Editing Opabinia (section)

==Theoretical significance==
{{main|Cambrian explosion}}
[[File:Walcott_Cambrian_Geology_and_Paleontology_II_plate_28.jpg|thumb|Top left: retouched image of ''Opabinia'' (''Walcott Cambrian Geology and Paleontology II'' by [[Charles Doolittle Walcott]])]] ''Opabinia'' made it clear how little was known about soft-bodied animals, which do not usually leave fossils.<ref name="Whittington1975" /> When Whittington described it in the mid-1970s, there was already a vigorous debate about the early evolution of [[animal]]s. [[Preston Cloud]] argued in 1948 and 1968 that the process was "explosive",<ref>{{cite journal | author=Cloud, P. E. | year=1948
| title=Some problems and patterns of evolution exemplified by fossil invertebrates | journal=Evolution | volume=2 | issue=4 | pages=322–350 | doi=10.2307/2405523 | pmid=18122310 | jstor=2405523}} and {{Cite book
| author=Cloud, P. E. | year= 1968 | contribution=Pre-metazoan evolution and the origins of the Metazoa.
| pages=1–72 | editor=Drake, E. T. | title=Evolution and Environment | publisher=Yale University Press
| location=New Haven, Conn.
}}</ref> and in the early 1970s [[Niles Eldredge]] and [[Stephen Jay Gould]] developed their theory of [[punctuated equilibrium]], which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.<ref>{{Cite book
| author1=Eldredge, N.
|contributor=Eldredge, N. |contributor2=Gould, S. J.
|contribution=APPENDIX: Punctuated Equilibria: An Alternative to Phyletic Gradualism | pages=193–224
| title=Time Frames: The Evolution of Punctuated Equilibria
|doi=10.1515/9781400860296.193
|isbn=9781400860296
|date=1989
}}</ref> On the other hand, around the same time Wyatt Durham and [[Martin Glaessner]] both argued that the animal kingdom had a long [[Proterozoic]] history that was hidden by the lack of fossils.<ref name="Bengtson2004">{{cite conference |last=Bengtson |first=Stefan |year=2004 |chapter=Early Skeletal Fossils |chapter-url=https://books.google.com/books?id=NNZZPwAACAAJ |editor1-last=Lipps |editor1-first=Jere H. |editor1-link=Jere H. Lipps |editor2-last=Waggoner |editor2-first=Benjamin M. |title=Neoproterozoic-Cambrian Biological Revolutions: Presented as a Paleontological Society Short Course at the Annual Meeting of the Geological Society of America, Denver, Colorado, November 6, 2004 |series=Paleontological Society Papers |volume=10 |location=New Haven, CT |publisher=Yale University Reprographics & Imaging Service; [[Paleontological Society]] |issn=1089-3326 |oclc=57481790 |access-date=2015-02-06}}</ref><ref>{{cite journal
|author=Durham, J. W. |year=1971 |title=The fossil record and the origin of the Deuterostomata |journal=Proceedings of the North American Paleontological Convention, Part H | pages=1104–1132 }} and {{Cite book |author=Glaessner, M. F. |year=1972 |contribution=Precambrian palaeozoology |pages=43–52 |editor=Jones, J. B. |editor2=McGowran, B. |title=Stratigraphic Problems of the Later Precambrian and Early Cambrian |volume=1 |publisher=University of Adelaide}}</ref> Whittington (1975) concluded that ''Opabinia'', and other [[taxa]] such as ''[[Marrella]]'' and ''[[Yohoia]]'', cannot be accommodated in modern groups. This was one of the primary reasons why Gould in his book on the [[Burgess Shale]], ''[[Wonderful Life (book)|Wonderful Life]]'', considered that Early Cambrian life was much more disparate and "experimental" than any later set of animals and that the [[Cambrian explosion]] was a truly dramatic event, possibly driven by unusual [[evolution]]ary mechanisms.<ref name=":0">{{cite book |author=Gould, S. J. |title=Wonderful Life |publisher=Hutchinson Radius |year=1989 |isbn=978-0-09-174271-3 |pages=124–136&nbsp;ff }}</ref> He regarded ''Opabinia'' as so important to understanding this phenomenon that he wanted to call his book ''Homage to Opabinia''.<ref>{{cite book |last=Knoll |first=A. H. |title=The First Three Billion Years of Evolution on Earth |publisher=Princeton University Press |year=2004 |page=192 |chapter=Cambrian Redux |chapter-url=https://books.google.com/books?id=Lq0S-C6zMHAC&q=%22burgess+shale%22&pg=PA192 |access-date=2009-04-22 |isbn=978-0-691-12029-4}}</ref>

