturally to around {{convert|29|C|F}} by night. Some giant tortoises seek out shade to avoid overheating on sunny days. On [[Grand Terre Island]]<!--part of Aldabra-->, food is scarce inland, shade is scarce near the coast, and the <!--Aldabra giant--> tortoises compete for space under the few trees on hot days. Large males may push smaller females out of the shade, and some then overheat and die.<ref name="Pough Janis 2019"/>

Adult sea turtles, too, have large enough bodies that they can to some extent control their temperature. The largest turtle, the leatherback, can swim in the waters off [[Nova Scotia]], which may be as cold as {{convert|8|C|F}}, while their body temperature has been measured at up to {{convert|12|C-change|F-change}} warmer than the surrounding water. To help keep their temperature up, they have a system of [[countercurrent heat exchange]] in the blood vessels between their body core and the skin of their flippers. The vessels supplying the head are insulated by fat around the neck.<ref name="Pough Janis 2019">{{cite book |last1=Pough |first1=F. Harvey |last2=Janis |first2=Christine M. |chapter=16. Turtles |title=Vertebrate Life |publisher=Sinauer Associates |publication-place=New York |year=2019 |edition=10th |isbn=978-1-60535-607-5 |oclc=1022979490 |pages=283–299}}</ref>

== Behavior==

===Diet and feeding===
[[File:Green Sea Turtle grazing seagrass.jpg|thumb|A [[green sea turtle]] grazing on seagrass |alt=Photograph of a green sea turtle on the seabed, feeding]]

Most turtle species are opportunistic omnivores; land-dwelling species are more [[Herbivore|herbivorous]] and aquatic ones more [[Carnivore|carnivorous]].{{sfn|Franklin|2011|p=28}} Generally lacking speed and agility, most turtles feed either on plant material or on animals with limited movements like mollusks, worms, and insect larvae.<ref name=Firefly/> Some species, such as the [[African helmeted turtle]] and snapping turtles, eat fish, amphibians, reptiles (including other turtles), birds, and mammals. They may take them by [[ambush predator|ambush]] but also scavenge.{{sfn|Orenstein|2012|p=231}} The [[alligator snapping turtle]] has a worm-like appendage on its tongue that it [[Aggressive mimicry|uses to lure fish]] into its mouth. Tortoises are the most herbivorous group, consuming grasses, leaves, and fruits.{{sfn|Franklin|2011|pp=29–30}} Many turtle species, including tortoises, supplement their diet with eggshells, animal bones, hair, and droppings for extra nutrients.{{sfn|Orenstein|2012|p=237}}

Turtles generally eat their food in a straightforward way, though some species have special feeding techniques.<ref name=Firefly/> The [[yellow-spotted river turtle]] and the [[painted turtle]] may [[filter feed]] by skimming the water surface with their mouth and throat open to collect particles of food. When the mouth closes, the throat constricts and water is pushed out through the nostrils and the gap in between the jaws.{{sfn|Orenstein|2012|p=235}} Some species employ a "gape-and-suck method" where the turtle opens its jaws and expands its throat widely, sucking the prey in.<ref name=Firefly/><ref name="Van Damme 1997">{{cite journal |last1=Van Damme |first1=Johan |last2=Aerts |first2=Peter |title=Kinematics and Functional Morphology of Aquatic Feeding in Australian Snake-necked Turtles (Pleurodira;Chelodina) |journal=Journal of Morphology |volume=233 |issue=2 |pages=113–125 |year=1997 |doi=10.1002/(SICI)1097-4687(199708)233:2<113::AID-JMOR3>3.0.CO;2-7 |pmid=9218349 |s2cid=32906130 }}</ref>{{sfn|Franklin|2011|p=30}}

The diet of an individual within a species may change with age, sex, and season, and may also differ between populations. In many species, juveniles are generally carnivorous but become more herbivorous as adults.<ref name=Firefly/>{{sfn|Orenstein|2012|p=239}} With [[Barbour's map turtle]], the larger female mainly eats mollusks while the male usually eats [[arthropod]]s.<ref name=Firefly/> [[Blanding's turtle]] may feed mainly on snails or crayfish depending on the population. The [[European pond turtle]] has been recorded as being mostly carnivorous much of the year but switching to [[Nymphaea alba|water lilies]] during the summer.{{sfn|Orenstein|2012|p=229}} Some species have developed [[Generalist and specialist species|specialized]] diets such as the hawksbill, which eats [[sponges]], the leatherback, which feeds on [[jellyfish]], and the [[Mekong snail-eating turtle]].{{sfn|Franklin|2011|p=28}}<ref name=Firefly/>

===Communication and intelligence===
[[File:Chelodina oblonga 1.jpeg|thumb|The [[oblong turtle]] has a sizable vocal repertoire.<ref name="Glies"/>|alt=Photograph of an oblong turtle]]
{{see also|Animal cognition}}

While popularly thought of as mute, turtles make various sounds to communicate.{{sfn|Pryke|2021|p=39}}<ref name="Ferrara"/> One study which recorded 53 species found that all of them vocalized.<ref>{{cite journal|last1=Jorgewich-Cohen|first1=G|display-authors=etal|year=2022|title=Common evolutionary origin of acoustic communication in choanate vertebrates|journal=Nature Communications|volume=13|issue=1|article-number=6089|doi=10.1038/s41467-022-33741-8|pmid=36284092|pmc=9596459|bibcode=2022NatCo..13.6089J|s2cid=253111242}}</ref> Tortoises may bellow when courting and mating.<ref name="Ferrara"/>{{sfn|Orenstein|2012|p=36}} Various species of both freshwater and sea turtles emit short, low-frequency calls from the time they are in the egg to when they are adults. These vocalizations may serve to create group cohesion when [[Animal migration|migrating]].<ref name="Ferrara">{{cite journal |last1=Ferrara |first1=Camila R. |last2=Vogt |first2=Richard C. |last3=Sousa-Lima |first3=Renata Santoro |year=2012 |title=Turtle Vocalizations as the First Evidence of Posthatching Parental Care in Chelonians |journal=Journal of Comparative Psychology |volume=127 |issue=1 |pages=24–32 |doi=10.1037/a0029656 |pmid=23088649 |url=http://www.seaturtle.org/PDF/FerraraCR_2013_JCompPsychol.pdf |access-date=September 1, 2017 |archive-date=September 2, 2017 |archive-url=https://web.archive.org/web/20170902055618/http://www.seaturtle.org/PDF/FerraraCR_2013_JCompPsychol.pdf |url-status=live }}</ref> The [[oblong turtle]] has a particularly large vocal range; producing sounds described as clacks, clicks, squawks, hoots, various kinds of chirps, wails, ''{{Not a typo|hooos}}'', grunts, growls, blow bursts, howls, and drum rolls.<ref name="Glies">{{cite journal |last1=Giles |first1=Jacqueline C. |last2=Davis |first2=Jenny |last3=McCauley |first3=Robert D. |last4=Kuchling |first4=Gerald |year=2009 |title=Voice of the Turtle: The Underwater Acoustic Repertoire of the Long-necked Freshwater Turtle, ''Chelodina oblonga'' |journal=The Journal of the Acoustical Society of America |volume=126 |issue=1 |pages=434–443 |doi=10.1121/1.3148209 |pmid=19603900 |bibcode=2009ASAJ..126..434G |doi-access=free }}</ref>

Play behavior has been documented in some turtle species.<ref>{{cite journal |last1=Burghardt |first1=Gordon M. |last2=Ward |first2=B. |last3=Rosscoe |first3=Roger |year=1996 |title=Problem of Reptile Play: Environmental Enrichment and Play Behavior in a Captive Nile Softshelled Turtle, ''Trionyx triunguis'' |journal=Zoo Biology |volume=15 |issue=3 |pages=223–238 |doi=10.1002/(SICI)1098-2361(1996)15:3<223::AID-ZOO3>3.0.CO;2-D }}</ref> In the laboratory, [[Florida red-bellied cooter]]s can learn novel tasks and have demonstrated a long-term memory of at least 7.5&nbsp;months.<ref name="Learning">{{cite journal |last1=Davis |first1=K. M. |last2=Burghardt |first2=Gordon M. |year=2007 |title=Training and Long-term Memory of a Novel Food Acquisition Task in a Turtle (''Pseudemys nelsoni'') |journal=Behavioural Processes |volume=75 |issue=2 |pages=225–230 |doi=10.1016/j.beproc.2007.02.021 |pmid=17433570 |s2cid=34130920 }}</ref> Similarly, giant tortoises can learn and remember tasks, and master lessons much faster when trained in groups.<ref>{{cite journal |date=November 29, 2019 |df=dmy-all |author=<!-- no author listed in ''Nature'' article --> |title=Reptiles Known as 'Living Rocks' Show Surprising Cognitive Powers |series=Animal Behaviour |journal=Nature |volume=576 |issue=7785 |page=10 |doi=10.1038/d41586-019-03655-5 |bibcode=2019Natur.576...10.|s2cid=208613023 |doi-access=free }}</ref> Tortoises appear to be able to retain [[operant conditioning]] nine years after their initial training.<ref>{{cite journal |last1=Gutnick |first1=Tamar |last2=Weissenbacher |first2=Anton |last3=Kuba |first3=Michael J. |date=November 13, 2019 |df=dmy-all |title=The Underestimated Giants: Operant Conditioning, Visual Discrimination and Long-term Memory in Giant Tortoises |journal=Animal Cognition |volume=23 |issue=1 |pages=159–167 |doi=10.1007/s10071-019-01326-6 |issn=1435-9456 |pmid=31720927 |s2cid=207962281 |url=http://id.nii.ac.jp/1394/00001487/ }}</ref> Studies have shown that turtles can navigate the environment using landmarks and a map-like system resulting in accurate direct routes towards a goal.<ref name=":1">{{Cite journal |last1=Salas |first1=Cosme |last2=Broglio |first2=Cristina |last3=Rodríguez |first3=Fernando |date=2003 |title=Evolution of Forebrain and Spatial Cognition in Vertebrates: Conservation across Diversity |url=https://www.karger.com/Article/FullText/72438 |journal=Brain, Behavior and Evolution |language=english |volume=62 |issue=2 |pages=72–82 |doi=10.1159/000072438 |issn=0006-8977 |pmid=12937346|s2cid=23055468 |url-access=subscription }}</ref> Navigation in turtles have been correlated to high cognition function in the medial cortex region of the brain.<ref name=":1" /><ref>{{Cite journal |last1=Reiter |first1=Sam |last2=Liaw |first2=Hua-Peng |last3=Yamawaki |first3=Tracy M. |last4=Naumann |first4=Robert K. |last5=Laurent |first5=Gilles |date=2017 |title=On the Value of Reptilian Brains to Map the Evolution of the Hippocampal Formation |url=https://www.karger.com/Article/FullText/478693 |journal=Brain, Behavior and Evolution |language=english |volume=90 |issue=1 |pages=41–52 |doi=10.1159/000478693 |issn=0006-8977 |pmid=28866680|s2cid=13763864 |url-access=subscription }}</ref>
[[File:Crested Caracara eating a turtle (16753759877).jpg|thumb|[[Crested caracara]] eating a turtle|alt=Photo of a large bird eating a turtle]]
===Defense===
{{see also|Anti-predator adaptation}}
When sensing danger, a turtle may flee, freeze or withdraw into its shell. Freshwater turtles flee into the water, though the [[Sonora mud turtle]] may take refuge on land as the shallow temporary ponds they inhabit make them vulnerable.{{sfn|Orenstein|2012|pp=252–253}} When startled, a softshell turtle may dive underwater and bury itself under the sea floor.{{sfn|Franklin|2011|p=40}} If a predator persists, the turtle may bite or discharge from its cloaca. Several species produce foul-smelling chemicals from musk glands. Other tactics include threat displays and [[Bell's hinge-back tortoise]] can [[Apparent death|play dead]]. When attacked, big-headed turtle hatchlings squeal, possibly startling the predator.{{sfn|Orenstein|2012|pp=252–253, 301}}

