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, 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 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. Those that were further back in the jaws replaced teeth that were worn out as the animal grew in size and the teeth were worn out because of a diet of very tough plants. In the end, they likely 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] These fossils were later reinterpreted as belonging to malerisaurine azendohsaurids.[5]
| List of rhynchosaur genera | |||||
|---|---|---|---|---|---|
| Genus | Species | Age | Location | Unit | Notes |
| Ammorhynchus | A. navajoi | Anisian | (Arizona) | Moenkopi Formation | A stenaulorhynchine. |
| Beesiiwo[6] | B. cooowuse | Carnian | (Wyoming) | Popo Agie Formation | A hyperodapedontine previously referred to Hyperodapedon. |
| Brasinorhynchus | B. mariantensis | Ladinian | Santa Maria Formation | A stenaulorhynchine, previously known as the "Mariante Rhynchosaur". | |
| Bentonyx | B. sidensis | late Anisian | (England) | Otter Sandstone Formation | A basal hyperodapedontid. |
| Elorhynchus | E. carrolli | late Ladinian? - earliest Carnian? |  Argentina | Chañares Formation (Tarjadia Assemblage Zone) | A stenaulorhynchine. |
| Eohyosaurus[7] | E. wolvaardti | early Anisian | Burgersdorp Formation (Cynognathus Assemblage Zone) | A basal (non-rhynchosaurid) rhynchosaur. | |
| Fodonyx | F. spenceri | late Anisian | (England) | Otter Sandstone Formation | A basal hyperodapedontid. |
| Howesia | H. browni | early Anisian | Burgersdorp Formation (Cynognathus Assemblage Zone) | A basal (non-rhynchosaurid) rhynchosaur. | |
| Hyperodapedon | H. gordoni | Carnian | (Scotland) | A hyperodapedontine, one of the most abundant and speciose rhynchosaur genera. Six valid species has been named, the most of any rhynchosaur. | |
| H. huenei | Carnian |  Brazil | Santa Maria Formation | ||
| H. huxleyi | Carnian |  India | Lower Maleri Formation | ||
| H. mariensis | Carnian | Brazil Argentina | Santa Maria Formation Ischigualasto Formation | ||
| H. sanjuanensis | Carnian | Argentina Brazil | Ischigualasto Formation Santa Maria Formation | ||
| H. tikiensis | Carnian | India | Tiki Formation | ||
| Isalorhynchus | I. genovefae | Carnian |  Madagascar | Makay Formation (Isalo II) | A hyperodapedontine occasionally referred to Hyperodapedon. |
| Mesodapedon | M. kuttyi | Anisian | Yerrapalli Formation | A stenaulorhynchine. | |
| Langeronyx | L. brodiei | Anisian | (England) | Bromsgrove Sandstone Formation | A basal hyperodapedontid. |
| Mesosuchus | M. browni | early Anisian | Burgersdorp Formation (Cynognathus Assemblage Zone) | A basal (non-rhynchosaurid) rhynchosaur. | |
| Noteosuchus | N. colletti | early Induan | Katberg Formation (Lystrosaurus Assemblage Zone) | A basal (non-rhynchosaurid) rhynchosaur. The earliest known species of rhynchosaur, and the only known Early Triassic representative. | |
| Oryctorhynchus | O. bairdi | latest Carnian?-earliest Norian? | (Nova Scotia) | Wolfville Formation | A hyperodapedontine previously referred to Hyperodapedon. |
| Rhynchosaurus | R. articeps | Anisian | (England) | Tarporley Siltstone Formation | A basal rhynchosaurid. |
| Stenaulorhynchus | S. stockleyi | late Anisian | Manda Formation | A stenaulorhynchine. | |
| Supradapedon | S. stockleyi | Middle - Late Triassic | Tunduru district | A hyperodapedontine previously referred to Hyperodapedon. | |
| Teyumbaita | T. sulcognathus | late Carnian - early Norian | Brazil Argentina | Caturrita Formation Ischigualasto Formation | The latest surviving species, and the only rhynchosaur known with confidence to have survived into the Norian stage. |
The Rhynchosauria included a single family, named Rhynchosauridae. All rhynchosaurs, apart from the four Early and Middle Triassic monospecific genera, Eohyosaurus, Mesosuchus, Howesia and Noteosuchus, are included in this family.[7] Hyperodapedontidae named by Lydekker (1885) was considered its junior synonym.[8] However, Langer et al. (2000) noted that Hyperodapedontidae was erected by Lydekker to include Hyperodapedon gordoni and H. huxleyi, clearly excluding Rhynchosaurus articeps, which was the only other rhynchosaur known at that time. Thus, they defined it as the stem-based taxon that includes all rhynchosaurs more closely related to Hyperodapedon than to Rhynchosaurus.[9]
Within Hyperodapedontidae, which is now a subgroup of Rhynchosauridae, two subfamilies have been named. Stenaulorhynchinae named by Kuhn (1933) is defined sensu Langer and Schultz (2000) to include all species more closely related to Stenaulorhynchus than to Hyperodapedon. Hyperodapedontinae named by Chatterjee (1969) was redefined by Langer et al. (2000) to include "all rhynchosaurs closer to Hyperodapedon than to "Rhynchosaurus" spenceri" (now Fodonyx).[10]
The cladogram below is based on Schultz et al. (2016) which is the most genera inclusive rhynchosaur phylogenetic analysis to date,[10] with the position of Noteosuchus taken from other recent analyses (since it was removed in Schultz et al. (2016)), all in consensus with one another.[7] [11]