Doushantuo Formation

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The Doushantuo Formation (formerly transcribed as Toushantuo or Toushantou,[1] from Template:Lang-zh) is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China.[2] It is known for the fossil Lagerstätten in Zigui in Hubei, Xiuning in Anhui, and Weng'an in Guizhou, as one of the oldest beds to contain minutely preserved microfossils, phosphatic fossils that are so characteristic [3] they have given their name to "Doushantuo type preservation". The formation, whose deposits date back to the Early and Middle Ediacaran,[4][5] is of particular interest because it covers the poorly understood interval of time between the end of the Cryogenian geological period and the more familiar fauna of the Late Ediacaran Avalon explosion,[6] as well as due to its microfossils' potential utility as biostratigraphical markers.[7] Taken as a whole, the Doushantuo Formation ranges from about 635 Ma (million years ago) at its base to about 551 Ma at its top, with the most fossiliferous layer predating by perhaps five Ma the earliest of the 'classical' Ediacaran faunas from Mistaken Point on the Avalon Peninsula of Newfoundland, and recording conditions up to a good forty to fifty million years before the Cambrian explosion at the beginning of the Phanerozoic.

Sedimentology

The whole sequence sits on an unconformity with the underlying Liantuo formation, which is free of fossils, an unconformity usually being interpreted as a period of erosion. On that unconformity lie tillites of the Nantuo formation - cemented glacial till formed of glacial deposits of cobbles and gravel laid down at the end of the Marinoan glaciation (also known as Varangian glaciation, this is the second and last of a series of very extensive glaciations during a period called the Cryogenian—named because 'Snowball Earth' conditions at the time). This latest Cryogenian glacial level is tentatively dated ca 654 (660 ± 5) — 635 Ma (million years ago).

The Doushantuo formation itself has three layers representing aquatic sediments that formed as sea levels rose with the melting of worldwide glaciation. Biomarkers indicate highly saline conditions, such as might be found in a lagoon, low oxygen levels, and very little sediment that had been washed off land surfaces.

The richest finds (the Lagerstätte itself) lie at the bottom of the middle stratum, with a date about 570 Ma, thus from some time after the great Gaskiers glaciation of [585 ± 1 - 582.1 ± 0.4 Ma].

Fossils

Script error: No such module "labelled list hatnote". Doushantuo fossils are all aquatic, microscopic, and preserved to a great degree of detail. The latter two characteristics mean that the structure of the organisms that made them can be studied at the cellular level, and considerable insight has been gained into the embryonic and larval stages of many early creatures. One contentious claim is that many of the fossils show signs of bilateral symmetry, a common feature in many modern-day animals which is usually assumed to have evolved later, during the Cambrian Explosion. A nearly microscopic fossil animal, Vernanimalcula ("springtime micro-animal") was announced in October 2005, with the claim that it was the oldest known bilateral animal.[8] However, the absence of adult forms of almost all animal types in the Doushantuo (there are microscopic adult sponges and corals) makes these claims difficult to prove: some argue that their lack suggests these finds are not larval and embryonic forms at all; supporters contend that some unidentified process "filtered out" all but the smallest forms from fossilization. An alternative interpretation suggests that it was created by non-biological rock-forming processes.[9] The team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found ten specimens (not illustrated) of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike.[10]

The discovery was made when the rich phosphate deposits were being mined, and was first reported in 1998. The finds offer direct evidence that confirms expectations that major evolutionary diversification of animals already had occurred before the onset of the Cambrian period, with its apparent 'explosion' of metazoan life-forms and, therefore, that more remote ancestral forms of the phyla recognizable in Cambrian macrofossils must have existed previously.Script error: No such module "Unsubst".