=== Migration ===
{{further|Sea turtle migration}}
[[File:Turtle golfina escobilla Oaxaca Mexico Claudio Giovenzana 2010.jpg|thumb|right|An olive ridley sea turtle nesting on Escobilla Beach, [[Oaxaca]], [[Mexico]]. Female sea turtles migrate long distances to nest on favored beaches.]]
Turtles are the only reptiles that migrate long distances, more specifically the marine species that can travel up to thousands of kilometers. Some non-marine turtles, such as the species of ''[[Geochelone]]'' (terrestrial), ''[[Chelydra]]'' (freshwater), and ''[[Malaclemys]]'' (estuarine), migrate seasonally over much shorter distances, up to around {{convert|27|km|mi|abbr=on}}, to lay eggs. Such short migrations are comparable to those of some lizards, snakes, and crocodilians.<ref name="Southwood Avens 2009">{{cite journal |last1=Southwood |first1=Amanda |last2=Avens |first2=Larisa |title=Physiological, Behavioral, and Ecological Aspects of Migration in Reptiles |journal=Journal of Comparative Physiology B |volume=180 |issue=1 |year=2009 |pages=1–23 |issn=0174-1578 |doi=10.1007/s00360-009-0415-8 |pmid=19847440 |s2cid=20245401 |url=https://www.researchgate.net/publication/38028266 |access-date=June 6, 2021 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105250/https://www.researchgate.net/publication/38028266_Physiological_behavioral_and_ecological_aspects_of_migration_in_reptiles |url-status=live }}</ref> Sea turtles nest in a specific area, such as a beach, leaving the eggs to hatch unattended. The young turtles leave that area, migrating long distances in the years or decades in which they grow to maturity, and then return seemingly to the same area every few years to mate and lay eggs, though the precision varies between species and populations. This "natal homing" has appeared remarkable to biologists, though there is now plentiful evidence for it, including from genetics.<ref name="Lohmann et al 2013">{{cite book |last1=Lohmann |first1=Kenneth J. |last2=Lohmann |first2=Catherine M. F. |last3=Brothers |first3=J. Roger |last4=Putman |first4=Nathan F. |chapter=Natal Homing and Imprinting in Sea Turtles |editor-last=Wyneken |editor-first=Jeanette |title=The Biology of Sea Turtles |publisher=CRC Press |year=2013 |isbn=978-1-4398-7308-3 |oclc=828509848 |pages=59–78}}</ref>

How sea turtles navigate to their breeding beaches remains unknown. One possibility is [[Imprinting (psychology)|imprinting]] as in [[salmon]], where the young learn the chemical signature, effectively the scent, of their home waters before leaving, and remember that when the time comes for them to return as adults. Another possible cue is the orientation of the [[Earth's magnetic field]] at the natal beach. There is experimental evidence that turtles have an effective magnetic sense, and that they use this in [[Animal navigation|navigation]]. Proof that homing occurs is derived from genetic analysis of populations of loggerheads, hawksbills, leatherbacks, and [[Olive ridley sea turtle|olive ridleys]] by nesting place. For each of these species, the populations in different places have their own [[mitochondrial DNA]] genetic signatures that persist over the years. This shows that the populations are distinct and that homing must be occurring reliably.<ref name="Lohmann et al 2013"/>

== Reproduction and life cycle==
[[File:Tortoise Snap (2 frames).jpg|alt=Frames from a film showing one desert tortoise biting the other desert tortoise|thumb|Desert tortoises fighting]]
Turtles have a wide variety of mating behaviors but do not form [[Pair bond|pair-bonds]] or social groups.<ref name="Pearse 2001">{{cite journal |last=Pearse |first=Devon E.<!--Brigham Young--> |title=Turtle Mating Systems: Behavior, Sperm Storage, and Genetic Paternity |journal=Journal of Heredity |volume=92 |issue=2 |pages=206–211 |year=2001 |doi=10.1093/jhered/92.2.206 |pmid=11396580 |doi-access=free}}</ref> In terrestrial species, males are often larger than females and fighting between males establishes a [[dominance hierarchy]] for access to mates. For most semi-aquatic and bottom-walking aquatic species, combat occurs less often. Males of these species instead may use their size advantage to [[Sexual coercion among animals|mate forcibly]]. In fully aquatic species, males are often smaller than females and rely on [[courtship display]]s to gain mating access to females.<ref name="Berry 1980">{{cite journal |doi=10.1007/bf00572678 |pmid=28310555 |title=Sexual Size Dimorphism and Sexual Selection in Turtles (order Testudines) |journal=Oecologia |volume=44 |issue=2 |pages=185–191 |year=1980 |last1=Berry |first1=James F. |last2=Shine |first2=Richard |bibcode=1980Oecol..44..185B |s2cid=2456783}}</ref> In [[green sea turtle]]s, females generally outnumber males.<ref name="Booth 1972">{{cite journal |doi=10.1016/S0003-3472(72)80155-6 |title=Behavioural Studies on the Green Turtle (''Chelonia mydas'') in the Sea |journal=Animal Behaviour |volume=20 |issue=4 |pages=808–812 |year=1972 |last1=Booth |first1=Julie |last2=Peters |first2=James A. |bibcode=1972AnBeh..20N9811B }}</ref>

=== Courtship and mounting===

<!--need to lead in here, section is not about one species-->Courtship varies between species, and with habitat. It is often complex in aquatic species, both marine and freshwater, but simpler in the semi-aquatic mud turtles and snapping turtles. A male tortoise bobs his head, then subdues the female by biting and butting her before mounting.<ref name=Firefly/> The male [[scorpion mud turtle]] approaches the female from the rear, and often resorts to aggressive methods such as biting the female's tail or hind limbs, followed by a mounting.<ref name="Berry Iverson 2011">{{Cite journal |last1=Berry |first1=James |last2=Iverson |first2=John |date=December 2011 |title=''Kinosternon scorpioides'' (Linnaeus 1766) – Scorpion Mud Turtle |journal=Conservation Biology of Freshwater Turtles and Tortoises |pages=063.1–063.15 |doi=10.3854/crm.5.063.scorpioides.v1.2011 |url=http://www.iucn-tftsg.org/wp-content/uploads/file/Accounts/crm_5_063_scorpioides_v1_2011.pdf |access-date=August 20, 2019 |isbn=978-0-9653540-9-7 |doi-access=free |archive-date=February 4, 2019 |archive-url=https://web.archive.org/web/20190204032917/http://www.iucn-tftsg.org/wp-content/uploads/file/Accounts/crm_5_063_scorpioides_v1_2011.pdf |url-status=live }}</ref>

Female choice is important in some species, and female green sea turtles are not always receptive. As such, they have evolved behaviors to avoid the male's attempts at copulation, such as swimming away, confronting the male followed by biting or taking up a refusal position with her body vertical, her limbs widely outspread, and her plastron facing the male. If the water is too shallow for the refusal position, the females resort to beaching themselves, as the males do not follow them ashore.<ref name="Booth 1972"/>

[[File:Three-toed Box Turtle (47930139991).jpg|thumb|Mounting behavior in the [[three-toed box turtle]] |alt=Photograph of a male turtle mounting a female]]

All turtles fertilize internally; mounting and copulation can be difficult. In many species, males have a concave plastron that interlocks with the female's carapace. In species like the [[Russian tortoise]], the male has a lighter shell and longer legs. The high, rounded shape of box turtles are particular obstacles for mounting. The male [[eastern box turtle]] leans backward and hooks onto the back of the female's plastron.{{sfn|Orenstein|2012|pp=270–271}} Aquatic turtles mount in water,{{sfn|Orenstein|2012|p=270}}{{sfn|Franklin|2011|p=33}} and female sea turtles support the mounting male while swimming and diving.<ref>{{cite book |last1=Ripple |first1=J. |year=1996 |title=Sea Turtles |publisher=Voyageur Press |page=26 |isbn=978-0-89658-315-3}}</ref> During copulation, the male turtle aligns his tail with the female's so he can insert his penis into her cloaca.{{sfn|Franklin|2011|pp=20–22}} Some female turtles can [[Female sperm storage|store sperm]] from multiple males and their [[Clutch (eggs)|egg clutches]] can have multiple sires.{{sfn|Orenstein|2012|p=260}}<ref name="Pearse 2001"/>

=== Eggs and hatchlings===
[[File:Snapping turtle eggs md.jpg|thumb|A female [[common snapping turtle]] with her eggs in a hole she dug|alt=Land turtle with eggs in a hole|left]]

Turtles, including sea turtles, lay their eggs on land, although some lay eggs near water that rises and falls in level, submerging the eggs. While most species build nests and lay eggs where they forage, some travel miles. The [[common snapping turtle]] walks {{convert|5|km|mi|0|abbr=on}} on land, while sea turtles travel even further; the leatherback swims some {{convert|12000|km|mi|abbr=on}} to its nesting beaches.<ref name=Firefly/>{{sfn|Franklin|2011|p=33}} Most turtles create a nest for their eggs. Females usually dig a flask-like chamber in the substrate. Other species lay their eggs in vegetation or crevices.{{sfn|Orenstein|2012|p=277}} Females choose nesting locations based on environmental factors such as temperature and humidity, which are important for developing embryos.{{sfn|Franklin|2011|p=33}} Depending on the species, the number of eggs laid varies from one to over 100. Larger females can lay eggs that are greater in number or bigger in size. Compared to freshwater turtles, tortoises deposit fewer but larger eggs. Females can lay multiple clutches throughout a season, particularly in species that experience unpredictable [[monsoon]]s.{{sfn|Orenstein|2012|pp=273, 276}}
[[File:Tortoise-Hatchling.jpg|thumb|[[Marginated tortoise]] emerges from its egg|alt=Tortoise hatching from egg]]
Most mother turtles do no more in the way of parental care than covering their eggs and immediately leaving, though some species guard their nests for days or weeks.{{sfn|Franklin|2011|p=37}} Eggs vary between rounded, oval, elongated, and between hard- and soft-shelled.{{sfn|Orenstein|2012|p=274}} Most species have their [[temperature-dependent sex determination|sex determined by temperature]]. In some species, higher temperatures produce females and lower ones produce males, while in others, milder temperatures produce males and both hot and cold extremes produce females.<ref name=Firefly/> There is experimental evidence that the embryos of ''[[Mauremys reevesii]]'' can move around inside their eggs to select the best temperature for development, thus influencing their sexual destiny.<ref name="YeMa2019">{{cite journal |last1=Ye |first1=Yin-Zi |last2=Ma |first2=Liang |last3=Sun |first3=Bao-Jun |last4=Li |first4=Teng |last5=Wang |first5=Yang |last6=Shine |first6=Richard |last7=Du |first7=Wei-Guo |title=The Embryos of Turtles can Influence their own Sexual Destinies |journal=Current Biology |volume=29 |issue=16 |year=2019 |pages=2597–2603.e4 |issn=0960-9822 |doi=10.1016/j.cub.2019.06.038 |pmid=31378606 |doi-access=free |bibcode=2019CBio...29E2597Y }}</ref> In other species, sex is [[Sex-determination system#Chromosomal systems|determined genetically]]. The length of incubation for turtle eggs varies from two to three months for temperate species, and four months to over a year for tropical species.<ref name=Firefly/> Species that live in warm temperate climates can [[embryonic diapause|delay their development]].{{sfn|Orenstein|2012|p=286}}

Hatching young turtles break out of the shell using an [[egg tooth]], a sharp projection that exists temporarily on their upper beak.<ref name=Firefly/>{{sfn|Orenstein|2012|p=294}} Hatchlings dig themselves out of the nest and find safety in vegetation or water. Some species stay in the nest for longer, be it for overwintering or to wait for the rain to loosen the soil for them to dig out.<ref name=Firefly/> Young turtles are highly vulnerable to predators, both in the egg and as hatchlings. Mortality is high during this period but significantly decreases when they reach adulthood. Most species grow quickly during their early years and slow down when they mature.{{sfn|Orenstein|2012|pp=301–302}}

