The classification and evolution of the Heterostraci

An outline classification is given of the Heterostraci, with diagnoses of the following orders and suborders: Astraspidiformes, Eriptychiiformes, Cyathaspidiformes (Cyathaspidida, Poraspidida, Ctenaspidida), Psammosteiformes (Tesseraspidida, Psarnmosteida), Traquairaspidiformes, Pteraspidiformes (Pteraspidida, Doryaspidida), Cardipeltiformes and Amphiaspidiformes (Amphiaspidida, Hibernaspidida, Eglonaspidida). It is shown that the various orders fall into four main evolutionary lineages cyathaspid, psammosteid, pteraspid and amphiaspid, and these are traced from primitive tessellated forms. A tentative phylogeny is proposed and alternatives are discussed