Rhynchocephalia is an order of lizard-like reptiles that includes only one living species, the tuatara of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a speciose group with high morphological and ecological diversity. The oldest record of the group is dated to the Middle Triassic around 238 to 240 million years ago, and they had achieved a worldwide distribution by the Early Jurassic. Most rhynchocephalians belong to the group Sphenodontia ('wedge-teeth'). Their closest living relatives are lizards and snakes in the order Squamata, with the two orders being grouped together in the superorder Lepidosauria.
Planocephalosaurus is an extinct genus of basal rhynchocephalian. Fossils of the genus are primarily known from fissure fill deposits from the Late Triassic of southwest Britain, with fragmentary remains possibly belonging to the genus also known from the Late Triassic of Texas.
Terrestrisuchus is an extinct genus of very small early crocodylomorph that was about 76 centimetres (30 in) long. Fossils have been found in Wales and Southern England and date from near the very end of the Late Triassic during the Rhaetian, and it is known by type and only known species T. gracilis. Terrestrisuchus was a long-legged, active predator that lived entirely on land, unlike modern crocodilians. It inhabited a chain of tropical, low-lying islands that made up southern Britain, along with similarly small-sized dinosaurs and abundant rhynchocephalians. Numerous fossils of Terrestrisuchus are known from fissures in limestone karst which made up the islands it lived on, which formed caverns and sinkholes that preserved the remains of Terrestrisuchus and other island-living reptiles.
Brachyrhinodon is an extinct genus of sphenodontian from the Late Triassic Lossiemouth Sandstone of Scotland.
Clevosaurus is an extinct genus of rhynchocephalian reptile from the Late Triassic and the Early Jurassic periods. Species of Clevosaurus were widespread across Pangaea, and have been found on all continents except Australia and Antarctica. Five species of Clevosaurus have been found in ancient fissure fill deposits in south-west England and Wales, alongside other sphenodontians, early mammals and dinosaurs. In regards to its Pangaean distribution, C. hadroprodon is the oldest record of a sphenodontian from Gondwana, though its affinity to Clevosaurus has been questioned.
The Magnesian Conglomerate is a geological formation in Clifton, Bristol in England, Gloucestershire and southern Wales, present in Tytherington, Durdham Down, Slickstones Quarry and Cromhall Quarry.
Theretairus is a Late Jurassic genus of sphenodont reptile from the Morrison Formation of western North America, present in stratigraphic zones 5 and 6.
Gephyrosaurus is a genus of early rhynchocephalian first described and named in 1980 by Susan E. Evans. They are distantly related to the extant Sphenodon with which they shared a number of skeletal features including a large tooth row along the side of the palatine bone and posterior process of the dentary bone. The type species, G. bridensis, lived during Early Jurassic in Wales, UK. Whiteside & Duffin (2017) described the second species, G. evansae, known from a partial maxilla recovered from Late Triassic (Rhaetian) fissure fills in Carboniferous Limestone in Somerset. Gephyrosaurus, other potential gephyrosaurids and Wirtembergia are the only rhynchocephalians to lie outside Sphenodontia in modern definitions of the group, and have been found to be more closely related to squamates in some phylogenetic analyses.
Sphenotitan is an extinct genus of rhynchocephalian reptile, known from the Late Triassic (Norian) Quebrada del Barro Formation of Argentina. It is the earliest known member of the herbivorous Elienodontinae, and the only one known from the Triassic. It was a large-sized sphenodontian, with an estimated skull length of over 10 centimetres (3.9 in). The skull is roughly triangular in shape, and had large upper temporal fenestrae. The region of the skull in front of the eye socket is short. The premaxillae forms beak, with a cutting edge similar to a chisel. The teeth of Sphenotitan, like other elienodontines, were large and wide, and designed for shredding vegetation, with blade-like palatal teeth on the roof of the mouth.