===Lifespan===
Turtles can live long lives. The oldest living turtle and land animal is said to be a [[Seychelles giant tortoise]] named [[Jonathan (tortoise)|Jonathan]], who turned 187 in 2019.<ref>{{cite web |url=https://www.guinnessworldrecords.com/news/2019/2/introducing-jonathan-the-worlds-oldest-animal-on-land-561882 |title=Introducing Jonathan, the World's Oldest Animal on Land at 187 Years Old |last=Millward |first=Adam |publisher=[[Guinness World Records]] |date=February 27, 2019 |access-date=July 14, 2020 |archive-date=August 9, 2020 |archive-url=https://web.archive.org/web/20200809153110/https://www.guinnessworldrecords.com/news/2019/2/introducing-jonathan-the-worlds-oldest-animal-on-land-561882 |url-status=live }}</ref> A [[Galápagos tortoise]] named [[Harriet (tortoise)|Harriet]] was collected by [[Charles Darwin]] in 1835; it died in 2006, having lived for at least 176 years. Most wild turtles do not reach that age. Turtles keep growing new scutes under the previous scutes every year, allowing researchers to estimate how long they have lived.{{sfn|Franklin|2011|pp=44–45}} They also [[negligible senescence|age slowly]].<ref name="oldage">{{cite journal |last1=Warner |first1=Daniel A. |last2=Miller |first2=David A. W. |last3=Bronikowski |first3=Anne M. |last4=Janzen |first4=Fredric J. |year=2016 |title=Decades of Field Data Reveal that Turtles Senesce in the Wild |journal=PNAS |volume=113 |issue=23 |pages=6502–6507 |doi=10.1073/pnas.1600035113 |pmid=27140634 |pmc=4988574 |bibcode=2016PNAS..113.6502W |doi-access=free }}</ref> The survival rate for adult turtles can reach 99% per year.<ref name=Firefly/>

==Systematics and evolution==
{{further |Turtle classification |List of Testudines families}}

=== Fossil history===
[[File:Origin of Turtle Body Plan.jpg|thumb|upright=1.2|Diagram of the origins of the turtle body plan through the [[Triassic]]: isolated bony plates evolved to [[Turtle shell|form a complete shell]], in a sequence involving ''[[Pappochelys]]'', ''[[Eorhynchochelys]]'', ''[[Odontochelys]]'', and ''[[Proganochelys]]''.<ref name="Schoch Sues 2019"/>|alt=Diagram of evolution of turtle shells showing four fossil species]]

Zoologists have sought to explain the evolutionary origin of the turtles, and in particular of their unique shells. In 1914, [[Jan Versluys]] proposed that bony plates in the dermis, called [[osteoderm]]s, fused to the ribs beneath them, later called the "Polka Dot Ancestor" by Olivier Rieppel.<ref name="Schoch Sues 2019"/><ref>{{Cite book |title=Turtles as Hopeful Monsters: Origins and Evolution<!--what a title, but Rieppel is ok actually--> |last=Rieppel |first=Olivier |isbn=978-0-253-02507-4 |publisher=Indiana University Press |page=195 |oclc=962141060 |year=2017}}</ref> The theory accounted for the evolution of fossil [[pareiasaur]]s from ''[[Bradysaurus]]'' to ''[[Anthodon (reptile)|Anthodon]]'', but not for how the ribs could have become attached to the bony dermal plates.<ref name="Schoch Sues 2019">{{cite journal |last1=Schoch |first1=Rainer R. |last2=Sues |first2=Hans-Dieter |last3=Benson |first3=Roger |title=The Origin of the Turtle Body Plan: Evidence from Fossils and Embryos |journal=Palaeontology |volume=63 |issue=3 |year=2019 |pages=375–393 |issn=0031-0239 |doi=10.1111/pala.12460 |doi-access=free }}</ref>

More recent discoveries have painted a different scenario for the evolution of the turtle's shell. The [[Crown group#Stem groups|stem]]-turtles ''[[Eunotosaurus]]'' of the Middle [[Permian]], ''[[Pappochelys]]'' of the [[Middle Triassic]], and ''[[Eorhynchochelys]]'' of the [[Late Triassic]] lacked carapaces and plastrons but had shortened torsos, expanded ribs, and lengthened dorsal vertebrae. Also in the Late Triassic, ''[[Odontochelys]]'' had a partial shell consisting of a complete bony plastron and an incomplete carapace. The development of a shell reached completion with the Late Triassic ''[[Proganochelys]]'', with its fully developed carapace and plastron.<ref name="Schoch Sues 2019"/><ref name="Lyson2020"/> Adaptations that led to the evolution of the shell may have originally been for digging and a [[fossorial]] lifestyle.<ref name="Lyson2020">{{cite journal |last1=Lyson |first1=Tyler R. |last2=Bever |first2=Gabriel S. |year=2020 |title=Origin and Evolution of the Turtle Body Plan |journal=Annual Review of Ecology, Evolution, and Systematics |volume=51 |issue=1 |pages=143–166 |doi=10.1146/annurev-ecolsys-110218-024746 |s2cid=225486775|doi-access=free }}</ref>

The oldest known members of the Pleurodira lineage are the [[Platychelyidae]], from the [[Late Jurassic]].<ref name=":02">{{Cite journal|last1=Cadena|first1=Edwin|last2=Joyce|first2=Walter G.|date=April 2015|title=A Review of the Fossil Record of Turtles of the Clades Platychelyidae and Dortokidae|url=http://www.bioone.org/doi/10.3374/014.056.0101|journal=Bulletin of the Peabody Museum of Natural History|language=en|volume=56|issue=1|pages=3–20|doi=10.3374/014.056.0101|bibcode=2015BPMNH..56....3C |s2cid=56195415|issn=0079-032X|url-access=subscription}}</ref> The oldest known unambiguous cryptodire is ''[[Sinaspideretes]],'' a close relative of softshell turtles, from the Late Jurassic of China.<ref>{{Cite journal|last1=Evers|first1=Serjoscha W.|last2=Benson|first2=Roger B. J.|date=January 2019|editor-last=Smith|editor-first=Andrew|title=A new phylogenetic hypothesis of turtles with implications for the timing and number of evolutionary transitions to marine lifestyles in the group|url=https://onlinelibrary.wiley.com/doi/10.1111/pala.12384|journal=Palaeontology|language=en|volume=62|issue=1|pages=93–134|doi=10.1111/pala.12384|bibcode=2019Palgy..62...93E |s2cid=134736808|url-access=subscription}}</ref> Turtles became highly diverse during the Cretaceous, as climatic conditions in this period were favourable for their global dispersal.<ref>{{Cite journal |last1=Nicholson |first1=David B. |last2=Holroyd |first2=Patricia A. |last3=Benson |first3=Roger B. J. |last4=Barrett |first4=Paul M. |date=3 August 2015 |title=Climate-mediated diversification of turtles in the Cretaceous |journal=[[Nature Communications]] |language=en |volume=6 |issue=1 |page=7848 |doi=10.1038/ncomms8848 |pmid=26234913 |pmc=4532850 |bibcode=2015NatCo...6.7848N |issn=2041-1723 }}</ref> During the [[Late Cretaceous]] and [[Cenozoic]], members of the pleurodire families [[Bothremydidae]] and [[Podocnemididae]] became widely distributed in the Northern Hemisphere due to their coastal habits.<ref>{{Cite journal |last=Pérez-García |first=Adán |date=September 2, 2017 |title=A new Turtle Taxon (Podocnemidoidea, Bothremydidae) Reveals the Oldest Known Dispersal Event of the Crown Pleurodira from Gondwana to Laurasia |url=https://www.tandfonline.com/doi/full/10.1080/14772019.2016.1228549 |journal=Journal of Systematic Palaeontology |volume=15 |issue=9 |pages=709–731 |doi=10.1080/14772019.2016.1228549 |bibcode=2017JSPal..15..709P |s2cid=88840423 |issn=1477-2019|url-access=subscription }}</ref><ref>{{Cite journal |last1=Ferreira |first1=Gabriel S. |last2=Bandyopadhyay |first2=Saswati |last3=Joyce |first3=Walter G. |date=November 15, 2018 |title=A Taxonomic Reassessment of Piramys auffenbergi, a Neglected Turtle from the late Miocene of Piram Island, Gujarat, India |journal=PeerJ |language=en |volume=6 |article-number=e5938 |doi=10.7717/peerj.5938 |pmid=30479901 |pmc=6240434|issn=2167-8359 |doi-access=free }}</ref> The oldest known soft-shelled turtles and sea turtles appeared during the [[Early Cretaceous]].<ref>{{cite book |last1=Hirayama |first1=Ren |chapter=Kappachelys okurai gen. et sp. nov., a New Stem Soft-Shelled Turtle from the Early Cretaceous of Japan |date=2013 |chapter-url=http://link.springer.com/10.1007/978-94-007-4309-0_12 |title=Morphology and Evolution of Turtles |pages=179–185 |editor-last=Brinkman |editor-first=Donald B. |location=Dordrecht |publisher=Springer |doi=10.1007/978-94-007-4309-0_12 |isbn=978-94-007-4308-3 |last2=Isaji |first2=Shinji |last3=Hibino |first3=Tsuyoshi |series=Vertebrate Paleobiology and Paleoanthropology |editor2-last=Holroyd |editor2-first=Patricia A. |editor3-last=Gardner |editor3-first=James D.}}</ref><ref>{{Cite journal |last1=Evers |first1=Serjoscha W. |last2=Barrett |first2=Paul M. |last3=Benson |first3=Roger B. J. |date=May 2019 |title=Anatomy of Rhinochelys pulchriceps (Protostegidae) and Marine Adaptation During the Early Evolution of chelonioids |journal=PeerJ |volume=7 |article-number=e6811 |doi=10.7717/peerj.6811 |pmid=31106054 |pmc=6500378 |issn=2167-8359 |doi-access=free }}</ref> Tortoises originated in Asia during the [[Eocene]].<ref>{{Cite journal |last1=Hofmeyr |first1=Margaretha D. |last2=Vamberger |first2=Melita |last3=Branch |first3=William |last4=Schleicher |first4=Alfred |last5=Daniels |first5=Savel R. |date=July 2017 |title=Tortoise (Reptilia, Testudinidae) Radiations in Southern Africa from the Eocene to the Present |url=http://doi.wiley.com/10.1111/zsc.12223|journal=Zoologica Scripta |volume=46 |issue=4 |pages=389–400 |doi=10.1111/zsc.12223 |s2cid=88712318|url-access=subscription }}</ref> A late surviving group of stem-turtles, the [[Meiolaniidae]], survived in Australasia into the [[Pleistocene]] and [[Holocene]].<ref>{{Cite journal |last1=Poropat |first1=Stephen F. |last2=Kool |first2=Lesley |last3=Vickers-Rich |first3=Patricia |last4=Rich |first4=Thomas H. |date=April 3, 2017 |title=Oldest Meiolaniid Turtle Remains from Australia: Evidence from the Eocene Kerosene Creek Member of the Rundle Formation, Queensland |url=https://www.tandfonline.com/doi/full/10.1080/03115518.2016.1224441 |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=41 |issue=2 |pages=231–239 |doi=10.1080/03115518.2016.1224441 |bibcode=2017Alch...41..231P |s2cid=131795055 |issn=0311-5518|url-access=subscription }}</ref>

=== External relationships===
<!--[[File:Haeckel Chelonia.jpg|thumb|upright|"Chelonia" from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'', 1904 |alt=Historic painting of examples of different families of turtles ]]-->

The turtles' exact ancestry has been disputed. It was believed they were the only surviving branch of the ancient [[evolutionary grade]] [[Anapsid]]a, which includes groups such as [[Procolophonidae|procolophonid]]s and pareiasaurs. All anapsid skulls lack a [[Temporal fenestra|temporal opening]] while all other living amniotes have temporal openings.<ref name="Rieppel">{{cite journal |last1=Rieppel |first1=Olivier |year=1996 |last2=DeBraga |first2=M. |title=Turtles as Diapsid Reptiles |journal=Nature |volume=384 |issue=6608 |pages=453–455 |doi=10.1038/384453a0 |bibcode=1996Natur.384..453R |s2cid=4264378 |url=http://doc.rero.ch/record/16242/files/PAL_E3477.pdf }}</ref> It was later suggested that the anapsid-like turtle skulls may be due to [[backward evolution]] rather than to anapsid descent.<ref name="Zardoya">{{Cite journal |last1=Zardoya |first1=Rafael |last2=Meyer |first2=Axel |title=Complete Mitochondrial Genome Suggests Diapsid Affinities of Turtles |year=1998 |journal=PNAS |volume=95 |issue=24 |pages=14226–14231 |issn=0027-8424 |doi=10.1073/pnas.95.24.14226 |pmid=9826682 |pmc=24355 |bibcode=1998PNAS...9514226Z |url=https://kops.uni-konstanz.de/bitstream/123456789/6719/1/Complete_mitochondrial_genome_1998.pdf |access-date=October 31, 2018 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105253/https://kops.uni-konstanz.de/bitstream/handle/123456789/6719/Complete_mitochondrial_genome_1998.pdf;jsessionid=CDA4382393A7DA330520DCB828E8B86D?sequence=1 |url-status=live|doi-access=free }}</ref> Fossil evidence has shown that early stem-turtles possessed small temporal openings.<ref name="Lyson2020"/>

