The species of Rhyncosaurus , a rhynchosaur (Reptilia, Diapsida) from the Middle Triassic of England

Abstract

The rhynchosaur Rhynchosaurus articeps Owen, 1842, from the Middle Triassic of Grinshill, northern Shropshire, England, was a small reptile, about 0.5 m long. About 17 individual animals are represented by skulls, complete skeletons and partial skeletons, and these have permitted detailed restorations. The skull (60-80 mm long) is low and broad at the back, and it shows all of the typical rhynchosaur features of beak-like premaxillae, single median naris, fused parietal, broad maxillary tooth plate and dentary, both with multiple rows of teeth, and a deep lower jaw. The skeleton shows adaptations for fast terrestrial locomotion with a semi-erect hindlimb posture and for scratch-digging with the hind-foot. The skeleton is relatively more slender than that of most other middle and late Triassic rhynchosaurs, but this is probably an allometric effect of its much smaller size (they are typically 1-2 m long). A further species of Rhynchosaurus from Warwick, named here R. brodiei , is represented by 15 specimens of partial skulls, tooth-bearing elements, and isolated postcranial bones. It was slightly larger than R. articeps , with a typical skull length of 90 mm, and estimated body length of 0.6 m, but the skull length ranged up to 140 mm. It differs from R. articeps in having a much larger jugal in the cheek area, and in the greater height and breadth of the skull. The isolated maxillary fragments from Bromsgrove probably also belong to R. brodiei . The third species of Rhynchosaurus from Devon, named here R. spenceri , is now known from numerous specimens of at least 25 individuals, most of which were collected recently. These show a range in estimated skull length from 40 to 170 mm, but most specimens are at the upper end of that range, with an average skull length of 140 mm, and an estimated total body length of 0.9-1.0 m R. spenceri differs from R. articeps and R. brodiei in having a skull that is broader than it is long (otherwise a character of late Triassic rhynchosaurs), and it shares the large jugal character with R. brodiei . Teeth are not well preserved in R. articeps, but several specimens of R. brodiei and R. spenceri give detailed information. The pattern of wear, and the nature of the jaw joint, suggest that Rhynchosaurus had a precision-shear bite, as in other rhynchosaurs, with no back and forwards motion. The maxilla had two grooves, a major and a minor one, which received two matching ridges of the dentary on occlusion. The multiple rows of teeth on maxilla and dentary, and the surrounding bone, wore down as uniform units. The diet was probably tough vegetation, which was dug up by scratch-digging, raked together with the hands or the premaxillary beak, and manipulated in the mouth by a strong tongue. Rhynchosaurus is found variously in fluvial-intertidal deposits with evidence of desiccation (Grinshill, Warwick, Bromsgrove), and fluvial-aeolian deposits laid down in arid conditions with occasional flash floods (Devon). The bones have generally been transported (Warwick, Bromsgrove, Devon), but the Grinshill specimens are largely complete and undisturbed. The associated floras and faunas at Warwick, Bromsgrove, and Devon include pteridophytes, gymnospermopsids, bivalves, scorpions, freshwater fish, temnospondyl amphibians and reptiles (macrocnemids, thecodontians, ?procolophonids). Rhynchosaurs are archosauromorph diapsids, possibly related to the enigmatic Trilophosaurus, and a sister group to Prolacertiformes + Archosauria. A cladistic analysis of Rhynchosauria reveals one major subgroup, the Hyperodapedontinae ( Hyperodapedonand and Scaphonyx ), which is late Triassic in age. The earlier rhynchosaurs, including the middle Triassic Stenaulorhynchus and Rhynchosaurus , appear to form successively closer outgroups to the Hyperodapedontinae. The three species of Rhynchosaurus share only one possible synapomorphy in comparison with Stenaluorhynchus : The dentary is well over half the length of the lower jaw. The ‘Rhynchosaurinae’ ( Stenaulorhynchus and Rhynchosaurus ) was not established as a monophyletic group in the present analysis. These two genera share two postulated synapomorphies: the occipital condyle lies well in front of the quadrates, and there are two grooves on the maxilla and two ridges on the dentary. A third postulated synapomorphy, the presence of a single row of teeth on the pterygoid, has not been confirmed in this study for either Rhynchosaurus or Stenaulorhynchus . However, these postulated synapomorphies are outweighed by the synapomorphies that Rhynchosaurus shares with the Hyperodapedontinae. The specimens of Rhynchosaurus have been used as biostratigraphic indicators for the English middle Triassic, indicating Anisian to early Ladinian ages. The three species can be arranged in a sequence from ‘most prim itive’ to ‘most advanced’, but this cannot be used confidently to give a stratigraphic sequence.