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.