Some early morphological [[phylogenetics|phylogenetic]] studies have placed turtles closer to [[Lepidosauria]] ([[tuatara]]s, [[lizard]]s, and [[snake]]s) than to [[Archosaur]]ia ([[crocodilian]]s and birds).<ref name="Rieppel"/> By contrast, several [[Molecular phylogenetics|molecular]] studies place turtles either within Archosauria,<ref name="Mannen">{{Cite journal |last1=Mannen |first1=Hideyuki |last2=Li |first2=Steven S.-L. |title=Molecular Evidence for a Clade of Turtles |date=October 1999 |journal=Molecular Phylogenetics and Evolution |volume=13 |issue=1 |pages=144–148 |doi=10.1006/mpev.1999.0640 |pmid=10508547 |bibcode=1999MolPE..13..144M }}</ref> or, more commonly, as a [[sister group]] to extant archosaurs,<ref name="Zardoya"/><ref name="Iwabe">{{Cite journal |last1=Iwabe |first1=Naoyuki |last2=Hara |first2=Yuichiro |last3=Kumazawa |first3=Yoshinori |last4=Shibamoto |first4=Kaori |last5=Saito |first5=Yumi |last6=Miyata |first6=Takashi |last7=Katoh |first7=Kazutaka |title=Sister Group Relationship of Turtles to the Bird-Crocodilian Clade Revealed by Nuclear DNA-coded Proteins |date=December 2004 |journal=Molecular Biology and Evolution |volume=22 |issue=4 |pages=810–813 |doi=10.1093/molbev/msi075 |pmid=15625185  |doi-access=free}}</ref><ref name="Roos">{{Cite journal |last1=Roos |first1=Jonas |last2=Aggarwal |first2=Ramesh K. |last3=Janke |first3=Axel |title=Extended Mitogenomic Phylogenetic Analyses Yield new Insight into Crocodylian Evolution and Their Survival of the Cretaceous–Tertiary Boundary |journal=[[Molecular Phylogenetics and Evolution]] |volume=45 |issue=2 |pages=663–673 |date=November 2007  |doi=10.1016/j.ympev.2007.06.018 |pmid=17719245 |bibcode=2007MolPE..45..663R }}</ref><ref name="Katsu">{{Cite journal |last1=Katsu |first1=Yoshinao |date=March 2010 |last2=Braun |first2=Edward L. |last3=Guillette |first3=Louis J. Jr. |last4=Iguchi |first4=Taisen |title=From Reptilian Phylogenomics to Reptilian Genomes: Analyses of c-Jun and DJ-1 Proto-oncogenes |journal=Cytogenetic and Genome Research |volume=127 |issue=2–4 |pages=79–93 |doi=10.1159/000297715 |pmid=20234127 |s2cid=12116018}}</ref> though an analysis conducted by Tyler Lyson and colleagues (2012) recovered turtles as the sister group of lepidosaurs instead.<ref>{{Cite journal |last1=Lyson |first1=Tyler R. |last2=Sperling |first2=Erik A. |last3=Heimberg |first3=Alysha M. |last4=Gauthier |first4=Jacques A. |last5=King |first5=Benjamin L. |last6=Peterson |first6=Kevin J. |title=MicroRNAs Support a Turtle + Lizard Clade |year=2012 |journal=[[Biology Letters]] |volume=8 |issue=1 |pages=104–107 |doi=10.1098/rsbl.2011.0477 |pmid=21775315 |pmc=3259949 |bibcode=2012BiLet...8..104L }}</ref> Ylenia Chiari and colleagues (2012) analyzed 248 [[nuclear gene]]s from 16 vertebrates and suggested that turtles share a [[most recent common ancestor|more recent common ancestor]] with birds and crocodilians. The date of separation of turtles and birds and crocodilians was estimated to be {{Ma|255}} during the Permian.<ref name="Chiari2012">{{Cite journal |last1=Chiari |first1=Ylenia |year=2012 |last2=Cahais |first2=Vincent |last3=Galtier |first3=Nicolas |last4=Delsuc |first4= Frédéric |title=Phylogenomic Analyses Support the Position of Turtles as the Sister Group of Birds and Crocodiles (Archosauria) |journal=BMC Biology |volume=10 |issue=65 |page=65 |doi=10.1186/1741-7007-10-65 |pmid=22839781 |pmc=3473239 |doi-access=free }}</ref> Through [[genomic]]-scale phylogenetic study of [[ultra-conserved element]]s (UCEs) to clarify the placement of turtles within reptiles, Nicholas Crawford and colleagues (2012) similarly found that turtles are closer to birds and crocodilians.<ref name="Crawford 2012">{{Cite journal |year=2012 |last1=Crawford |first1=Nicholas G. |last2=Faircloth |first2=Brant C. |last3=McCormack |first3=John E. |last4=Brumfield |first4=Robb T. |last5=Winker |first5=Kevin |last6=Glen |first6=Travis C. |title=More than 1000 Ultraconserved Elements Provide Evidence that Turtles are the Sister Group to Archosaurs |journal=Biology Letters |volume=8 |issue=5 |pages=783–786 |url=http://faculty.oxy.edu/mccormack/McCormack/picks/Crawford_2012_BioLett.pdf |access-date=September 21, 2014 |doi=10.1098/rsbl.2012.0331 |pmid=22593086 |pmc=3440978 |archive-date=August 10, 2012 |archive-url=https://web.archive.org/web/20120810100341/http://faculty.oxy.edu/mccormack/McCormack/picks/Crawford_2012_BioLett.pdf |url-status=live }}</ref>

Using the draft (unfinished) genome sequences of the green sea turtle and the [[Chinese softshell turtle]], Zhuo Wang and colleagues (2013) concluded that turtles are likely a sister group of crocodilians and birds.<ref name="Wang 2013">{{cite journal |doi=10.1038/ng.2615 |pmid=23624526 |pmc=4000948 |title=The Draft Genomes of Soft-shell Turtle and Green Sea Turtle Yield Insights into the Development and Evolution of the Turtle-specific Body Plan |journal=Nature Genetics |volume=45 |issue=6 |pages=701–706 |year=2013 |last1=Wang |first1=Zhuo |last2=Pascual-Anaya |first2=Juan |last3=Zadissa |first3=Amonida |last4=Li |first4=Wenqi |last5=Niimura |first5=Yoshihito |last6=Huang |first6=Zhiyong |last7=Li |first7=Chunyi |last8=White |first8=Simon |last9=Xiong |first9=Zhiqiang |last10=Fang |first10=Dongming |last11=Wang |first11=Bo |last12=Ming |first12=Yao |last13=Chen |first13=Yan |last14=Zheng |first14=Yuan |last15=Kuraku |first15=Shigehiro |last16=Pignatelli |first16=Miguel |last17=Herrero |first17=Javier |last18=Beal |first18=Kathryn |last19=Nozawa |first19=Masafumi |last20=Li |first20=Qiye |last21=Wang |first21=Juan |last22=Zhang |first22=Hongyan |last23=Yu |first23=Lili |last24=Shigenobu |first24=Shuji |last25=Wang |first25=Junyi |last26=Liu |first26=Jiannan |last27=Flicek |first27=Paul |last28=Searle |first28=Steve |last29=Wang |first29=Jun |last30=Kuratani |first30=Shigeru |display-authors=3}}</ref> The external phylogeny of the turtles is shown in the [[cladogram]] below.<ref name="Crawford 2012"/>

{{clade
|label1=[[Diapsida]]
|1={{clade
   |label1=[[Lepidosauromorpha]]
   |1=[[Squamata]] (lizards, snakes) [[File:Zoology of Egypt (1898) (Varanus exanthematicus).png|70px]]
   |label2=[[Archosauromorpha]]
   |2={{clade
      |1='''Testudines''' [[File:Psammobates geometricus 1872 white background.jpg|60px]]
      |2={{clade
         |1=[[Crocodilia]] (crocodiles, alligators) <span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|100px]]</span>
         |2=[[Aves]] (birds) [[File:Spot-billed pelican takeoff white background.jpg|80px]]
         }}
      }}
   }}
}}

===Internal relationships===
Modern turtles and their extinct relatives with a complete shell are classified within the clade [[Testudinata]].<ref>{{Cite journal |last1=Joyce |first1=Walter G. |last2=Anquetin |first2=Jérémy |last3=Cadena |first3=Edwin-Alberto |last4=Claude |first4=Julien |last5=Danilov |first5=Igor G. |last6=Evers |first6=Serjoscha W. |last7=Ferreira |first7=Gabriel S. |last8=Gentry |first8=Andrew D. |last9=Georgalis |first9=Georgios L. |last10=Lyson |first10=Tyler R. |last11=Pérez-García |first11=Adán |date=December 2021 |title=A Nomenclature for Fossil and Living Turtles Using Phylogenetically Defined Clade Names |journal=[[Swiss Journal of Palaeontology]] |language=en |volume=140 |issue=1 |page=5 |doi=10.1186/s13358-020-00211-x |s2cid=229506832 |issn=1664-2376 |doi-access=free|bibcode=2021SwJP..140....5J |hdl=11336/155192 |hdl-access=free }}</ref> The most recent common ancestor of living turtles, corresponding to the split between Pleurodira (side-necked species) and [[Cryptodira]] (hidden necked species), is estimated to have occurred around {{Ma|210}} during the Late Triassic.<ref>{{cite journal |last1=Böhmer |first1=Christine |last2=Werneburg |first2=Ingmar |year=2017 |title=Deep Time Perspective on Turtle Neck Evolution: Chasing the ''Hox'' Code by Vertebral Morphology |journal=Scientific Reports |volume=7 |issue=1 |page=8939 |doi=10.1038/s41598-017-09133-0|pmid=28827543 |pmc=5566328 |bibcode=2017NatSR...7.8939B }}</ref> Robert Thompson and colleagues (2021) comment that living turtles have low diversity, relative to how long they existed. Diversity has been stable, according to their analysis, except for a single rapid increase around the [[Eocene-Oligocene boundary]] some 30 million years ago, and a large regional extinction at roughly the same time. They suggest that global climate change caused both events, as the cooling and drying caused the land to become arid and turtles to become extinct there, while new continental margins opened up by the climate change provided habitats for other species to evolve.<ref name="Thomson Spinks Shaffer 2021">{{cite journal |last1=Thomson |first1=Robert C. |last2=Spinks |first2=Phillip Q. |last3=Shaffer |first3=H. Bradley |title=A Global Phylogeny of Turtles Reveals a Burst of Climate-associated Diversification on Continental Margins |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=7 |date=February 8, 2021 |issn=0027-8424 |doi=10.1073/pnas.2012215118 |article-number=e2012215118 |pmid=33558231 |pmc=7896334 |bibcode=2021PNAS..11812215T |doi-access=free }}</ref>

The cladogram, from Nicholas<!--2nd use--> Crawford and colleagues 2015, shows the internal phylogeny of the Testudines down to the level of [[Family (biology)|families]].<ref name="Crawford Parham 2015">{{cite journal |last1=Crawford |first1=Nicholas G. |last2=Parham |first2=James F. |last3=Sellas |first3=Anna B. |last4=Faircloth |first4=Brant C. |last5=Glenn |first5=Travis C. |last6=Papenfuss |first6=Theodore J. |last7=Henderson |first7=James B. |last8=Hansen |first8=Madison H. |last9=Simison |first9=W. Brian |title=A Phylogenomic Analysis of Turtles |journal=Molecular Phylogenetics and Evolution |volume=83 |year=2015 |pages=250–257 |issn=1055-7903 |doi=10.1016/j.ympev.2014.10.021|pmid=25450099 |bibcode=2015MolPE..83..250C }}</ref><ref name="Knauss Joyce Lyson Pearson pp. 125–142"><!--for Chelydroidea-->{{cite journal |last1=Knauss |first1=Georgia E. |last2=Joyce |first2=Walter G. |last3=Lyson |first3=Tyler R. |last4=Pearson |first4=Dean |title=A New Kinosternoid from the Late Cretaceous Hell Creek Formation of North Dakota and Montana and the Origin of the ''Dermatemys mawii'' Lineage |journal=Paläontologische Zeitschrift |publisher=Springer Science and Business Media LLC |volume=85 |issue=2 |date=September 21, 2010 |issn=0031-0220 |doi=10.1007/s12542-010-0081-x |pages=125–142|s2cid=129123961 }}</ref> The analysis by <!--Robert--> Thompson and colleagues in 2021 supports the same structure down to the family level.<ref name="Thomson Spinks Shaffer 2021"/>

