Morphology and ontogeny of the plagiosaurid temnospondyl Plagiosternum granulosum from the Middle Triassic of Germany

Abstract

Plagiosaurids form a small but highly disparate clade of Triassic temnospondyls that are characterized by extremely flattened and wide skulls, large orbits and a knobby to pustular ornamentation. The largest European taxon is Plagiosternum granulosum from the Middle Triassic of Germany. Originally known from fragmentary bonebed material only, recent finds add well-preserved specimens that reveal the structure of the skull, mandible and pectoral girdle in great detail. The humerus and atlas of P. granulosum are described for the first time, and a putative cleithrum is identified. Ontogenetic changes include a proportional extension of the orbit, a more convex occipital margin, elongation of the parietal, proportional increase of the occipital condyles and formation of pustular ornament at nodal points of reticulate ridges. The presence of gastral scales can be inferred from the structure of the interclavicle, but there is no evidence of osteoderms. Plagiosternum granulosum was the most salt-tolerant temnospondyl in the Ladinian palaeoenvironments, where it dwelled brackish lagoons and lakes and formed a community with nothosaurids and other euhaline taxa.