Conodont

Conodonts Temporal range: 495–199.6Ma PreꞒ Ꞓ O S D C P T J K Pg N Late Cambrian to Late Triassic
Reconstruction of a conodont
Two conodont "teeth" and a reconstruction of a conodont
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Craniata
Subphylum:	Vertebrata
Class:	†Conodonta Eichenberg 1930
Groups
Protoconodonta Conodonta Paraconodonta Euconodonta Panderodontida Prioniodontida Prioniodinina Ozarkodinina
Synonyms
†Conodontophorida

Conodonts (Greek kōnos, "cone", + odont, "tooth") are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils found in isolation and now called conodont elements. Knowledge about soft tissues remains limited. The animals are also called Conodontophora (conodont bearers) to avoid ambiguity.

Conodont teeth are the earliest found in the fossil record. Neil Shubin writes: "For a long time conodonts were enigmas: scientists disagreed over whether they were animalian, plant or mineral. Conodonts were claimed to be pieces of clams, sponges, vertebrates, even worms. The speculation ended when whole animals started to show up in the fossil record. For years, paleontologists have argued about why hard skeletons, those containing hydroxyapatite, arose in the first place. For those who believed that skeletons began with the backbones, or body armor, conodonts provide an 'inconvenient tooth' if you will. The first hydroxyapatite-containing body parts were teeth. Hard bones arose not to protect animals but to eat them."^[1]

Description

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

The 11 known fossil imprints of conodont animals record an eel-like creature with 15, or more rarely, 19 elements forming a bilaterally symmetrical array in the head. This array constituted a feeding apparatus radically different from the jaws of modern animals. The three forms of teeth, coniform cones, ramiform bars, and pectiniform platforms, may have performed different functions.

The organisms range from a centimeter or so to the large Promissum, 40 cm in length.^[2] It is now widely agreed that conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.

The entire class of conodonts is postulated to have been wiped out in the Triassic–Jurassic extinction event, which occurred roughly 200 million years ago.^[3]

Ecology

The "teeth" of some conodonts have been interpreted as filter-feeding apparatuses, filtering plankton from the water and passing it down the throat. Others have been interpreted as a "grasping and crushing array".^[2] The lateral position of the eyes makes it unlikely that conodonts were active predators. The preserved musculature suggests that some conodonts (Promissum at least) were efficient cruisers, but incapable of bursts of speed.^[2]

Classification and phylogeny

As of 2012 scientists classify the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord.^[4]

Milsom and Rigby envision them as vertebrates similar in appearance to modern hagfish and lampreys,^[5] and phylogenetic analysis suggests they are more derived than either of these groups.^[6] This analysis, however, comes with one caveat: early forms of conodonts, the protoconodonts, appear to form a distinct clade from the later paraconodonts and euconodonts. The protoconodonts likely represent a stem group to the phylum containing chaetognath worms, indicating they are not close relatives of true conodonts.^[7] Moreover, some analyses do not regard conodonts as either vertebrates or craniates, because they lack the main characteristics of these groups.^[8]

Elements

For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometers to 5 millimeters in length^[11]), which occur commonly, but not always in isolation, and were not associated with any other fossil. These phosphatic microfossils are now termed "conodont elements" to avoid confusion. They are widely used in biostratigraphy.

Conodont elements are also used as paleothermometers, a proxy for thermal alteration in the host rock, 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.

Until the early 1980s, conodont teeth had not been found in association with fossils of the host organism, in a konservat lagerstätte.^[12] This is because most of the conodont animal was soft-bodied, thus everything but the teeth was unsuited for preservation under normal circumstances.

The conodont apparatus may comprise a number of discrete elements, including the spathognathiform, ozarkodiniform, trichonodelliform, neoprioniodiform, and other forms.^[13]

See also

• Lau event - mass extinction event with major impact on conodonts

Notes

References

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 (1294): 405–421. doi:10.1098/rstb.1993.0082. 
• Aldridge, R. J.; Purnell, M. A. (1996). "The conodont controversies". Trends in Ecology and Evolution 11 (11): 463–468. doi:10.1016/0169-5347(96)10048-3. PMID 21237922. 
• Donoghue, P. C. J.; Forey, P. L.; Aldridge, R. J. (2000). "Conodont affinity and chordate phylogeny". Biological Reviews 75 (2): 191–251. doi:10.1017/S0006323199005472. PMID 10881388. 
• Gould, Stephen Jay (1985). "Reducing Riddles". In The Flamingo's Smile, 245-260. New York, W.W. Norton and Company. ISBN 0-393-30375-6.
• Janvier, P (1997). "Euconodonta". The tree of life web project, http://tolweb.org''. Retrieved 2007-09-05. 
• Knell, Simon J. The Great Fossil Enigma: The Search for the Conodont Animal (Indiana University Press; 2012) 440 pages
• Sweet, Walter. The Conodonta.
• Sweet, W. C.; Donoghue, P. C. J. (2001). "Conodonts: past, present and future". Journal of Paleontology 75 (6): 1174–1184. doi:10.1666/0022-3360(2001)075<1174:CPPF>2.0.CO;2. ISSN 0022-3360. 

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 2013-07-01. 
• Jim Davison (2002-10-15). "Ordovician conodonts". Retrieved 2009-07-07. 

Evolution of fish Forerunners Chordata Lancelet Haikouichthys Jawless fish Conodonts †Prioniodontida †Promissum Ostracoderms †Pteraspida †Thelodonti †Anaspida †Galeaspida †Pituriaspida †Osteostraci Jawed fish Spiny sharks †Climatiiformes †Ischnacanthiformes Placoderms †Antiarchi †Arthrodira †Brindabellaspida †Petalichthyida †Phyllolepida †Ptyctodontida †Rhenanida †Acanthothoraci † †Pseudopetalichthyida? †Stensioellida? Cartilaginous Elasmobranchii Holocephali Bony Lobe-finned Actinistia Coelacanth Lungfish Tetrapodomorpha Ray-finned Chondrostei Neopterygii Holostei Teleostei Lists Lists of prehistoric fish spiny sharks placoderms cartilaginous bony lobe-finned List of transitional fossils Related Prehistoric life Transitional fossils Vertebrate paleontology