Conodonts (Greek kōnos, "cone", + odont, "tooth") are extinct agnathan 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.
|Reconstruction of a conodont|
|Two conodont "teeth" and a reconstruction of a conodont|
Conodonts are considered index fossils, fossils used to define and identify geological periods.
The conodonts first appeared during the Cambrian Stage 2 (also referred as Tommotian). 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 entire class is postulated to have been wiped out in the Triassic–Jurassic extinction event, which occurred roughly 200 million years ago. 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, along with 34% of other marine genera.
The last conodont species to appear, Neohindeodella detrei, existed at the very end of the Rhaetian. The youngest conodont specimen of this species was found in the earliest Hettangian of Hungary, when the final extinction of conodonts occurred.
Discovery and understanding of conodontsEdit
Conodonts, that is, the teeth-like fossils, were first discovered by Heinz Christian Pander, the results published, in Saint Petersburg, Russia, in 1856. The name pander is a common part, in scientific names of conodonts.
It was only in the early 1980s that the first fossil evidence was found of the rest of the animal (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.
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[verification needed] to 40 cm (Promissum) in length. It is now widely agreed[by whom?] that conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.
Conodont teeth are the earliest found in the fossil record. The evolution of mineralized tissues has 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 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), 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. This is because the conodont animal was soft-bodied, thus everything but the teeth was unsuited for preservation under normal circumstances.
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.
The conodont apparatus may comprise a number of discrete elements, including the spathognathiform, ozarkodiniform, trichonodelliform, neoprioniodiform, and other forms.
Elements of ozarkodinidsEdit
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.
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". 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.
Classification and phylogenyEdit
Milsom and Rigby envision them as vertebrates similar in appearance to modern hagfish and lampreys, and phylogenetic analysis suggests they are more derived than either of these groups. 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. Moreover, some analyses do not regard conodonts as either vertebrates or craniates, because they lack the main characteristics of these groups.
Conodonta Pander 1856 non Eichenberg 1930 sensu Sweet & Donoghue 2001 [Conodontia; Conodontophorida Eichenberg 1930; Conodontochordata]
- Paraconodonta Müller 1962 [Paraconodontida]
- Conodontophora Eichenberg 1930
- Caviodonti Sweet 1988
- Proconodontiformes Sweet 1988
- Pseudooneotodidae Wang & Aldridge 2010
- Proconodontidae Lindström 1981
- Cordylodontidae Lindström 1970 [Cyrtoniodontinae Hass 1959]
- Belodellina Sweet 1988
- Proconodontiformes Sweet 1988
- Conodonti Pander 1856 non Branson 1938
- Oneotodontidae Miller 1981 [Teridontidae Miller 1981]
- Protopanderodontida Sweet 1988 [Panderodontida]
- ?Pronodontidae Lindström 1970
- ?Cornuodontidae Faohraeus1966
- ?Protopanderodontidae Lindström 1970 [Juanognathidae Bergström 1981]
- ?Strachanognathidae Bergström 1981
- Clavohamulidae Lindström 1970
- Drepanoistodontidae Faohraeus 1978
- Acanthodontidae Lindström 1970
- Scolopodontidae Bergström 1981
- Panderodontidae Lindström 1970
- Prioniodontida Dzik 1976 [Distacodontida] (paraphyletic)
- ?Acodontidae Dzik 1993 [Tripodontinae Sweet 1988]
- ?Cahabagnathidae Stouge & Bagnoli 1999
- ?Distacodontidae Bassler 1925 emend. Ulrich & Bassler 1926 [Drepanodontinae Fahraeus & Nowlan 1978; Lonchodininae Hass 1959]
- ?Gamachignathidae Wang & Aldridge 2010
- ?Jablonnodontidae Dzik 2006
- ?Nurrellidae Pomešano-Cherchi 1967
- ?Paracordylodontidae Bergström 1981
- ?Playfordiidae Dzik 2002
- ?Ulrichodinidae Bergström 1981
- Rossodus Repetski & Ethington 1983
- Multioistodontidae Harris 1964 [Dischidognathidae]
- Oistodontidae Lindström 1970
- Periodontidae Lindström 1970
- Rhipidognathidae Lindström 1970 sensu Sweet 1988
- Prioniodontidae Bassler 1925
- Phragmodontidae Bergström 1981
- Plectodinidae Sweet 1988
- Prioniodinida Sweet 1988
- Ozarkodinida Dzik 1976 [Polygnathida]
- ?Anchignathodontidae Clark 1972
- ?Archeognathidae Miller 1969
- ?Belodontidae Huddle 1934
- ?Coleodontidae Branson & Mehl 1944 [Hibbardellidae Müller 1956; Loxodontidae]
- ?Eognathodontidae Bardashev, Weddige & Ziegler 2002
- ?Francodinidae Dzik 2006
- ?Gladigondolellidae (Hirsch 1994) [Sephardiellinae Plasencia, Hirsch & Márquez-Aliaga 2007; Neogondolellinae Hirsch 1994; Cornudininae Orchard 2005; Epigondolellinae Orchard 2005; Marquezellinae Plasencia et al. 2018; Paragondolellinae Orchard 2005; Pseudofurnishiidae Ramovs 1977]
- ?Iowagnathidae Liu et al. 2017
- ?Novispathodontidae (Orchard 2005)
- ?Trucherognathidae Branson & Mehl 1944
- ?Vjalovognathidae Shen, Yuan & Henderson 2015
- ?Wapitiodontidae Orchard 2005
- Cryptotaxidae Klapper & Philip 1971
- Spathognathodontidae Hass 1959 [Ozarkodinidae Dzik 1976]
- Pterospathodontidae Cooper 1977 [Carniodontidae]
- Kockelellidae Klapper 1981 [Caenodontontidae]
- Polygnathidae Bassler 1925 [?Eopolygnathidae Bardashev, Weddige & Ziegler 2002]
- Palmatolepidae Sweet 1988
- Hindeodontidae (Hass 1959)
- Elictognathidae Austin & Rhodes 1981
- Gnathodontidae Sweet 1988
- Idiognathodontidae Harris & Hollingsworth 1933
- Mestognathidae Austin & Rhodes 1981
- Cavusgnathidae Austin & Rhodes 1981
- Sweetognathidae Ritter 1986
- Caviodonti Sweet 1988
- Here, the hagfish are treated as a separate clade, as in Sweet and Donoghue's 2001 tree produced without cladistic analysis. However, it has been recognised by some  that the hagfish and lampreys may be closer to one another in their own clade, the Cyclostomata.
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