Thelodonti
| Thelodonti Temporal range: 453–359 Ma Middle–Late Ordovician to Late Devonian |
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| Among the flat-bodied forms are Lanarkia (top left), provided with long, spine-shaped scales, and Loganellia (top right and middle). Other thelodonts, such as Furcacauda from the Devonian of Canada (bottom) are deep-bodied, with lateral gill openings and a very large, forked tail. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Subphylum: | Vertebrata |
| Class: |
Thelodonti Jaekel, 1911 |
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Thelodonti (from Greek: "feeble teeth") is a class of extinct jawless fishes with distinctive scales instead of large plates of armor.
There is much debate over whether the group of Palaeozoic fish known as the Thelodonti (formerly coelolepids) represent a monophyletic grouping, or disparate stem groups to the major lines of jawless and jawed fish.
Thelodonts are united in possession of "thelodont scales". This defining character is not necessarily a result of shared ancestry, as it may have been evolved independently by different groups. Thus the thelodonts are generally thought to represent a polyphyletic group, although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence regarding their ancestral state.
"Thelodonts" were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their covering of distinctive, small, spiny scales. These scales were easily dispersed after death; their small size and resilience makes them the most common vertebrate fossil of their time.
The fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing during the Frasnian–Famennian extinction event of the Late Devonian. They were predominantly deposit-feeding bottom dwellers, although there is evidence to suggest that some species took to the water column to be free-swimming organisms.
Very few complete thelodont specimens are known; fewer still are preserved in three dimensions. This is due in part to the lack of an internal ossified (i.e. bony) skeleton; it does not help that the scales are poorly, if at all, attached to one another, and that they readily detach from their owners upon death.
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Wikipedia