SKULL MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF A NEW MIDDLE TRIASSIC PLAGIOSAURID TEMNOSPONDYL FROM GERMANY, AND THE EVOLUTION OF PLAGIOSAURID EYES

A partial skull from the Lower Keuper (Middle Triassic) of Germany is recognized as belonging to a new genus and species of plagiosaurid temnospondyl. It is readily identified by the following autapomorphies: (1) extremely large orbits medially extended to give very thin interorbital region and cheek; (2) posterior skull table abbreviated, with splint-like supratemporals, postparietals and parietals; (3) supraorbital lateral line sulcus absent on frontal, with blind ending on parietal, and continued at the anterior margin of the postorbital; (4) occiput sloping posteriorly, with subtympanic fossa exposed in ventral view; (5) cultriform process and basicranial suture extremely narrow; (6) mandibular and maxillary teeth very long with crowns markedly curved inwards. The new taxon shares the following derived character states with Plagiosternum: (1) the pentagonal shape of orbits; (2) the slender interorbital region and cultriform process; (3) the small-scale, polygonal pit-and-ridge ornament on posterior skull table, with single prominent tubercles rising from ridges. Phylogenetic analysis finds plagiosaurids to be monophyletic within a broad range of temnospondyls, nesting within a clade of short-skulled stereospondyls. Plagio-suchus is the most basal plagiosaurid, and Plagiosternum forms the sister group to the new taxon. The biological significance of the large orbits of plagiosaurids and their relationships to the eyeballs is discussed. The only type of eyeball that would be feasible for all known plagiosaurids would be a small spherical structure possibly situated near the anterior edge of the orbit. For those plagiosaurs with extremely large shallow orbits like Plagiosternum and Megalophthalma, a possible adaptation would be a lens-less eye comprising a flat retinal plate across the entire orbit.