Pteraspidomorphi

Pteraspidomorpha (also frequently spelled Pteraspidomorphi) is an extinct class of early jawless fish. They have long been regarded as closely related or even ancestral to jawed vertebrates, but the few characteristics they share with the latter are now considered as basal traits for all vertebrates.

thumb|left|Sacabambaspis, the best known arandaspid, from the Ordovician of Bolivia. Shows the characteristic, frontally positioned eyes, like car head lamps Arandaspida is a taxon of very early, jawless prehistoric fish which lived during the Ordovician period. Arandaspids represent some of the oldest known vertebrates. The group represents a subclass within the class Pteraspidomorphi, and contains only one order, the Arandaspidiformes. The oldest known genus of this group is Sacabambaspis found in South America.

Astraspida, or astraspids, are a small group of extinct armored jawless vertebrates, which lived in the Late Ordovician (about 450 million years ago) in North America. They are placed among the Pteraspidomorphi because of the large dorsal and ventral shield of their head armor. They are represented by a single genus, Astraspis, including possibly two species, A. desiderata and A. splendens but their remains are fairly abundant in Ordovician sandstones of the USA (Colorado, Arizona, Oklahoma, Wyoming) and Canada (Quebec). The head armor of Astraspis is rather massive, with a series of ten gill