Hyperodapedon

Abstract: The Rosario do Sul Group is one of the better-known tetrapod-bearing continental deposits of Triassic age. It crops out in central Rio Grande do Sul, south Brazil, and has yielded a fauna of more than 40 valid species, including temnospondyl, procolophonoideans, dicynodonts, cynodonts, sphenodontian, rhynchosaurs, and archosaurs. Its fossil record is herein briefly assessed, along with the stratigraphy of its bearing sequences. This includes the Early (perhaps earliest) Triassic Sanga do Cabral Formation, which correlates to the Lystrosaurus/ "Impoverished" zones of the Karoo Basin, and a second major stratigraphic sequence of Mid-Late Triassic age that broadly corresponds to the Santa Maria and Caturrita formations. The lower, cynodont-dicynodont dominated fauna of that sequence is partially Chanarian (Ladinian) in age, and encompasses the Dinodontosaurus Assem- blage-Zone. The Late Triassic fauna includes the Hyperodapedon and Ictidosaur assemblage-zones. The former, dominated by rhynchosaurs, includes some of the oldest known dinosaurs, and can be given an Ischigualastian (Carnian) age. The younger fauna contains advanced cynodonts and procolophonids, as well sphenodontians, most probably corresponding to the latest Triassic.

Abstract: Three newly discovered skeletons from the Carnian red beds of the Santa Maria Formation, south Brazil, represent one of the oldest dinosaurs ever found. The new taxon Saturnalia tupiniquim, is equivalent in age to the earliest dinosaurs from northwestern Argentina, being the oldest sauropodomorph dinosaur known from plentiful skeletal material. The record of Saturnalia, a 1.5-m-long gracile plant-eating animal, indicates that, like other major dinosaur lineages, the first representatives of the mainly heavy-built sauropodomorphs were gracile animals.

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.

Abstract: The history of South American therapsids and archosaurs during the Early, Middle and Late Triassic is analyzed. Improvements in the locomotor apparatus are discernible in seven groups of archosaurs and indicate that a great adaptive radiation took place before the oldest recorded fauna dominated by archosaurs appeared. Competition between therapsids and archosaurs began at the outset of the Triassic, when the proterosuchids were cosmopolitan and ceased 10 m.y. before the end of the Triassic. The rich Dicroidium flora and the variety of types of both therapsids and archosaurs were synchronous. The flora probably triggered the radiation in both reptilian groups. It is assumed that the extinction of therapsids was caused by: (a) the locomotor superiority of the Middle and Late Triassic archosaurs; (b) the capability of archosaurs to increase their body size during the Triassic; (c) the appearance of herbivorous archosaurs at the outset of the Late Triassic; and (d) the competitive pressure imposed b...