View source for Halwaxiida

{{Short description|Proposed clade of extinct Lophotrochozoa}}
{{Taxobox
| fossil_range = {{fossil range|Early Cambrian|Middle Cambrian}}
| image = Halkieria uncropped.jpg
| image_caption = ''[[Halkieria]]'' sp.
| regnum = [[Animal]]ia
| superphylum = [[Lophotrochozoa]]
| unranked_phylum = '''Halwaxiida'''
| subdivision_ranks = Families
| subdivision = {{Linked taxon list
  |†Halkieriidae|
  |†Orthrozanclidae|Merged with Halkieriidae by Zhao et al,<ref name="ReferenceA">{{cite journal|doi=10.1038/s41598-017-16304-6|pmid=29176685|pmc=5701144|title=''Orthrozanclus elongata'' n. sp. And the significance of sclerite-covered taxa for early trochozoan evolution|journal=Scientific Reports|volume=7|issue=1|pages=16232|year=2017|last1=Zhao|first1=Fangchen|last2=Smith|first2=Martin R|last3=Yin|first3=Zongjun|last4=Zeng|first4=Han|last5=Li|first5=Guoxiang|last6=Zhu|first6=Maoyan|bibcode=2017NatSR...716232Z}}</ref>
  |†Wiwaxia{{!}}Wiwaxiidae|
  }}
}}

'''Halwaxiida''' or '''halwaxiids''' is a proposed [[clade]] equivalent to the older orders '''Sachitida''' He 1980<ref name="Bengtson1985">{{Cite journal | last1 = Bengtson | first1 = S. | title = Redescription of the Lower Cambrian ''Halkieria obliqua ''Poulsen | doi = 10.1080/11035898509452621 | journal = Geologiska Föreningen i Stockholm Förhandlingar | volume = 107 | issue = 2 | pages = 101–106 | year = 1985 }}</ref> and '''Thambetolepidea''' Jell 1981,<ref name="Jell1981">{{Cite journal| last1 = Jell | first1 = P.| title = ''Thambetolepis delicata'' gen. et sp. nov., an enigmatic fossil from the Early Cambrian of South Australia| journal = Alcheringa: An Australasian Journal of Palaeontology| volume = 5| issue = 2| pages = 85–89| year = 1981| doi = 10.1080/03115518108565423| bibcode = 1981Alch....5...85J}}</ref> loosely uniting scale-bearing [[Cambrian]] animals, which may lie in the [[Crown group#Stem groups|stem group]] to molluscs or lophotrochozoa.  Some palaeontologists question the validity of the Halwaxiida clade.<ref>{{Cite book |last=Minelli |first=Alessandro |url=https://books.google.com/books?id=jIASDAAAQBAJ&pg=PA61 |title=Perspectives in Animal Phylogeny and Evolution |date=2009 |publisher=Oxford University Press |isbn=978-0-19-856620-5 |pages=61 |language=en}}</ref>

==Definition and members==
{{ further | Halkieriid | Wiwaxia | Orthrozanclus | Odontogriphus }}
{| align="center" cellspacing="0" cellpadding="2"
|- valign="top"
| [[Image:Wiwaxia corrugata.jpg| thumb | 150px | A reconstruction of ''[[Wiwaxia]]'' ]]
| [[Image:Odontogriphus 01.png| thumb | 160px | Diagrammatic reconstruction of top and underside of ''[[Odontogriphus]]''<ref name="CaronScheltemaSchanderRudkin2006" /> ]]
| [[Image:Orthrozanclus reburrus E.jpg| thumb | 150px | Artist's impression of ''[[Orthrozanclus]]'']]
|}
{{Annotated image | float=right | caption=''[[Halkieria]]'' sclerite zones| image=Halkieria scale zones color coded.png | width=250 | height=262| image-width=85 | image-left=0 | image-top=0
 | annotations =
 {{Annotation|75|3|Top view|font-weight=bold|font-style=italic|font-size=15}}
 {{Annotation|75|202|Front / back view|font-weight=bold|font-style=italic|font-size=15}}
 {{Annotation|80|45|{{legend2|#d50000|{{inline-block|style=vertical-align:text-top|Palmate sclerites<br />– dorsal}}}}}}
 {{Annotation|80|95|{{legend2|#d4d400|{{inline-block|style=vertical-align:text-top|Cultrate sclerites<br />– lateral}}}}}}
 {{Annotation|80|145|{{legend2|#008440|{{inline-block|style=vertical-align:text-top|Siculate sclerites<br />– ventro-lateral}}}}}}
 }}
The name "halwaxiid" was formed by combining the names of two members of the proposed group, ''[[Halkieria]]'' and ''[[Wiwaxia]]''. The group was defined as a set of Early to Mid [[Cambrian]] animals that had: a "chain mail" coat of three [[concentric]] bands of small armor plates that are called [[sclerite]]s; in some cases, a small cap-like shell at the front end and in some cases both ends.<ref name="ConwayMorrisCaron2007" /> Some scientists are unhappy with this loose definition, arguing that such traits may have arisen [[convergent evolution|convergently]] rather than being inherited from a common ancestor. This objection implies that the group may not be [[monophyletic]],<ref name=ButterfieldPalAss>{{cite conference
| first = Butterfield | last = N.J. | date = 2007-12-18 | title = Lophotrochozoan roots and stems
| conference = Palaeontological Association Annual Meeting 2007
| conference-url = http://www.palass.org/modules.php?name=annual_meeting&page=1
| book-title = Programme with Abstracts
| editor = Budd, G.E. |editor2=Streng, M. |editor3=Daley, A.C. |editor4=Willman, S.
| location = Uppsala, Sweden | pages = 26–7
}}</ref> and that their similarities are not biologically equivalent but evolved separately.<ref name=Butterfield1990>{{Cite journal| last1 = Butterfield | first1 = N. J.| title = A reassessment of the enigmatic Burgess Shale fossil ''Wiwaxia corrugata'' (Matthew) and its relationship to the polychaete ''Canadia spinosa'' Walcott| jstor = 2400789| journal = Paleobiology| volume = 16| issue = 3| pages = 287–303| year = 1990| doi = 10.1017/S0094837300010009| bibcode = 1990Pbio...16..287B| s2cid = 88100863}}</ref> In fact the originators of the term "halwaxiid" prefer an evolutionary "family tree" in which the halwaxiids are not monophyletic.<ref name="ConwayMorrisCaron2007" />  The possibility of convergence is reinforced by the presence of a scleritome in a derived gastropod mollusc.<ref>{{Cite journal| doi = 10.1126/science.1087696| pmid = 14605361| year = 2003| last1 = Warén | first1 = A.| last2 = Bengtson | first2 = S.| last3 = Goffredi | first3 = S.| last4 = Van Dover | first4 = C.| title = A hot-vent gastropod with iron sulfide dermal sclerites| volume = 302| issue = 5647| pages = 1007| journal = Science | s2cid = 38386600}}</ref>

