The Triassic reptile Hyperodapedon from Elgin: functional morphology and relationships

Abstract

The rhynchosaur Hyperodapedon gordoni from the late Triassic of Elgin, northeast Scotland, was a squat 1.3 m long reptile with a specialized skull. The partial remains of about 35 individuals were studied largely from casts. There is a broad size range (? age), and two morphs may be distinguished on the basis of proportions (? sexual dimorphism). Hyperodapedon had a large nasal capsule and a large eye with sclerotic plates. These indicate a good sense of smell and diurnal vision. There was no tympanum, but Hyperodapedon could probably detect sound via skin behind the quadrate, and via its throat and large hyoids. Evidence from the jaw articulation and tooth wear shows that Hyperodapedon had a precision-shear bite. Food was probably scraped up with the premaxillary beak and manipulated by a powerful tongue. The batteries of teeth on maxilla and dentary retained open roots throughout their functional life with no normal reptilian replacement. The hindlimb was adapted for scratch-digging, from a comparison of its claws with those of modern mammals. The diet probably consisted of tough, but not hard, plant material. The forelimb may have been held in a semi-sprawling pose with humerus rotation im portant in locomotion. The hindlimb was adapted for a semi-erect gait and the femur moved back and forwards with little rotation. Hyperodapedon is found in aeolian sands, and its skeletons show little physical disturbance, although there is evidence of scavenging. The late Triassic rhynchosaurs from Elgin, India, Brazil and Argentina form a close taxonomic group quite distinct from those of the middle Triassic, on the basis of univariate and multivariate analyses. The Indian form is returned to the genus Hyperodapedon as H. huxleyi since it is so similar to the Elgin form. Hyperodapedon differs from the South American Scaphonyx in the presence of lingual teeth on the dentary. The early Triassic Mesosuchus probably belongs to a separate family rather distinct from the true Rhynchosauridae. Rhynchosaurs have no special relationship with the sphenodontids, and they form a distinct, and important, lineage within the PermoTriassic diapsid radiation.