The brain in the early fossil jawless vertebrates: evolutionary information from an empty nutshell.

Various 535-365 million year-old extinct jawless vertebrates taxa provide either direct or indirect information about brain and cranial nerve morphology. The paraphyletic group referred to as "ostracoderms", includes some forms in which the braincase closely encapsulated the brain, thereby providing relatively accurate data about its overall external morphology. Current morphology-based phylogenies suggests that "ostracoderms" are in fact jawless stem gnathostomes, and the closely similar aspect of their brain cavity suggests that it illustrates the ancestral condition of the gnathostome brain and fills the morphological gap between the brain condition of the extant cyclostomes and that of the extant jawed vertebrates.