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Conodont

(Redirected fromConodonts)
Not to be confused withConodonorCynodont.

Conodonts(Greekkōnos,"cone",+odont,"tooth") are anextinctgroup ofjawless vertebrates,classified in theclassConodonta.They are primarily known from their hard, mineralised tooth-like structures called "conodont elements" that in life were present in the oral cavity and used to process food. Rare soft tissue remains suggest that they had elongate eel-like bodies with large eyes. Conodonts were a long-lasting group with over 300 million years of existence from theCambrian(over 500 million years ago) to the beginning of theJurassic(around 200 million years ago). Conodont elements are highly distinctive to particular species and are widely used inbiostratigraphyas indicative of particular periods of geological time.

Conodonts
Conodont elements
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: †Conodonta
Pander,1856
Subgroups
Synonyms

Discovery and understanding of conodonts

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The teeth-like fossils of the conodont were first discovered byHeinz Christian Panderand the results published inSaint Petersburg, Russia,in 1856.[2]

It was only in the early 1980s that the first fossil evidence of the rest of the animal was found (see below). In the 1990s exquisite fossils were found in South Africa in which the soft tissue had been converted to clay, preserving even muscle fibres. The presence of muscles for rotating the eyes showed definitively that the animals were primitive vertebrates.[3]

Nomenclature and taxonomic rank

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Through their history of study, "conodont" is a term which has been applied to both the individual fossils and to the animals to which they belonged. The original German term used by Pander was "conodonten", which was subsequentlyanglicizedas "conodonts", though no formallatinizedname was provided for several decades. MacFarlane (1923) described them as anorder,Conodontes (a Greek translation), which Huddle (1934) altered to the Latin spelling Conodonta.[4]A few years earlier, Eichenberg (1930) established another name for the animals responsible for conodont fossils: Conodontophorida ( "conodont bearers" ).[1]A few other scientific names were rarely and inconsistently applied to conodonts and their proposed close relatives during 20th century, such as Conodontophoridia, Conodontophora, Conodontochordata, Conodontiformes,[5]and Conodontomorpha.

Conodonta and Conodontophorida are by far the most common scientific names used to refer to conodonts, though inconsistencies regarding theirtaxonomic rankstill persist. Bengtson (1976)'s research on conodont evolution identified three morphological tiers of early conodont-like fossils:protoconodonts,paraconodonts,and "true conodonts" (euconodonts).[5]Further investigations revealed that protoconodonts were probably more closely related tochaetognaths(arrow worms) rather than true conodonts. On the other hand, paraconodonts are still considered a likely ancestral stock orsister groupto euconodonts.

The 1981Treatise on Invertebrate Paleontologyvolume on the conodonts (Part W revised, supplement 2) lists Conodonta as the name of both aphylumand aclass,with Conodontophorida as a subordinate order for "true conodonts". All three ranks were attributed to Eichenberg, and Paraconodontida was also included as an order under Conodonta.[6]This approach was criticized by Fåhraeus (1983), who argued that it overlooked Pander's historical relevance as a founder and primary figure in conodontology. Fåhraeus proposed to retain Conodonta as a phylum (attributed to Pander), with the single class Conodontata (Pander) and the single order Conodontophorida (Eichenberg).[4][7]Subsequent authors continued to regard Conodonta as a phylum with an ever-increasing number of subgroups.[8]

With increasingly strong evidence that conodonts lie within the phylum Chordata, more recent studies generally refer to "true conodonts" as the class Conodonta, containing multiple smaller orders.[9][10][11]Paraconodonts are typically excluded from the group, though still regarded as close relatives.[9][10][11]In practice, Conodonta, Conodontophorida, and Euconodonta are equivalent terms and are used interchangeably.

Conodont elements

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Lone elements

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Conodont elements consist of mineralised teeth-like structures of varying morphology and complexity. The evolution ofmineralized tissueshas been puzzling for more than a century. It has been hypothesized that the first mechanism of chordate tissue mineralization began either in the oral skeleton of conodonts or the dermal skeleton of earlyagnathans.

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 Enigma tic tooth-like microfossils (200 micrometers to 5 millimeters in length[12]), 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 akonservat lagerstätte.[13]This is because the conodont animal was soft-bodied, thus everything but the teeth was unsuited for preservation under normal circumstances.

These microfossils are made ofhydroxylapatite(a phosphatic mineral).[14]The conodont elements can be extracted from rock using adequate solvents.[15][16][17]

They are widelyused in biostratigraphy.Conodont elements are also used aspaleothermometers,a proxy for thermal alteration in the host rock, because under higher temperatures, the phosphate undergoes predictable and permanent color changes, measured with theconodont alteration index.This has made them useful forpetroleum explorationwhere they are known, in rocks dating from theCambrianto the LateTriassic.

