Conodont
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Conodonts are extinct worm-like forms with distinctive conical or multi-denticulate teeth made of apatite (calcium phosphate). The animals are sometimes referred to as conodontophora ("bearers of conodonts"), taking the word "conodont" to refer to the teeth themselves, but the strictly correct name for the animal group is 'Conodonta'. These tiny teeth are quite common in Paleozoic rocks and sands (250 to 500 million years old), but body fossils were not found until the early 1980s. The teeth (or more formally "elements") show complex, specialized structures, and survived through the ages and the fossilization process due to their resilient phosphatic chemical composition.
Conodonts and their presumed relatives are known from the Cambrian to the Late Triassic. The earliest forms are identified as protoconodonts, followed by paraconodonts, followed by euconodonts (or "true conodonts").
Following the discovery of eleven body fossils in Scotland and South Africa, most paleontologists think conodonts — which turn out to have fins with fin rays, chevron shaped muscles, a stiffening chord that is the precursor of the backbone, and eyes — are in the phylum Chordata. There are a lot of opinions about where the conodonts belong amongst the chordates/vertebrates. Most paleontologists (following Szaniawski) place the protoconodonts in a phylum along with the chaetognath worms, indicating that they are not close relatives of the true conodonts. Complete fossils are rare, but the eleven imprints that have been found show an eel-like creature with 15 or, more rarely, 19 elements forming a bilaterally symmetrical array in the head, comprising a feeding apparatus radically different from the jaws of modern animals.
Cladistic analyses by Donoghue et. al (1998, 2000) suggest that conodonts are vertebrates. The paraconodonts (known only from teeth) are thought to be related, but the relationship is unclear. According to Donoghue, protoconodonts are not related to the rest.
External links and references
- http://animaldiversity.ummz.umich.edu/site/accounts/information/Vertebrata.html
- http://tolweb.org/tree?group=Euconodonta&contgroup=Vertebrate
- http://www.le.ac.uk/geology/map2/map2stuff/model.html
- http://www.geocities.com/CapeCanaveral/Hall/1383/2TopCone.htm
- http://geology.csusb.edu/faculty/wbl/cdont_art.htm
- http://www.geo.ucalgary.ca/~macrae/
- http://www.nhm.ac.uk/palaeontology/micro/collections/conodont/
- Aldridge, R. J., Briggs, D. E. G., Smith, M. P., Clarkson, E. N. K. & Clark, N. D. L. (1993), The anatomy of conodonts. "Philosophical Transactions of the Royal Society of London, Series B", 340, 405-421.
- Aldridge, R. J. & Purnell, M. A. (1996). The conodont controversies. "Trends in Ecology and Evolution", 11, 463-468.
- Donoghue, P. C. J., Purnell, M. A., & Aldridge, R. J. (1998), Conodont anatomy, chordate phylogeny and vertebrate classification. Lethaia, 31, 211-219.
- Donoghue, P. C. J., Forey, P. L. & Aldridge, R. J. (2000), Conodont affinity and chordate phylogeny. "Biological Reviews". 75, 191-251.de:Conodonten