Riojasaurus was a herbivorous

Herbivore

Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

 prosauropod dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 named after La Rioja Province in Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

 where it was found by José Bonaparte

José Bonaparte

José Fernando Bonaparte, Ph.D. , is an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists like Rodolfo Coria...

. It lived during the Late Triassic

Late Triassic

The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...

 and grew to about 10 metres (32.8 ft) long. Riojasaurus is the only known riojasaurid

Riojasauridae

Riojasauridae is a family of sauropod-like dinosaurs from the Upper Triassic. It is known primarily from the genera Riojasaurus and Eucnemesaurus...

 to live in South America.

Description

Riojasaurus had a heavy body, bulky legs, and a long neck and tail. Its leg bones were dense and massive for a prosauropod. By contrast, its vertebrae were lightened by hollow cavities, and unlike most prosauropods, Riojasaurus had four sacral vertebrae instead of three. 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 is also suggestive of an obligatorily quadrupedal gait.

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 ring

Sclerotic ring

Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...

s of Riojasaurus and modern birds and reptiles suggest that it may have been cathemeral

Cathemeral

A cathemeral organism is one that has sporadic and random intervals of activity during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are performed...

, active throughout the day at short intervals.

Classification

Many scientists think that Riojasaurus was closely related to Melanorosaurus

Melanorosaurus

Melanorosaurus , is a genus of basal sauropodomorph dinosaur that lived during the Late Triassic period. A herbivore from South Africa, it had a large body and sturdy limbs, suggesting it moved about on all fours...

, the largest prosauropod known from the Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

-Early Jurassic

Early Jurassic

The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...

 period. However, studies at Bristol University, England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

, suggest that it is unique in some key ways, such as the longer bones in its neck. It is certainly quite different from other prosauropods found in the Los Colorados Formation

Los Colorados Formation

The Los Colorados Formation is a sedimentary rock formation found in La Rioja, Argentina. It has a Norian age , and is notable for its fossils of early dinosaurs, including the coelophysoid Zupaysaurus and the "prosauropods" Coloradisaurus, Lessemsaurus, and Riojasaurus.-Dinosaurs:Indeterminate...

 of Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

.
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.