Extinct jawless vertebrates from the Ordovician, Silurian, and Devonian periods. Thelodonts had bodies covered by tiny scales, each a single toothlike unit (odontode). The scales varied in shape over the body. Each scale had a bony base pierced by a pulp cavity, with pulp canals extending into a dentinous crown. Thelodonts have been thought to be related to the Heterostraci or to the jawed vertebrates (Gnathostomata).