Conodont

Conodonts
Temporal range: 520–200 Ma

Late Cambrian to Late Triassic (Rhaetian)

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[1]
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.

Conodonts are considered index fossils, fossils used to define and identify geologic periods.

Geological history

The conodonts first appeared during the Cambrian Stage 2 (also referred as Tommotian).[2] The still unnamed Cambrian Stage 10 can be defined as the first appearance of Eoconodontus notchpeakensis. The upper boundary is defined as the appearance of Iapetognathus fluctivagus which marks the beginning of the Tremadocian and is radiometrically dated as 485.4 ± 1.9 million years ago.

The Cambrian–Ordovician extinction event occurred approximately 488 million years ago. This early Paleozoic extinction event extirpated many conodonts.

The Lau event, about 420 million years ago, a relatively minor mass extinction during the Silurian period, had a major impact on conodont populations.

The Kačák Event was a period of significant extinctions. The group most affected was the Ammonoidea, although there were also faunal turnovers amongst conodonts and dacryoconarids.[3]

The entire class is postulated to have been wiped out in the Triassic–Jurassic extinction event, which occurred roughly 200 million years ago.[4] Near the end of the Triassic deadly marine biocalcification began to occur, along with oceanic acidification, sea-level fluctuations and the Central Atlantic Magmatic Province (CAMP) releasing carbon dioxide, sulfur dioxide and aerosols. These environmental catastrophes caused the extinction of the conodonts and 34% of other marine genera to disappear.[5]

Description

life restoration of Promissum pulchrum

The 11 known fossil imprints of conodont animals record an eel-like creature with 15, or more rarely, 19 elements that form a bilaterally symmetrical array in the head.

The organisms range from a centimeter or so to the large Promissum, 40 cm in length.[6] It is now widely agreed that conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.

Elements

Conodont elements from the Deer Valley Member of the Mauch Chunk Formation in Pennsylvania, Maryland, and West Virginia, USA.

Conodont teeth are the earliest found in the fossil record.[7] The evolution of mineralized tissues has been puzzling for more than a century. It has been hypothesized that the first mechanism of mammalian tissue mineralization began either in the oral skeleton of conodont or the dermal skeleton of early agnathans.

The element array constituted a feeding apparatus that is radically different from the jaws of modern animals. They are now termed "conodont elements" to avoid confusion. The three forms of teeth, i.e., coniform cones, ramiform bars, and pectiniform platforms, probably performed different functions.

For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometers to 5 millimeters in length[8]), which occur commonly, but not always in isolation, and were not associated with any other fossil. Until the early 1980s, conodont teeth had not been found in association with fossils of the host organism, in a konservat lagerstätte.[9] This is because most of the conodont animal was soft-bodied, thus everything but the teeth was unsuited for preservation under normal circumstances.

These microfossils are made of hydroxylapatite (a phosphatic mineral).[10] The conodont elements can be extracted from rock using adequate solvents.[11][12][13]

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.

Multielement conodonts

Model of elements of Manticolepis subrecta - a conodont from the Upper Frasnian of Poland. Photography taken in the Geological Museum of the Polish Geological Institute in Warsaw.

The conodont apparatus may comprise a number of discrete elements, including the spathognathiform, ozarkodiniform, trichonodelliform, neoprioniodiform, and other forms.[14]

In the 1930s, the concept of conodont assemblages was described by Hermann Schmidt[15] and by Harold W. Scott in 1934.[16][17][18][19]

Elements of ozarkodinids

The feeding apparatus of ozarkodinids is composed at the front of an axial Sa element, flanked by two groups of four close-set elongate Sb and Sc elements which were inclined obliquely inwards and forwards. Above these elements lay a pair of arched and inward pointing (makellate) M elements. Behind the S-M array lay transversely oriented and bilaterally opposed (pectiniform, i.e. comb-shaped) Pb and Pa elements.[20]

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".[6] 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.[6]

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.[21]

Milsom and Rigby envision them as vertebrates similar in appearance to modern hagfish and lampreys,[22] and phylogenetic analysis suggests they are more derived than either of these groups.[23] However, this analysis comes with one caveat: early forms of conodonts, the protoconodonts, appear to form a distinct clade from the later paraconodonts and euconodonts. Protoconodonts likely represent a stem group to the phylum that includes chaetognath worms; this conclusion suggests that chaetognaths are not close relatives of true conodonts.[24] Moreover, some analyses do not regard conodonts as either vertebrates or craniates, because they lack the main characteristics of these groups.[25]

 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





Taxonomy

Conodonta taxonomy based on Sweet & Donoghue[26][28] and Mikko's Phylogeny Archive[29]

Conodonta Pander 1856 non Eichenberg 1930 sensu Sweet & Donoghue 2001 [Conodontia; Conodontophorida Eichenberg 1930; Conodontochordata]

See also

Notes

  1. ↑ Here, the hagfish are treated as a separate clade, as in Sweet and Donoghue's 2001 tree produced without cladistic analysis.[26] However, it has been recognised by some [27] 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,[26] although this is not widely used.