{{clade
|label1='''Testudines'''
|1={{clade
   |label1=[[Pleurodira]]
   |sublabel1='' (Side-necked turtles) ''
   |1={{clade
      |1={{clade <!--extra dummy clade-->
         |label1=[[Chelidae]]
         |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Chelus fimbriata).jpg|70px]]
         }}
      |2={{clade
         |label1=[[Pelomedusidae]]
         |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Pelomedusa subrufa).jpg|70px]]
         |label2=[[Podocnemididae]]
         |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Podocnemis expansa).jpg|70px]]
         }}
      }}
   |label2=[[Cryptodira]]
   |sublabel2='' (Hidden-necked turtles) ''
   |2={{clade
      |1={{clade
         |label1=[[Trionychia]]
         |1={{clade
            |label1=[[Carettochelyidae]] 
            |sublabel1='' (Pig-nosed turtle) ''
            |1=[[File:Pig-nosed turtle (Carettochelys insculpta) (cropped).jpg|65px]]
            |label2=[[Trionychidae]]
            |sublabel2='' (Softshell turtles) ''
            |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Lissemys punctata).jpg|65px]]
            }}
         }}
      |2={{clade <!--extra dummy clade-->
         |label1=<!--Durocryptodira--Crawford 2015-->
         |sublabel1='' (Hardshell turtles) ''
         |1={{clade
            |1={{clade
               |label1=[[Testudinoidea]]
               |1={{clade
                  |1={{clade
                     |label1=[[Emydidae]]
                     |sublabel1='' (Terrapins) ''
                     |1=[[File:Emydoidea blandingiiHolbrookV1P03A flipped.jpg|70px]]
                     |label2=[[Platysternidae]] 
                     |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Platysternon megacephalum).jpg|70px]]
                     |sublabel2='' (Big-headed turtle) ''
                     }}
                  |2={{clade
                     |label1=[[Geoemydidae]] 
                     |sublabel1='' (Box and pond turtles) ''
                     |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Morenia ocellata).jpg|70px]]
                     |label2=[[Testudinidae]]
                     |sublabel2='' (Tortoises) ''
                     |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Centrochelys sulcata).jpg|70px]]
                     }}
                  }}
               }}
            |2={{clade
               |label1=[[Chelonioidea]]
               |sublabel1='' (Sea turtles) ''
               |1={{clade
                  |label1=[[Cheloniidae]]
                  |1=&nbsp;[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Chelonia mydas).jpg|90px]]
                  |label2=[[Dermochelyidae]]
                  |sublabel2='' (Leatherback) ''
                  |2=&nbsp;[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Dermochelys coriacea).jpg|90px]]
                  }}
               |label2=[[Chelydroidea]]<!--Crawford 2015-->
               |2={{clade
                  |1={{clade <!--extra dummy clade-->
                     |label1=[[Chelydridae]]
                     |1=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Chelydra serpentina).jpg|70px]]
                     |sublabel1='' (Snapping turtles) ''
                     }}
                  |label2x=[[Kinosternoidea]] <!-- Reptile DB now uses Chelydroidea -->
                  |2={{clade
                     |label1=[[Dermatemydidae]]
                     |sublabel1='' (Hickatee) ''
                     |1=[[File:ChloremysAbnormisFord white background.jpg|70px]]
                     |label2=[[Kinosternidae]]
                     |sublabel2='' (Musk turtles) ''
                     |2=[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Sternotherus odoratus).jpg|70px]]
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}

=== Differences between the two suborders ===

{{Multiple image
|header = Neck retraction
|total_width = 410
|perrow = 2
|image1 = T.h. hermanni con speroni 5.JPG
|caption1 = [[Cryptodira]] retract their necks backward.
|alt1 = Photograph of a cryptodiran with its head pulled back straight into its shell
|image2 = Pelomedusa subrufa (cropped).JPG
|caption2 = [[Pleurodira]] retract their necks sideways.
|alt2 = Photograph of a pleurodiran with its head and neck folded toward the side
|image3 = Turtle neck retraction.svg
|caption3 = The different mechanisms of neck retraction in the two suborders of turtles
|alt3=Diagrams of the top-down bending of the neck of cryptodirans, and the left-right bending of the neck in pleurodirans
}}

Turtles are divided into two living suborders: Cryptodira and Pleurodira.<ref name="Joyce 2021">{{Cite journal |last1=Joyce |first1=Walter G. |last2=Anquetin |first2=Jérémy |last3=Cadena |first3=Edwin-Alberto |last4=Claude |first4=Julien |last5=Danilov |first5=Igor G. |last6=Evers |first6=Serjoscha W. |last7=Ferreira |first7=Gabriel S. |last8=Gentry |first8=Andrew D. |last9=Georgalis |first9=Georgios L. |last10=Lyson |first10=Tyler R. |last11=Pérez-García |first11=Adán |date=February 9, 2021 |title=A Nomenclature for Fossil and Living Turtles Using Phylogenetically Defined Clade Names |journal=[[Swiss Journal of Palaeontology]] |volume=140 |issue=1 |page=5 |doi=10.1186/s13358-020-00211-x |issn=1664-2384 |display-authors=3 |doi-access=free |bibcode=2021SwJP..140....5J |hdl=11336/155192 |hdl-access=free }}</ref> The two groups differ in the way the neck is retracted for protection. Pleurodirans retract their neck to the side and in front of the shoulder girdles, whereas cryptodirans retract their neck backward into their shell. These motions are enabled by the morphology and arrangement of neck vertebrae.<ref name="Werneburg 2015">{{cite journal |last1=Werneburg |first1=Ingmar |last2=Wilson  |first2=Laura A. B. |last3=Parr |first3=William C. H. |last4=Joyce |first4=Walter G. |date=March 1, 2015 |title=Evolution of Neck Vertebral Shape and Neck Retraction at the Transition to Modern Turtles: an Integrated Geometric Morphometric Approach |journal=Systematic Biology |volume=64 |issue=2 |pages=187–204 |doi=10.1093/sysbio/syu072 |pmid=25305281 |issn=1063-5157 |doi-access=free|hdl=1959.4/unsworks_54973 |hdl-access=free }}</ref><ref name="Herrel 2008">{{cite book |last1=Herrel |first1=Anthony |last2=Van Damme |first2=Johan |last3=Aerts |first3=Peter |chapter=Cervical Anatomy and Function in Turtles |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2 |oclc=144570900 |pages=163–186}}</ref> Sea turtles (which belong to Cryptodira) have mostly lost the ability to retract their heads.{{sfn|Orenstein|2012|p=110}}

The adductor muscles in the lower jaw create a pulley-like system in both subgroups. However, the bones that the muscles articulate with differ. In Pleurodira, the pulley is formed with the [[pterygoid bone]]s of the [[palate]], but in Cryptodira the pulley is formed with the otic capsule. Both systems help to vertically redirect the [[Anatomical terms of motion#Abduction and adduction|adductor]] muscles and maintain a powerful bite.<ref name="Ferreira Lautenschlager Evers Pfaff 2020">{{cite journal |last1=Ferreira |first1=Gabriel S. |last2=Lautenschlager |first2=Stephan |last3=Evers |first3=Serjoscha W. |last4=Pfaff |first4=Cathrin |last5=Kriwet |first5=Jürgen |last6=Raselli |first6=Irena |last7=Werneburg |first7=Ingmar |title=Feeding Biomechanics Suggests Progressive Correlation of Skull Architecture and Neck Evolution in Turtles |journal=Scientific Reports |volume=10 |issue=1 |date=March 26, 2020 |issn=2045-2322 |doi=10.1038/s41598-020-62179-5 |pmid=32218478 |pmc=7099039 |bibcode=2020NatSR..10.5505F |display-authors=3 |at=article 5505 |doi-access=free }}</ref>

A further difference between the suborders is the attachment of the pelvis. In Cryptodira, the pelvis is free, linked to the shell only by ligaments. In Pleurodira, the pelvis is [[Suture (anatomy)|sutured]], joined with bony connections, to the carapace and to the plastron, creating a pair of large columns of bone at the back end of the turtle, linking the two parts of the shell.<ref name="Wise Stayton 2017">{{cite journal |last1=Wise |first1=Taylor B. |last2=Stayton |first2=C. Tristan |title=Side-necked Versus Hidden-necked: Comparison of Shell Morphology Between Pleurodiran and Cryptodiran Turtles |journal=Herpetologica |date=2017 |volume=73 |issue=1 |pages=18–29 |doi=10.1655/HERPETOLOGICA-D-15-00038 |jstor=26534349 |s2cid=90226667 }}</ref>

== Distribution and habitat==

Turtles are widely distributed across the world's continents, oceans, and islands with terrestrial, fully aquatic, and semi-aquatic species.<!--, being absent mainly from the polar regions, the northern parts of North America and Eurasia, and the driest regions of the Sahara, Arabia and Australia--> Sea turtles are mainly tropical and subtropical, but leatherbacks can be found in colder areas of the Atlantic and Pacific.<ref name="Pough 2001 pp. 146–151"/> Living Pleurodira all live in freshwater and are found only in the Southern Hemisphere.<ref name="Ferreira Bronzati Langer Sterli 2018">{{cite journal |last1=Ferreira |first1=Gabriel S. |last2=Bronzati |first2=Mario |last3=Langer |first3=Max C. |last4=Sterli |first4=Juliana |title=Phylogeny, Biogeography and Diversification Patterns of Side-necked Turtles (Testudines: Pleurodira) |journal=Royal Society Open Science |volume=5 |issue=3 |year=2018 |issn=2054-5703 |doi=10.1098/rsos.171773 |article-number=171773|pmid=29657780 |pmc=5882704 |bibcode=2018RSOS....571773F }}</ref> The Cryptodira include terrestrial, freshwater, and marine species, and these range more widely.<ref name="Pough 2001 pp. 146–151">{{cite book |last=Pough |first=F. Harvey |title=Encyclopedia of Biodiversity |chapter=Reptiles, Biodiversity of |publisher=Elsevier |year=2001 |doi=10.1016/b0-12-226865-2/00233-9 |pages=146–151|isbn=978-0-12-226865-6 }}</ref> The world regions richest in non-marine turtle species<!--"most speciose"--> are the Amazon basin, the [[Gulf of Mexico]] [[Drainage basin|drainages]] of the United States, and parts of South and Southeast Asia.<ref name="Buhlmann et al 2009">{{cite journal |last1=Buhlmann |first1=Kurt A. |first2=Thomas S. B. |last2=Akre |first3=John B. |last3=Iverson |first4=Deno |last4=Karapatakis |first5=Russell A. |last5=Mittermeier |first6=Arthur |last6=Georges |first7=Anders G. J. |last7=Rhodin |first8=Peter Paul |last8=van Dijk |first9=J. Whitfield |last9=Gibbons |title=A Global Analysis of Tortoise and Freshwater Turtle Distributions with Identification of Priority Conservation Areas |journal=Chelonian Conservation and Biology |volume=8 |issue=2 |year=2009 |pages=116–149 |doi=10.2744/CCB-0774.1 |s2cid=85942804 |display-authors=3 |url=https://iucn-tftsg.org/wp-content/uploads/file/Articles/Buhlmann_etal_2009.pdf |access-date=May 9, 2021 |archive-date=May 9, 2021 |archive-url=https://web.archive.org/web/20210509195346/https://iucn-tftsg.org/wp-content/uploads/file/Articles/Buhlmann_etal_2009.pdf |url-status=live }}</ref>

