Development of the gnathostome neurocranium includes the origin and fusion of the occiput and trabeculae to the primitive vertebrate braincase, evidenced by the anterior fissure among placoderms and the otic-occipital fissure in chondrichthyans and teleostomes. Placoderms retain plesiomorphic short trabeculae with the palatoquadrate articulating with the parachordals.
Fusion among anterior vertebrae to form a synarcual in placoderms mimics the anatomical transformations associated with misexpression of segment-related genes in mice and humans. Thus, one group�s maladaptive misexpression may be the source of an adaptive change in other taxa where a synarcual is found.
Goodrich suggested a paired and neural crest origin for basibranchial elements of the visceral skeleton. This is confirmed in Cowralepis mclachlani (Phyllolepida, Placodermi) where these elements are paired throughout ontogeny and possess significant attachment sites for hypobranchial musculature (documenting their neural crest origin).
Renewed interest in placoderms is providing important evidence to establish character polarity for a number of anatomical systems. It also documents the presence in adults of characters that are now only glimpsed as transitory features in ontogeny thus a peramorphic shift among extant taxa relative to the gnathostome primitive condition.