Ordovician - Devonian ||regnum = Animalia |phylum = Chordata |classis = Heterostraci | subdivision_ranks = Orders | subdivision = Cyathaspidiformes Pteraspidiformes }} Heterostraci ("Different scales")'s an extinct class of jawless vertebrate that lived primarily in marine and estuary environments. They arose during the Ordovician, and all, save for the Psammosteids, became extinct during the late Devonian. This last group of heterostracans died out in the extinction event at the end of the De… (More on Heterostraci)