Heterostraci

thumb|254px|right|Among the most primitive heterostracans are Tolypelepidida|tolypelepids (top right). Most heterostracans are pteraspidiforms, such as the pteraspidids (bottom right), doryaspidids (bottom left) and the huge psammosteids (top left), which are the youngest known members of the group.

Pteraspidiformes is an extinct order of heterostracan agnathan vertebrates known from extensive fossil remains primarily from Early Devonian strata of Europe and North America, and from Upper Silurian Canada.

Psammosteidae is an extinct family of flattened, benthic heterostracan vertebrates that lived in marine and estuary environments in Europe, Russia & North America. They arose during the Early Devonian, with the first (and best) known genus, Drepanaspis from the Hunsrück lagerstätte. The Psammosteids were the only heterostracans that survived the Upper Frasnian extinction event during the Late Devonian, dying out in the extinction event at the very end of the Devonian. ==References== Palaeos Vertebrates