Other fossil organisms became involved in the debate as there were plausible arguments for considering them closely related to the halkieriids or ''Wiwaxia'' or both. ''[[Orthrozanclus]]'', first described in 2007, looked an intermediate between the halkieriids and ''[[Wiwaxia]]'' as it had a shell like that of halkieriids, and [[Biomineralization|unmineralized]] sclerites and long spines like those of ''Wiwaxia'' – in fact the article which first described ''Orthrozanclus'' introduced the term "halwaxiid".<ref name="ConwayMorrisCaron2007" /> New finds of ''[[Odontogriphus]]'', reported in 2006, put this animal into play as well – despite its lack of sclerites or shells, its feeding apparatus looks very like ''Wiwaxia''’s.<ref name="ConwayMorrisCaron2007" /><ref name="Butterfield2006" /> The [[Siphogonuchitida|siphogonuchitids]], a very Early Cambrian group known only from isolated sclerites among the [[small shelly fossils]], also appear in analyses as their sclerites suggest this group  may have been close to the ancestors of halkieriids.<ref name="ConwayMorrisCaron2007" /><ref name=Genera group=note>The original, possibly incomplete, list includes ''[[Australohalkieria]], [[Drepanochites]], [[Eohalobia]], [[Halkieria]], [[Lomasulcachites]], [[Ninella]], [[Ocruranus]], [[Oikozetetes]], [[Orthrozanclus]], [[Sinosachites]], [[Siphogonuchites]], [[Thambetolepis]]'', and ''[[Wiwaxia]]'' — {{cite journal
| author = Conway Morris, S. |author2=Caron, J.B. | year = 2007
| title = Halwaxiids and the Early Evolution of the Lophotrochozoans
| journal = Science | volume = 315 | issue = 5816 | pages = 1255–8 | doi=10.1126/science.1137187
| pmid = 17332408
|bibcode = 2007Sci...315.1255M |s2cid=22556453 }}</ref>

== Ecology ==
Halwaxiid sclerites were not able to grow once they had been formed, but must rather have been either moulted or resorbed to make way for new, larger sclerites; the smallest ''Wiwaxia'' specimens, 3.4&nbsp;mm in length, appear to bear the same number of sclerites (minus the spines) as adults 5&nbsp;cm long.<ref name=Bengtson1984>{{cite journal | doi = 10.1111/j.1502-3931.1984.tb02022.x | volume=17 | issue=4 | title=A comparative study of Lower Cambrian Halkieria and Middle Cambrian Wiwaxia | journal=Lethaia | pages=307–329| year=2007 | last1=Bengtson | first1=Stefan | last2=Morris | first2=Simon Conway }}</ref>  The new sclerites cannot simply have poked between existing sclerites; rather they must have been shed or resorbed, either one at a time or all at once; presumably the latter, as the close-fitting nature of the armour does not seem compatible with the easy loss of individual sclerites.<ref name=Bengtson1984/>

