Kimberella
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2008-07-16T21:45:35Z
Philcha
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/* Occurrence */ timeline - again!
{{Taxobox
|image = Kimberella_blue.jpg
|image_caption = Cast of a partial ''Kimberella'' fossil. The bulges at the top of the image are the sides of the organism, and surrounded the rim of the creature; these are further flanked with a line which may represent the extent of a muscular foot.<ref name=Fedonkin1997/>
|fossil_range = [[Ediacaran]] (558-5Ma)
|regnum = [[Animal]]ia
|phylum = [[Mollusca]]?
|genus = '''''Kimberella'''''
|genus_authority = Glaessner & Wade 1966<ref name=Fedonkin2007>{{The Rise and Fall of the Ediacaran Biota|New data on ''Kimberella'', the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications|Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.|157|179|12}}</ref>
|species = '''''K. quadrata'''''
|binomial = ''Kimberella quadrata''
|binomial_authority = Glaessner & Wade 1966<ref name=Fedonkin2007/>
}}
'''''Kimberella''''' is a [[genus]] of [[fossil]]s known only from rocks of the [[Ediacaran]] period, and only one [[species]], '''''Kimberella quadrata''''', has been recognized. Specimens were first found in Australia's [[Ediacara Hills]], but recent research has concentrated on the numerous finds near the [[White Sea]] in [[Russia]], which cover an interval of time from {{ma|555|558}}.<ref name=Fedonkin2007/> As with many [[Ediacara biota|fossils from this time]], its evolutionary relationships to other organisms is hotly debated. Paleontologists initially classified ''Kimberella'' as a type of [[jellyfish]], but since 1997 features of its anatomy and its association with scratch marks resembling those made by a [[radula]] have been interpreted as signs that it may have been a [[mollusc]]. Although some paleontologists dispute its classification as a mollusc, it is generally accepted as being at least a [[bilaterian]].
The classification of ''Kimberella'' is important for scientific understanding of the [[Cambrian explosion]]: if it was a mollusc or at least a [[protostome]], the protostome and deuterostome lineages must have diverged significantly before {{ma|555}}. Even if it was a bilaterian but not a mollusc, its age would indicate that [[animal]]s were diversifying well before the start of the [[Cambrian]].
==Occurrence==
{{Graphical timeline
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|legend1=[[Neoproterozoic]]<br>(last [[Era (geology){{!}}era]] of the [[Precambrian]])
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|note21=[[Doushantuo Formation]]
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|note22=[[Ediacara biota]]
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|note25=? [[Cambrian explosion]], '''if''' it was sudden
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<!--Notes-->
|note7=''[[Kimberella]]''
|note7-at=-555
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|caption=Axis scale: millions of years ago.
<br>References: Waggoner 1998;<ref name="Waggoner1998t">{{cite journal
| author = Waggoner, Ben
| year = 1998
| title = Interpreting the Earliest Metazoan Fossils: What Can We Learn?
| journal = Integrative and Comparative Biology
| volume = 38
| issue = 6
| pages = 975—982
| issn = 1540-7063
| doi = 10.1093/icb/38.6.975
| url = http://intl-icb.oxfordjournals.org/cgi/content/abstract/38/6/975
| accessdate = 2007-03-08
}}
</ref> Hofmann 1990;<ref>{{cite journal
| title=Ediacaran remains from intertillite beds in northwestern Canada
| author=Hofmann, H.J.
| coauthors= Narbonne, G.M., Aitken, J.D.
| journal=Geology
| volume=18
| issue=12
| pages=1199-1202
| year=1990
| url=http://geology.geoscienceworld.org/cgi/content/abstract/18/12/1199
}}</ref> Martin ''et al'' 2000<ref name=Martin2000 />
}}
''Kimberella'' has been found both in the [[Ediacara Hills]] of [[South Australia]]<ref name=Glaessner1959>{{ cite journal | author=Glaessner, M., and Daily, B. | date=1959 | url=http://www.samuseum.sa.gov.au/Journals/RSAM/RSAM_v013/rsam_v013_p369p402.pdf | title=The Geology and Late Precambrian Fauna of the Ediacara Fossil Reserve | journal=Records of the South Australian Museum | volume=13 |pages=369–401 | accessdate=2008-07-16 }}</ref> and in the [[Ust’ Pinega Formation]] in the [[White Sea]] region of [[Russia]]. The White Sea fossils are often associated with the [[Ediacaran biota | Ediacaran "animals"]] ''[[Tribrachidium]]'' and ''[[Dickinsonia]]''; meandering [[trace fossil]] trails, possibly made by ''Kimberella'' itself; and [[algae]]. Beds in the the White Sea succession have been dated to {{Ma|555.3|error=0.3|million years of age}} and {{Ma|558}} by [[radiometric dating]], using uranium-lead ratios in [[zircon]]s found in volcanic ash layers that are sandwiched between layers that contain ''Kimberella'' fossils.<ref name=Martin2000>{{cite journal
| author = Martin, M.W.
