The neodiapsid <i>Thadeosaurus colcanapi</i> from the upper Permian of Madagascar

Abstract The enigmatic neodiapsid Thadeosaurus colcanapi (Lower Sakamena Formation, southwestern Madagascar), sole species of the genus Thadeosaurus , is revised here. The attribution of 12 of the 21 referred specimens is confirmed, spanning all ontogenetic stages, and the anatomy of Thadeosaurus is redescribed in detail with comments on ontogenetical differences. This new anatomical information is included in an expanded phylogenetical dataset tailored to examine the relationships of Permo‐Triassic diapsids. A stem‐saurian neodiapsid position is confirmed here for all ‘younginiforms’, which are here recovered paraphyletic, with Youngina representing an earlier‐diverging taxon. However, this topology is extremely labile, and the monophyly or paraphyly of ‘younginiforms’ could not be unequivocally supported. In contrast, our analyses provide good support for a monophyletic Tangasauridae including all other ‘younginiforms’. Thadeosaurus is here recovered as a member of the Tangasauridae and as the sister‐group to the putative semi‐aquatic Tangasaurinae with a moderate degree of support, despite the large amounts of missing data in lesser‐known tangasaurids partially obscuring our understanding of tangasaurid interrelationships. Last, Thadeosaurus is considered to have inhabited a nearshore, probably riparian, environment, although it remains unclear whether it was semi‐aquatic or fully terrestrial. Further examinations of lesser‐known tangasaurids, as well as a novel morphotype identified here in the Lower Sakamena Formation of Madagascar, could provide new evidence to deepen our understanding of the evolution and palaeoecology of the Tangasauridae.