==Suggested phylogenies==

Since 1995 several [[phylogeny|phylogenies]], or evolutionary "family trees", have been suggested for these organisms. The position of ''[[Wiwaxia]]'' is highlighted, since the position of this organism has been central to the debate since 1990.<ref name="CaronScheltemaSchanderRudkin2006">{{cite journal
 | author = Caron, J.B. |author2=Scheltema, A. |author3=Schander, C. |author4=Rudkin, D.  | date=2006-07-13
 | title = A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale
 | journal = Nature | volume = 442 | issue = 7099 | pages = 159–163| doi = 10.1038/nature04894
 | pmid = 16838013 | bibcode=2006Natur.442..159C
 |hdl=1912/1404 |s2cid=4431853 | hdl-access = free }}</ref><ref name="ConwayMorrisCaron2007">{{cite journal 
|author1=Conway Morris, S. |author2=Caron, J.-B. 
| title=Halwaxiids and the Early Evolution of the Lophotrochozoans | journal=Science 
| date=March 2007 | volume=315 | issue=5816 | pages=1255–1258 | doi=10.1126/science.1137187 
| pmid=17332408 
|bibcode = 2007Sci...315.1255M |s2cid=22556453 
}}</ref><ref name="Butterfield1990"/><ref name="Butterfield2006">{{cite journal
 | author = Butterfield, N.J. | year = 2006 
 | title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
 | journal = BioEssays | volume = 28 | issue = 12 | pages = 1161–6 | doi = 10.1002/bies.20507 
 | url=http://www3.interscience.wiley.com/journal/113471993/abstract?CRETRY=1&SRETRY=0
 | archive-url=https://archive.today/20110813124946/http://www3.interscience.wiley.com/journal/113471993/abstract?CRETRY=1&SRETRY=0
 | url-status=dead
 | archive-date=2011-08-13
 | access-date=2008-08-06
 | pmid = 17120226
 | s2cid = 29130876 
 | url-access = subscription}}</ref><ref name="ConwayMorrisPeel1995">{{cite journal 
| title=Articulated Halkieriids from the Lower Cambrian of North Greenland and their Role in Early Protostome Evolution
|author1=Conway Morris, S. |author2=Peel, J. S. 
| journal=[[Philosophical Transactions of the Royal Society B]] | volume=347 
| issue=1321 | pages=305–358 | doi=10.1098/rstb.1995.0029
| url=https://dx.doi.org/10.1098/rstb.1995.0029 | access-date=2008-07-31 
| year=1995 | bibcode=1995RSPTB.347..305C 
| url-access=subscription}}</ref><ref name="EibyeJacobsen2004">{{Cite journal| last1 = Eibye-Jacobsen | first1 = D.| title = A reevaluation of ''Wiwaxia'' and the polychaetes of the Burgess Shale| journal = Lethaia| volume = 37| issue = 3| pages = 317–335| year = 2004| doi = 10.1080/00241160410002027| bibcode = 2004Letha..37..317E}}</ref><ref name="www3.interscience.wiley.com">{{cite journal 
| title=Original Molluscan Radula: Comparisons Among Aplacophora, Polyplacophora, Gastropoda, and the Cambrian Fossil ''Wiwaxia corrugata''
| author=Scheltema, A.H., Kerth, K., and Kuzirian, A.M. 
| journal=Journal of Morphology | volume=257 |pages=219–245 | year=2003 | doi= 10.1002/jmor.10121 
| url=http://www3.interscience.wiley.com/journal/104528759/abstract?CRETRY=1&SRETRY=0
| archive-url=https://archive.today/20121208160026/http://www3.interscience.wiley.com/journal/104528759/abstract?CRETRY=1&SRETRY=0
| url-status=dead
| archive-date=2012-12-08
| access-date=2008-08-05 
| pmid=12833382 
| issue=2 
| s2cid=32940079 
| url-access=subscription}}</ref>
{{div flex row|justify-content=space-evenly}}
{{thumb|align=left|content=<div style="text-align:left">
{{clade|style=font-size:90%
 |1={{clade
    |1=[[Siphogonuchitida]]
    |2={{clade
      |1=[[Mollusca|MOLLUSCA]]<br />([[crown group]])
      |2={{clade
         |1="Siberian halkieriid"
         |2={{clade
            |1={{clade
               |1={{clade
                  |1={{clade
                     |1=''[[Canadia (annelid)|Canadia]]''
                     |2=[[Annelida|ANNELIDA]]<br />([[crown group]])
                  }}
                  |2={{bg|#ffffcc|'''''[[Wiwaxia]]'''''}}
               }}
               |2=''[[Thambetolepis]]''<br />(halkieriid)
            }}
            |2={{clade
               |1=''Halkieria evangelista''
               |2=[[Brachiopoda|BRACHIOPODA]]<br />([[crown group]])
               }}
            }}
         }}
      }}
  }}
}}
</div>|caption=Cladogram: Conway Morris & Peel (1995)<ref name="ConwayMorrisPeel1995" />}}

{{thumb|align=left|content=<div style="text-align:left">
{{clade|style=font-size:90%
 |1={{clade
    |1=[[Annelida|ANNELIDA]]
    |2={{clade
      |1=? ''[[Kimberella]]''
      |2={{clade
         |1=''[[Odontogriphus]]''
         |2={{clade
            |1={{clade
               |1={{clade
                  |1={{bg|#ffffcc|'''''[[Wiwaxia]]'''''}}
                  |2={{clade
                     |1=[[Halkieriid]]s
                     |2=[[Neomeniomorpha]] / [[Solenogaster]]s<br />([[Aplacophora]])
                     |3=[[Polyplacophora]]
                     |4=Other [[crown group]] molluscs
                  }}
               }}
            }}
         }}
      }}
  }}
}}
}}
</div>|caption=Cladogram: Caron, Scheltema, ''et al.'' (2006)<ref name="CaronScheltemaSchanderRudkin2006" />}}

{{thumb|align=left|content=
{{div flex row|justify-content=space-evenly}}
<!-- Conway Morris and Caron (2007) Hypothesis 1 -->
{{clade|style=font-size:90%
 |1={{clade 
     |1={{clade 
         |1=''[[Kimberella]]''
         |2={{clade
             |1=''[[Odontogriphus]]''
             |2={{clade
                 |1={{clade
                     |1={{bg|#ffffcc|'''''[[Wiwaxia]]'''''}}
                     |2={{clade
                         |1=[[Siphogonuchitida]]
                         |2={{clade
                             |1=''[[Orthrozanclus]]''
                             |2=[[Halkieriid]]s
                            }}
                        }}
                    }}
                 |2={{clade
                     |1=[[Mollusca|MOLLUSCA]]<br />([[crown group]])
                   }}
                }}
            }}
        }}
     |2={{clade
         |1={{clade
             |1=''[[Canadia (annelid)|Canadia]]''
             |2=[[Annelida|ANNELIDA]]<br />([[crown group]])
            }}
         |2={{clade
             |1=''[[Micrina]]''
             |2=[[Brachiopoda|BRACHIOPODA]]<br />([[crown group]])
            }}
        }}
    }}
}}
<!-- Conway Morris and Caron (2007) Hypothesis 2 -->
{{clade 
     |1=''[[Kimberella]]''
     |2={{clade
         |1=''[[Odontogriphus]]''
         |2={{clade
             |1=[[Mollusca|MOLLUSCA]]<br />([[crown group]])
             |2={{clade
                 |1={{clade
                     |1=[[Siphogonuchitida]]
                     |2={{clade
                         |1=[[Halkieriid]]s
                         |2={{clade
                             |1=''[[Orthrozanclus]]''
                             |2={{bg|#ffffcc|'''''[[Wiwaxia]]'''''}}
                            }}
                        }}
                    }}
                 |2={{clade
                     |1={{clade
                         |1=''[[Canadia (annelid)|Canadia]]''
                         |2=[[Annelida|ANNELIDA]]<br />([[crown group]])
                        }}
                     |2={{clade
                         |1=''[[Micrina]]''
                         |2=[[Brachiopoda|BRACHIOPODA]]<br />([[crown group]])
                        }}
                    }}
                }}
            }}
        }}
    }}
{{div flex row end}}|caption=Cladograms: Conway Morris and Caron (2007) — Hypothesis 1 (left) and 2 (right).<br />The authors regard Hypothesis 1 as better supported by the evidence, but not robust.<ref name="ConwayMorrisCaron2007" />}}