| coauthors = Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.
| year = 2000
| date = [[2000-05-05]]
| title = Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution
| journal = Science
| volume = 288
| issue = 5467
| pages = 841
| doi = 10.1126/science.288.5467.841
| accessdate = 2007-05-10
| format = abstract
| pmid = 10797002
}}</ref> ''Kimberella'' fossils are also known from beds older and younger than this precisely dated range.<ref name=Fedonkin2007/> The fossils from the Ediacara Hills have not been dated precisely.
==Description==
Over 800 specimens, representing organisms of all stages of maturity, have now been found in the [[White Sea]] area at the bottom of fine-grained sandstone layers.<ref name=Fedonkin2007/> The large number of specimens, the small grain of the sediments and the variety of circumstances in which specimens were preserved provide detailed information about ''Kimberella''′s external form, internal anatomy, locomotion and feeding style.<ref name="FedonkinEtAl2007NewDataOnKimberella">{{ cite journal | title=New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications | author=Fedonkin, M.A., Simonetta, A. and Ivantsov, A.Y. | journal=Geological Society, London, Special Publications | date=2007 | volume-286 | pages=157-179 | doi=10.1144/SP286.12 | url=http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/fedonkin2.pdf | accessdate=2008-07-10 }}</ref>
All of the fossils are oval in outline, and larger specimens are more elongated. The only type of [[Symmetry (biology) | symmetry]] visible in the White Sea specimens is bilateral; there is no sign of any of the kinds of radial symmetry that are normal in the [[Cnidaria]], the group that includes [[jellyfish]], [[sea anemone]]s and [[Hydrozoa | hydra]]s. The Australian fossils were originally described as a type of jellyfish, but this is inconsistent with the bilateral symmetry in the fossils. The White Sea fossils and the surrounding sediments also show that ''Kimberella'' lived on the [[benthos |surface of the sea-floor]].<ref name=Fedonkin1997/>
''Kimberella'' had a single, [[Anatomical_terms_of_location#Dorsal_and_ventral | dorsal]] shell; in the larger specimens this reached up to 15 cm in length, 5 to 7cm in width, and was 3 to 4 cm high;<ref name="FedonkinEtAl2007NewDataOnKimberella" /> the smallest specimens are only about 2–3 mm long.<ref name=Fedonkin2007/> The shell was stiff but flexible, and appears to have been non-mineralized, becoming tougher as it grew larger (and presumably thicker) in more mature specimens.<ref name=Fedonkin2007/> At its highest point was a hood-like structure, forming what is thought to be the front.<ref name=Fedonkin1997/><ref name="FedonkinEtAl2007NewDataOnKimberella" /> In some specimens, the shell bears stripes spanning the width of the creature; these may represent the attachment sites of muscles.<ref name=Fedonkin2007/> Similar stripes around the edge of the shell may have been connected to muscles involved in retracting the muscular foot into the shell.<ref name=Fedonkin2007/>
[[Image:Kimberella recon 01.png | thumb | right| 250px | Reconstruction showing how far ''Kimberella''′s "skirt" extended beyond the shell.]]
The long axis of the organism is marked by a raised ridge; the middle axis is slightly humped. Kimberella′s body had no visible [[Segmentation (biology)|segmentation]] but had a series of repeated "[[Modularity (biology)|modules]]." Each module included a well-developed band of dorso-ventral muscles running from the top to the single, broad, muscular "foot", and smaller transverse ventral muscles from side to side on the underside of the body. The combination of the bands of dorso-ventral and transverse ventral muscles enabled Kimberella to move by making the foot ripple.<ref name=Fedonkin1997/><ref name="FedonkinEtAl2007NewDataOnKimberella" />
The body also had a frilled fringe which may have been part of the animal's respiratory system, performing a function similar to that of [[gills]]. The fact that the fringe extended well beyond the shell may indicate that ''Kimberella''′s "gills" were inefficient and needed a large area, or that there were no effective predators on ''Kimberella'' and the shell's main function was to provide a platform for the muscles.