Bharatagama is an extinct genus of lepidosaur from the Early Jurassic of India. It has been suggested to be one of the oldest known lizards and the oldest known iguanian. The type and only species is Bharatagama rebbanensis, named in 2002. Over one hundred fossils of Bharatagama have been found in the Kota Formation, which outcrops in the Pranhita–Godavari Basin and dates back to about 190 million years ago (Ma). Despite its abundance, Bharatagama is known only from isolated jaw bones mixed together in microvertebrate assemblages with equally fragmentary remains of fish, sphenodontians, dinosaurs, crocodylomorphs, and mammals. These fossils represent all stages of development, from hatchlings to adults. The total length of the skull in adult specimens is estimated to have been about 15 millimetres (0.59 in). Later analysis suggested that the taxon might be a member of Rhynchocephalia.
Whitakersaurus is a genus of sphenodontid rhynchocephalian reptile dated to be late Triassic in age and is from the Ghost Ranch fossil quarry in New Mexico, USA. It is named after the discoverer of the Ghost Ranch quarry, George O. Whitaker. The fossil was described in 2007.
Opisthodontia is a proposed clade of sphenodontian reptiles, uniting Opisthias from the Late Jurassic-earliest Cretaceous of Europe and North America with the Eilenodontinae, a group of herbivorous sphenodontians known from the Late Triassic to Late Cretaceous.
Clevosaurs are an extinct group of rhynchocephalian reptiles from the Triassic and Jurassic periods.
The Klettgau Formation is a geological formation in Switzerland. It is Late Triassic in age, covering most of the mid to late Norian, the Carnian, and into the Rhaetian, spanning a period of 26-30 million years.
The Avon Fissure Fill, also known as the Bristol Fissure Fill or Tytherington Fissure Fill, is a fissure fill in Avon, England which dates variously from the Norian and Rhaetian stages of the Late Triassic, or possibly as late as the Hettangian stage of the Early Jurassic. The fissure fill at Avon was a sinkhole formed by the dissolution of Lower Carboniferous limestones.
Kawasphenodon is an extinct genus of sphenodontian reptile, known from the Late Cretaceous and Paleocene of Patagonia in South America. The type species, K. expectatus, was described in 2005 from jaw fragments found in late Campanian aged sediments in the Los Alamitos Formation, the jaw when complete was estimated to be 11 cm long, making it among the largest known sphenodontians. A second species, K. peligrensis, around 1/3 the size of the type species, was described in 2014 also from jaw fragments in early Paleocene (Danian) sediments of the Salamanca Formation, making it the youngest known definitive representative of Rhynchocephalia outside of New Zealand. In the original description, it was found to be a member of Sphenodontidae, in some other subsequent analyses it was found to be a member of Opisthodontia. A 2020 analysis of rhyncocephalian relationships found it to be outside Opisthodontia, and instead a member of the Sphenodontinae as the closest known relative of the tuatara, with an estimated divergence between the two genera in the Early Cretaceous. Other subsequent studies have endorsed its placement as a member of Sphenodontidae. Like most other rhynchocephalians, the teeth are acrodont, with a deep dentary, and it probably had an omnivorous habit.
Microsphenodon is an extinct genus of sphenodontian from the Late Triassic of Brazil. The type species is Microsphenodon bonapartei. It is a small sphenodontian with a skull roughly 20 mm long, and represents a unique mosaic of characteristics shared by both early diverging sphenodontians and eusphenodontian characters. Specimens of this taxon were first identified by Bonaparte and Sues (2006) but were misidentified as juvenile Clevosaurus brasiliensis, characteristics such as differences in the configuration of the teeth on the palate, and the unique form of tooth implantation seen in C. brasiliensis, helped differentiate these two co-occurring sphenodontian taxa.
Cryptovaranoides is an extinct genus of reptile from the Late Triassic Magnesian Conglomerate of England. It contains a single species, Cryptovaranoides microlanius.
Wirtembergia is an extinct genus of rhynchocephalian reptile known from the Middle Triassic (Ladinian) of Germany. It is the earliest known rhynchocephalian.
Parvosaurus is an extinct species of rhynchocephalian reptile known from the Late Triassic of Germany.