{{thumb|align=left|content=<div style="text-align:left">
{{clade|style=font-size:90%
|1={{clade
|1=''[[Kimberella]]''
|2=''[[Odontogriphus]]''
|3={{clade
 |1={{clade|1={{clade|"'''Halsiphonids'''"<br />(halkieriids + <br />siphogonuchitids) }}}}
 |2={{clade|1={{clade|[[Mollusca|MOLLUSCA]]<br />([[crown group]]) }}}}
 |3={{clade
  |1={{bg|#ffffcc|'''''[[Wiwaxia]]'''''}}
  |2={{clade
   |1=''[[Canadia (annelid)|Canadia]]''
   |2=[[Annelida|ANNELIDA]]<br />([[crown group]])
   }}
  }}
 |4={{clade|1={{clade|''[[Micrina]]''|[[Brachiopoda|BRACHIOPODA]]<br />([[crown group]])}}}}
 }}
}}
}}
</div>|caption=Butterfield's alternative.<ref name=ButterfieldPalAss/>
}}
{{div flex row end}}

Not depicted: Sun et al. (2018)<ref>{{Cite journal |last1=Sun |first1=Haijing |last2=Smith |first2=Martin R. |last3=Zeng |first3=Han |last4=Zhao |first4=Fangchen |last5=Li |first5=Guoxiang |last6=Zhu |first6=Maoyan |date=2018-09-26 |title=Hyoliths with pedicles illuminate the origin of the brachiopod body plan |journal=Proceedings of the Royal Society B: Biological Sciences |volume=285 |issue=1887 |pages=20181780 |doi=10.1098/rspb.2018.1780 |pmc=6170810 |pmid=30257914}}</ref> propose ''Halkieria'' as a stem brachiopod, and ''Wiwaxia'' as a stem mollusc.

==Decline==
Wiwaxia and Halkieria did not thrive for a long period of time and were extinct before the Cambrian was over. A possible reason for this is that during the [[Cambrian substrate revolution]], the microbial mat's disappearance triggered an alternating pattern in the fauna that were present for the Halwaxiids to eat.

==''Wiwaxia'' and ''Odontogriphus''==
{{further|Wiwaxia|Odontogriphus}}
[[Image:Wiwaxia feeding apparatus 01.png| thumb | Conway Morris' reconstruction of ''[[Wiwaxia]]'' feeding apparatus (1985):<ref name="ConwayMorris1985" /> opened for feeding (left); folded (right). In both cases the front of the animal is at the top. ]]
Ever since the debate started in 1990, the most intense part has centered round ''[[Wiwaxia]]''. Until 2006 only one, poor-preserved fossil of ''Odontogriphus'' had been described; but in that year a description of many new, well-preserved specimens highlighted similarities between the two organisms, making the classification of ''Odontogriphus'' equally controversial.