<ref name="FedonkinEtAl2007NewDataOnKimberella" />
==Ecology==
''Kimberella'' dwelt in shallow (up to tens of metres depth) waters, sharing the quiet<!--is "calm" a better word to use?-->, well oxygenated sea floor with photosynthesic organisms and microbial mats.<ref name=Fedonkin2007/> Assemblages bearing ''Kimberella'' often also bear fossils of ''Yorgia'', ''Dickinsonia'', ''Tribrachidium'' and ''Charniodiscus'', suggesting that it lived along side these organisms.<ref name=Fedonkin2007/>
''Kimberella'' probably grazed on microbial mats, but a selective predatory habit cannot be ruled out.<ref name=Fedonkin2007/> As it ate, it moved "backwards"; the trail thus created was destroyed by the subsequent grazing activity.<ref name=Fedonkin2007/> The lack of evidence to the contrary suggests that the organisms reproduced sexually.<ref name=Fedonkin2007/>
The waters in which ''Kimberella'' dwelt were occasionally disturbed by sandy currents, caused when sediments were whipped up by storms or meltwater discharge, and washed over the creatures. In response to this stress, the organisms appear to have retracted their soft parts into their shells; apparently they could not move fast enough to outrun the currents.<ref name=Fedonkin2007/> Some organisms survived the current, and attempted to burrow out of the sand that had been deposited above them; some unsuccessful attempts can be seen where juveniles were fossilised at the end of a burrow a few centimetres long.<ref name=Fedonkin2007/>
==Taphonomy==
All ''Kimberella'' fossils are preserved as a depression in the base of a bed, implying that the organism, although not mineralised, was firm enough to resist being crushed as sediment accumulated above it; as the soft parts of the organism decayed, the soft muds underneath would be squeezed up into the shell, preserving the form of the organism.<ref name=Fedonkin2007/>
Preservation of most specimens was made possible by the fast sedimentation which quickly cut the organism off from seawater; it may also have been enhanced by the decay products of the rotting organism, which could have helped the overlying sediment to mineralise and harden.<ref name=Fedonkin2007/> It has been suggested that a mucus trail deposited by the organism may have assisted its preservation,<ref name=Fedonkin2007/> but experiments suggest that mucus disintegrates too easily to play a role in binding sediment together.<ref name=Getty2006>{{cite journal
| author = Getty, P.R.
| year = 2006
| title = Producing And Preserving ''Climactichnites''
| journal = 2006 Philadelphia Annual Meeting
| url = http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_111068.htm
| accessdate = 2008-06-02
}}</ref>
==Classification==
{{see|Triploblasty|Bilateria}}
All the ''Kimberella'' fossils found so far are assigned to one [[species]], ''K. quadrata''. The first specimens were discovered in Australia in 1959. They were originally classified as [[jellyfish]] by [[Martin Glaessner]] and [[Mary Wade (Paleontologist) | Mary Wade]] in 1966,<ref name=Glaessner1966>{{cite journal
| author = Glaessner, M.F.
| coauthors = Wade, M.
| year = 1966
| title = The late Precambrian fossils from Ediacara, South Australia
| journal = Palaeontology
| volume = 9
| issue = 4
| pages = 599
}}</ref> and then as [[cubozoa|box jellyfish]] by Wade in 1972,<ref name="Wade1972MedusoidsFromEdiacaraFauna">{{cite journal | author=Wade, M. | title=Hydrozoa and Scyphozoa and other medusoids from the Precambrian Ediacara fauna, South Australia | journal=Palaeontology | volume=15 | pages=197–225 | date=1972 }}</ref> a view which remained popular until the fossils of the White sea region were discovered; these prompted a reinterpretation.<ref name=Fedonkin2007/> Research on these specimens by [[Mikhail A. Fedonkin]], initially with Benjamin M. Waggoner in 1997,<ref name=Fedonkin1997/> led to ''Kimberella'' being recognised as the oldest well-documented [[triploblastic]] [[bilateria]]n organism - not a jellyfish at all.<ref name=Erwin20002>{{cite journal|author=Erwin, Douglas H.; Eric H. Davidson|year=2002|title=The last common bilaterian ancestor|journal=Development|volume=129|pages=3021–3032|url=http://dev.biologists.org/cgi/content/full/129/13/3021|pmid=12070079}}</ref>
So far ''Kimberella'' fossils show no sign of a [[radula]], the toothed [[chitinous]] "tongue" which is the diagnostic feature of modern [[molluscs]], excluding [[bivalves]]. Since radulae are very rarely preserved in fossil molluscs, its absence does not necessarily mean that ''K. quadrata'' did not have one. The rocks in the immediate vicinity of ''Kimberella'' fossils bear scratch marks which are very similar those made by the radulae of molluscs as they graze on [[microbial mat]]s. These [[trace fossil|trace]]s, named ''[[Radulichnus]]'', have been interpreted as circumstantial evidence for the presence of a radula. In conjunction with the univalve shell, this has been taken to indicate ''Kimberella'' was a mollusc or very closely related to molluscs.<ref name=Fedonkin1997>{{cite journal
| author = Fedonkin, M.A.
