Gerrothorax pulcherrimus from the upper Triassic Fleming Fjord formation of East Greenland and a reassessment of head lifting in temnospondyl feeding

Abstract

The plagiosaurine Gerrothorax pulcherrimus, a ubiquitous faunal component of the Fleming Fjord Formation, is recognized by tubercular ornamentation, contact between the postfrontal and supratemporal, at least two tooth rows on the posterior coronoid, and a posttemporal fenestra that is small or absent. Gerrothorax pulcherrimus also possesses a derived pectoral morphology that includes an interlocking claviculocleithral complex and an interclavicle with paired posterolateral projections and a transversely truncated posterior margin. Gerrothorax pustuloglomeratus is a junior synonym of G. pulcherrimus; other Gerrothorax species cannot be differentiated from G. pulcherrimus. Flat-headed temnospondyls have been interpreted as having achieved a large gape by raising the skull rather than by lowering the jaw. No detailed, corroborating anatomical evidence from the atlanto-occipital joint, however, has ever been adduced. In G. pulcherrimus the widths of the atlantal and condylar facets are comparable, but dorsoventrally the condylar facets are 45% longer than the comparable dimension of the atlantal facets. Elevation of the skull occurred by atlanto-occipital rotation, and was facilitated by a radius of curvature of the dorsal part of the condylar facets that is shorter than that of the inferior part. From a resting, closed mouth position, G. pulcherrimus was capable of elevating the skull through an excursion of approximately 50°, a movement that rotated the quadrate forward and protruded the lower jaw. An elongate, broadly open neural canal in the atlanto-occipital region, and dorsal displacement of the occipital condyles, are related to relieving the spinal medulla of the sharp angular deformation that head lifting might entail.