Thelodonts are an ensemble of fossil, presumably jawless vertebrates, distinguished from other jawless vertebrate taxa by the structure of their exoskeleton, which is entirely made up by minute scales. These scales are hollowed by a pulp cavity and superficially resemble the placoid scales of sharks. They are known from the Lowermost Silurian (and possibly the Late Ordovician) to the Late Devonian (430 to 370 million years ago).
Thelodonts lived in shallow-water marine environments. They seem to have survived longer in the Gondwana than in Euramerica, where they disappear from the fossil record after the early Middle Devonian.
There are two competing theories regarding the affinities of the thelodonts. Some consider them as clade, characterized by:
- Scales with a growing base and special processes to anchor them in the dermis.
Others consider them as paraphyletic; that is, an ensemble of primitive forms respectively related to other major vertebrate groups. Some thelodonts would thus be relatives of the pteraspidomorphs, others of anaspids, galeaspids, osteostracans, and the gnathostomes.
The overall morphology of the thelodonts is only known from a few forms but seems to be relatively homogeneous. Most of them have a dorsoventrally flattened head with lateral flaps (or paired fins?), small eyes, and eight, ventrally placed gill slits. There is one dorsal fin, one anal fin, and the caudal fin is slightly hypocercal, with a large dorsal web strengthened by radials. Other thelodonts are deep-bodied, with larger eyes, laterally placed gill slits, and a large fork-shaped tail.
There are very few information on the internal anatomy of thelodonts, although some distinctly show imprints of paired olfactory organs. Some thelodonts have minute, internal denticles which cover the pharynx and possibly a large, median, inhalent duct comparable to the median dorsal opening of the Galeaspida. The presence of such denticles is unique among jawless vertebrates and recalls the condition in the gnathostomes. Well preserved thelodonts from Canada show the impression of a large, barrel-shaped stomach, a characteristic which is shared with the gnathostomes.
When thelodonts are regarded as clade, they are placed as the sister-group of either the Pteraspidomorphi, or the Gnathostomata (see Vertebrata page). When considered as being paraphyletic, they may include stem forms of all "ostracoderm" groups (pteraspidomorphs, galeaspids, anaspids, osteostracans, and pituriaspids) and the gnathostomes. In particular, one group of thelodonts, the Katoporida, shares with the osteostracans, the presence of a special type of dentine, called mesodentine, in the scales. It is probable that the fork-tailed thelodonts (unamed to date) are more closely related to the Heterostraci (or all the Pteraspidomorphi) than to any other vertebrate taxon, as they share the same tail structure. Thelodonts with internal denticles, such as Loganellia, may be more closely related to the gnathostomes than to any group of jawless vertebrate.
Thelodont taxa have been defined on the basis of scale histology and morphology, but recent studies on articulated thelodonts show that scale morphology can be misleading, as a wide range of scale morphologies can occur in the same individual. Thelodonts are currently classified into four groups: Achanolepida, Loganiida, Turiniida, and Katoporida, on the basis of scale histology and morphology, but this is merely a phenetic classification.
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