Full apparatus

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  • Complete element set of the conodontHindeodus parvus
  • Preserved articulated association of conodont elements belonging to the speciesArcheognathusprimus(Ordovician, North America)

The conodont apparatus may comprise a number of discrete elements, including the spathognathiform, ozarkodiniform, trichonodelliform, neoprioniodiform, and other forms.[18]

In the 1930s, the concept of conodont assemblages was described by Hermann Schmidt[19]and by Harold W. Scott in 1934.[20][21][22][23]

Elements of ozarkodinids

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The feeding apparatus ofozarkodinidsis composed of an axial Sa element at the front, 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.[24]

The conodont animal

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  • Life restoration ofPromissum pulchrum
  • Restoration ofPanderodus unicostatus
  • A body fossil ofPanderodus unicostatus
  • A size comparison of the three conodont species with preserved body fossils.
  • Fossils ofTyphloesus,at one time considered the first conodont body fossil.

Although conodont elements are abundant in the fossil record, fossils preserving soft tissues of conodont animals are known from only a few deposits in the world. One of the first possible body fossils of a conodont were those ofTyphloesus,an Enigma tic animal known from theBear Gulch limestoneinMontana.[25]This possible identification was based on the presence of conodont elements with the fossils ofTyphloesus.This claim was disproved, however, as the conodont elements were actually in the creature'sdigestivearea.[26]That animal is now regarded as a possiblemolluskrelated togastropods.[26]As of 2023, there are only three described species of conodonts that have preserved trunk fossils:Clydagnathus windsorensisfrom theCarboniferousagedGranton Shrimp BedinScotland,Promissum pulchrumfrom theOrdovicianagedSoom ShaleinSouth Africa,andPanderodus unicostatusfrom theSilurianagedWaukesha BiotainWisconsin.[9][27][28]There are other examples of conodont animals that only preserve the head region, including eyes, of the animals known from the Silurian agedEramosasite inOntarioandTriassicagedAkkamori sectioninJapan.[29][30]

According to these fossils, conodonts had large eyes, fins with fin rays,chevron-shaped musclesand axial line, which were interpreted asnotochordor thedorsal nerve cord.[27][31]WhileClydagnathusandPanderodushad lengths only reaching 4–5 cm (1.6–2.0 in),Promissumis estimated to reach 40 cm (16 in) in length, if it had the same proportions asClydagnathus.[27][28]

Ecology

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Model of elements ofManticolepis subrecta– a conodont from the Upper Frasnian of Poland – photography taken in the Geological Museum of the Polish Geological Institute in Warsaw

The "teeth" of some conodonts have been interpreted as filter-feeding apparatuses, filtering plankton from the water and passing it down the throat.[32]Others have been interpreted as a "grasping and crushing array".[28]Wear on some conodont elements suggests that they functioned like teeth, with both wear marks likely created by food as well as byocclusionwith other elements.[33]Studies have concluded that conodonts taxa occupied bothpelagic(open ocean) andnektobenthic(swimming above the sediment surface) niches.[33]The preserved musculature suggests that some conodonts (Promissumat least) were efficient cruisers, but incapable of bursts of speed.[28]Based on isotopic evidence, some Devonian conodonts have been proposed to have been low-level consumers that fed onzooplankton.[33]

A study on the population dynamics ofAlternognathushas been published. Among other things, it demonstrates that at least this taxon had short lifespans lasting around a month.[34]A studySr/CaandBa/Ca ratios of a population of conodonts from a carbonate platform from the Silurian of Sweden found that the different conodont species and genera likely occupied differenttrophic niches.[33]

Some species of the genusPanderodushave been speculated to be venomous, based on grooves found on some elements.[35]

Classification and phylogeny

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Affinities

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As of 2012,scientists classify the conodonts in thephylumChordataon the basis of their fins with fin rays,chevron-shaped muscles andnotochord.[36]

Milsom andRigbyenvision them as vertebrates similar in appearance to modern hagfish and lampreys,[37]andphylogeneticanalysis suggests they are morederivedthan either of these groups.[9]However, this analysis comes with one caveat: the earliest conodont-like fossils, theprotoconodonts,appear to form a distinct clade from the laterparaconodontsandeuconodonts.Protoconodonts are probably not relatives of true conodonts, but likely represent a stem group toChaetognatha,an unrelated phylum that includes arrow worms.[38]

Moreover, some analyses do not regard conodonts as eithervertebratesorcraniates,because they lack the main characteristics of these groups.[39]More recently it has been proposed that conodonts may be stem-cyclostomes,more closely related tohagfishandlampreysthan tojawed vertebrates.[40]

Ingroup relations

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Individual conodont elements are difficult to classify in a consistent manner, but an increasing number of conodont species are now known from multi-element assemblages, which offer more data to infer how different conodont lineages are related to each other. The following is a simplified cladogram based on Sweet and Donoghue (2001),[10]which summarized previous work by Sweet (1988)[8]and Donoghue et al. (2000):[9]

