Conodonta

Conodonts Fossil range: Late Cambrian to Late Triassic
Reconstruction of a Conodont
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Conodonta
Groups
Protoconodonta Paraconodonta Euconodonta

Conodont elements from the Deer Valley Member of the Mauch Chunk Formation

Conodonts are extinct chordates that form the class Conodonta. For many years, they were known only from tooth-like microfossils now called cododont elements, found in isolation. The animal is also called conodontophora (conodont bearers) to avoid ambiguity.

Description

The eleven known fossil imprints of conodont animals depict an eel-like creature with 15 or, more rarely, 19 elements forming a bilaterally symmetrical array in the head. This array comprised a feeding apparatus radically different from the jaws of modern animals. There are three forms of teeth, coniform cones, ramiform bars, and pectiniform platforms, which may have performed different roles. Conodonts are thought to have been herbivores. Despite their ferocious appearance, the "teeth" were probably used to filter out plankton and pass it down the throat.^{[citation needed]} It is possible that some were used as biting teeth, but the lateral position of the eyes makes a carnivorous role unlikely.

Once thought to only exist on the millimetre scale, a well-preserved and unusually large genus, Promissum, was found in 1994. Gabbott, S.E.; R. J. Aldridge, J. N. Theron (1995). "A giant conodont with preserved muscle tissue from the Upper Ordovician of South Africa". Nature 374: 800-803. doi:10.1038/374800a0. Retrieved on 2007-09-06.  It is now widely agreed that conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.

Classification

The conodonts are currently classified in the phylum Chordata because their fins with fin rays, chevron-shaped muscles and notochord are characteristic of Chordata. Briggs, D. (1992). "Conodonts: a major extinct group added to the vertebrates". Science 256: 1285-1286. Retrieved on 2007-09-06. 

But such a classification is far from secure: while Milsom and Rigby (2004) consider them to be vertebrates similar in appearance to modern hagfish and lampreys, an opinion supported by cladistic analysis done by Donoghue et al. (2000).^[vague] But this analysis comes with one caveat: early forms of conodonts, the protoconodonts, appear to form a distinct clade from the later paracononts and euconodonts: most paleontologists^{[verification needed][vague]} (following Szaniawski)^{[citation needed]} deem the protoconodonts to represent a stem group to the phylum containing chaetognath worms, indicating that they are not close relatives of true conodonts.

Conodont teeth fossils

For many years, conodonts were known only from enigmatic tooth-like microfossils, which occur commonly but always in isolation, and were not associated with any other fossil. These phosphatic microfossils are now termed "conodont elements" to avoid confusion (e.g. Zhuravlev 2007). They are widely used in biostratigraphy.

Conodont elements are also used as paleothermometers, a proxy for thermal alteration in the host rock. This is because under higher temperatures the phosphate undergoes predictable and permanent color changes, measured with the conodont alteration index. This has made them useful for petroleum exploration where they are known, in rocks dating from the Cambrian to the Late Triassic.

It was not until early 1980s that the conodont teeth were found in association with fossils of the host organism, in a konservat lagerstätte. Briggs, D.E.G.; E.N.K. Clarkson, R.J. Aldridge (1983). "The conodont animal". Lethaia 16: 1-14. Retrieved on 2007-09-06.  This is because most of the conodont animal was soft-bodied, thus everything but the teeth were not suited for preservation under normal circumstances. A few exceptionally preserved fossils have revealed the full conodont animal; new discoveries have expanded the known body fossil record, and a more consistent impression of the animals has emerged.

External links

• Mark Purnell. An oblique anterior view of a model of the apparatus of the Pennsylvanian conodont Idiognathodus.
• "The Jaws That Catch": an Introduction to the Conodonta. Palæos. Retrieved on 2007-09-05.
• Jim Davison (2002-10-15). Ordovician conodonts. Retrieved on 2007-09-05.

References

• 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., Forey, P. L. & Aldridge, R. J. (2000), Conodont affinity and chordate phylogeny. "Biological Reviews". 75, 191-251.
• Janvier, P (1997). Euconodonta. The tree of life web project, http://tolweb.org. Retrieved on 2007-09-05.
• Milsom, C. & Rigby, S (2004). Fossils at a Glance. Victoria, Australia: Blackwell Publishing, 155 pp.

• Sweet, Walter. The Conodonta.
• Zhuravlev, A. (in press, 2007). "Ecological control on micro-ornamentation of conodont elements". Nature. Retrieved on 2007-09-05. 

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