Riojasaurus (meaning "lizard from La Rioja") was a herbivorous sauropodomorph dinosaur named after La Rioja Province in Argentina where it was found in the Los Colorados Formation in the Ischigualasto-Villa Unión Basin by José Bonaparte. It lived during the Late Triassic (Norian stage) and grew to about long. Riojasaurus is the only known riojasaurid to live in South America.
Discovery and naming
Riojasaurus incertus was named by Bonaparte (1969) and was based on the holotype specimen, PVL 3808, which was discovered in 1966 and consists of a postcranial skeleton which lacks the skull and mandibles, but preserves 6 presacral vertebrae, diverse caudals, both scapulae, the ischia, and the bones of the hand. As of 2023, the whereabouts of the stolen fossils are unknown.
Description
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Riojasaurus had a heavy body, bulky legs, and a long neck and tail. Its leg bones were dense and massive for an early sauropodomorph. Large individuals were estimated to have reached long and weighed . By contrast, its vertebrae were lightened by hollow cavities, and unlike most early sauropodomorphs, Riojasaurus had four sacral vertebrae instead of three. The nearly equal length of the fore and hindlimbs has also been interpreted as suggestive of an obligatorily quadrupedal gait, although a well-preserved skull attributed to Riojasaurus was found later. The teeth of Riojasaurus were leaf shaped and serrated. The upper jaw contained 5 teeth at the front, with 24 more behind them in a row that ended under the eyes.
Comparisons between the scleral rings of Riojasaurus and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day at short intervals.
Classification
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Many scientists think that Riojasaurus was closely related to Melanorosaurus,
Due to their size and limb anatomy Riojasaurus and the possibly related Melanorosaurus have been considered close relatives of the earliest sauropods.