Only a few studies approach the question of conodont ingroup relationships from acladisticperspective, as informed byphylogenetic analyses.One of the broadest studies of this nature was the analysis of Donoghue et al. (2008), which focused on "complex" conodonts (Prioniodontida and other descendant groups):[11]

Evolutionary history

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Conodont elements from the Deer Valley Member of theMauch Chunk Formationin Pennsylvania, Maryland, and West Virginia, US
detail
Figures 1, 2. Conodonts from the Deer Valley Member of the Mauch Chunk Formation, Keystone quarry, Pa. This collection (93RS–79c) is from the lower 10 cm of the Deer Valley Member. Note the nonabraded, although slightly broken, conodont elements of the high-energy oolitic marine facies of the Deer Valley Member.
1.Kladognathussp.,Sa element, posterior view, X140 2.Cavusgnathusunicornis,gamma morphotype, Pa element, lateral view, X140
3–9. Conodonts from the uppermost Loyalhanna Limestone Member of the Mauch Chunk Formation, Keystone quarry, Pa. This collection (93RS–79b) is from the upper 10 cm of the Loyalhanna Member. Note the highly abraded and reworked aeolian forms.
3, 4.Kladognathussp.,Sa element, lateral views, X140
5.Cavusgnathusunicornis,Alpha morphotype, Pa element, lateral view, X140
6, 7.Cavusgnathussp.,Pa element, lateral view, X140
8.Polygnathussp.,Pa element, upper view, reworked Late Devonian to Early Mississippian morphotype, X140
9.Gnathodustexanus?,Pa element, upper view, X140
10–14. Conodonts from the basal 20 cm of the Loyalhanna Limestone Member of the Mauch Chunk Formation, Keystone quarry, Pa. (93RS–79a), and Westernport, Md. (93RS–67), note the highly abraded and reworked aeolian forms
10.Polygnathussp.,Pa element, upper view, reworked Late Devonian to Early Mississippian morphotype, 93RS–79a, X140
11.Polygnathus sp.,Pa element, upper view, reworked Late Devonian to Early Mississippian morphotype, 93RS–67, X140
12.Gnathodussp.,Pa element, upper view, reworked Late Devonian(?) through Mississippian morphotype, 93RS–67, X140
13.Kladognathussp.,M element, lateral views, 93RS–67, X140
14.Cavusgnathussp.,Pa element, lateral view, 93RS–67, X140

The earliest fossils of conodonts are known from the Cambrian period. Conodonts extensively diversified during the early Ordovician, reaching their apex of diversity during the middle part of the period, and experienced a sharp decline during the late Ordovician and Silurian, before reaching another peak of diversity during the mid-late Devonian. Conodont diversity declined during theCarboniferous,with an extinction event at the end of the middleTournaisian[41]and a prolonged period of significant loss of diversity during thePennsylvanian.[42][43]Only a handful of conodont genera were present during the Permian, though diversity increased after the P-T extinction during the Early Triassic.

Diversity continued to decline during the Middle and Late Triassic, culminating in their extinction soon after the Triassic-Jurassic boundary. Much of their diversity during the Paleozoic was likely controlled by sea levels and temperature, with the major declines during the Late Ordovician and Late Carboniferous due to cooler temperatures, especiallyglacial eventsand associatedmarine regressionswhich reducedcontinental shelfarea. However, their final demise is more likely related tobiotic interactions,perhaps competition with new Mesozoic taxa.[44]

Taxonomy

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Conodonta taxonomy based on Sweet (1988),[8]Sweet & Donoghue (2001),[10]and Mikko's Phylogeny Archive.[45][clarification needed]

See also

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References

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  1. ^abEichenberg, W. (1930). "Conodonten aus dem Culm des Harzes".Paläontologische Zeitschrift.12(3–4): 177–182.doi:10.1007/BF03044446.S2CID129519805.
  2. ^Sweet, Walter C.; Cooper, Barry J. (December 2008)."C.H. Pander's introduction to conodonts, 1856".Retrieved3 January2019.
  3. ^Jan ZalasiewiczandSarah Gabbott(Jun 5, 1999)."The quick and the dead".New Scientist.
  4. ^abFåhraeus, Lars E. (1983)."Phylum Conodonta Pander, 1856 and Nomenclatural Priority".Systematic Zoology.32(4): 455–459.doi:10.2307/2413175.JSTOR2413175.
  5. ^abBengtson, Stefan (1976)."The structure of some Middle Cambrian conodonts, and the early evolution of conodont structure and function".Lethaia.9(2): 185–206.doi:10.1111/j.1502-3931.1976.tb00966.x.ISSN0024-1164.
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  7. ^Fåhraeus, Lars E. (1984)."A critical look at the Treatise family-group classification of Conodonta: an exercise in eclecticism".Lethaia.17(4): 293–305.doi:10.1111/j.1502-3931.1984.tb00675.x.ISSN0024-1164.
  8. ^abcSweet, 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.ISBN978-0-19-504352-5.
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Further reading

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