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Rhynchosaurs Temporal range: Induan-early Norian PreꞒ Ꞓ O S D C P T J K Pg N
Mounted skeleton model of Hyperodapedon
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Crocopoda
Order:	†Rhynchosauria Osborn 1903
Subgroups
†Colobops? †Eohyosaurus †Howesia †Mesosuchus †Noteosuchus †Rhynchosauridae †Eifelosaurus †Rhynchosaurus †Hyperodapedontidae †Bentonyx †Fodonyx †Langeronyx †Hyperodapedontinae †Stenaulorhynchinae †Ammorhynchus †Brasinorhynchus †Elorhynchus †Mesodapedon †Stenaulorhynchus

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Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria.^[1] Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs first appeared in the Early Triassic, reaching their broadest abundance and a global distribution during the Carnian stage of the Late Triassic.

Rhynchosaurs were herbivores, and at times abundant (in some fossil localities accounting for 40 to 60% of specimens found), with stocky bodies and a powerful beak. Early primitive forms, like Mesosuchus and Howesia, were generally small and more typically lizard-like in build, and had skulls rather similar to the early diapsid Youngina, except for the beak and a few other features. Later and more advanced genera grew to medium to medium large size, up to two meters in length. The skull in these forms were short, broad, and triangular, becoming much wider than long in the most advanced forms like Hyperodapedon (= Scaphonyx), with a deep cheek region, and the premaxilla extending outwards and downwards to form the upper beak. The broad skull would have accommodated powerful jaw muscles. The lower jaw was also deep, and when the mouth was closed it clamped firmly into the maxilla (upper jaw), like the blade of a penknife closing into its handle. This scissors-like action would have enabled rhynchosaurs to cut up tough plant material. Rhynchosaur teeth had a unique condition known as ankylothecodonty, similar to the acrodonty of modern tuataras and some lizards but differing in the presence of deep roots.^[2]

The teeth were unusual; those in the maxilla and palate were modified into broad tooth plates. The hind feet were equipped with massive claws, presumably for digging up roots and tubers by backwards scratching of the hind limbs. Similar to elephants they had a fixed number of teeth where those further back in the jaws replaced those who were worn out as the animal grew in size and the teeth was worn out because of a diet of very tough plants. In the end they probably starved to death.^[3]

Like many animals of this time, they had a worldwide distribution, being found across Pangea. These abundant animals might have died out suddenly at the end of the Carnian (Middle of the Late Triassic period), perhaps as a result of the extinction of the Dicroidium flora on which they may have fed. On the other hand, Spielmann, Lucas and Hunt (2013) described three distal ends of humeri from early-mid Norian Bull Canyon Formation in New Mexico, which they interpreted as bones of rhynchosaurs belonging to the species Otischalkia elderae; thus, the fossils might indicate that rhynchosaurs survived until the Norian.^[4]