Fossil Wiki
Don't have an account?
Register
Advertisement
Register
in: Pages with reference errors, Pages with DOIs broken since 2009, Pages using ISBN magic links, Chordates

Chordata

Edit
Chordata
Fossil range: Early Cambrian – Recent
Scientific classification

Kingdom

Animalia

Subkingdom

Eumetazoa

(Unranked)

Bilateria

Superphylum

Deuterostomia

Phylum

Chordata
Bateson, 1885

Classes

See text.



Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates.

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses, but the current consensus is that chordates are monophyletic, in other words contain all and only the descendants of a single common ancestor which is itself a chordate, and that craniates' nearest relatives are cephalochordates. All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, which implies that these are vertebrates. Because the fossil record of chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. However the use of molecular phylogenetics for dating evolutionary transitions is controversial.

Origins[]

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, and chordates are deuterostomes.[1] It seems very likely that 555 million years old Kimberella was a member of the protostomes.[2][3] If so, this means that the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least 558 million years ago, and hence well before the start of the Cambrian 542 million years ago.[1] The Ediacaran fossil Ernettia, from about 549-543 Ma, may represent a deuterostome animal.[4]

Haikouichthys4

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[5]

Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids) are quite common from the start of the Cambrian, 542 million years ago.[6] The Middle Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[7] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[8][9] Another Chenjiang fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes — although it also had short tentacles round its mouth.[9] Haikouichthys and Myllokunmingia, also from the Chenjiang fauna, are regarded as fish.[5][10] Pikaia, discovered much earlier but from the Mid Cambrian Burgess Shale, is also regarded as a primitive chordate.[11] On the other hand fossils of early chordates are very rare, since non-vertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.[12]

Deuterostomes

Xenoturbellida






Hemichordates



Echinoderms



Chordates

Tunicates (Urochordates)




Cephalochordates



Craniates







A consensus family tree of the chordates[13][14]

The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected. Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but open the possibility that tunicates (urochordates) are "basal deuterostomes", in other words surviving members of the group from which echinoderms, hemichordates and chordates evolved.[15] Most researchers agree that, within the chordates, craniates are most closely related to cephalochordates, but there also reasons for regarding tunicates (urochordates) as craniates' closest relatives.[13][16] One other phylum, Xenoturbellida, appears to be basal within the deuterostomes, in other words closer to the original deuterostomes than to the chordates, echinoderms and hemichordates.[14]

Since chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques, in other words by analysing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years ago and the earliest chordates around 896 million years ago.[16] However molecular estimates of dates often disagree with each other and with the fossil record,[16] and their assumption that the molecular clock runs at a known constant rate has been challenged.[17][18]

Classification[]

Taxonomy[]

The following schema is from the third edition of Vertebrate Palaeontology.[19] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

  • Phylum Chordata
    • Subphylum Tunicata (Urochordata) — (tunicates, 3,000 species)
    • Subphylum Cephalochordata (Acraniata) — (lancelets, 30 species)
    • Subphylum Vertebrata (Craniata) (vertebrates — animals with backbones; 57,674 species)
      • Class 'Agnatha'Paraphyletic (jawless vertebrates; 100+ species)
        • Subclass Myxinoidea (hagfish; 65 species)
        • Subclass Petromyzontida (Lampreys)
        • Subclass Conodonta
        • Subclass Pteraspidomorphi (Paleozoic jawless fish)
          • Order Anaspida
          • Order Thelodonti (Paleozoic jawless fish)
      • Infraphylum Gnathostomata (jawed vertebrates)
        • Class Placodermi (Paleozoic armoured forms)
        • Class Chondrichthyes (cartilaginous fish; 900+ species)
        • Class Acanthodii (Paleozoic "spiny sharks")
        • Class Osteichthyes (bony fishes; 30,000+ species)
        • Superclass Tetrapoda (four-legged vertebrates; 18,000+ species)
          • Class Amphibia (amphibians; 6,000 species)
          • Series Amniota (with amniotic egg)
            • Class Reptilia — (reptiles; 8,225+ species)
            • Class Aves (birds; 8,800–10,000 species)
            • Class Mammalia (mammals; 5,800 species)

References[]

