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Hyperodapedon (from , 'above' and , 'pavement') is an extinct genus of rhynchosaur reptiles which lived during Triassic period. Like other rhynchosaurs, it was a heavily built Archosauromorpha, distantly related to such as and . Hyperodapedon in particular was part of the subfamily Hyperodapedontinae, a specialized rhynchosaurian subgroup with broad skulls, beaked snouts, and crushing tooth plates on the roof of the mouth.
Hyperodapedon remains one of the most widespread and well-understood rhynchosaurs due to its abundance of fossils on several continents. It was named and discovered by Thomas Henry Huxley in 1859, based on H. gordoni, a species from Scotland. It has also been reported from Africa, Asia (India), and North and South America, though some species were later split off into their own genera. An Indian species, H. huxleyi, is also known by the genus name Paradapedon. Some of the early South American finds were described under the name Scaphonyx, which is often considered a junior synonym of Hyperodapedon. Hyperodapedon fossils are abundant and Biostratigraphy significant in Stratum of the late Carnian stage, such as the Ischigualasto Formation of Argentina and the Upper Santa Maria Formation of Brazil. It is generally considered a herbivore that used its beaked premaxilla and hindlimbs to dig for plants on land.
T.H. Huxley found many series of subcylindrical palatal teeth which was the main trait of Hyperodapedon. Huxley was able to distinguish Hyperodapedon from Rhynchosaurus articeps by the maxillary tooth rows. Later on, Richard Lydekker realized that Hyperodapedon have more than two rows of teeth in both the maxilla and palatine.
For much of the 20th century, Paradapedon huxleyi was entangled with debates over the validity of Parasuchus hislopi, a species which was first mentioned by Huxley (1870) and formally described by Lydekker (1885). One of the syntype fossils of Parasuchus hislopi was a collection of bones including a partial braincase, (bony scutes), teeth, and other associated material. The braincase was later identified as belonging to a rhynchosaur, but the other bones in the syntype are from a carnivorous phytosaur. This would make the syntype a chimera, comprising fossil material from two unrelated animal species.
To remedy this issue, von Huene (1940) elected to abandon the name Parasuchus hislopi, as he considered the name to apply to the rhynchosaur braincase first and foremost, which was certainly referrable to Paradapedon huxleyi. Chatterjee (1974) disagreed, noting that the braincase had no special status relative to the other fossils, as it was merely a syntype rather than a holotype. He separated the braincase from Parasuchus hislopi and named a phytosaur snout fragment as a new lectotype for the species. Assisted by the discovery of new complete skeletons, Parasuchus hislopi is still considered a valid phytosaur taxon to the present day.
Benton (1983) concluded that "Paradapedon" huxleyi should once again be considered a species of Hyperodapedon, thus rendering Paradapedon a junior synonym of Hyperodapedon.
Arthur Smith Woodward (1907) named a new genus and species, Scaphonyx fischeri, for a small number of reptile vertebrae and phalanges from the Upper Santa Maria Formation, in the Brazilian state of Rio Grande do Sul. The genus name Scaphonyx (meaning canoe claw) referred to the scoop-like shape of the reptile's claws, while the species name honors Dr. Jango Fischer, who discovered the fossils in 1902. At the time Woodward considered Scaphonyx fischeri to be a short-necked dinosaur closely related to Euskelosaurus. Further collection from the same area by Friedrich von Huene produced more extensive fossil material. Huene (1926) informally proposed multiple new names to describe the reptile fossils he had collected: Cephalonia, Cephalastron, Cephalostronius, and Scaphonychimus. By 1942, he had lumped all of these remains into either Cephalonia lotziana (a gracile form) or Scaphonyx fischeri (a robust form), while recognizing their rhynchosaurian affinities.
The name Scaphonyx sanjuanensis was established by Sill (1970) in reference to rhynchosaur fossils from the Ischigualasto Formation of Argentina. Sill also demonstrated the synonymy of Cephalonia lotziana and Scaphonyx fischeri, with fossils of the latter simply having been recrystallized into a more inflated state via Diagenesis.
H. sanjuanensis is regarded as the most abundant fossil organism preserved in the Ischigualasto Formation, making up the majority of fossils found within the first of the formation.