For turtles in colder climates, their distribution is limited by constraints on reproduction, which is reduced by long hibernations. North American species barely range above the southern Canadian border.<ref name="Ultsch p=339">{{cite journal |last=Ultsch |first=Gordon R. |title=The Ecology of Overwintering Among Turtles: Where Turtles Overwinter and its Consequences |journal=Biological Reviews |volume=81 |issue=3 |date=15 May 2006 |pages=339–367 |issn=1464-7931 |doi=10.1017/s1464793106007032 |pmid=16700968 |s2cid=32939695 }}</ref> Some turtles are found at high altitudes, for example, the species ''[[Terrapene ornata]]'' occurs up to {{convert|2000|m|ft|abbr=on}} in New Mexico.<ref>{{cite journal |last1=Degenhardt |first1=William G. |last2=Christiansen |first2=James L. |title=Distribution and Habitats of Turtles in New Mexico |journal=The Southwestern Naturalist |year=1975 |volume=19 |issue=1 |pages=21–46 |doi=10.2307/3669787 |jstor=3669787}}</ref> Conversely, the leatherback sea turtle can dive over {{convert|1200|m|ft|abbr=on}}.<ref>{{cite journal |last1=Dodge |first1=Kara L. |last2=Galuardi|first2=Benjamin |last3=Miller |first3=Timothy J. |last4=Lutcavage |first4=Molly E. |title=Leatherback Turtle Movements, Dive Behavior, and Habitat Characteristics in Ecoregions of the Northwest Atlantic Ocean |journal=PLOS ONE |year=2014 |volume=9 |issue=3 |article-number=e91726 |doi=10.1371/journal.pone.0091726 |pmid=24646920 |pmc=3960146 |bibcode=2014PLoSO...991726D |doi-access=free }}</ref> Species of the genus ''[[Gopherus]]'' can tolerate both below freezing and over {{convert|40|C|F|0|abbr=on}} in body temperature, though they are most active at {{convert|26|–|34|C|F|0|abbr=on}}.<ref>{{cite web |last=Meyer |first=Rachelle |title=Gopherus spp. |url=https://www.fs.fed.us/database/feis/animals/reptile/goph/all.html |publisher=[[U.S. Department of Agriculture]], Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory |access-date=May 9, 2021 |year=2008 |archive-date=September 16, 2020 |archive-url=https://web.archive.org/web/20200916021457/https://www.fs.fed.us/database/feis/animals/reptile/goph/all.html |url-status=live }}</ref>

== Conservation ==
[[File:Loggerhead ted-noaa.jpg|thumb|left|Many turtles have been killed accidentally in fishing nets.<ref name="Wallace Lewison McDonald McDonald 2010"/> Some trawlers now use nets fitted with [[Turtle excluder device|turtle excluders]].<ref name="Montgomery 2021"/> Seen here, a loggerhead escapes a net so fitted.|alt=Photograph of a marine turtle escaping from a specially-designed fishing net]]

Among vertebrate orders, turtles are second only to [[primates]] in the percentage of threatened species. 360 modern species have existed since 1500 AD. Of these, 51–56% are considered threatened and 60% considered threatened or extinct.<ref name="Rhodin2018">{{cite journal |last1=Rhodin |first1=Anders G. J. |last2=Stanford |first2=Craig B. |last3=van Dijk |first3=Peter Paul |last4=Eisemberg |first4=Carla Camilo |display-authors=etal |year=2018 |title=Global Conservation Status of Turtles and Tortoises (order Testudines) |journal=Chelonian Conservation and Biology |volume=17 |issue=2 |pages=135–161 |doi=10.2744/CCB-1348.1 |bibcode=2018CConB..17..135R |s2cid=91937716|url=http://ir.unimas.my/id/eprint/23198/3/Global.pdf }}</ref> Turtles face many threats, including [[habitat destruction]], harvesting for consumption, the pet trade,<ref name="Rhodin 2011">{{Cite book |editor1-last=Rhodin |editor1-first=Anders G. J. |year=2011 |editor2-last=Walde |editor2-first=Andrew D. |editor3-last=Horne |editor3-first=Brian D. |editor4-last=van Dijk |editor4-first=Peter Paul |editor5-last=Blanck |editor5-first=Torsten |editor6-last=Hudson |editor6-first=Rick |title=Turtles in Trouble: The World's 25+ Most Endangered Tortoises and Freshwater Turtles – 2011 |publisher=Turtle Conservation Coalition |url=http://www.iucn-tftsg.org/top-25-2011/ |access-date=March 5, 2011 |archive-date=March 5, 2011 |archive-url=https://web.archive.org/web/20110305215104/http://www.iucn-tftsg.org/top-25-2011/ |url-status=live |oclc=711692023 }}</ref>{{sfn|Pryke|2021|pp=160–166}} [[light pollution]],{{sfn|Pryke|2021|p=156}} and [[climate change]].{{sfn|Pryke|2021|p=157}} Asian species have a particularly high extinction risk, primarily due to their long-term unsustainable exploitation for food and medicine,<ref name=Firefly2>{{cite book |last=van Dijk |first=Peter Paul |year=2002 |contribution=The Asian Turtle Crisis |title=The Firefly Encyclopedia of Reptiles and Amphibians |editor=Halliday, Tim |editor2=Adler, Kraig |publisher=Firefly Books |pages=134–135 |isbn=978-1-55297-613-5 }}</ref> and about 83% of Asia's non-marine turtle species are considered threatened.<ref name="Rhodin2018"/> As of 2021, turtle extinction is progressing much faster than during the [[Cretaceous-Tertiary extinction]]. At this rate, all turtles could be extinct in a few centuries.<ref>{{cite journal |last=McCallum |first=Malcolm |year=2021 |title=Turtle Biodiversity Losses Suggest Coming Sixth Mass Extinction |journal=Biodiversity and Conservation |volume=30 |issue=5 |pages=1257–1275 |doi=10.1007/s10531-021-02140-8 |bibcode=2021BiCon..30.1257M |issn=0960-3115  |s2cid=233903598 }}</ref>

Turtle [[hatchery|hatcheries]] can be set up when protection against flooding, erosion, predation, or heavy [[poaching]] is required.<ref>{{cite magazine |last=Draven |first=James |url=https://www.nationalgeographic.co.uk/travel/2018/05/are-turtle-hatcheries-unethical |title=Are Turtle Hatcheries Unethical? |magazine=National Geographic |date=30 May 2018 |access-date=10 May 2021 |archive-date=June 13, 2019 |archive-url=https://web.archive.org/web/20190613214349/https://www.nationalgeographic.co.uk/travel/2018/05/are-turtle-hatcheries-unethical |url-status=live }}</ref><ref>{{cite book |title=Sea Turtle Conservation Beach Management and Hatchery Programmes |url=http://www.seaturtle.org/documents/hatchery_manual.pdf |publisher=Centre for Herpetology/ Madras Crocodile Bank Trust, Tamil Nadu |year=2003 |access-date=April 7, 2021 |archive-date=November 12, 2020 |archive-url=https://web.archive.org/web/20201112030749/http://www.seaturtle.org/documents/hatchery_manual.pdf |url-status=live }}</ref><ref>{{cite book |last1=Chacon |first1=Didiher |last2=Sanchez |first2=Juan |last3=Calvo |first3=Jose Joaquin |last4=Ash |first4=Jenny |title=Manual Para el Manejo y la Conservación de las Tortugas Marinas en Costa Rica; con énfasis en la operación de proyectos en playa y viveros |language=Spanish |url=http://www.latinamericanseaturtles.com/archivos/documentos/Manual_Viveros_Impreso.pdf |trans-title=Manual for the management and conservation of sea turtles in Costa Rica; with emphasis on the operation of beach and nursery projects |year=2007 |publisher=Latin American Sea Turtles and WIDECAST (Wider Caribbean Sea Turtle Network) |access-date=April 7, 2021 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105248/http://www.latinamericanseaturtles.com/archivos/documentos/Manual_Viveros_Impreso.pdf |url-status=live }}</ref> Chinese markets have sought to satisfy an increasing demand for turtle meat with farmed turtles. In 2007 it was estimated that over a thousand turtle farms operated in China.<ref name="Fishfarmer">{{Cite web |title=Turtle Farms Threaten Rare Species, Experts Say |publisher=Fish Farmer |date=March 30, 2007 |url=http://www.fishfarmer-magazine.com/news/fullstory.php/aid/993/Turtle_farms_threaten_rare_species,_experts_say.html |access-date=November 1, 2012 |archive-date=February 18, 2012 |archive-url=https://web.archive.org/web/20120218191219/http://www.fishfarmer-magazine.com/news/fullstory.php/aid/993/Turtle_farms_threaten_rare_species%2C_experts_say.html }}</ref> All the same, wild turtles continue to be caught and sent to market in large numbers, resulting in what conservationists have called "the Asian turtle crisis".<ref>{{Cite journal |last1=Cheung |first1=Sze Man |date=November–December 2006 |last2=Dudgeon |first2=David |title=Quantifying the Asian Turtle Crisis: Market Surveys in Southern China, 2000–2003 |journal=Aquatic Conservation: Marine and Freshwater Ecosystems |volume=16 |issue=7 |pages=751–770 |doi=10.1002/aqc.803 |bibcode=2006ACMFE..16..751C }}</ref><ref name=Firefly2/> In the words of the biologist George Amato, the hunting of turtles "vacuumed up entire species from areas in Southeast Asia", even as biologists still did not know how many species lived in the region.<ref>{{Cite video |last=Amato |first=George |year=2007 |title=A Conversation at the Museum of Natural History |url=https://www.pbs.org/pov/chancesoftheworld/special_video.php#.UKeS2BJ-qzc |format=video |medium=.flv |publisher=POV25 |access-date=November 1, 2012 |archive-date=November 12, 2012 |archive-url=https://web.archive.org/web/20121112150908/http://www.pbs.org/pov/chancesoftheworld/special_video.php#.UKeS2BJ-qzc }}</ref> In 2000, all the [[Asian box turtle]]s were placed on the [[CITES]] list of endangered species.<ref name=Firefly2/>

Harvesting wild turtles is legal in some American states,<ref>{{cite web |last1=Bennett |first1=Elise |title=Report: Refusal to Ban Trapping Threatens Turtles in Nine States, Enables Illegal Trade |url=https://biologicaldiversity.org/w/news/press-releases/report-refusal-ban-trapping-threatens-turtles-10-states-enables-illegal-trade-2020-02-20/ |publisher=Center for Biological Diversity |access-date=5 August 2021 |date=February 20, 2020}}</ref> and there has been a growing demand for American turtles in China.<ref name=hylton/><ref name="Pittman"/> The [[Florida Fish and Wildlife Conservation Commission]] estimated in 2008 that around 3,000 pounds of softshell turtles were exported weekly via [[Tampa International Airport]].<ref name="Pittman">{{Cite news |last=Pittman |first=Craig |author-link=Craig Pittman (writer) |date=October 9, 2008 |title=China Gobbling Up Florida Turtles |newspaper=St. Petersburg Times |url=http://www.theledger.com/article/20081009/NEWS/810090272?p=all&tc=pgall |access-date=August 18, 2016 |archive-date=September 20, 2016 |archive-url=https://web.archive.org/web/20160920180105/http://www.theledger.com/article/20081009/NEWS/810090272?p=all&tc=pgall }}</ref> However, the great majority of turtles exported from the US between 2002 and 2005 were farmed.<ref name="hylton">{{Cite magazine |last=Hylton |first=Hilary |title=Keeping U.S. Turtles Out of China |magazine=[[Time (magazine)|Time]] |date=May 8, 2007 |url=http://www.time.com/time/health/article/0,8599,1618565,00.html |archive-url=https://web.archive.org/web/20070512064953/http://www.time.com/time/health/article/0,8599,1618565,00.html |archive-date=May 12, 2007 |access-date=November 1, 2012 }}</ref>