The documented biota now includes phosphatized microfossils of algae, multicellular thallophytes (seaweeds), acritarchs, ciliates,[11] and cyanophytes, besides adult sponges and adult cnidarians (coelenterates; these may be early forms of tabulate corals (tetracorallians)).Script error: No such module "Unsubst". There also seem to be what scientists cautiously report as bilateral animal embryos, termed Parapandorina, and eggs (Megasphaera). Some of the possible animal embryos are in an early stage of cellular division (that was first interpreted as spores or algal cells), including eggs and embryos which are most probably of sponges or cnidarians, as well as adult sponges and a variety of adult cnidarians.

An alternative possibility is that the "embryos" and "eggs" are in fact fossils of giant sulfur bacteria resembling Thiomargarita, a bacterium so large that it is visible to the naked eye.[12] The interpretation would also provide a mechanism for phosphatic fossilization through microbially mediated phosphate precipitation by the bacteria, which has been observed in modern environments. If dark spots in the fossil transpire to be fossilised nuclei - an unlikely claim[13] - this would refute the Thiomargarita hypothesis. That being said, recent comparisons of the Doushantuo fossils to modern decaying Thiomargarita and expired sea urchin embryos shows little similarity between the fossils and decaying bacterial cells.[14]

Only about one-twentieth of the site's fossils have been excavated. The fossil beds are threatened by increasing intensity of phosphate mining operations in the area. A workshop led in protest by local paleontologists resulted in a temporary halt to the mining in 2017.[15]

Paleobiota

While here they are placed in the Doushantuo Formation, most of the algae are found in the Miaohe biota, which may instead belong to the lower part of the Dengying Formation.[16]

Algae

Algae
Genus Species Higher taxon Notes Images
Anhuiphyton A. lineatum Eukaryota Essentially identical to specimens from the Lantian Formation[17]
Anomalophyton A. zhangzhongyingi Eukaryota Similar to Doushantuophyton, but branches less frequently[18]
Baculiphyca B. taeniata, B. brevistipitata Eukaryota Species differ in stipe length[19]
Beltanelliformis B. brunsae Cyanobacteria Originally interpreted as a polypoid cnidarian[18]
File:Anthozoa - Nemiana simplex.JPG
Beltanelliformis fossil from Ukraine
Crassitubus C. costata Cyanobacteria Originally interpreted as a tubular animal,[20] before being reinterpreted as a cyanobacterium[21]
Cobios C. rubro Rhodophyta Preserves evidence of multiple life stages[22]
Doushantuophyton D. lineare, D. rigidulum, D. quyuani, D. cometa, D. laticladus Eukaryota Relatively small and thin[19]
Enteromorphites E. siniansis, E. magnus Eukaryota One of the larger "branching algae" from Doushantuo[19]
Gemmaphyton G. taoyingensis Eukaryota Bears a very long stipe and round holdfast[17]
Globusphyton G. lineare Eukaryota Fairly unusual morphology suggests it likely crept along the seafloor[23]
Glomulus G. filamentum Cyanobacteria Resembles modern cyanobacteria like Microcoleus[19]
Gremiphyca G. corymbiata Rhodophyta Likely a stem-rhodophyte,[24] resembles coralline algae[25]
Grypania G. spiralis Eukaryota? One of the most common large Proterozoic fossils[19]
File:Grypania spiralis.JPG
Grypania fossils from the Negaunee Iron Formation in the USA
Jiuqunaoella J. simplicis Eukaryota Resembles Grypania, but differs in presence of constrictions[19]
Jixiania J. lineata Eukaryota? Also known from the Mesoproterozoic, probably a filamentous alga[26]
Konglingiphyton K. erecta, K. laterale Rhodophyta? Likely a rhodophyte, a formerly separate genus ("Ramalga") was actually a specimen of this genus overlaid by one of Doushantuophyton[18]
Latiortenuiphyton L. robusta Eukaryota Has large vein-like structures in its thallus[17]
Liulingjitaenia L. alloplecta Eukaryota Unusually consists of a set of braided filaments[19]
Longifuniculum L. dissolutum Eukaryota Similar to Liulingjitaenia, but its filaments fray out at each end almost like a braided rope[19]
Maxiphyton M. stipitatum Eukaryota Has an unusually thick stipe[19]
Megaspirellus M. houi Eukaryota Holotype reinterpreted as a coprolite due to sediment grains[19]
Miaohephyton M. bifurcatum Phaeophyta? Fossils likely represent shed fragments from larger thalli[27]
Paramecia P. incognata Corallinales? Formerly interpreted as a marine lichen[25]
Paratetraphycus P. gigantea Bangiophyceae? Resembles early developmental stages of the alga Porphyra[25]
Pseudodoushantuophyton P. wenghuiensis Eukaryota Bears cluster-like filaments on its branches[17]
Quadratitubus Q. orbigoniatus Cyanobacteria Originally interpreted as a tubular animal,[20] before being reinterpreted as a cyanobacterium[21]
Ramitubus R. increscens, R. decrescens Eukaryota Originally interpreted as a tubular animal,[20] then found to be an alga similar to Epiphyton[21]
Sinocylindra S. linearis, S. yunnanensis Eukaryota Resembles the cyanobacteria Siphonophycus[19]
File:Sinocylindra yunnanensis, Chengjiang biota 2.jpg
S. yunnanensis fossil from the Chengjiang biota
Sinocyclocyclicus S. guizhouensis Cyanobacteria Formerly thought to be a larval or tube-dwelling metazoan alongside other tubular microfossils[21][25]
Siphonophycus S. solidum, S. septatum, S. kestron, S. typicum, S. robustum[26] Cyanobacteria Relatively common cyanobacteria[18]
Thallophyca T. corrugata Rhodophyta Resembles the Solenoporaceae[25]
Thallophycoides T. phloeatus Eukaryota May be a rhodophyte?[25]
Tongrenphyton T. komma Eukaryota Has several filaments along its thallus, showing a clear eukaryotic affinity[17]
Vendotaenia V. sp Vendotaenid Resembles specimens from the lower Cambrian[19]
File:Vendotaenid indet.jpg
Indeterminate vendotaenid fossil
Wengania W. globosa, W. exquisita, W. minuta[28] Eukaryota Resembles various algae in its parenchymatous structure, especially green algae[25]

