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Thelodonti (from Greek: "nipple teeth")Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996. is a class of extinct Palaeozoic with distinctive Fish scale instead of large plates of armor. These fish lived in both freshwater and marine environments, first appearing during the Ordovician, and perishing shortly after the Frasnian–Famennian extinction event in the Late Devonian.
There is much debate over whether the group represents a Monophyly, or disparate stem groups to the major lines of Agnatha and Gnathostome.
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 sometimes thought to represent a polyphyletic group, although there is no firm agreement on this point. On the basis that they are monophyletic, they are reconstructed as being ancestrally marine and invading freshwater on multiple occasions.
"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 early and mid Silurian. See
Traditionally thelodonts were considered predominantly deposit-feeding bottom dwellers, but more recent studies have shown they occupied various ecological roles in various parts of the water column, much like modern bony fishes and sharks. In particular, a large variety of species preferred reef ecosystems, and it has been suggested that this preference was the reason for the development of their unique scales, protecting against abrasion and allowing for the development of more flexible bodies than other jawless fish, which had inflexible armors and were restricted to open habitats.
The exoskeleton is composed of many tooth-like scales, usually around 0.5–1.5mm in size. These scales did not overlap, and 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. 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.
The scaly covering contrasts them with most other jawless fishes, 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. 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.
Despite the rarity of complete fossils, these very rare intact specimens do allow us to gain an insight into the internal organ arrangement 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 flattened 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.
The fins of the thelodonts are useful in reconstructing their mode of life. Their paired were combined with a single, usually well-developed, Dorsal fin and ; these and the hypercercal and much larger hypocercal lobes forming a heterocercal tail resemble features of modern fish that are associated with their deftness at predation and evasion.
Bone – being one of the most resistant materials to the process of fossilisation – often preserves internal detail, which allows the histology and growth of the scales to be studied in detail. The scales consist of a non-growing "crown" composed of dentine, with a sometimes-ornamented Tooth enamel upper surface and an aspidine (acellular bony tissue) base. Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish. Beyond that, there appear to be five types of bone-growth, which may represent five natural groupings within the thelodonts – or a spectrum ranging between the end members, meta- (or ortho-) dentine and mesodentine tissues. Each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish.
Scale morphology, alone, has limited value for distinguishing thelodont species. Within each organism, scale shape varies greatly according to body area, with intermediate scale forms appearing between different areas; furthermore, scale morphology may not even be constant within a given body area. To confuse things further, scale morphologies are not unique to specific taxa, and may be indistinguishable on the same area of two different species.
The morphology and histology of the thelodonts provides the main tool for quantifying their diversity and distinguishing between species – although ultimately using such convergent traits is prone to errors. Nonetheless, a framework of three groups has been proposed, based upon scale morphology and histology.
A recent assessment of thelodont taxonomy by Wilson and Märss in 2009 merges the orders Loganelliiformes, Katoporiida and Shieliiformes into Thelodontiformes, places families Lanarkiidae and Nikoliviidae into Furcacaudiformes on the basis of scale morphology, and establishes Archipelepidiformes as the basal-most order.
A newer taxonomy based on the work of Nelson, Grande and Wilson 2016 (2025). 9781118342336, John Wiley & Sons. ISBN 9781118342336 and van der Laan 2016.
Most thelodonts were considered deposit feeders, but more recent studies have shown that several species were active swimmers and thus more pelagic. A large variety of species in particular preferred reef ecosystems. They are mainly known from open shelf environments, but are also found nearer the shore and in some freshwater settings. The appearance of the same species in fresh- and salt-water settings has led to suggestions that some thelodonts migrated into fresh water, perhaps to spawn. However, the transition from fresh- to salt- water should be observable, as the scales' composition would change to reflect the different environment. This compositional change has not yet been found.
The oldest named thelodonts are from the Sandbian stage of the Late Ordovician: Larolepis (from St. Joseph Island, Canada) and Stroinolepis (from Severnaya Zemlya, Russia). Both belong to the family Loganelliidae. This family is not the earliest-diverging group of thelodonts, suggesting that their origin goes back further. The two earliest-diverging thelodont groups, the archipelepidiforms and furcacaudiforms, are mainly distributed in Laurentia (the ancient tropical continent roughly equivalent to North America), with only the latter dispersing to nearby continents a few times. Thelodontiforms, on the other hand, may have originated from Baltica (equivalent to much of Eastern Europe). Thelodontiforms attained a much wider distribution than the other two orders, despite not appearing to be adapted for active swimming as adults. Their ability to reach new regions relate to new dispersal adaptations in their Larva. Thelodonts reached their maximum diversity in the Early Devonian only to decline sharply afterwards. The last named thelodont is Arianalepis, a Turiniidae which managed to survive until the Famennian (the last stage of the Devonian) in Iran and Australia.
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