Diagnostic features of H. huenei include the absence of an infraorbital foramen, a single dentary blade, and fusion between the supraoocipital and opisthotic bones of the braincase. In addition, the rear portion of the maxillary tooth plate has a secondary wear groove alongside the main longitudinal groove, and the medial portion of the tooth plate is broader than the lateral portion. Both conditions are similar to more basal rhynchosaurs but unlike other species of Hyperodapedon.
The upper temporal bar faces dorsally and is raised above the level of the ventral margin of the orbit (eye socket). The braincase of Hyperodapedon had a longitudinal stapedial canal on the posterior side of the spatulate paroccipital process which the lagenar crest extended laterally to limit the posterior end. The pterygoid of Hyperodapedon lacks a pair of ridges present in other rhynchosaurs, as well as palatal dentition as a whole. The prefrontal is deeply concave on the dorsal side.
Langer et al. (2000) defined Hyperodapedon as a stem-based taxon that includes all closer to Hyperodapedon gordoni than to Scaphonyx sulcognathus (a species which was renamed to Teyumbaita in 2010). This definition for the genus is reliant on the assumption that Teyumbaita is the sister taxon to Hyperodapedon, rather than deeply nested within it. The cladogram below is based on the phylogenetic analysis of Mukherjee & Ray (2014), a study which supports the definition provided by Langer et al. (2000):
Valid species that were first assigned to ''[[Scaphonyx]]''.
Other studies have nested Teyumbaita deep within the clade of Hyperodapedon species. If this is the case, then Hyperodapedon in its broadest form would be a Paraphyly genus rather than an exclusive monophyletic clade. As a result, it has been proposed to split up the genus into multiple genera, with one species each. Several of the new genera had been used in the past ( Scaphonyx for H. sanjuanensis, Macrocephalosaurus for H. mariensis, Paradapedon for H. huxleyi, Supradapedon for H. stockleyi) while others are newly named ( Beesiiwo, Oryctorhynchus). The type species H. gordoni would be the only species left in the genus under this narrow interpretation.
The following cladogram is based on the phylogenetic analysis by Fitch et al. (2023):
H. gordoni likely preferred a semi-sprawling limb posture, with the hindlimbs more powerful and flexible than the forelimbs. During movement of the forelimb, the humerus could travel along an arc of about 100°. This range of motion was enabled by a narrow glenoid (shoulder joint) which allowed the humerus to rock against the shoulder girdle rather than sliding. At its maximum forward extension, the upper arm was sprawled out sideways and slightly forwards and downwards. In its maximum rearward position, the humerus points nearly straight back and lies flat against the body, twisting the forearm inwards and narrowing the stance of the forelimb as a whole. The forelimbs were likely secondary to the hindlimbs during locomotion, since the shoulder joint was small and weak while its associated muscles were poorly positioned for strong movement.
The hip joint consists of a broad acetabulum (hip socket) and an indistinct femoral head, so it would have been broadly flexible, even accounting for the presence of cartilage to fill in the gaps. The ideal posture to maximize stride length was semi-erect, with the thigh about 45° below the horizontal (when seen from both the front and the side) at the start and end of its 90° arc. The knee and ankle joints were rather simple and hinge-like, so much of the hindlimb's movement relied on the rolling hip joint. Despite its heavyset body, the limb proportions of Hyperodapedon were similar to active and relatively agile dinosaurs such as Protoceratops. Regardless, Hyperodapedon was certainly fully Quadrupedalism, owing to its narrow hip, short tail, and bulky torso.
Several lines of evidence support the idea that Hyperodapedon and other rhynchosaurs used their hindlimbs for scratch digging, a versatile type of burrowing behavior utilized by animals such as Turtle, Ground squirrel, Armadillo, and Pangolin, among others. Scratch-diggers combine a strong lower limb with large claws to loosen and scrape dirt backwards. Like living scratch-diggers, Hyperodapedon had a large foot with tall, narrow claws, though these adaptations occur on the hind feet rather than the front feet. The pelvis and tibia had room for dense musculature, and the stout interlocking toe phalanges would have been reinforced with strong Ligament. Both forelimbs and one hindlimb could act to brace the body while the other hindlimb engages in digging.
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