| Riojasaurus | |
|---|---|
 | |
| Riojasaurus skull cast | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Riojasauridae |
| Genus: | †Riojasaurus Bonaparte 1969 |
| Type species | |
| †Riojasaurus incertus Bonaparte 1969 | |
Riojasaurus (meaning "lizard fromLa Rioja") was aherbivoroussauropodomorphdinosaur named afterLa Rioja Province inArgentina where it was found in theLos Colorados Formation in theIschigualasto-Villa Unión Basin byJosé Bonaparte. It lived during theLate Triassic (Norian stage) and grew to about 6.6 metres (22 ft) long.[1]Riojasaurus is the only knownriojasaurid to live in South America.
Riojasaurus incertus was named byBonaparte (1969) and was based on theholotype 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.[2] In total, at least twenty specimens have been assigned toR. incertus, and the first cranial material ofR. incertus was discovered in 1987 and described in 1995.[3]
In 1994, 56 caudal vertebrae from one specimen, and a cast of the skull ofRiojasaurus incertus, along with several other specimens, were stolen from theNational University of La Rioja in Argentina.[4] As of 2023, the whereabouts of the stolen fossils are unknown.
Riojasaurus had a heavy body, bulky legs, and a long neck and tail. Its leg bones were dense and massive for an early sauropodomorph.[1] In 2010Gregory S. Paul estimated its length at 6.6 metres (22 ft) and its weight at 800 kilograms (1,800 lb).[5] Large individuals were estimated to have reached 10 metres (33 ft) long and weighed 3 metric tons (3.3 short tons).[6] By contrast, its vertebrae were lightened by hollow cavities, and unlike most early sauropodomorphs,Riojasaurus had foursacral vertebrae instead of three.[1] It has been thought it probably moved slowly on all fours and was unable to rear up on its back legs.[1][7] The nearly equal length of the fore and hindlimbs has also been interpreted as suggestive of an obligatorilyquadrupedal gait,[1] and the relative robustness of the forelimbs and hindlimbs is in the range of quadrupedal animals.[8] However, the morphology of the hand and shoulder girdle has been interpreted as inconsistent with a quadrupedal gait.[9]
No skull was found with the first skeleton ofRiojasaurus,[2] although a well-preserved skull attributed toRiojasaurus was found later.[3] The teeth ofRiojasaurus 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 thescleral rings ofRiojasaurus and modern birds and reptiles suggest that it may have beencathemeral, active throughout the day at short intervals.[10]
Many scientists think thatRiojasaurus was closely related toMelanorosaurus,[1] known from theTriassic-Early Jurassic period. However, studies atBristol 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 theLos Colorados Formation ofArgentina.[11]
Due to their size and limb anatomyRiojasaurus and the possibly relatedMelanorosaurus have been considered close relatives of the earliestsauropods.[1]
