Riojasaurus

*Rioarribasaurus colberti*

Riojasaurus (meaning "Rioja lizard") is 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 10 meters (33 feet) long. Riojasaurus is the only known riojasaurid to live in South America.

## Description

*Riojasaurus* had a heavy body (~1 tonne), bulky legs, and a long neck and tail. It is thought to be around 10 metres (33 feet) in length. Its leg bones were dense and massive for an early sauropodomorph. By contrast, its vertebrae were lightened by hollow cavities, and unlike most early sauropodomorphs, *Riojasaurus* had four sacral vertebrae instead of three. It has been thought it probably moved slowly on all fours and was unable to rear up on its back legs. The nearly equal length of the fore and hindlimbs has also been interpreted as suggestive of an obligatorily quadrupedal gait. However in 2016, Scott Hartman found the hand anatomy, relatively straight back and largely immobile shoulder girdle of *Riojasaurus* supported it being a biped. No skull was found with the first skeleton of *Riojasaurus*, 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

Many scientists think that *Riojasaurus* was closely related to *Melanorosaurus*, known from the Triassic-Early Jurassic period. However, studies at Bristol University, England, suggest that it is unique in some key ways, such as the longer bones in its neck. It is certainly quite different from other sauropodomorphs found in the Los Colorados Formation of Argentina. Due to their size and limb anatomy, *Riojasaurus* and the possibly related *Melanorosaurus* have been considered close relatives of the earliest sauropods. However, if the hypothesis proposed by Peter Galton and Paul Sereno is correct -- namely, that prosauropods and true sauropods share a common ancestor rather than the former giving rise to the latter -- then commonalities shared by *Riojasaurus* and true sauropods are likely due to convergent evolution.

## JPInstitute.com Description

Riojasaurus was one of the very earliest plant-eating dinosaurs and was a member of the family of dinosaurs that became the long-necked sauropod. For such an early dinosaur, scientists know a great deal about this animal as parts of more than 20 different individuals have been found. It grew very large for such an early dinosaur, as long as a school bus!

A large and heavily built prosauropod, Riojasaurus was first discovered in 1967. It exhibits characteristics that led to the evolution of the sauropods, including successively lengthening cervical vertebrae. Almost no cranial material has been found, but its neck suggests that it had a small skull.

## Links

https://web.archive.org/web/20030803051844/http://www.jpinstitute.com/dinopedia/dinocards/dc\_rioja.html