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| Pikaia Fossil range: Mid Cambrian |
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Pikaia gracilens is an extinct animal known from the Middle Cambrian fossil found near Mount Pika in the Burgess Shale of British Columbia. It was discovered by Charles Walcott and was first described by him in 1911. Based on the obvious and regular segmentation of the body, Walcott classified it as a Polychaete worm. It resembles a living chordate commonly known as the lancelet and perhaps swam much like an eel.
During his re-examination of the Burgess Shale fauna in 1979, paleontologist Simon Conway Morris placed P. gracilens in the chordates, making it perhaps the oldest known ancestor of modern vertebrates, because it seemed to have a very primitive, proto-notochord. Further, the status of Pikaia as a chordate is not universally accepted; its preservational mode suggests that it had cuticle, which is uncharacteristic of the vertebrates;1 further, its tentacles are unknown from other vertebrate lineages.1 The presence of earlier vertebrates in the Chengjiang, including Haikouichthys and Myllokunmingia, appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,2 but the presence of tentacles is still intriguing, and the organism cannot be conclusively assigned even to the vertebrate stem group.3
Averaging about 1½ inches (5 cm) in length, Pikaia swam above the sea floor using its body and an expanded tail fin. Pikaia may have filtered particles from the water as it swam along.citation needed Its "tentacles" may be comparable to those in the present-day hagfish, a jawless chordate.citation needed Only 60 specimens have been found to date.
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See also
Further reading
Gould, Stephen Jay. 1989. Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton, New York, NY.
Conway Morris, Simon. 1998. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press, New York, New York.
Referenes
- ^ a b Butterfield, N.J. (1990), "Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale", Paleobiology 16 (3): 272–286, http://www.jstor.org/stable/pdfplus/2400788.pdf
- ^ 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, http://www.bioone.org/perlserv/?request=get-document
- ^ Donoghue, P.C.J.; Purnell, M.A. (2005), "Genome duplication, extinction and vertebrate evolution", Trends in Ecology & Evolution 20 (6): 312–319, http://palaeo.gly.bris.ac.uk/Publs/donoghue/Donoghue_and_Purnell_2005.pdf
External links
- An artist's rendering
- Fossils of the Burgess Shale - Middle Cambrian
- http://www.ucm.es/info/zoo/Vertebrados/JADiaz/evolucion.htm (Spanish)
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- This page was last modified on 21 November 2008, at 04:39.
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