Article: The braincase of the anthracosaur Archeria crassidisca with comments on the interrelationships of primitive tetrapods
The braincase of the embolomere Archeria crassidisca from the Lower Permian of Texas is described, and provides much new detail about a primitive tetrapod braincase. It is a solidly ossified unit as in other embolomeres and like them has no separate dorsal ossification of the occipital arch (supraoccipital). It retains evidence of primitive braincase characters (ventral cranial and lateral otic fissures) found in the embryonic stages of recent jawed vertebrates, and seen in the adult stages of primitive fish groups. A survey of the braincases in early tetrapods shows that no taxonomic weight can be accorded to the degree to which they are ossified. Certain characters (parasphenoidal tubera for the insertion of subvertebral (hypaxial) muscles, and a median retractor pit in the basisphenoid) resemble those in Seymouria. The latter is also seen in the anthracosaur Eoherpeton and the reptile Eocaptorhinus, and is proposed as a shared derived character uniting a reptiliomorph ramus of tetrapods. Another character which this group also shares (true synovial basal articulation) may represent a further derived character. However, the condition of both these characters is unknown in some of the earliest tetrapods (e.g. Crassigyrinus and the loxommatids), and so may be primitive for all tetrapods.