Conodont
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| Conodonts Fossil range: Late Cambrian to Late Triassic |
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 Reconstruction of a Conodont
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Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. The animal is also called conodontophora (conodont bearers) to avoid ambiguity.
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[edit] 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.
The organisms range from a centimeter or so[verification needed] to the giant Promissum, 40cm in length.[1] It is now widely agreed that conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.
[edit] Ecology
The "teeth" of some conodonts have been interpreted as filter-feeding apparatuses, filtering out plankton from the water and passing it down the throat.[citation needed] Others have been interpreted as a "grasping and crushing array".[1]
The lateral position of the eyes makes a predatory role unlikely.[citation needed]
The preserved musculature hints that some conodonts (Promissum at least) were efficient cruisers but incapable of bursts of speed.[1]
[edit] 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.[2]
They are considered by Milsom and Rigby to be vertebrates similar in appearance to modern hagfish and lampreys,[3] and phylogenetic analysis suggests that they are more derived than either of these groups.[4] This analysis, however, comes with one caveat: early forms of conodonts, the protoconodonts, appear to form a distinct clade from the later paracononts and euconodonts. It appears likely that the protoconodonts represent a stem group to the phylum containing chaetognath worms, indicating that they are not close relatives of true conodonts.[5]
[edit] 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.[6] 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.[7] This is because most of the conodont animal was soft-bodied, thus everything but the teeth were not suited for preservation under normal circumstances.
[edit] Further reading
- 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.
- Janvier, P (1997). Euconodonta. The tree of life web project, http://tolweb.org. Retrieved on 2007-09-05.
- Sweet, Walter. The Conodonta.
[edit] 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.
[edit] References
- ^ a b c 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:.
- ^ Briggs, D. (1992). "Conodonts: a major extinct group added to the vertebrates". Science 256: 1285-1286.
- ^ Milsom, C. & Rigby, S (2004). Fossils at a Glance. Victoria, Australia: Blackwell Publishing, 155 pp.
- ^ Donoghue, P.C.J.; Forey, P.L.; Aldridge, R.J. (2000). "Conodont affinity and chordate phylogeny". Biological Reviews 75 (02): 191-251. doi:.
- ^ Szaniawski, H. (2002). "New evidence for the protoconodont origin of chaetognaths". Acta Palaeontologica Polonica 47 (3): 405.
- ^
- Zhuravlev, A. (in press, 2007). "Ecological control on micro-ornamentation of conodont elements". Nature.
- ^ Briggs, D.E.G.; E.N.K. Clarkson, R.J. Aldridge (1983). "The conodont animal". Lethaia 16: 1-14.
