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Conodont

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Title: Conodont  
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Conodont

Conodonts
Temporal range: 495–199.6Ma
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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
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 that we find 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]

 Craniata 

Hagfish[Note 1]

 Vertebrata 
 Hyperoartia  Lampreys


 Conodonta 

Paraconodontida



Proconodontida[Note 2]

 Euconodonta[Note 3] 


Protopanderodontida


Panderontida


 Prioniodontida 

Paracordylodus



Balognathidae



Prioniodinida


Ozarkodinida








Heterostracans, osteostracans and gnathostomes




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

  1. ^ Here, the hagfish are treated as a separate clade, as in Sweet and Donoghue's 2001 tree produced without cladistic analysis.[9] However, it has been recognised by some [10] that the hagfish and lampreys may be closer to one another in their own clade, the Cyclostomata.
  2. ^ The clade Proconodontida is also known as Cavidonti.
  3. ^ Euconodonta is referred to as "Conodonti" by Sweet and Donoghue,[9] although this is not widely used.

References

  1. ^ Shubin, Neil (2009). Your Inner Fish: A Journey into the 3.5 Billion Year History of the Human Body (reprint ed.). New York: Pantheon Books. pp. 85–86.  
  2. ^ 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 (6525): 800–803.  
  3. ^ The extinction of conodonts —in terms of discrete elements— at the Triassic-Jurassic boundary
  4. ^ Briggs, D. (May 1992). "Conodonts: a major extinct group added to the vertebrates". Science 256 (5061): 1285–1286.  
  5. ^ Milsom, Clare; Rigby, Sue (2004). "Vertebrates". Fossils at a Glance. Victoria, Australia: Blackwell Publishing. p. 88.  
  6. ^ Donoghue, P.C.J.; Forey, P.L.; Aldridge, R.J. (2000). "Conodont affinity and chordate phylogeny". Biological Reviews 75 (2): 191–251.  
  7. ^ Szaniawski, H. (2002). "New evidence for the protoconodont origin of chaetognaths". Acta Palaeontologica Polonica 47 (3): 405. 
  8. ^ Turner, S., Burrow, C.J., Schultze, H.P., Blieck, A., Reif, W.E., Rexroad, C.B., Bultynck, P., Nowlan, G.S.; Burrow; Schultze; Blieck; Reif; Rexroad; Bultynck; Nowlan (2010). "False teeth: conodont-vertebrate phylogenetic relationships revisited". Geodiversitas 32 (4): 545–594.  
  9. ^ a b Sweet, W. C.; P. C. J Donoghue (2001). "Conodonts: past, present, future". Journal of Paleontology 75 (6): 1174–1184.  
  10. ^ Bourlat, S. J; T. Juliusdottir, C. J Lowe, R. Freeman, J. Aronowicz, M. Kirschner, E. S Lander, M. Thorndyke, H. Nakano, A. B Kohn, others (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida". Nature 444 (7115): 85–88.  
  11. ^ MIRACLE. "Conodonts". Retrieved 26 August 2014. 
  12. ^ Briggs, D. E. G.; Clarkson, E. N. K.; Aldridge, R. J. (1983). "The conodont animal". Lethaia 16 (1): 1–14.  
  13. ^ Bergström, S. M.; Carnes, J. B.; Ethington, R. L.; Votaw, R. B.; Wigley, P. B. (1974). "Appalachignathus, a New Multielement Conodont Genus from the Middle Ordovician of North America". Journal of Paleontology (Paleontological Society) 48 (2): 227–235.  

Further reading

External links

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