However, other discoveries and analyses soon followed, revealing similar-looking animals such as ''[[Anomalocaris]]'' from the Burgess Shale and ''[[Kerygmachela]]'' from [[Sirius Passet]].<ref name="Bergström1986 "/><ref name="Budd1993" /> Another Burgess Shale animal, ''Aysheaia'', was considered very similar to modern [[Onychophora]],<ref>{{cite journal |title=Affinities of ''Aysheaia'' (Onychophora), with Description of a New Cambrian Species |author=Robison, R. A. |journal=Journal of Paleontology |volume=59 |issue=1 |date=January 1985 |pages=226–235 |jstor=1304837}}</ref> which are regarded as close relatives of arthropods.<ref>{{cite journal |author1=Jacobs, D. K. |author2=Wray, C. G. |author3=Wedeen, C. J. |author4=Kostriken, R. |author5=DeSalle, R. |author6=Staton, J. L. |author7=Gates, R. D. |author8=Lindberg, D. R. |title=Molluscan engrailed Expression, Serial Organization, and Shell Evolution |year=2000 |journal=Evolution and Development |volume=2 |issue=6 |pages=340–347 |doi=10.1046/j.1525-142x.2000.00077.x |pmid=11256378|s2cid=25274057 }}</ref> Paleontologists defined a group called [[lobopodia]]ns to include fossil panarthropods that are thought to be close relatives of onychophorans, tardigrades and arthropods but lack jointed limbs. This group was later widely accepted as a paraphyletic grade that led to the origin of extant panarthropod phyla.

[[File:Crown n Stem Groups.svg|thumb|Concept of [[stem group]]s<ref name="CraskeJefferies1989" />{{colbegin}}{{ubl
|{{legend2|text={{resize|120%|'''—'''}}||{{nowrap|{{=}} Lines of descent}}}}
|{{legend2|black|border=1px solid gray|{{=}} Basal node}}
|{{legend2|white|border=1px solid gray|{{=}} Crown node}}
|{{legend2|#8080ff|border=1px solid gray|{{=}} Total group}}
|{{legend2|#faada7|border=1px solid gray|{{=}} Crown group}}
|{{legend2|#fdefa4|border=1px solid gray|{{=}} Stem group}}
}}{{colend}}]]
While this discussion about specific fossils such as ''Opabinia'' and ''Anomalocaris'' was going on in the late 20th century, the concept of [[stem group]]s was introduced to cover evolutionary "aunts" and "cousins". A [[crown group]] is a group of closely related living animals plus their last common ancestor plus all its descendants. A stem group contains offshoots from members of the lineage earlier than the last common ancestor of the crown group; it is a ''relative'' concept, for example [[tardigrade]]s are living animals that form a crown group in their own right, but Budd (1996) regarded them also as being a stem group relative to the arthropods.<ref name="Budd1996" /><ref name="CraskeJefferies1989">{{cite journal |author1=Craske, A. J. |author2=Jefferies, R. P. S. |year=1989 |title=A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion |journal=Palaeontology |volume=32 |pages=69–99
}}</ref> Viewing strange-looking organisms like ''Opabinia'' in this way makes it possible to see that, while the Cambrian explosion was unusual, it can be understood in terms of normal evolutionary processes.<ref name=Budd2003>{{cite journal |author=Budd, G. E. |year=2003 |title=The Cambrian Fossil Record and the Origin of the Phyla |journal=Integrative and Comparative Biology |volume=43 |issue=1 |pages=157–165 |doi=10.1093/icb/43.1.157 |doi-access=free |pmid=21680420}}</ref>

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