Large numbers of sea turtles are accidentally killed in [[Longline fishing|longlines]], [[gillnetting|gillnets]], and [[trawling]] nets as [[bycatch]]. A 2010 study suggested that over 8 million had been killed between 1990 and 2008; the Eastern Pacific and the Mediterranean were identified as among the areas worst affected.<ref name="Wallace Lewison McDonald McDonald 2010">{{cite journal |last1=Wallace |first1=Bryan P. |last2=Lewison |first2=Rebecca L. |last3=McDonald |first3=Sara L. |last4=McDonald |first4=Richard K. |last5=Kot |first5=Connie Y. |last6=Kelez |first6=Shaleyla |last7=Bjorkland |first7=Rhema K. |last8=Finkbeiner |first8=Elena M. |last9=Helmbrecht |first9=S'rai |last10=Crowder |first10=Larry B. |title=Global Patterns of Marine Turtle Bycatch |journal=Conservation Letters |publisher=Wiley |volume=3 |issue=3 |date=April 5, 2010 |issn=1755-263X |doi=10.1111/j.1755-263x.2010.00105.x |pages=131–142 |doi-access=free |bibcode=2010ConL....3..131W }}</ref> Since the 1980s, the United States has required all [[Shrimp trawling|shrimp trawlers]] to fit their nets with [[turtle excluder device]]s that prevent turtles from being entangled in the net and drowning.<ref name="Montgomery 2021">{{cite web |last=Montgomery |first=Madeline |date=April 15, 2021 |title=Environmentalists Fight Against New Law that Could Kill Thousands of Sea Turtles |url=https://cbs12.com/news/local/environmentalists-fight-against-new-law-that-could-kill-thousands-of-sea-turtles |access-date=May 11, 2021 |website=WPEC |archive-date=April 17, 2021 |archive-url=https://web.archive.org/web/20210417055217/https://cbs12.com/news/local/environmentalists-fight-against-new-law-that-could-kill-thousands-of-sea-turtles |url-status=live }}</ref>
More locally, other human activities are affecting marine turtles. In Australia, [[Queensland]]'s [[shark culling]] program, which uses [[shark nets]] and [[drum line (shark control)|drum lines]], has killed over 5,000 turtles as bycatch between 1962 and 2015; including 719 loggerhead turtles and 33 hawksbill sea turtles, which are listed as critically endangered.<ref name="QLDcull">{{cite web |last=Thom |first=Mitchell |url=http://www.afd.org.au/news-articles/queenslands-shark-control-program-has-snagged-84000-animals |publisher=Action for Dolphins |title=Queensland's Shark Control Program Has Snagged 84,000 Animals |date=November 20, 2015 |access-date=December 25, 2018 |archive-date=December 24, 2020 |archive-url=https://web.archive.org/web/20201224053505/https://www.afd.org.au/news-articles/queenslands-shark-control-program-has-snagged-84000-animals |url-status=live }}</ref>

Native turtle populations can also be threatened by [[Invasive species|invasive ones]]. The central North American red-eared slider turtle has been listed among the "[[100 of the World's Worst Invasive Alien Species|world's worst invasive species]]", pet turtle having been released globally. They appear to compete with native turtle species in eastern and western North America, Europe, and Japan.<ref>{{cite journal |last1=Lambert |first1=Max R. |display-authors=etal |year=2019 |title=Experimental Removal of Introduced Slider Turtles Offers New Insight into Competition with a Native, Threatened Turtle |journal=PeerJ |volume=7 |article-number=e7444 |doi=10.7717/peerj.7444 |pmid=31435491 |pmc=6698372 |doi-access=free }}</ref>{{sfn|Pryke|2021|p=107}}

==Human uses ==
===On space flights===
Two tortoises were on the Soviet Union's September 1968 [[Zond 5]] [[Circumlunar trajectory|circumlunar flight]], making them the first earthly living things to travel to the vicinity of the Moon. Turtles were also on the [[Zond 6]] (1968) and the [[Zond 7]] (1969) circumlunar flights.<ref>{{Cite news|url=http://blogs.discovermagazine.com/crux/2018/09/18/zond-5-soviet-tortises-around-the-moon/|title=The First Earthlings Around the Moon Were Two Soviet Tortoises|last=Betz|first=Eric|date=18 September 2018|work=[[Discover (magazine)|Discover]]|access-date=14 July 2019|archive-date=28 September 2019|archive-url=https://web.archive.org/web/20190928192410/http://blogs.discovermagazine.com/crux/2018/09/18/zond-5-soviet-tortises-around-the-moon/}}</ref><ref>{{cite book|url=https://www.nasa.gov/sites/default/files/atoms/files/beyond-earth-tagged.pdf|title=Beyond Earth: A Chronicle of Deep Space Exploration, 1958–2016|last1=Siddiqi|first1=Asif|publisher=NASA History Program Office|edition=second|year=2018}}</ref>

=== In culture ===
{{main article|Cultural depictions of turtles}}
{{further|World Turtle<!--|List of fictional turtles-->}}
<gallery mode="packed" heights="155">
File:Kurmavatara (tortoise incarnation of Vishnu), from Garhwa, Allahabad District.jpg|alt=Photograph of temple sculpture in India|4th-century sculpture of [[Kurma|turtle avatar]] of [[Vishnu]]. [[Garhwa|Garhwa, India]]
File:PSM V10 D562 The hindoo earth.jpg|alt=Lithograph drawing of world resting on 4 elephants standing on a giant turtle|World resting on four elephants on the back of the [[World Turtle]]. Western depiction of "The Hindu Earth", 1877
File:Kangxi-Lugou-rebuilding-stele-3581.jpg|alt=Chinese funeral stone held up by a stone tortoise|[[Bixi (tortoise)|Bixi]] supporting [[Kangxi Emperor]]'s [[stele]], [[Beijing]], 1698
File:Alice par John Tenniel 34.png|alt=Children's book illustration with turtle figure standing on hind legs|The [[Mock Turtle]] in [[Lewis Carroll]]'s 1865 ''[[Alice's Adventures in Wonderland]]''
File:The Turtle 1898 Manhattan Theatre poster.jpg|alt=Painting of a turtle standing on hind legs, with top hat and cane, on theatre poster|Poster for 1898 production of ''The Turtle'' at the Manhattan Theatre, Broadway
File:Terrapin Shell Leg Rattles Worn by Lead Woman Dancer, Oconaluftee Village, NC. - NARA - 281630.jpg|alt=Photograph of cloth with four terrapin shell rattles to be tied around a dancer's leg|Terrapin shell leg rattles worn by lead [[Cherokee]] woman dancer, 20th century
</gallery>

Turtles have featured in human cultures across the world since ancient times. They are generally viewed positively despite not being "cuddly" or flashy; their association with the ancient times and old age have contributed to their endearing image.{{sfn|Pryke|2021|pp=9–10}}

In [[Hindu mythology]], the [[World Turtle]], named [[Kurma|Kurma or Kacchapa]], supports four elephants on his back; they, in turn, carry the weight of the whole world on their backs.{{sfn|Pryke|2021|pp=63–68}}<ref name="WWF">{{cite web |last1=McLellan |first1=Liz |last2=Nickson |first2=Amanda |last3=Benn |first3=Jo |title=Marine turtle conservation in the Asia Pacific region |publisher=WWF |date=June 2005 |url=https://wwfint.awsassets.panda.org/downloads/marine_turtles_ap.pdf |access-date=July 22, 2021 |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105248/https://wwfint.awsassets.panda.org/downloads/marine_turtles_ap.pdf |url-status=live }}</ref> The turtle is one of the ten [[avatar]]s or incarnations of the god [[Vishnu]].{{sfn|Pryke|2021|pp=63–68}} The [[yoga]] pose Kurmasana is named for the avatar.<ref name="Mallinson 2011">{{cite journal |last=Mallinson |first=James |author-link=James Mallinson (author) |title=A Response to Mark Singleton's ''Yoga Body'' by James Mallinson |url=https://www.academia.edu/1146607 |access-date=4 January 2019 |date=9 December 2011 |website=Academia |archive-date=July 24, 2021 |archive-url=https://web.archive.org/web/20210724105251/https://www.academia.edu/1146607 |url-status=live }} revised from American Academy of Religions conference, San Francisco, 19 November 2011.</ref><ref>{{cite book |last=Iyengar |first=Bellur K. S. |author-link=B. K. S. Iyengar |year=1979 |orig-date=1966 |title=[[Light on Yoga|Light on Yoga: Yoga Dipika]] |publisher=Thorsons |isbn=978-1-85538-166-7 |pages=288–291}}</ref> World Turtles are found in Native American cultures including the [[Algonquian peoples|Algonquian]], [[Iroquois]], and [[Lenape]]. They tell many versions of the [[creation story]] of [[Turtle Island (Native American folklore)|Turtle Island]]. One version has Muskrat pile up earth on Turtle's back, creating the continent of North America. An [[Iroquois]] version has the pregnant [[Sky Woman]] fall through a hole in the sky between a tree's roots, where she is caught by birds who land her safely on Turtle's back; the Earth grows around her. The turtle here is altruistic, but the world is a heavy burden, and the turtle sometimes shakes itself to relieve the load, causing earthquakes.{{sfn|Pryke|2021|pp=63–68}}<ref name="Converse1908">{{cite book |last1=Converse |first1=Harriet Maxwell |author1-link=Harriet Maxwell Converse |last2=Parker |first2=Arthur Caswell |author2-link=Arthur Caswell Parker |title=Myths and Legends of the New York State Iroquois |url=https://books.google.com/books?id=S6y6uxjhf60C&pg=PA33 |year=1908 |publisher=University of the State of New York |page=33 |access-date=May 22, 2021 |archive-date=May 22, 2021 |archive-url=https://web.archive.org/web/20210522092758/https://books.google.com/books?id=S6y6uxjhf60C&pg=PA33 |url-status=live }}</ref><ref name="Canadian Encyclopedia">{{cite web |last=Filice |first=Michelle |title=Turtle Island |url=https://www.thecanadianencyclopedia.ca/en/article/turtle-island |website=The Canadian Encyclopedia |access-date=May 22, 2021 |date=November 6, 2018 |archive-date=May 20, 2021 |archive-url=https://web.archive.org/web/20210520010504/https://thecanadianencyclopedia.ca/en/article/turtle-island |url-status=live }}</ref>

A turtle was the symbol of the Ancient Mesopotamian god [[Enki]] from the 3rd millennium BCE onward.{{sfn|Pryke|2021|pp=44–48}} An ancient Greek [[origin myth]] told that only the tortoise refused the invitation of the gods [[Zeus]] and [[Hera]] to their wedding, as it preferred to stay at home. Zeus then ordered it to carry its house with it, ever after.{{sfn|Pryke|2021|p=56}} Another of their gods, [[Hermes]], invented a seven-stringed [[chelys|lyre]] made with the shell of a tortoise.<ref>{{Cite web |last2=Evelyn-White |first2=Hugh G. |title=Hymn 4 to Hermes |last1=Anonymous |publisher=Harvard University Press |at=Lines 26–65 |year=1914 |publication-place=Cambridge, Massachusetts |url=https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.01.0138:hymn=4 |access-date=May 11, 2021 |archive-date=May 11, 2021 |archive-url=https://web.archive.org/web/20210511094022/https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.01.0138:hymn=4 |url-status=live }}</ref> In the [[Shang dynasty]] [[China|Chinese]] practice of [[plastromancy]], dating back to 1200 BCE, oracles were obtained by inscribing questions on turtle plastrons using the [[Oracle bone script|oldest known form of Chinese characters]], burning the plastron, and interpreting the resulting cracks. Later, the turtle was one of the [[Four Symbols|four sacred animals]] <!--along with the Phoenix, Qulin, and Dragon--> in [[Confucianism]], while in the [[Han period]], [[stele]]s were mounted on top of stone turtles, later linked with [[Bixi]], the turtle-shelled son of the Dragon King.{{sfn|Pryke|2021|pp=49–52}} Marine turtles feature significantly in [[Australian Aboriginal]] art.<ref name="WWF"/> The army of [[Ancient Rome]] used the {{lang|la|testudo}} [[Testudo formation|("tortoise") formation]] where soldiers would form a [[shield wall]] for protection.{{sfn|Pryke|2021|p=107}}

