Abstract

Seymouriamorphs are a group of early Permian stem amniotes with both terrestrial and aquatic members. Since their initial discovery, they have been recognized as being approximately intermediate between temnospondyls and crown amniotes, and are frequently used in phylogenetic analyses of both early amniotes and Palaeozoic tetrapods more broadly. We utilised neutron computed tomography (nCT) to study the morphology of the first cranial material of Seymouria from the Dolese Brothers Limestone Quarry near Richards Spur, Oklahoma, with particular emphasis on the braincase and otic capsule. Our data reveal additional information regarding the neurocranium, including an unpaired sphenethmoid that expands dorsally to abut the overlying frontals. The prootic and opisthotic are complex structures that preserve clear impressions and features of all three semicircular canals and that frame the laterally extensive otic tube. We identified the presence of an ossification of the synotic tectum, although it was previously believed to be absent in this group. This raises the possibility that similar ossifications in other taxa may be also unrecognized, and highlights the ongoing question regarding the homology of such ossifications across Palaeozoic tetrapods. The presence of characters found in the better known species of Seymouria, S. baylorensis and S. sanjuanensis prevents referral to either species; the potential for ontogenetic variation in many of these characters highlights the need to reevaluate the taxonomy of these species.