THELODUS

extinct genus of early and primitive jawless vertebrates that is known from impressions on rocks of Silurian age in Europe and North America, although the genus persisted into the Early Devonian epoch (the Silurian period began 438 million years ago and ended 408 million years ago). The structure of Thelodus and its relatives, the coelolepids, is poorly known; but it is probable that the body was encased by tiny bony denticles. Thelodus was more agile than its contemporaries, who were encased in thick bony armour.