Euconodonta

Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which are usually found in isolation and are now called conodont elements. Knowledge about soft tissues remains limited. They existed in the world's oceans for over 300 million years, from the Cambrian to the beginning of the Jurassic. Conodont elements are widely used as index fossils, fossils used to define and identify geological periods. The animals are also called Conodontophora (conodont bearers) to avoid ambiguity.

Discovery and understanding of conodonts

The teeth-like fossils of the conodont were first discovered by Heinz Christian Pander and the results published in Saint Petersburg, Russia, in 1856. The name ''pander'' is commonly used in scientific names of conodonts.

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.

Description

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 ranged from 1–40 cm (''Promissum'') in length. Conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.

Elements

Conodont elements consist of mineralised teeth-like structures of varying morphology and complexity. 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 conodonts 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.

These microfossils are made of hydroxylapatite (a phosphatic mineral). The conodont elements can be extracted from rock using adequate solvents.

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

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

In the 1930s, the concept of conodont assemblages was described by Hermann Schmidt and by Harold W. Scott in 1934.

Elements of ozarkodinids

The feeding apparatus of ozarkodinids is 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.

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". Wear on some condont elements suggests that they functioned like teeth, with both wear marks likely created by food as well as by occlusion with other elements. Studies have concluded that conodonts taxa occupied both pelagic (open ocean) and nektobenthic (swimming above the sediment surface) niches. The preserved musculature suggests that some conodonts (''Promissum'' at least) were efficient cruisers, but incapable of bursts of speed. Based on isotopic evidence, some Devonian conodonts have been proposed to have been low-level consumers that fed on zooplankton.

A study on the population dynamics of '' Alternognathus'' has been published. Among other things, it demonstrates that at least this taxon had short lifespans lasting around a month. A study Sr/ Ca and Ba/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 different trophic niches.

The some species of the genus ''Panderodus'' have been speculated to be venomous, based on grooves found on some elements..

Classification and phylogeny

, scientists classify the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord.

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. More recently it has been proposed that conodonts may be stem-cyclostomes, more closely related to hagfish and lampreys than other living vertebrates.

Evolutionary history

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 the Carboniferous, with an extinction event at the end of the middle Tournaisian and a prolonged period of significant loss of diversity during the Pennsylvanian. 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 at 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, especially glacial events and associated marine regressions which reduced continental shelf area. However, their final demise is more likely related to biotic interactions, perhaps competition with new Mesozoic taxa.

Taxonomy

Conodonta taxonomy based on Sweet & Donoghue, Mikko's Phylogeny ArchiveMikko's Phylogeny Archiv

and Fish classification 2017.

Conodonta Pander, 1856 non Eichenberg, 1930 sensu Sweet & Donoghue, 2001 onodontia; Conodontophorida Eichenberg, 1930; Conodontochordata* Paraconodonta Müller, 1962 araconodontida** Amphigeisiniformes
*** Amphigeisinidae Miller, 1981
** Westergaardodiniformes Lindström, 1970
***Westergaardodinidae Müller, 1959 hosonodinidae/small>
***Furnishinidae Müller & Nogami, 1971
* Conodontophora Eichenberg, 1930
** Cavidonti Sweet, 1988
*** Proconodontiformes Sweet, 1988
**** Pseudooneotodidae Wang & Aldridge, 2010
**** Proconodontidae Lindström, 1981
**** Cordylodontidae Lindström, 1970 yrtoniodontinae Hass, 1959****Fryxellodontina
***** Fryxellodontidae Miller, 1981
***** Pygodontidae Bergstrom, 1981
****Belodellina Sweet, 1988
***** Ansellidae Faohraeus & Hunter, 1985
***** Dapsilodontidae Sweet, 1988
***** Belodellidae Khodalevich & Tschernich, 1973 ambropustulidae** Conodonti Pander, 1856 non Branson, 1938
*** Oneotodontidae Miller, 1981 eridontidae Miller, 1981
*** Protopanderodontida Sweet, 1988 anderodontida**** ? Pronodontidae Lindström, 1970
**** ? Cornuodontidae Faohraeus, 1966
**** ? Protopanderodontidae Lindström, 1970 uanognathidae Bergström, 1981****? Strachanognathidae Bergström, 1981
****? Pseudooneotodidae
**** Clavohamulidae Lindström, 1970
**** Drepanoistodontidae Faohraeus, 1978
**** Acanthodontidae Lindström, 1970
**** Scolopodontidae Bergström, 1981
**** Panderodontidae Lindström, 1970
*** Prioniodontida Dzik, 1976 istacodontida(paraphyletic)
**** ? Acodontidae Dzik, 1993 ripodontinae Sweet, 1988****? Cahabagnathidae Stouge & Bagnoli 1999
****?Distacodontidae Bassler, 1925 emend. Ulrich & Bassler, 1926 repanodontinae 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 ischidognathidae**** Oistodontidae Lindström, 1970
**** Periodontidae Lindström, 1970
****Rhipidognathidae Lindström, 1970 sensu Sweet, 1988
**** Prioniodontidae Bassler, 1925
**** Phragmodontidae Bergström, 1981
**** Plectodinidae Sweet, 1988
****Icriodontacea
***** Balognathidae (Hass, 1959)
***** Polyplacognathidae Bergström, 1981
***** Distomodontidae Klapper, 1981
***** Icriodellidae Sweet, 1988
***** Icriodontidae Müller & Müller, 1957
****Prioniodinida Sweet, 1988
***** ? Oepikodontidae Bergström, 1981
*****? Xaniognathidae Sweet, 1981
*****Chirognathidae Branson & Mehl, 1944
***** Prioniodinidae Bassler, 1925 ibbardellidae Mueller, 1956***** Bactrognathidae Lindström, 1970
***** Ellisoniidae Clark, 1972
*****Gondolellidae Lindström, 1970
**** Ozarkodinida Dzik, 1976 olygnathida***** ?Anchignathodontidae Clark, 1972
*****? Archeognathidae Miller, 1969
*****? Belodontidae Huddle, 1934
*****? Coleodontidae Branson & Mehl, 1944 ibbardellidae 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 /small>Ozarkodinidae Dzik, 1976*****Pterospathodontidae Cooper, 1977 arniodontidae
***** Kockelellidae Klapper, 1981 aenodontontidae*****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

See also

* Timeline of the evolutionary history of life
* Micropaleontology
* List of conodont genera
* Conodont biostratigraphy
* Conodont alteration index

References

Further reading

*
*
*
* Gould, Stephen Jay (1985). "Reducing Riddles". In ''The Flamingo's Smile'', 245-260. New York, W.W. Norton and Company. .
*
*Knell, Simon J. ''The Great Fossil Enigma: The Search for the Conodont Animal'' (Indiana University Press; 2012) 440 pages
* Sweet, Walter (1988). ''The Conodonta: morphology, taxonomy, paleoecology, and evolutionary history of a long-extinct animal phylum.'' Oxford, Clarendon Press.
*
*

External links

*
*
*

{{Authority control

Furongian first appearances
Late Triassic extinctions
Cambrian chordates