Conway Morris (1985) originally dismissed the earliest classification of ''[[Wiwaxia]]'' as a [[polychaete]] worm, because he thought there was little structural similarity between a polychaete's scale-like [[elytra]] and ''Wiwaxia''’s [[sclerite]]s, and because the arrangement of the sclerites, with quite different numbers in each band, showed no sign of the regular [[Segmentation (biology)|segmentation]] that is a feature of polychaetes. Instead he thought ''Wiwaxia'' was very similar to shell-less [[aplacophora]]n [[mollusc]]s, that it must have moved on a mollusc-like muscular foot, and that its feeding apparatus looked like a primitive form of the molluscan [[radula]], a tooth-bearing [[chitin]]ous "tongue". Hence he classified it as a "[[cladistics|sister]]" of the molluscs.<ref name="ConwayMorris1985">{{Cite journal | last1 = Conway Morris | first1 = S.  | title = The Middle Cambrian metazoan ''Wiwaxia corrugata'' (Matthew) from the Burgess Shale and Ogygopsis Shale, British Columbia, Canada | jstor = 2396338 | journal = Philosophical Transactions of the Royal Society of London B| volume = 307 | issue = 1134 | pages = 507–582 | year = 1985 | doi = 10.1098/rstb.1985.0005|bibcode = 1985RSPTB.307..507M | doi-access = free }}</ref> When he briefly described the first articulated specimens of ''[[Halkieria]]'' in 1990, Conway Morris wrote of "the halkieriid-wiwaxiid body plan" and that the halkieriids might be close relatives of molluscs.<ref name="ConwayMorrisPeel1990">{{cite journal 
| title=Articulated halkieriids from the Lower Cambrian of north Greenland 
|author1=Conway Morris, S. |author2=Peel, J.S. | journal=Nature | volume=345 | pages=802–805 
| date=June 1990 | doi=10.1038/345802a0 
| issue=6278
|bibcode = 1990Natur.345..802M |s2cid=4324153 }} A short but free account is given at {{cite web 
| url=http://www.stephenjaygould.org/library/naturalhistory_cambrian.html 
| title=Showdown on the Burgess Shale 
| access-date=2008-07-31 
| archive-url=https://web.archive.org/web/20101210200047/http://www.stephenjaygould.org/library/naturalhistory_cambrian.html 
| archive-date=2010-12-10 
| url-status=dead 
}}</ref>
{{Clear}}
{{Annotated image | float=right | caption=''[[Wiwaxia]]'' spine, seen from front and side.<ref name="ConwayMorris1985" /> They had relatively soft bases and were rooted in pockets in the skin<ref name="Butterfield1990" />| image=Wiwaxia spines 01.png | width=220 | height=124| image-width=200 | image-left=0 | image-top=0
 | annotations =
 {{Annotation|80|40|"Blade"}}
 {{Annotation|80|100|{{legend2|#ff8080}} {{=}} Root}}
}}
{{Annotated image | float=right | caption=[[Halkieriid]] sclerite structure<ref name="Porter2008ChancelloriidsHalkieriids">{{cite journal 
| title=Skeletal microstructure indicates Chancelloriids and Halkieriids are closely related 
| author=Porter, S.M | journal=Palaeontology | volume=51 | issue=4 | year=2008 | pages=865–879 
| doi=10.1111/j.1475-4983.2008.00792.x 
| url=http://www.geol.ucsb.edu/faculty/porter/Papers/Coeloscleritophora.pdf | access-date=2008-08-07 
| doi-access=free | bibcode=2008Palgy..51..865P }}</ref>| image=Halkieriid sclerite structure 300.png | width=220 | height=96 | image-width=200 | image-left=0 | image-top=0
| annotations =
{{Annotation|141|44| {{=}} skin}}
{{Annotation|141|64| {{=}} [[aragonite]]}}
{{Annotation|141|84| {{=}} flesh}}
}}
Shortly after this in 1990 Butterfield published his first paper on ''Wiwaxia''. He argued that, since ''Wiwaxia''’s sclerites appeared  to be solid, they were not similar to the hollow sclerites of halkieriids.  In fact he thought they were more similar to the chitinous bristles ([[setae]]) that project from the bodies of modern annelids and in some [[genus|genera]] form leaf-like scales that cover the back like roof tiles - in composition, in detailed structure, in how they were attached to the body via follicle-like pockets in the skin, and in overall appearance. He also contended that Wiwaxia’s feeding apparatus, instead of being mounted in the middle of its "head", was just as likely to be mounted in two parts on the sides of the "head", an arrangement that is common in polychaetes. He therefore classified ''Wiwaxia'' as a polychaete.<ref name="Butterfield1990"/>

Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated ''Wiwaxia'' as an ancestor or "aunt" of the polychaetes. However they also argued that Wiwaxia was fairly closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller, more numerous and hollow. They wrote that in 1994 Butterfield had found ''Wiwaxia'' sclerites that were clearly hollow.  They noted that one specimen of Wiwaxia showed traces of a small shell, possibly a vestige left over from an earlier stage in the animal's evolution, and noted that one group of modern polychaetes also has what may be a vestigial shell. However they maintained that Wiwaxia’s feeding apparatus was much more like a molluscan radula. The cladogram they presented showed the halkieriids split into three groups: one as "aunts" of [[brachiopod]]s, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the [[lophophore]]; the second group of halkieriids as "aunts" of both ''Wiwaxia'' and [[annelid]] worms; and the earliest halkieriids as "great aunts" of all of these.<ref name="ConwayMorrisPeel1995" />

Marine biologist Amélie H. Scheltema  ''et al.'' (2003) argued that ''Wiwaxia''’s feeding apparatus was very similar to the [[radula]]s of some modern shell-less [[aplacophora]]n [[mollusc]]s, and that the sclerites of the two groups are very similar. They concluded that ''Wiwaxia'' was a member of a [[clade]] that includes molluscs.<ref name="www3.interscience.wiley.com"/>

Danish zoologist Danny Eibye-Jacobsen (2004) regarded bristles as a feature shared by molluscs, annelids and brachiopods. Hence even if ''Wiwaxia''’s sclerites closely resembled bristles, which he doubted, this would not prove that ''Wiwaxia''’s closest relative were annelids. He also pointed out that the very different numbers of sclerites in the various zones of ''Wiwaxia''’s body do not correspond to any reasonable pattern of segmentation. Since in his opinion ''Wiwaxia'' lacked other clearly polychaete features, he regarded this as an argument against classifying ''Wiwaxia'' as a polychaete. In his opinion there were no strong grounds for classifying ''Wiwaxia'' as a proto-annelid or a proto-mollusc, although he thought the objections against classification as a proto-annelid were the stronger.<ref name="EibyeJacobsen2004" />

{{Annotated image/Snail radula working}}
Caron, Scheltema ''et al.'' (2006) interpreted ''Odontogriphus''’s feeding apparatus as a forerunner of the molluscan [[radula]], on the grounds that: the occasional and less distinct third tooth-row looked like evidence that the animals grew replacement tooth-rows at the rear of their mouths and shed worn-out ones from the front, as happens with molluscan radulae; the isolated pairs of tooth-rows they found, not associated with body fossils but in the same relative positions as in the more complete fossils, suggested they were mounted on a fairly tough surface, like the [[chitin]]ous "belt" of a modern radula; they even found signs that discarded tooth-rows were sometimes eaten by the animals. They thought the teeth on the feeding apparatus of both ''Wiwaxia'' and ''Odontogriphus'' strongly resembled those of a modern group of molluscs, [[Neomeniomorpha]]. On the other hand, they thought ''Wiwaxia'' bore little resemblance to polychaetes as it showed no signs of segmentation, [[appendage]]s in front of the mouth, or "[[parapodia|legs]]" – all of which are typical polychaete features. Hence they classified both ''Odontogriphus'', and ''[[Wiwaxia]]'' with its "nearly identical" feeding apparatus, as primitive relatives of molluscs.<ref name="CaronScheltemaSchanderRudkin2006" />