Microorganisms

Microorganisms
Genus Species Higher taxon Notes Images
Aggregatosphaera A. miaoheensis Eukaryota? Spheroidal cell clusters with similarities to algal cysts[18]
Annularidens A. inconditus Acritarch Bears numerous hollow processes on its outer surface,[26] giving it a gearwheel-like appearance in cross-section[29]
Apodastoides A. basileus Acritarch Bears processes with plugs at their base[25]
Appendisphaera A. anguina, A. clava, A. grandis, A. fragilis, A. longispina, A. setosa, A. tabifica, A. tenuis, A. helicaea,[29] A. brevispina, A. hemisphaerica[30] Acritarch Bears numerous long, flexible processes[26]
Archaeophycus A. yunnanensis Eukaryota? Either a cyanobacteria or the early developmental stage of an alga[26]
Asterocapsoides A. sinicus Acritarch Original holotype was damaged by the surrounding rock breaking[25]
Bacatisphaera B. baokangensis Acritarch Bears large pimple-like processes[31]
Baltisphaeridium B. rigidum Acritarch Bears short, triangular processes[32]
Bispinosphaera[30] B. vacua Acritarch Bears two separate types of process; one is conical and hollow, the other is solid and hair-like[29]
Botominella B. lineata Eukaryota? Known from small trichome-like structures[26]
Castaneasphaera C. speciosa Acritarch Resembles Phanerozoic "mazuelloids"[31]
Cavaspira C. acuminata, C. basiconica Acritarch Bears short, conical processes[26]
Cerionopora C. ordinata Acritarch May be a resting cyst of a multicellular alga[25]
Comasphaeridium C. magnum Acritarch Bears long hair-like processes[32]
Crassimembrana C. crispans, C. multitunica Acritarch Bears a thick vesicle wall[29]
Cyanonema C. attenuatum Cyanobacteria Similar to forms from the Bitter Springs formation[30]
Cymatiosphaeroides C. forabilatus, C. yinii[32] Acritarch Similar to Membranosphaera, bears cylindrical processes with pointed tips[26]
Dicrospinasphaera D. zhangii Acritarch Bears thin, branching processes[32]
Distosphaera D. jinguadunensis, D. speciosa[25] Acritarch Bears two layers of cylindrical processes[29]
Doushantuonema D. peatii Cyanobacteria Earliest reported cyanobacteria from the formation[33]
Duospinosphaera D. shennongjiaensis, D. biformis Acritarch Bears two layers of processes; an inner cylindrical one likened to hanging icicles and a larger conical one on the outer wall[26]
Echinosphaeridium E. maximum Acritarch Formerly placed in Ericiasphaera due to a misinterpretation of the spines as solid[25]
Eotylotopalla E. apophysa, E. dactylos, E. delicata[25] Acritarch Formerly placed within Timanisphaera[26]
Ericiasphaera E. fibrila, E. magna, E. rigida?, E. densispina,[30] E. sparsa[25] Acritarch Bears dense cylindrical processes,[26] badly preserved acritarchs likely belonging to this genus were misinterpreted as ciliates[34]
Goniosphaeridium G. acuminatum, G. conoideum, G. cratum Acritarch Similar to Baltisphaeridium[25]
Granitunica G. mcfaddenae Acritarch Differs from other sphaeromorph acritarchs in its granular wall[30]
