Period: Late Triassic
Order, Suborder, Family: Saurischia, Sauropodomorpha, Melanorosauridae
Location: South America (Argentina)
Length: 33 feet (10 meters)
Riojasaurus and the related South African genus Melanorosaurus are prosauropod giants. Both had a number of features in common with the sauropod dinosaurs. These include vertebrae with hollow spaces that made them lighter, and dense, massive limb bones.
Riojasaurus was a quadrupedal (it walked on all four legs) dinosaur; it could not rear up on its back legs like some of its relatives, such as Plateosaurus. Its long, massive body needed to be supported by all four limbs. The front limbs of Riojasaurus were nearly as long as its back limbs. It had four vertebrae connecting the pelvis to the trunk; most other small prosauropods had only three.
Some scientists consider Riojasaurus and Melanorosaurus the closest relatives of the sauropods, because of their large size and some features of their limbs. Recently, however, Peter Galton and Paul Sereno have argued that the prosauropods and sauropods had a common ancestor sometime in the Triassic. If this is correct, the similarities probably are because both groups were large animals. More light will be shed on this question when newly discovered material of Riojasaurus, including at least one good skull, is described by scientists.
Riojasaurus was an early and short-lived prosauropod that lived at the end of the Late Triassic. In the Early Jurassic, smaller animals such as Anchisaurus become the dominant prosauropods, and the first sauropods, Vulcanodon and Barapasaurus, appeared.