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.
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 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.
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 conodonts
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 commonly used 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 1-40 cm (Promissum) in length. Conodonts have large eyes, fins with fin rays, chevron-shaped muscles and a notochord.
Conodont elements from the Deer Valley Member of the Mauch Chunk Formation
in Pennsylvania, Maryland, and West Virginia, USA
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. Kladognathus sp.
, Sa element, posterior view, X140 2. Cavusgnathus unicornis
, 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. Kladognathus sp.
, Sa element, lateral views, X140
5. Cavusgnathus unicornis
, alpha morphotype, Pa element, lateral view, X140
6, 7. Cavusgnathus sp.
, Pa element, lateral view, X140
8. Polygnathus sp.
, Pa element, upper view, reworked Late Devonian to Early Mississippian morphotype, X140
9. Gnathodus texanus?
, 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. Polygnathus sp.
, 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. Gnathodus sp.
, Pa element, upper view, reworked Late Devonian(?) through Mississippian morphotype, 93RS–67, X140
13. Kladognathus sp.
, M element, lateral views, 93RS–67, X140
14. Cavusgnathus sp.
, Pa element, lateral view, 93RS–67, X140
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.
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.
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.
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 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. The preserved musculature suggests that some conodonts (Promissum at least) were efficient cruisers, but incapable of bursts of speed.
Others have been interpreted as a "grasping and crushing array".
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.
Classification and phylogeny
As of 2012phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord.
, scientists classify the conodonts in the
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 taxonomy based on Sweet & Donoghue, Mikko's Phylogeny Archive and Fish classification 2017.
Conodonta Pander 1856 non Eichenberg 1930 sensu Sweet & Donoghue 2001 [Conodontia; Conodontophorida Eichenberg 1930; Conodontochordata]
- Paraconodonta Müller 1962 [Paraconodontida]
- Conodontophora Eichenberg 1930
- ^ 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.
- ^ The clade Proconodontida is also known as Cavidonti.
- ^ Euconodonta is referred to as "Conodonti" by Sweet and Donoghue, although this is not widely used .
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