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a subclass of agnathous fossil vertebrates. Heterostraci were common during the Paleozoic era, from the Ordovician to the end of the Devonian. The shell was made of a varying number of schistoid plates of acellular bone, with a single (caudal) fin, lateral apophyses serving as supporting planes, and dorsal ridges, or spines, for the maintenance of equilibrium. The length of Heterostraci ranged approximately from several cm to 1 m. They inhabited both ocean and fresh waters, feeding on small organisms of the benthic and middle depths. Heterostraci are of great significance in the comparison of continental and marine deposits of the Middle Paleozoic.
REFERENCEObruchev, D. V. “Vetv’ Agnatha: Bescheliustnye.” In Osnovy paleontologii: Bescheliustnye, ryby. Moscow, 1964.
D. V. OBRUCHEV