In [[Aesop's Fables]], "[[The Tortoise and the Hare]]" tells how an unequal race may be won by the slower partner.<ref>{{cite web |title=The Tortoise and the Hare |website=Aesopica: Aesop's Fables in English, Latin, and Greek |url=http://mythfolklore.net/aesopica/perry/226.htm |access-date=11 May 2021 |archive-date=August 16, 2019 |archive-url=https://web.archive.org/web/20190816021223/http://mythfolklore.net/aesopica/perry/226.htm |url-status=live }}</ref>{{sfn|Pryke|2021|p=139}} [[Lewis Carroll]]'s 1865 ''[[Alice's Adventures in Wonderland]]'' features a [[Mock Turtle]], named for [[Mock turtle soup|a soup meant to imitate]] the expensive soup made from real turtle meat.<ref>{{cite book |last=Carroll |first=Lewis |title=Alice's Adventures in Wonderland |chapter=The Mock-Turtle's Story |date=1901 |orig-date=1865 |publisher=Harper & Brothers |page=128 |oclc=1049742993}}</ref>{{sfn|Pryke|2021|page=135}}<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/mock+turtle+soup |title=Mock Turtle Soup |publisher=[[Merriam-Webster]] |access-date=December 22, 2020 |archive-date=October 20, 2012 |archive-url=https://web.archive.org/web/20121020034920/http://www.merriam-webster.com/dictionary/mock+turtle+soup |url-status=live }}</ref> In 1896, the French playwright [[Léon Gandillot]] wrote a comedy in three acts named {{lang|fr|La Tortue}} that was "a Parisian sensation"<ref name="Brooklyn Life 1899">{{cite news |last1=Anon |title=Brooklyn Life [Theater] |url=https://www.newspapers.com/newspage/83148023/ |access-date=22 May 2021 |work=Brooklyn Life |date=April 1, 1899 |page=31 |quote=it is primarily a very amusing farce. The plot is slight, and concerns chiefly the proverbial fickle-mindedness of woman. |archive-date=May 22, 2021 |archive-url=https://web.archive.org/web/20210522075832/https://www.newspapers.com/newspage/83148023/ |url-status=live }}</ref> in its run in France, and came to the [[Manhattan Theatre]], Broadway, New York, in 1898 as ''The Turtle''.{{sfn|Pryke|2021|p=137}} A "cosmic turtle" and the island motif reappear in [[Gary Snyder]]'s 1974 novel ''[[Turtle Island (book)|Turtle Island]]'', and again in [[Terry Pratchett]]'s ''[[Discworld]]'' series as Great A'Tuin, starting with the 1983 novel ''[[The Colour of Magic]]''. It is supposedly of the species ''Chelys galactica'', the galactic turtle, complete with four elephants on its back to support Discworld.{{sfn|Pryke|2021|pp=118–120}} A giant fire-breathing turtle called [[Gamera]] is the star of a series of Japanese monster movies in the ''[[kaiju]]'' genre and has had twelve films from 1965 to 2006.{{sfn|Pryke|2021|pp=146–148}} Turtles have been featured in comic books and animations such as the 1984 [[Teenage Mutant Ninja Turtles]].<ref>{{cite news |last=Greenberg |first=Harvey R. |title=Just How Powerful Are Those Turtles? |work=[[The New York Times]] |date=April 15, 1990 |url=https://www.nytimes.com/1990/04/15/movies/just-how-powerful-are-those-turtles.html?scp=8&sq=ninja%20turtles&st=cse |access-date=May 11, 2021 |archive-date=June 12, 2018 |archive-url=https://web.archive.org/web/20180612182757/https://www.nytimes.com/1990/04/15/movies/just-how-powerful-are-those-turtles.html?scp=8&sq=ninja%20turtles&st=cse |url-status=live }}</ref>{{sfn|Pryke|2021|pp=148–151}}

=== As pets===

Some turtles, particularly small terrestrial and freshwater species, are kept as [[pet]]s.<ref name="Reid 2017"/>{{sfn|Pryke|2021|p=181}}  The demand for pet turtles increased in the 1950s, with the US being the main supplier, particularly of farm-bred [[red-eared slider]]s. The popularity for exotic pets has led to an increase in illegal [[Wildlife trade|wildlife trafficking]]. Around 21% of the value of live animal trade is in reptiles, and turtles are among the more popularly traded species.{{sfn|Pryke|2021|pp=181–183}} Poor husbandry of tortoises can cause chronic [[rhinitis]] (nasal swelling), overgrown beaks, [[hyperparathyroidism]] (which softens their skeleton), [[constipation]], various reproductive problems, and injuries from dogs.<ref name="Reid 2017">{{Cite journal |last=Reid |first=Siuna A. |title=Current Trends in the Husbandry and Veterinary Care of Tortoises |url=http://www.britishcheloniagroup.org.uk/sites/default/files/u8/v8n4Reid.pdf |journal=Testudo |volume=8 |issue=4 |pages=58–68 |year=2017 |access-date=July 31, 2019 |archive-date=July 31, 2019 |archive-url=https://web.archive.org/web/20190731152314/http://www.britishcheloniagroup.org.uk/sites/default/files/u8/v8n4Reid.pdf |url-status=live }}</ref> In the early 20th century, people in the United States have organized and gambled on [[Turtle racing|turtle races]].{{sfn|Pryke|2021|pp=120–122}}

=== As food and other uses===

The flesh of captured wild turtles continues to be eaten in Asian cultures,<ref>{{Cite web |last=Barzyk |first=James E. |date=November 1999 |title=Turtles in Crisis: The Asian Food Markets |publisher=Tortoise Trust |url=http://www.tortoisetrust.org/articles/asia.html |access-date=November 1, 2012 |archive-date=February 22, 2015 |archive-url=https://web.archive.org/web/20150222091358/http://www.tortoisetrust.org/articles/asia.html |url-status=live }}</ref> while [[turtle soup]] was once a popular dish in [[English cuisine]].<ref name=":0">{{Cite book |last=Clarkson |first=Janet |title=Soup: a global history|date=2010 |publisher=Reaktion |isbn=978-1-86189-774-9 |page=115 |oclc=642290114}}</ref> Gopher tortoise stew has been popular with some groups in Florida.<ref>{{Cite magazine |title=Recipes from Another Time |date=October 2001 |url=http://www.smithsonianmag.com/people-places/recipes-from-another-time-52824959/?all |magazine=[[Smithsonian (magazine)|Smithsonian]] |access-date=August 19, 2016 |archive-date=August 19, 2016 |archive-url=https://web.archive.org/web/20160819163959/http://www.smithsonianmag.com/people-places/recipes-from-another-time-52824959/?all |url-status=live }}</ref> The supposed aphrodisiac or medicinal properties of turtle eggs created a large trade for them in Southeast Asia.<ref name="WWF"/> Hard-shell turtle plastrons and soft-shell carapaces are widely used in [[traditional Chinese medicine]]; [[Taiwan]] imported nearly 200 metric tons of hard-shells from its neighbors yearly from 1999 to 2008.<ref>{{Cite journal |last1=Chen |first1=Tien-Hsi |year=2009 |last2=Chang |first2=H.-C. |last3=Lue |first3=Kuang-Yang |title=Unregulated Trade in Turtle Shells for Chinese Traditional Medicine in East and Southeast Asia: the Case of Taiwan |journal=Chelonian Conservation and Biology |volume=8 |issue=1 |pages=11–18 |doi=10.2744/CCB-0747.1 |s2cid=86821249 |doi-access=free }}</ref> A popular medicinal preparation based on herbs and turtle shells is ''[[guilinggao]]'' jelly.<ref>{{cite journal |last1=Zhang |first1=Huan |last2=Wu |first2=Min-Yi |last3=Guo |first3=De-Jian |display-authors=etal |title=Gui-ling-gao (turtle jelly), a traditional Chinese functional food, exerts anti-inflammatory effects by inhibiting iNOS and pro-inflammatory cytokine expressions in splenocytes isolated from BALB/c mice |journal=Journal of Functional Foods |year=2013 |volume=5 |issue=2 |pages=625–632 |doi=10.1016/j.jff.2013.01.004 |hdl=10397/16357 |hdl-access=free}}</ref> The substance [[tortoiseshell]], usually from the hawksbill turtle, has been used for centuries to make jewelry, tools, and ornaments around the Western Pacific.<ref name="WWF"/> Hawksbills have accordingly been hunted for their shells.<ref name="Cox 2018">{{cite news |last1=Cox |first1=Lisa |title=Hawksbill Turtle Poaching to be Fought with DNA Technology |url=https://www.theguardian.com/environment/2018/nov/13/hawksbill-turtle-poaching-to-be-fought-with-dna-technology |access-date=7 August 2021 |work=[[The Guardian]] |date=November 12, 2018 |quote=Hawksbills are the only sea turtles hunted for their shells, despite international trade in hawksbill products being banned more than 20 years ago.}}</ref> The trading of tortoiseshell was internationally banned in 1977 by CITES.<ref>{{cite web |title=Global Status of the Hawksbill Sea Turtle: The Tortoiseshell Trade |publisher=Sea Turtle Conservancy |date=2007 |url=https://www.conserveturtles.org/11503-2/}}</ref> Some cultures have used turtle shells to make music: Native American [[shaman]]s made them into ceremonial rattles, while [[Aztec]]s, [[Maya civilization|Maya]]s, and [[Mixtec]]s made ''{{lang|az|ayotl}}'' drums.{{sfn|Pryke|2021|pp=58–60}}

<gallery mode="packed" heights="155px">
File:StateLibQld 2 395489 Catching turtles, wood engraving, 1875.jpg|Catching turtles in Australia, 1875|alt=Historic engraving of men catching turtles on a beach
File:TurtleSeafood.jpg|Turtles on sale as food in Canada, 2007|alt=photo of turtles on sale as food in a shop
File:Turtle plastrons as TCM in Xi'an market.jpg|Turtle plastrons for traditional Chinese medicine|alt=Photograph of a box of turtle plastrons in a market
File:Peigne en écaille.jpg|A tortoiseshell comb; the material was expensive and decorative, and widely used for small items.<ref name="Strieker 2001">{{cite web |last=Strieker |first=Gary |title=Tortoiseshell Ban Threatens Japanese Tradition |url=http://archives.cnn.com/2001/TECH/science/04/10/japan.turtles/ |publisher=CNN |access-date=11 May 2021 |archive-url=https://web.archive.org/web/20061215162652/http://archives.cnn.com/2001/TECH/science/04/10/japan.turtles/ |archive-date=December 15, 2006 |date=April 10, 2001}}</ref>|alt=Photograph of a decoratively ridged comb made of tortoiseshell
File:Sheldonbasking.JPG|A pet red-eared slider basking on a floating platform under a sun lamp|alt=Photograph of a pet turtle in a terrarium
File:Prohibited from capturing and consuming tutles, Bluefields, Nicaragua.jpg|A notice in [[Bluefields]], [[Nicaragua]] in 2023 indicating that the capturing and consuming of turtles is prohibited
</gallery>

==See also==
* [[World Turtle Day]]

==References==
=== Citations ===
{{Reflist|25em}}

=== Cited sources===
* {{Cite book |last=Franklin |first=Carl J. |year=2011 |title=Turtle: A Extraordinary Natural History 245 Million Years in the Making |publisher=Crestline |isbn=978-0-7858-2775-7}}
* {{Cite book |last=Orenstein |first=Ronald |year=2012 |title=Turtles, Tortoises and Terrapins: a Natural History |publisher=Firefly Books |isbn=978-1-77085-119-1 |oclc=791162481}}
* {{Cite book |last=Pryke |first=Louise |year=2021 |title=Turtle |publisher=[[Reaktion Books]] |isbn=978-1-78914-336-2 |oclc=1223025640}}

==External links==
{{Wikiquote|Turtles}}
{{Commons category|Turtles}}
{{Wikibooks|Animal Care|Turtle}}
* [https://turtlesurvival.org/ Turtle Survival Alliance]
* [https://www.turtleconservancy.org/ Turtle Conservancy]
* [https://web.archive.org/web/20230328020135/https://webofmeetings.com/2020/earthsciences/paleontology/turtle-evolution-symposium-2021/ Symposium on Turtle Evolution]

{{Testudines}}
{{Named turtles}}
{{Turtles in human activities}}
{{Chordata}}
{{Taxonbar|from=Q223044}}
{{Authority control}}

[[Category:Turtles|Turtles]]
[[Category:Articles containing video clips]]
[[Category:Kimmeridgian first appearances]]
[[Category:Extant Late Jurassic first appearances]]
[[Category:Turtle taxonomy]]