The recumbirostran <i>Hapsidopareion lepton</i> from the early Permian (Cisuralian: Artinskian) of Oklahoma reassessed using <scp>HRμCT</scp>, and the placement of Recumbirostra on the amniote stem

Abstract Recumbirostra is a clade of heavily modified, superficially lizard‐like tetrapods that were originally interpreted as ‘microsaurian lepospondyls’ unrelated to the amniote crown. However, recent work has placed Recumbirostra within Reptilia, based on many similarities between the braincase and postcranium of recumbirostrans with early reptiles. Here, the early Permian hapsidopareiid recumbirostran Hapsidopareion lepton is re‐described using high‐resolution μCT data of three individuals across distinct ontogenetic stages, including the holotype specimen. These data reveal a suite of similarities with the hapsidopareiid Llistrofus pricei , suggesting that the latter is a subjective junior synonym of Hapsidopareion lepton . Furthermore, we highlight derived features present in Recumbirostra and Amniota that are otherwise absent in early reptiliomorphs, including: a single supraoccipital element that contributes to the endosseous labyrinths, the absence of paired endolymphatic fossae, and the presence of a distinct ampullary fossa between the semicircular canals. We also identify plesiomorphies of the braincase and skull roof of Hapsidopareion that are present in recumbirostrans and early stem‐amniotes but lacking in unambiguous crown amniotes. This suggests that features previously uniting recumbirostrans with reptiles are possible symplesiomorphies of Amniota, and a new phylogenetic analysis places Recumbirostra as a crownward group along the amniote stem, more derived than traditionally recognized reptiliomorphs such as Seymouria . Our findings highlight the need for further anatomical and descriptive studies of both stem‐ and crown‐group amniotes, and specifically the need for further revisions to those taxa originally regarded as ‘microsaurs’.