Rhynchosaurus

Rhynchosaurus articeps was a small plant-eating archosauromorph from Middle Triassic England. At Grinshill, remains representing about 17 individuals include skulls and partial to nearly complete skeletons. This sample documents an early stage in the evolution of a beak and tooth plates, before those structures became more extreme in later rhynchosaurs. The profile is part of the other fossil reptile catalogue; rhynchosaurs were not dinosaurs.

The number of specimens makes comparisons possible, but a fossil concentration does not prove that the animals formed a herd. Jaw mechanics support plant eating, while the exact plants in the diet, social behaviour and soft covering of the beak are not preserved. Those distinctions matter when a complete-looking reconstruction is based on bones alone.

Quick facts

The first named rhynchosaur

Richard Owen described Rhynchosaurus articeps in 1842. Its material came from quarries at Grinshill, north of Shrewsbury. The lectotype, SHYMS 1, and other museum specimens preserve enough anatomy for detailed comparison. Michael Benton's 1990 monograph reviewed the genus and its British material.

Owen's genus was once used for several British species, including R. brodiei and R. spenceri. Later analyses assigned those species to other genera, including Bentonyx and Fodonyx. A current account of Rhynchosaurus should therefore focus on the type species R. articeps, rather than combine distinct sets of bones under an old broad label.

A body about half a metre long

R. articeps was about half a metre long. Its skull widened towards the back, while the toothless front portions of the jaws formed a downturned beak. The body was low, the limbs relatively short and set out to the sides, and the long tail balanced the trunk.

Interpretations of the lower temporal bar have changed after researchers re-examined the specimens. The modern review found no secure contact between the jugal and quadratojugal bones, although older diagrams sometimes showed one. Such a modest anatomical correction matters because phylogenetic analyses code individual bone contacts as characters.

Tooth plates and plant processing

The upper jaw carried tooth fields separated by a groove. A row of lower teeth worked against these surfaces. New teeth were added as the animal grew, allowing the functional edge to persist despite wear. The arrangement was less specialised than the broad, multi-row grinding system found in some later hyperodapedontine rhynchosaurs.

The beak could crop vegetation and the tooth plates could break it down. That conclusion comes from the mechanics of the jaws as a whole. No gut contents identify particular plants, and there are no unambiguous coprolites that define the diet of this species. Plant fossils found nearby are not by themselves proof that Rhynchosaurus ate those exact species.

Where it fits among rhynchosaurs

Azendohsaurus was another plant-eating archosauromorph, but it lacked rhynchosaur tooth plates. R. articeps has its own position among rhynchosaurs and need not have been the direct ancestor of later forms. The two examples show how herbivory evolved in separate Triassic reptile lineages.

Rhynchosaurs were near the base of Archosauromorpha and were not dinosaurs. Some similarities between their beaked jaws and those of other plant-eaters evolved independently. The Triassic Period contains many such branches, while the Euparkeria profile describes a different small archosauriform from the same broad period. Both appear in the other fossil reptile catalogue because they are useful subjects, not because they form one close group.

Grinshill and the limits of the sample

The bones came from the Tarporley Siltstone Formation, part of a Middle Triassic continental river system. Sandstones and siltstones record shifting channels and floodplain deposits. The site preserves enough individuals to compare size and preservation, but an accumulation at one locality does not show that the animals lived together as a permanent herd.

The beak's keratin covering, skin, colour and soft tissues are unknown. The fossils do not reveal the sex of each individual, a breeding season or social organisation. A full-body image must restore those missing features, and the close grouping of fossil remains cannot supply them.