Hocosphaeridium H. dilatatum, H. scaberfacium Acritarch Bears numerous hooked processes[26]
Knollisphaeridium K. maximum, K. coniformum, K. denticulatum, K. longilatum, K. obtusum, K. parvum[30] Acritarch Bears conical processes with a long filament-like tip[26]
Leiosphaeridia L. tenuissima, L. crassa, L. minutissima[26] Acritarch Incredibly abundant[30]
Megasphaera M. inornata Protista Thought represent an animal egg, then revealed to likely be an encysting protist[35]
Meghystricosphaeridium M. chadianensis, M?. densum, M. gracilentum, M. perfectum, M. magnificum, M. wenganensis[32] Acritarch Somewhat unclear which species belong to this genus[25]
Mengeosphaera M. angusta?, M. chadianensis, M. constricta, M. gracilis, M. mamma, M. minima, M. stegosauriformis, M. bellula, M. cuspidata, M. grandispina, M. latibasis, M. spicata, M. spinula, M. triangula, M. uniformis, M. membranifera[36] Acritarch The species M. stegosauriformis is named after the dinosaur Stegosaurus, due to the acritarch's processes resembling the dinosaur's plates in shape[30]
Obruchevella O. minor Cyanobacteria Relatively rare[26]
Oscillatoriopsis O. amadeus, O. longa, O. obtusa, O. majuscula[26] Cyanobacteria Common cyanobacterial genus[30]
Osculosphaera O. arcelliformis, O. hyalina, O. membranifera Acritarch Resembles arcellinids[30]
Papillomembrana P. compta Acritarch Formerly placed within Dasycladales[25]
Pustulisphaera P. membranacea Acritarch Bears three wall layers, with the middle one bearing pimple-like processes[25]
Salome S. hubeiensis Cyanobacteria A fossil identified as a "microburrow" was placed in this genus before being reidentified as an oblique section of a tubular fossil[30]
Sarcinophycus S. radiatus, S. papilloformis Eukaryota? Only known from Doushantuo, consists of bundles of sarcinoid cell packets[26]
Schizofusa S. zangwenlongii Acritarch Similar to Leiosphaeridia[30]
Sinosphaera S. asteriformis, S. rupina[25] Acritarch Bears small conical spines and a few larger ones[30]
Symphysosphaera S. basimembrana Acritarch Similar to Eosphaera, but larger[30]
Tanarium T. acus, T. conoideum, T. elegans, T. longitubulare, T. minimum, T. obesum, T. pilosiusculum, T. pycnacanthum, T. varium Acritarch Spine shapes vary from small and triangular to long and needle-like[30]
Tianzhushania T. spinosa Acritarch Bears a many-layered wall with spines penetrating through it[25]
Urasphaera U. fungiformis, U. nupta Acritarch Bears mushroom-shaped spines, hence the specific name fungiformis[30]
Variomargosphaeridium V. floridum Acritarch Bears processes which branch at their tips[30]
Vulcanosphaera V. phacelosa Acritarch Bears bundled hair-like processes[32]
Weissiella W. grandistella Acritarch Doushantuo fossils resemble Russian specimens, but are smaller[30]
Xenosphaera X. liantuoensis Acritarch Bears extremely thin processes, which have a tendency to not preserve[30]
Yushengia Y. ramispina Acritarch Resembles Weissiella, but has longer spines[30]

