Heterostraci (Pteraspida; heterostracans; superclass Agnatha) Oldest known vertebrate order of jawless, heavily armoured fish-like forms, ranging from the Upper Cambrian to the Devonian. The dermal plates of the body lacked true bone cells and no internal skeleton has been preserved, so this presumably was cartilaginous. The anterior part of the body was covered by large dorsal and ventral plates, with smaller plates to the side. There was a single lateral gill opening, eyes at the sides of the head, and the impression of paired nasal sacs on the inner surface of the rostral shield (which protruded in front of the mouth). The rest of the body was covered by scales; the tail was hypocercal (tilted downwards). Heterostracans were usually only a few centimetres long, although some species reached 1.5m. The body in typical forms, such as the Devonian Pteraspis, was rounded, but the group included dorsoventrally flattened, bottom-dwelling forms such as Drepanaspis.

Heterostraci Alternative name for Pteraspida, an order of fossil jawless vertebrates. See PTERASPIDOMORPHA.