Butterfield (2006) accepted that ''Wiwaxia'' and ''Odontogriphus'' were closely related  but wrote, "…it certainly does not require that they be [[shoehorn]]ed into the same lineage. …the seemingly trivial distinction between these two taxa is exactly what is expected at the divergence points leading from a last common ancestor to extant phyla." In his opinion the feeding apparatus of these organisms, with consisted of two or at most four rows of teeth, could not perform the functions of the "belt-like" molluscan radula with its numerous tooth-rows; the different tooth-rows in both ''Wiwaxia'' and ''Odontogriphus'' tooth-rows also have noticeably different shapes, while those of molluscan radulae are produced one after the other by the same group of "factory" cells and therefore are almost identical. He also regarded lines running across the middle region of ''Odontogriphus'' fossils as evidence of external segmentation, since the lines are evenly spaced and run exactly at [[right angle]]s to the long axis of the body. As in his earlier papers, Butterfield emphasized the similarities of internal structure between ''Wiwaxia''’s sclerites and the bristles of polychaetes, and the fact that polychates are the only modern organisms in which some of the bristles form a covering over the back. He therefore concluded that both ''Wiwaxia'' and ''Odontogriphus'' were stem-group polychaetes rather than stem-group molluscs.<ref name=Butterfield2006/>

In 2007 Conway Morris and Caron described a new fossil, ''[[Orthrozanclus]]'', which had a [[Biomineralization|mineralized]] shell like that of halkieriids, and unmineralized sclerites and long spines like those of ''Wiwaxia'' – and, like both of these, a soft underside which they interpreted as a muscular foot, and a similar arrangement of the sclerites into three [[concentric]] bands. Some of ''Orthrozanclus''’s sclerites appear to have been hollow, as the specimen includes what look like internal [[casting]]s. They took these features as evidence that the halkieriids, ''Wiwaxia'' and ''[[Orthrozanclus]]'' were very closely related and formed the group "halwaxiids". However the simplest "family tree" faces an obstacle: the siphogonuchitids appear in earlier rocks and had ''mineralized'' sclerites. Hence Conway Morris and Caron found it necessary to consider two more complex family trees, concluding that "Hypothesis 1" fitted the available data better, but was sensitive to [[sensitivity analysis|minor changes]] in the characteristics used:<ref name="ConwayMorrisCaron2007" />
# Conway Morris and Caron considered this hypothesis the more likely, although it is not robust:<ref name="ConwayMorrisCaron2007" />
#*''Kimberella'' and ''Odontogriphus'' were early, primitive molluscs, without sclerites or any kind of mineralized armor.
#*''Wiwaxia'', the siphogonuchitids, ''Orthrozanclus'' and ''Halkeria'' form a side-branch of the mollusc family tree, and diverged from it in that order. This would mean that: ''Wiwaxia'' was the first of them to have sclerites, which were unmineralized; the siphogonuchitids were the first to have mineralized sclerites, although their scleritomes were simpler; halkieriids then developed more complex scleritomes, and a shell at each end; in ''Orthrozanclus'' the scleritome became unmineralized again and the rear shell vanished or became so  small that it has not been seen in fossils. This hypothesis faces the difficulty that siphogonuchitids appear in earlier rocks and have simpler scleritomes than the other three groups.<ref name="ConwayMorrisCaron2007" />
#*The annelids and brachiopods evolved from the other main branch of the family tree, which did not include the molluscs.
# The alternative view is:
#*''Kimberella'' and ''Odontogriphus'' are early, primitive lophotrochozoans.
#*The siphogonuchitids, ''Halkeria'', ''Orthrozanclus'' and ''Wiwaxia'' form a group that is closer to the shared ancestor of annelids and brachiopods than it is to the molluscs. The siphogonuchitids are the first of the group to become distinctive, with ''two'' types of mineralized sclerites and a "shell" made of fused sclerites. Halkieriids had three types of sclerites  and two one-piece shells. In ''Orthrozanclus'' the sclerites became unmineralized and in ''Wiwaxia'' the shells were lost.<ref name="ConwayMorrisCaron2007" />

In 2008 Butterfield described a set of micro-fossils dated to between {{ma|515}} and {{ma|510}}, found in the Mahto Formation in [[Alberta]]'s [[Jasper National Park]] – this fossil bed is 5 – 10 M years older than the [[Burgess Shale]] in which the only known specimens of ''Odontogriphus'' and ''Wiwaxia'' were found. Some groupings of these micro-fossils showed a consistent arrangement that he interpreted as an "articulated apparatus" with tens of closely spaced tooth rows, apparently mounted on an organic base, and with noticeable signs of wear in the rows at one end. The rows were not quite identical, but he noted that some modern aplacophoran molluscs show similar variations. He concluded that the "articulated apparatus" was a genuine molluscan radula, most similar to those of modern aplacophorans or [[gastropod]]s. He then commented on the contrast between this apparatus and the two or rarely three widely spaced and more heterogeneous tooth-rows found in fossils of ''Odontogriphus'' and ''Wiwaxia'', and concluded that ''Odontogriphus'' and ''Wiwaxia'' were unlikely to be molluscs. He also noted that ''Wiwaxia'' lacked some polychaete features which he would expect to be easily preserved in fossils, and therefore classified ''Wiwaxia'' as a [[stem group|stem-group]] annelid, in other words an evolutionary "aunt" of modern annelids.<ref name="Butterfield2008EarlyCambrianRadula">{{cite journal 
| author=Butterfield, N.J. | title=An Early Cambrian Radula 
| journal=Journal of Paleontology | date=May 2008 | volume=82 | issue=3 | pages=543–554 
| doi=10.1666/07-066.1
| bibcode=2008JPal...82..543B 
| s2cid=86083492 
| url=https://dx.doi.org/10.1666/07-066.1 
| access-date=2008-08-20
| url-access=subscription}}</ref>