Miscellaneous taxa

Miscellaneous taxa
Genus Species Higher taxon Notes Images
Cucullus C. fraudulentus incertae sedis Originally interpreted as a sponge, then reinterpreted as a microbialite[37]
Paragraptobranca P. curvus incertae sedis Shares features with both graptolites and macroalgae[17]
Calyptrina C. striata Annelida? Almost certainly a metazoan[18]
File:Calyptrina striata sketch.jpg
Calyptrina reconstruction
Eoandromeda E. octobrachiata Ctenophora? Originally interpreted as the adult stage of various embryos,[38] then as a pelagic discoid ctenophore,[39] then an umbrella-shaped demersal form[40]
File:Eoandromeda.jpeg
Artist restoration of the "discoid ctenophore" hypothesis
Eocyathispongia E. qiania Porifera Earliest likely sponge fossil[41]
File:Eocyathispongia qiani.jpg
Reconstruction of Eocyathispongia
Linbotulitaenia

L. globosa

Metazoa Likely a trace fossil of a wriggling mucus-covered animal, possibly from Wenghuiia[17]
Protoconites P. minor Cnidaria? Likely a cnidarian-grade organism[42] similar to Cambrorhytium[18]
Sinospongia S. chenjunyuani, S. typica Porifera? May also be a vermiform alga like the Huainan biota[18]
Trilobozoa indet. Unapplicable Metazoa Known from an undescribed triradial fossil[43]
Vernanimalcula V. guizhouensis Bilateria? Originally interpreted as a bilaterian,[8] then found to be more likely an indeterminate acritarch,[9] then again found to be a likely bilaterian[44][10]
Wenghuiia W. jiangkouensis Annelida? Putatively the earliest known annelid (and also the earliest known bilaterian), even though it already seems very derived[45]

Palaeogeography

The formation was laid down on a carbonate shelf, whose rim enclosed a lagoon between tidal flats on the shore, and the deeper ocean. This lagoon was periodically anoxic or euxinic (containing hydrogen sulfide); variations in the chemistry in the lagoon can be detected in isotopic and elemental abundance cycles in the rock and possibly contributed to the fossil preservation.[5]

Geochemistry

The most recent Doushantuo rocks show a sharp decrease in the 13C/12C carbon isotope ratio. Since this change appears to be worldwide but its timing does not match that of any other known major event such as a mass extinction, it may represent "possible feedback relationships between evolutionary innovation and seawater chemistry" in which metazoans (multi-celled organisms) removed carbon from the water, which increased the concentration of oxygen, and the increased oxygen level made possible the evolution of new metazoans.[46]

See also

Footnotes

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References

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  • Knoll, A. H., 2003. Life on a Young Planet. Princeton Univ. Press.
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External links

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