Riojasaurus: a large sauropodomorph before the true sauropods

Riojasaurus incertus was a large Late Triassic sauropodomorph from what is now La Rioja Province in north-western Argentina. Its fossils come from the upper Los Colorados Formation, generally assigned to the Norian and dated broadly to about 220–213 million years ago. Several partial skeletons are known. The holotype lacks a skull, while a separate articulated specimen preserves most of one. The combination of a heavy body and powerful limbs makes Riojasaurus an important early experiment in large-bodied plant-eating dinosaurs. It belongs in the dinosaur catalogue among sauropodomorphs, not among true sauropods.

Quick facts

Name, discovery and the skull specimen

Riojasaurus means “lizard from La Rioja”, the Argentine province where the fossils were found. The species epithet incertus means “uncertain” or “doubtful” in Latin. It records uncertainty at the time the name was chosen; it does not mean that the genus is automatically invalid today.

José Bonaparte found the holotype PVL 3808 in 1966 and named Riojasaurus incertus in 1969. The specimen preserves much of the skeleton behind the skull: vertebrae, parts of the shoulder and pelvic girdles, and bones of both pairs of limbs, hands and feet. Because the skull is absent, early restorations had to borrow its shape from other sauropodomorphs.

A 1986 expedition to La Esquina near Paganzo found the articulated skeleton ULR 56. It preserves most of a skull and lower jaws, a major addition to the genus's anatomy. Bonaparte and José Pumares described the skull in 1995. They referred it to Riojasaurus because its neck and back vertebrae, limb girdles and leg bones matched the type material. ULR 56 was about one-third smaller than the holotype individual.

The referral is supported by anatomical overlap, but the two skeletons are different individuals. Fossils from one formation could include closely related animals. A full modern revision of the referred material would help test whether all of it belongs to one species; current uncertainty should not be turned into a claim that the genus is definitely a composite.

Species and evolutionary relationships

Only R. incertus is recognised. Older works called it a “prosauropod” and placed it in Melanorosauridae. “Prosauropod” was a broad informal label for early sauropod relatives, not a clear modern branch. Current analyses usually treat Riojasaurus as a non-sauropod sauropodomorph, often within Massopoda and Riojasauridae.

Many family trees pair it with the South African Eucnemesaurus. The position of riojasaurids relative to massospondylids and the sauropod line varies among studies. Differences can reflect the taxa sampled, the characters scored and the incompleteness of individual skeletons. These alternatives matter for the early evolution of sauropodomorphs, but they do not make Riojasaurus a direct ancestor of later giants.

A partial foot, PVL 4087, was once assigned to the genus from the Quebrada del Barro Formation. A later examination found no diagnostic features that securely identify it as Riojasaurus. It is now treated more cautiously as an indeterminate non-sauropod sauropodomorph. The reliable range of the genus is therefore centred on the Los Colorados Formation.

What the bones preserve

PVL 3808, the holotype, preserves a substantial postcranial skeleton but no skull or lower jaws. Other referred skeletons vary in completeness and size. ULR 56 is especially useful because its articulated skull can be compared with overlapping parts of the type material. Its skull is about 25 centimetres long, low and elongated, with a large antorbital opening and a jaw joint near the level of the upper tooth row. The upper jaw carried about 24 nearly cylindrical, conical teeth, some with fine serrations.

The skull's assignment relies on the matching vertebrae, girdles and leg bones, not on being attached to PVL 3808. Some authors have noted that the genus diagnosis is assembled from several individuals and that similar animals could occur in the same formation. This is a reason for careful reassessment, not proof that the named species contains more than one taxon.

The Los Colorados record is stronger than the rejected Quebrada del Barro referral. Distinguishing which bones are diagnostic prevents the apparent range of a genus from expanding every time a large early sauropodomorph foot is found. The fossils also do not preserve every soft part that appears in a life restoration.

Size, body and support

Length estimates range from about 6–7 metres in more compact reconstructions to around 10 metres in older or larger models. Mass estimates similarly span roughly 0.8–1.5 tonnes to about 3 tonnes. There is no fully measurable holotype skeleton, and researchers have used different body proportions and different referred individuals. A single exact length or mass would conceal those choices.

The body was deep and heavy, with a long neck and tail. Tall vertebrae had accessory articulations that stiffened parts of the spinal column. The tail balanced the trunk. The number of sacral vertebrae has been reported inconsistently: the original description identified three, with the front two tightly joined, while some later summaries state four. Without a renewed examination, that disagreement should not be converted into a diagnostic certainty.

The humerus was large and carried a strong deltopectoral crest for muscle attachment. The arms were shorter than the hind limbs, but the difference was smaller than in many bipedal sauropodomorphs. The femora, tibiae and feet were robust, supporting substantial weight. These proportions make regular four-legged locomotion plausible.

Massive forelimbs alone do not prove that the animal could only walk on four legs. Its hand had not become the semicircular, column-like support seen in later sauropods. It probably moved confidently on four limbs and could change posture, but how often it reared or walked bipedally has not been resolved by a trackway or a complete biomechanical model.

Food, growth and daily life

Riojasaurus was probably primarily herbivorous. Its small head, numerous teeth and possible horny edge at the front of the jaws fit a plant-eating interpretation. The teeth were relatively rounded and weakly serrated rather than strongly leaf-shaped. They do not identify a preferred food such as ferns, seed ferns or conifer shoots.

There is no direct evidence that it ate animals. Some early sauropodomorphs have been described as omnivorous, but that general possibility is not proof for this genus. Its tooth form was less specialised than that of later sauropods, and a large part of food processing may have happened in the digestive tract rather than through prolonged chewing.

Histological sections of two femora show periodic lines of slowed growth. A smaller individual appears to have reached skeletal maturity while a larger one may still have been growing. The sample is too small to distinguish flexible growth, sex-related differences or the possibility that some referred bones belong to a very close form. It does not establish a full growth curve or the exact age at maturity.

No nest, eggs, trackway, herd deposit or parental-care evidence is securely linked to Riojasaurus. Several individuals in one formation do not establish that they lived together. Its neck posture, skin, colour and soft cheeks remain reconstructions rather than preserved observations.

Los Colorados landscapes

The Los Colorados Formation consists mainly of red sandstones, siltstones and mudstones deposited in river channels and broad floodplains within Pangaea. The local climate was seasonal and ranged from semi-humid to semi-arid. It was neither a modern desert nor a permanent tropical swamp. Channel shifts and floodplain deposits preserve a changing landscape rather than one uniform habitat.

The upper Triassic beds contain other sauropodomorphs, including Coloradisaurus and Lessemsaurus, as well as the predator Zupaysaurus, crocodylomorphs, herbivorous therapsids and small cynodonts. Not every taxon necessarily lived at the same time on the same stretch of river, but together the fossils record a complex terrestrial community. The Triassic Period overview gives the larger environmental and chronological setting.

What the reconstruction should not imply

It is misleading to portray Riojasaurus as a smaller copy of a Jurassic sauropod. It was large, but its limbs, hand, neck and skull retained a different combination of early features. At the other extreme, calling it an ordinary lightweight biped ignores its robust forelimbs and the weight they could support.

The best-supported reconstruction is a large sauropodomorph that likely used all four limbs much of the time, without assuming strict quadrupedality. Its food was probably plant-based, but the exact plants are unknown. The catalogue's sauropodomorph profiles place it in an evolutionary context without suggesting a straight ancestor-to-giant line.