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 microCT data of three individuals, including the holotype specimen. The neurocranium of Hapsidoparieon is described for the first time, that is heavily integrated with the skull roof, suggesting a fossorial lifestyle in this taxon. These scans reveal a suite of surprising plesiomorphies of the braincase and skull roof of Hapsidoparieon that are present in recumbirostrans and early stem-amniotes but lacking in unambiguous amniotes, including; the presence of a paired supraoccipital and a broad contact between the otic capsules and skull roof. These results suggest that features previously uniting recumbirostrans with reptiles are symplesiomorphies of Amniota, and a new phylogenetic analysis places Recumbirostra as a crownward group along the amniote stem. 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 conceptualized as ‘microsaurs’. 

This phylogenetic matrix is a modified version of Mann et al. (2023) on recumbirostran relationships, with additional characters and taxa (notably amniotes) added to the matrix.