  1. ^ a b Erwin, Douglas H.; Eric H. Davidson (01 Jul 2002). "The last common bilaterian ancestor". Development 129 (13): 3021–3032. PMID 12070079. http://dev.biologists.org/cgi/content/full/129/13/3021. 
  2. ^ New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications (2007), "Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications, 286, London: Geological Society, pp. 157–179, doi:10.1144/SP286.12, ISBN 9781862392335 
  3. ^ Butterfield, N.J. (2006). "Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale". Bioessays 28 (12): 1161–6. doi:10.1002/bies.20507. 
  4. ^ Dzik , J. (June 1999). "Organic membranous skeleton of the Precambrian metazoans from Namibia". Geology 27 (6): 519–522. doi:10.1130/0091-7613(1999)027<0519:OMSOTP>2.3.CO;2. http://geology.geoscienceworld.org/cgi/content/abstract/27/6/519. Retrieved on 2008-09-22.  Ernettia is from the Kuibis formation, approximate date given by Waggoner, B. (2003). "The Ediacaran Biotas in Space and Time". Integrative and Comparative Biology 43 (1): 104–113. doi:10.1093/icb/43.1.104. http://icb.oxfordjournals.org/cgi/content/full/43/1/104. Retrieved on 2008-09-22. 
  5. ^ a b Shu, D-G., Conway Morris, S., and Han, J., et al. (January 2003). "Head and backbone of the Early Cambrian vertebrate Haikouichthys". Nature 421: 526–529. doi:10.1038/nature01264; (inactive 2009-03-14). http://www.nature.com/nature/journal/v421/n6922/abs/nature01264.html. Retrieved on 2008-09-21. 
  6. ^ Bengtson, S. (2004), Early skeletal fossils, in Lipps, J.H., and Waggoner, B.M., "Neoproterozoic-Cambrian Biological Revolutions" (PDF), Palentological Society Papers 10: 67–78, http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf, retrieved on 2008-07-18 
  7. ^ Bengtson, S., and Urbanek, A. (October 2007). "Rhabdotubus, a Middle Cambrian rhabdopleurid hemichordate". Lethaia 19 (4): 293–308. doi:10.1111/j.1502-3931.1986.tb00743.x. http://www3.interscience.wiley.com/journal/120025616/abstract. Retrieved on 2008-09-23. 
  8. ^ Shu, D., Zhang, X. and Chen, L. (April 1996). "Reinterpretation of Yunnanozoon as the earliest known hemichordate". Nature 380: 428–430. doi:10.1038/380428a0. http://www.nature.com/nature/journal/v380/n6573/abs/380428a0.html. Retrieved on 2008-09-23. 
  9. ^ a b Chen, J-Y., Hang, D-Y., and Li, C.W. (December 1999). "An early Cambrian craniate-like chordate". Nature 402: 518–522. doi:10.1038/990080. http://www.nature.com/nature/journal/v402/n6761/abs/402518a0.html. Retrieved on 2008-09-23. 
  10. ^ Shu, D-G., Conway Morris, S., and Zhang, X-L. (November 1999). "Lower Cambrian vertebrates from south China" (PDF). Nature 402: 42. doi:10.1038/46965. http://www.bios.niu.edu/davis/bios458/Shu1.pdf. Retrieved on 2008-09-23. 
  11. ^ Shu, D-G., Conway Morris, S., and Zhang, X-L. (November 1996). "A Pikaia-like chordate from the Lower Cambrian of China". Nature 384: 157–158. doi:10.1038/384157a0. http://www.nature.com/nature/journal/v384/n6605/abs/384157a0.html. Retrieved on 2008-09-23. 
  12. ^ Conway Morris, S. (2008), "A Redescription of a Rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada", Journal of Paleontology 82 (2): 424–430, doi:10.1666/06-130.1, http://jpaleontol.geoscienceworld.org/cgi/content/extract/82/2/424, retrieved on 2009-04-28 
  13. ^ a b Cite error: Invalid <ref> tag; no text was provided for refs named Ruppert2005
  14. ^ a b Perseke M, Hankeln T, Weich B, Fritzsch G, Stadler PF, Israelsson O, Bernhard D, Schlegel M. (2007) "The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis". Theory Biosci. 126(1):35–42. Available online at [1]
  15. ^ Winchell, C.J., Sullivan, J., Cameron, C.B., Swalla, B.J., and Mallatt, J. (01 May 2002). "Evaluating Hypotheses of Deuterostome Phylogeny and Chordate Evolution with New LSU and SSU Ribosomal DNA Data". Molecular Biology and Evolution 19 (5): 762–776. PMID 11961109. http://mbe.oxfordjournals.org/cgi/content/full/19/5/762#MBEV-19-05-09-SWALLA1. Retrieved on 2008-09-23. 
  16. ^ a b c Blair, J.E., and S. Blair Hedges, S.B. (2005). "Molecular Phylogeny and Divergence Times of Deuterostome Animals". Molecular Biology and Evolution 22 (11): 2275–2284. doi:10.1093/molbev/msi225. PMID 16049193. http://mbe.oxfordjournals.org/cgi/content/full/22/11/2275. Retrieved on 2008-09-23. 
  17. ^ Ayala, F.J. (1999). "Molecular clock mirages". BioEssays 21 (1): 71–75. doi:10.1002/(SICI)1521-1878(199901)21:1<71::AID-BIES9>3.0.CO;2-B. http://www3.interscience.wiley.com/cgi-bin/abstract/60000186/ABSTRACT?CRETRY=1&SRETRY=0. 
  18. ^ Schwartz, J. H. and Maresca, B. (2006). "Do Molecular Clocks Run at All? A Critique of Molecular Systematics". Biological Theory 1: 357–371. doi:10.1162/biot.2006.1.4.357. 
  19. ^ Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing, 472 pp. The classification scheme is available online


External links[]

Community content is available under CC-BY-SA unless otherwise noted.
Advertisement

Fan Feed

More Fossil Wiki