Thelodonts (from Greek: "nipple teeth") are a group of small, 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[3]) 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,[4] although there is no firm agreement on this point; if they are monophyletic, there is no firm evidence on what their ancestral state was.[5]:206 "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.[6][7] 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. Consequently, we are best able to describe the exoskeleton, which was composed of many tooth-like scales, usually around 0.5-1.5mm in size. These scales did not overlap,[8] were aligned to point backwards along the fish, in the most streamlined direction, but beyond that often appear haphazard in their orientation. The scales themselves approximate the form of a teardrop mounted on a small, bulky base

with the base often containing a small rootlet with which the scale was attached to the fish. The "teardrop" often contains lines, ridges, furrows and spikes running down its length in an array of sometimes complex patterns.[9] Scales found around the gill region were generally smaller than the larger, bulkier scales found on the dorsal/ventral sides of the fish; some genera display rows of longer spikes.[10] The scaly covering contrasts them with most other jawless fish (Cephalaspidomorphi), which were armor-plated with large, flat scales. Aside from scattered scales, some specimens do appear to display imprints, giving an indication of the structure of the whole animal - which appeared to reach 15–30 cm in length.[11] Tentative studies appear to suggest that the fish possessed a more developed braincase than the lampreys, with an almost shark-like outline. Internal scales have also been recovered, some fused into plates resembling gnathostome tooth-whorls to such a degree that some researchers favour a close link between the families.[9] Despite the rarity of complete fossils, very rarely specimens do allow us to gain an insight into the innards of the Thelodonts. Some specimens described in 1993 were the first to be found with a significant degree of three-dimensionality, ending speculations that the Thelodonts were flat fish. Further, these fossils allowed the gut morphology to be interpreted, which generated much excitement: their guts were unlike those of any other agnathans, and a stomach was clearly visible: this was unexpected, as it was previously thought that stomachs evolved after jaws. Distinctive fork-shaped tails - usually characteristic of the jawed fish (gnathostomes) - were also found, linking the two groups to an unexpected degree.[12] The fins of the thelodonts are useful in reconstructing their mode of life. Their paired pectoral fins combined with single, usually well-developed, dorsal and anal fins;[11] these and the prolonged anterior tube-like handle, followed by a heterocercal tail resemble features of modern fish that associated with their deftness at predation and evasion.