Osteology and phylogenetic position of Plagiosaurus depressus (Temnospondyli: Plagiosauridae) from the Late Triassic of Germany and the repeated loss of dermal bones in plagiosaurids

Abstract

The skull and postcranium of the Late Triassic plagiosaurid temnospondyl Plagiosaurus depressus from Halberstadt (Germany) are redescribed in detail. Plagiosaurus possesses two autapomorphies, the abbreviated tabular and the broad contact between the postorbital and parietal. A comprehensive phylogenetic analysis of Plagiosauridae finds a clade Plagiosaurinae consisting of Plagiosaurus and Gerrothorax. Among other characters, both taxa share the solid box-like sides of the low pectoral girdle, the short but robust humerus with small deltopectoral crest and well-developed supinator process, and an anterodorsally directed lateral line sulcus close to the mandibular symphysis. Compared with Gerrothorax, the skull of Plagiosaurus is deeper and more slender, and the postfrontal is absent, meaning that the parietal forms the posteromedial margin of the enlarged orbit. This new interpretation of the bone configuration shows that reduction of circumorbital bones associated with orbital enlargement occurred three times independently within Plagiosauridae, albeit probably in different functional contexts. Our phylogenetic analysis further reveals the Plagiosuchinae (Plagioscutum + Plagiosuchus) as the most basal plagiosaurid clade, which, in turn, forms the sister group to Plagiosaurinae and Plagiosterninae (Plagiorophus + (Plagiosternum + Megalophthalma)). The mentioned shared derived characters of Plagiosaurinae in the pectoral girdle, forelimbs, and mandibular lateral lines suggest a similar benthonic lifestyle.