| coauthors = Waggoner, B.M.
| year = 1997
| title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
| journal = Nature
| volume = 388
| issue = 6645
| pages = 868
| doi = 10.1038/42242
}}</ref> In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable proboscis with hook-like organs at its end.<ref name="FedonkinEtAl2007NewDataOnKimberella" />
However, sceptics feel that the available evidence is not enough to reliably identify ''Kimberella'' as a mollusc or near-mollusc, considering it presumptuous to call it anything more than a "possible" mollusc,<ref name=Martin2000/> or even just a "probable bilaterian".<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
| accessdate = 2007-05-21
}}</ref> Nicholas J. Butterfield argues that ''Kimberella'''s association with ''Radulichnus'' marks is not strong evidence that it was a mollusc, as other groups of organisms bear structures capable of making similar marks.<ref name=Butterfield2006/><ref name=Butterfield2008>{{cite journal | author = Butterfield | year = 2008 | url = http://jpaleontol.geoscienceworld.org/cgi/content/full/82/3/543
| doi = 10.1666/07-066.1
| title = An Early Cambrian Radula
| journal = Journal of Paleontology
| volume = 82
| pages = 543
}}</ref>
==Theoretical importance==
{{main|Cambrian explosion}}
The [[Cambrian explosion]] is an apparently rapid increase in the variety of basic body structures of animals in the Early [[Cambrian]] period, starting after {{ma|543}} and finishing before {{ma|518}}.<ref name="CowenHistLife">{{ cite book | author=Cowen, R. | title=History of Life | publisher=Blackwell Science | edition=3rd | page=63 | date=2000 | isbn=0-632-04444-6 }}</ref> A few of the Early Cambrian fossils were already known in the mid-19th century, and [[Charles Darwin]] saw the apparently sudden appearance and [[evolutionary radiation | diversification]] of [[animal]]s as one of the main objections that could be made against his theory of evolution by [[natural selection]].<ref name=OriginOfSpecies>{{cite book
| title=On the Origin of Species by Natural Selection
| author=Darwin, C
| authorlink=Charles Darwin
| year=1859
| pages=315–316
| publisher=Murray, London, United Kingdom
}}</ref>
<!--
In animals at least as complex as an [[earthworm]], the [[embryo]] forms a dent on one side, which deepens to become the [[gut]]. In [[deuterostome]]s, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The other main group of complex animals is called [[protostome]]s because it used to be thought that in their embryos the dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the dent close up in the middle, leaving openings at the ends which become the mouth and anus.<ref>{{cite journal | title=Evolution of the bilaterian larval foregut | author=Arendt, D., Technau, U., and Wittbrodt, J. | journal=Nature | volume=409 | pages=81-85 | date=4 January 2001 | doi=10.1038/35051075 | url=http://www.nature.com/nature/journal/v409/n6816/full/409081a0.html | accessdate=2008-07-14}}</ref>
-->
The majority of animals more complex than [[jellyfish]] and other [[Cnidarians]] are split into two groups, the [[protostome]]s and [[deuterostome]]s.<ref name=Erwin20002 /> The mollusc-like features of ''Kimberella'' strongly suggest that it was a member of the protostomes.<ref name=Fedonkin1997 /><ref name="FedonkinEtAl2007NewDataOnKimberella" /> If so, this means that the protostome and deuterostome lineages must have split some time before ''Kimberella'' - appeared at least {{ma|558}}, and hence well before the start of the Cambrian {{ma|Cambrian}}. Even if it is not a protostome, it is widely accepted as a member of the more inclusive [[bilaterian]] clade.<ref name=Erwin20002 /><ref name=Butterfield2006 /> Since fossils of rather modern-looking [[Cnidaria]]ns have been found in the [[Doushantuo formation | Doushantuo]] [[lagerstätte]], the Cnidarian and bilaterian lineages would have diverged well over {{Ma|580}}.<ref name=Erwin20002 />
==References==
{{reflist}}
==External links==
*[http://www.ucmp.berkeley.edu/vendian/kimberella2.html image from UCMP]
{{Creatures of the slime}}
[[Category:Prehistoric animals]]
[[Category:Ediacaran biota]]
[[Category:White Sea]]