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The Pteraspidomorphi, or pteraspidomorphs, is a group of fossil jawless vertebrates which lived from the Early Ordovician to the Late Devonian (i.e. from 470 to 370 million years ago). They possess and extensive dermal armor covering the head, and a scaly tail. They have no other fin than the caudal fin. Their head armor, which always comprises large, median dorsal and ventral plates (or shields), is generally oblong, and this gives them the aspect of a tadpole. Pteraspidormorphs 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 primitive for all vertebrates.
Most pteraspidomorphs were marine, but lived very near to the shore, in lagoons and deltas. Some groups are thought to have been fresh water. They were certainly bottom-dewellers, as shown by traces of abrasion of the ventral surface of their headshield.