Recent finds indicate that ''Orthrozanclus'' is likely a halkieriid, but has little affinity to Wiwaxia.<ref name="ReferenceA">{{cite journal|doi=10.1038/s41598-017-16304-6|pmid=29176685|pmc=5701144|title=''Orthrozanclus elongata'' n. sp. And the significance of sclerite-covered taxa for early trochozoan evolution|journal=Scientific Reports|volume=7|issue=1|pages=16232|year=2017|last1=Zhao|first1=Fangchen|last2=Smith|first2=Martin R|last3=Yin|first3=Zongjun|last4=Zeng|first4=Han|last5=Li|first5=Guoxiang|last6=Zhu|first6=Maoyan|bibcode=2017NatSR...716232Z}}</ref>

==Halkieriids and siphogonuchitids==
{{further|Halkieriid}}
Most of the debate has been about halkieriids' relationship to [[mollusc]]s, [[annelid]]s and [[brachiopod]]s. However a paper published in 2008 argued that halkieriids were closely related to [[chancelloriidae|chancelloriid]]s, which have been generally regarded as [[sponge]]s.

===Relationship to Molluscs, Annelids and Brachiopods===
Halkieriid [[sclerite]]s were known for a long time as elements of the [[small shelly fossils]]. The first  articulated specimen, with all their hard parts together, were collected in 1989 and were described briefly in 1990 by Conway Morris and Peel (1990).<ref name="ConwayMorrisPeel1990"/> The same authors produced a more detailed analysis in 1995, which divided the halkieriids into three groups: one as "aunts" of [[brachiopod]]s, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the [[lophophore]]; the second group of halkieriids as "aunts" of both ''[[Wiwaxia]]'' and annelid worms; and the earliest halkieriids as "great aunts" of all of these.<ref name="ConwayMorrisPeel1995" />

In 2003 Cohen, Holmer and Luter supported the halkieriid-brachiopod relationship, suggesting that brachiopods may have arisen from a halkieriid lineage that developed a shorter body and larger shells, and then folded itself and finally grew a stalk out of what used to be the back.<ref>{{cite journal 
| author=Cohen, B. L. and Holmer, L. E. and Luter, C. | year=2003 
| title=The brachiopod fold: a neglected body plan hypothesis
| url=http://eprints.gla.ac.uk/2920/01/Cohen_2920.pdf | access-date=2008-08-07 | journal=Palaeontology 
| volume=46 | issue=1 | pages=59–65 | doi=10.1111/1475-4983.00287
| doi-access=free | bibcode=2003Palgy..46...59C 
}}</ref>

Vinther and Nielsen (2005) proposed instead that ''Halkieria'' was a [[crown group]] mollusc, in other words more similar to modern molluscs that to annelids, brachiopods or any intermediate groups. They argued that: ''Halkieria''’s sclerites resembled those of the modern [[solenogaster]] [[aplacophora]]n shell-less molluscs (see <ref name="Scheltema2002">{{Cite journal | last1 = Scheltema | first1 = A. H. | last2 = Ivanov | first2 = D. L. | doi = 10.1111/j.1744-7410.2002.tb00124.x | title = An aplacophoran postlarva with iterated dorsal groups of spicules and skeletal similarities to Paleozoic fossils | journal = Invertebrate Biology | volume = 121 | pages = 1–10| year = 2002| issue = 1 | bibcode = 2002InvBi.121....1S }}</ref>), of some modern [[polyplacophora]]n molluscs, which have several shell plates, and of the [[Ordovician]] polyplacophoran ''[[Echinochiton]]''; ''Halkieria''’s shells are more similar to the shells of [[conchifera]]n molluscs, since shells of both of these groups show no trace of the canals and pores seen in polyplacophoran shell plates; the bristles of brachiopods and annelids are similar to each other but not to ''Halkieria''’s sclerites.<ref>{{Cite journal | last1 = Vinther | first1 = J. | last2 = Nielsen | first2 = C. | title = The Early Cambrian ''Halkieria'' is a mollusc | journal = Zoologica Scripta | volume = 34 | pages = 81–89 | year = 2005 | doi = 10.1111/j.1463-6409.2005.00177.x| s2cid = 84493997 }}</ref>

However Conway Morris (2006) criticized Vinther and Nielsen's classification of ''Halkieria'' as a crown group mollusc, on the grounds that the growth of the spicules in the aplacophorans and polyplacophorans is not similar to the method of growth deduced for the complex halkieriid sclerites; in particular, he said, the hollow spines of various molluscs are not at all like the halkieriid sclerites with their complex internal channels. Conway Morris repeated his earlier conclusion that halkieriids were close to the ancestors of both molluscs and brachiopods.<ref>{{cite journal
| author=Conway Morris, S. | date=June 2006 | title=Darwin's dilemma: the realities of the Cambrian 'explosion' 
| journal=[[Philosophical Transactions of the Royal Society B]] | volume=361 | issue=1470| pmid=16754615
| doi=10.1098/rstb.2006.1846
| pages=1069–83
| pmc=1578734 
}}</ref>

In their description of the newly discovered ''[[Orthrozanclus]]'' (2007), which has similarities to both ''Wiwaxia'' and the halkieriids, Conway Morris and Caron also took account of the siphogonuchitids, a group known only from isolated mineralized sclerites that resemble those of the halkieriids. They proposed the two "family trees" described above:
# The halkieriids formed part of a "sister" group to the molluscs.  This hypothesis implies that the halwaxiids were not [[monophyletic]], since the siphogonuchitids appear between ''Wiwaxia'' and the halkieriids. Nevertheless, Conway Morris and Caron found that this [[phylogeny]] fitted the available data better, although it was not robust.
# The siphogonuchitids plus the halwaxiids formed a "sister" group to the annelids and brachiopods, while the molluscs were "aunts" of all of these.<ref name="ConwayMorrisCaron2007" />