References

  1. ↑ Conodonten aus dem Culm des Harzes. W Eichenberg, Paläontologische Zeitschrift, 1930, page 177-182, doi:10.1007/BF03044446
  2. ↑ Early Cambrian (Tommotian) conodonts and other shelly microfauna from the Upper Krol of Mussoorie Syncline. RJ Azmi, VP Pancholi, Lesser Himalaya, with remarks on the Precambrian, 1983
  3. ↑ DeSantis, M.K.; Brett C.E. (2011). "Late Eifelian (Middle Devonian) biocrises: Timing and signature of the pre-Kačák Bakoven and Stony Hollow Events in eastern North America". Palaeogeography, Palaeoclimatology, Palaeoecology. Elsevier. 304 (1–2): 113–135. doi:10.1016/j.palaeo.2010.10.013. Retrieved 4 March 2012.
  4. ↑ The extinction of conodonts —in terms of discrete elements— at the Triassic-Jurassic boundary
  5. ↑ Graham Ryder; David E. Fastovsky; Stefan Gartner (1996). The Cretaceous-Tertiary Event and Other Catastrophes in Earth History. Geological Society of America. p. 19. ISBN 9780813723075.
  6. 1 2 3 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. doi:10.1038/374800a0.
  7. ↑ 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. ISBN 9780307277459.
  8. ↑ MIRACLE. "Conodonts". Retrieved 26 August 2014.
  9. ↑ Briggs, D. E. G.; Clarkson, E. N. K.; Aldridge, R. J. (1983). "The conodont animal". Lethaia. 16 (1): 1–14. doi:10.1111/j.1502-3931.1983.tb01993.x.
  10. ↑ Chemical systematics of conodont apatite determined by laser ablation ICPMS. Julie A. Trotter and Stephen M. Eggins, Chemical Geology, Volume 233, Issues 3–4, 15 October 2006, Pages 196–216, doi:10.1016/j.chemgeo.2006.03.004
  11. ↑ A Buffered Formic Acid Technique for Conodont Extraction. Lennart Jeppsson and Rikard Anehus, Journal of Paleontology, Vol. 69, No. 4 (Jul., 1995), pages 790-794 (Stable URL)
  12. ↑ Extraction Techniques for Phosphatic Fossils. Owen R. Green in A Manual of Practical Laboratory and Field Techniques in Palaeobiology. pages 318-330, doi:10.1007/978-94-017-0581-3_27
  13. ↑ Effects of extraction protocols on the oxygen isotope composition of conodont elements. Page C. Quinton, Stephen A. Leslie, Achim D. Herrmann and Kenneth G. MacLeod, Chemical Geology, Volume 431, 1 August 2016, Pages 36–43, doi:10.1016/j.chemgeo.2016.03.023
  14. ↑ 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. doi:10.2307/1303249 (inactive 2015-02-15). JSTOR 1303249.
  15. ↑ Conodonten-Funde in ursprünglichem Zusammenhang. Hermann Schmidt, Palaeontologische Zeitschrift, 1934, volume 16, Issue 1-2 , pages 76-85, doi:10.1007/BF03041668
  16. ↑ The Zoological Relationships of the Conodonts. Harold W. Scott, Journal of Paleontology, Vol. 8, No. 4 (Dec., 1934), pages 448-455 (Stable URL)
  17. ↑ Conodont Assemblages from the Heath Formation, Montana. Harold W. Scott, Journal of Paleontology, Vol. 16, No. 3 (May, 1942), pages 293-300 (Stable URL)
  18. ↑ Late Mississippian conodonts from the Bird Spring Formation in Nevada. David L. Dunn, Journal of Paleontology, November 1965, volume 39, issue 6 (abstract)
  19. ↑ A Questionable Natural Conodont Assemblage from Middle Ordovician Limestone, Ottawa, Canada. Christopher R. Barnes, Journal of Paleontology, Vol. 41, No. 6 (Nov., 1967), pages 1557-1560 (Stable URL)
  20. ↑ Purnell, M. A.; Donoghue, P. C. (1997). "Architecture and functional morphology of the skeletal apparatus of ozarkodinid conodonts". Philosophical Transactions of the Royal Society B: Biological Sciences. 352 (1361): 1545–1564. doi:10.1098/rstb.1997.0141. PMC 1692076Freely accessible.
  21. ↑ Briggs, D. (May 1992). "Conodonts: a major extinct group added to the vertebrates". Science. 256 (5061): 1285–1286. doi:10.1126/science.1598571. PMID 1598571.
  22. ↑ Milsom, Clare; Rigby, Sue (2004). "Vertebrates". Fossils at a Glance. Victoria, Australia: Blackwell Publishing. p. 88. ISBN 0-632-06047-6.
  23. ↑ 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. Retrieved 2008-04-07.
  24. ↑ Szaniawski, H. (2002). "New evidence for the protoconodont origin of chaetognaths" (PDF). Acta Palaeontologica Polonica. 47 (3): 405.
  25. ↑ 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" (PDF). Geodiversitas. 32 (4): 545–594. doi:10.5252/g2010n4a1.
  26. 1 2 3 Sweet, W. C.; P. C. J Donoghue (2001). "Conodonts: past, present, future". Journal of Paleontology. 75 (6): 1174–1184. doi:10.1666/0022-3360(2001)075<1174:CPPF>2.0.CO;2.
  27. ↑ 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. doi:10.1038/nature05241. ISSN 0028-0836. PMID 17051155.
  28. ↑ Sweet, W. C. (1988). "The Conodonta: morphology, taxonomy, paleoecology and evolutionary history of a long-extinct animal phylum". Oxford monographs on geology and geophysics (10): 1–211.
  29. ↑ Mikko's Phylogeny Archive Haaramo, Mikko (2007). "Conodonta - conodonts". Retrieved 30 December 2015.

Further reading

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