Temporal range: 495–199.6Ma Late Cambrian to Late Triassic
|Reconstruction of a conodont|
|Two conodont "teeth" and a reconstruction of a conodont|
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.
Conodont teeth are the earliest that we find in the fossil record. Neil Shubin writes: "For a long time conodonts were enigmas: scientists disagreed over whether they were animalian, plant or mineral. Conodonts were claimed to be pieces of clams, sponges, vertebrates, even worms. The speculation ended when whole animals started to show up in the fossil record. For years, paleontologists have argued about why hard skeletons, those containing hydroxyapatite, arose in the first place. For those who believed that skeletons began with the backbones, or body armor, conodonts provide an 'inconvenient tooth' if you will. The first hydroxyapatite-containing body parts were teeth. Hard bones arose not to protect animals but to eat them."
The 11 known fossil imprints of conodont animals record an eel-like creature with 15, or more rarely, 19 elements forming a bilaterally symmetrical array in the head. This array constituted a feeding apparatus radically different from the jaws of modern animals. The three forms of teeth, coniform cones, ramiform bars, and pectiniform platforms, may have performed different functions.
The organisms range from a centimeter or so[verification needed] to the large Promissum, 40 cm in length. It is now widely agreed that conodonts had large eyes, fins with fin rays, chevron-shaped muscles and a notochord.
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 phylogeny
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. This analysis, however, comes with one caveat: early forms of conodonts, the protoconodonts, appear to form a distinct clade from the later paraconodonts and euconodonts. The protoconodonts likely represent a stem group to the phylum containing chaetognath worms, indicating they 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.
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. These phosphatic microfossils are now termed "conodont elements" to avoid confusion. 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.
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 most of the conodont animal was soft-bodied, thus everything but the teeth were not suited for preservation under normal circumstances.
The conodont apparatus may comprise a number of discrete elements, including the spathognathiform, ozarkodiniform, trichonodelliform, neoprioniodiform, and other forms.
- Lau event - mass extinction event with major impact on conodonts
- 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[original research?].
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- The extinction of conodonts —in terms of discrete elements— at the Triassic-Jurassic boundary
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