===Relationship to chancelloriids===
{{further|Halkieriid}}
<!-- Deleted image removed: [[Image:Chancelloriid scleritomes 01.png| thumb | right | 250px | [[Chancelloriidae|Chancelloriid]] [[sclerite|scleritomes]] ]] -->
Porter (2008) revived an early 1980s idea that the sclerites of ''Halkieria'' are extremely similar to those of [[chancelloriidae|chancelloriid]]s. These were sessile, bag-like, [[Radial symmetry#Radial symmetry|radially symmetric]] organisms with an opening at the top.<ref name="Porter2008ChancelloriidsHalkieriids" />

Since their fossils show no signs of a gut or other organs, chancelloriids were originally classified as some kind of [[sponge]]. Butterfield and Nicholas (1996) argued that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites is similar to that of fibers of [[spongin]], a [[collagen]] [[protein]], in  modern keratose (horny) [[demosponge]]s.<ref>{{cite journal 
|author1=Butterfield, N. J. |author2=C. J. Nicholas | year=1996
| title=Burgess Shale-type preservation of both non-mineralizing and "shelly" Cambrian organisms from the Mackenzie Mountains, northwestern Canada 
| journal=Journal of Paleontology | volume=70 | pages=893–899
 | issue=6
|jstor=1306492 |doi=10.1017/S0022336000038579 |bibcode=1996JPal...70..893B |s2cid=133427906 }}</ref> However Janussen, Steiner and Zhu (2002) denied that chancellorids were sponges on the grounds that: chancellorids' hollow sclerites, probably made of [[aragonite]], were quite different from the solid, [[silica]]-based spines of demosponges; sponges have simple skins that are only one cell thick, while chancellorids'  thicker, more complex skins resemble those of more complex animals. In their opinion chancellorids were at least members of the [[Epitheliazoa]], a grade more complex than the [[Porifera]], to which sponges belong.<ref name="JanussenSteinerZhu2002">{{cite journal 
| author=Janussen, D., Steiner, M., and Zhu, M-Y. 
| title=New Well-preserved Scleritomes of Chancelloridae from the Early Cambrian Yuanshan Formation (Chengjiang, China) and the Middle Cambrian Wheeler Shale (Utah, USA) and paleobiological implications 
| journal=Journal of Paleontology | date=July 2002 | volume=76 |issue=4 | pages=596–606 
| doi=10.1666/0022-3360(2002)076<0596:NWPSOC>2.0.CO;2 
| bibcode=2002JPal...76..596J 
| s2cid=129127213 
| url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/76/4/596 | access-date=2008-08-04 
| issn=0022-3360 
| url-access=subscription 
}} Free full text without images at {{cite news | url=http://findarticles.com/p/articles/mi_qa3790/is_200207/ai_n9134583/pg_1?tag=artBody;col1 | title=(as above) | access-date=2008-08-04 | work=Journal of Paleontology | first=Dorte | last=Janussen | year=2002
}}</ref>

Porter (2008) found that the sclerites of halkieriids and chancelloriids resemble each other at all levels: both have an internal "pulp cavity" and a thin external organic layer; the walls are made of the same material, [[aragonite]]; the arrangement of the aragonite fibers is in each is the same, running mainly from base to tip but with each being closer to the surface at the end nearest the tip. It is extremely improbable that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related. This dilemma may be resolved in various ways:<ref name="Porter2008ChancelloriidsHalkieriids"/>
* One possibility is that chancelloriids evolved from [[bilaterian]] ancestors but then adopted a sessile lifestyle and rapidly lost all unnecessary features. However the gut and other internal organs have not been lost in other bilaterians that lost their external bilateral symmetry.<ref name="Porter2008ChancelloriidsHalkieriids" />
* On the other hand, perhaps chancelloriids are similar to the organisms from which bilaterians evolved, and the earliest bilaterians had similar sclerites. However, there are no fossils of such sclerites before {{ma|542}}, while ''[[Kimberella]]'' from {{ma|555}} was almost certainly a bilaterian,<ref name="Fedonkin1997">{{Cite journal | last1 = Fedonkin | first1 = M. A. | last2 = Waggoner | first2 = B. M. | title = The Late Precambrian fossil ''Kimberella'' is a mollusc-like bilaterian organism| journal = Nature | volume = 388 | issue = 6645| pages = 868–871 | year = 1997 | doi = 10.1038/42242| bibcode = 1997Natur.388..868F| s2cid = 4395089 | doi-access = free }}</ref> but shows no evidence of sclerites.<ref name="Porter2008ChancelloriidsHalkieriids" />
* One solution to this [[dilemma]] may be that preservation of [[small shelly fossils]] by coatings of [[phosphate]] was common only  for a relatively short time, during the Early Cambrian, and that coelosclerite-bearing organisms were alive several million years before and after the time of phosphatic preservation.<ref name="Porter2008ChancelloriidsHalkieriids" />
* Alternatively, perhaps the common ancestor of both chancelloriids and halkieriids had very similar but unmineralized coelosclerites, and some intermediate groups independently incorporated aragonite into these very similar structures.<ref name="Porter2008ChancelloriidsHalkieriids" /><ref>{{ Cite book
| author=Bengtson, S. | contribution=Mineralized skeletons and early animal evolution | pages=288 
| editor=Briggs, D.E.G. | title=Evolving form and function: fossils and development 
| publisher=Peabody Museum of Natural History, Yale University | location=New Haven, CT 
}}</ref>

==Notes and references==
<references group=note/>
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{{Reflist}}

{{Taxonbar|from=Q5643790}}

[[Category:Lophotrochozoa]]
[[Category:Protostome unranked clades]]
[[Category:Burgess Shale animals]]
[[Category:Cambrian animals]]