Carnotaurus: horns, skin and an unusual abelisaurid

Carnotaurus was a large abelisaurid theropod from what is now Patagonia, Argentina. Its only recognised species, Carnotaurus sastrei, is known from one nearly complete adult skeleton found in the La Colonia Formation. The specimen preserves an unusual combination of a short, deep skull with paired brow horns, extremely reduced forelimbs and patches of fossil skin.

The single skeleton makes Carnotaurus unusually informative, but it also sets strict limits. Researchers can examine much of one individual's anatomy and several areas of skin. They cannot use one animal to measure normal variation, sex differences, top speed or social behaviour. Its dinosaur profile and related genera can be compared in the dinosaur catalogue.

Quick facts

Name, discovery and type specimen

Carnotaurus is formed from Latin roots for flesh and bull and is often translated as “meat-eating bull”. The name refers to the pair of prominent bony cores above the eyes. The species name sastrei honours Ángel Sastre, owner of the ranch where the fossil was found.

José Bonaparte led the Argentine expedition that excavated the skeleton in 1984 near Bajada Moreno in Chubut Province. The specimen, MACN-CH 894, lay on its right side and retained the natural position of many bones inside a hard iron-rich concretion. Preparation took considerable time. Bonaparte named the genus and species in a preliminary description in 1985; a fuller anatomical account by Bonaparte, Fernando Novas and Rodolfo Coria appeared in 1990. Later studies revised details of the skull, forelimbs, tail, braincase and skin.

The skull was deformed after burial, and the two sides were not compressed identically. This matters because early drawings sometimes treated a distorted outline as the undisturbed shape. Modern comparisons account for the damage rather than assuming that every old reconstruction was exact.

Classification and species

Carnotaurus is securely placed among theropods, ceratosaurs and Abelisauridae. Its short, high skull, reduced arms and features of the vertebrae, pelvis and hind limbs support that position. It is often included in the derived South American branch Brachyrostra and the narrower Furileusauria, although relationships within the family change across analyses.

A 2024 analysis grouped Carnotaurus with Aucasaurus, Niebla and Koleken in Carnotaurini. That result does not settle a single closest relative. Different matrices use different characters, and many abelisaurids are known from incomplete skeletons. The family-level assignment is stronger than a precise statement about its nearest branch.

Only one species is recognised: Carnotaurus sastrei. The abelisaurid Koleken inakayali comes from the same broad formation, but it is not considered a juvenile Carnotaurus or a second sex of the same animal. Differences involve skull bones, vertebrae and the femur, not simply the presence or absence of horns. On present evidence they are closely related but distinct genera.

What was preserved

MACN-CH 894 includes a nearly complete skull and lower jaws, much of the vertebral column, ribs, shoulder and pelvic elements, both reduced forelimbs and a substantial part of the hind limbs. Weathering removed the rear part of the tail, much of the lower legs and the feet. The specimen is still far more complete than many large abelisaurid fossils.

Skin impressions occur around the lower jaw, neck, shoulder region, trunk and front of the tail. They preserve small, non-overlapping scales together with larger conical elements. A 2021 study found that the larger scales were scattered irregularly rather than set in neat rows along the body. The impressions are local patches, not a continuous skin covering, so they do not tell us exactly what every part of Carnotaurus looked like.

No feathers were found in the preserved skin patches. That is direct evidence about those sampled areas, not proof that every unpreserved part of the animal lacked filamentous covering. The distinction between what is present in a fossil and what is absent from an unpreserved region is important when reconstructing soft tissue.

Size and body structure

Because part of the tail and the feet are missing, total length must be restored. Common reconstructions place Carnotaurus at about 7.5–8 metres. Mass estimates range roughly from 1.3 to 2.1 tonnes, depending on how researchers model the body volume, tail and soft tissues. One exact mass would imply a level of measurement the fossil does not provide.

The skull was about 60 centimetres long, short and deep compared with the skulls of many other large theropods. The paired brow horns were formed by the frontal bones. Their bases are fossil evidence; the horn sheath's exact length, curve and surface are unknown. The face and skull roof also carried pronounced sculpturing.

The forelimbs were extremely short. The upper arm was robust, while the forearm and hand were strongly reduced. Four rays remained in the hand, but the fingers were shortened and did not make an effective grasping apparatus. They could not seize prey in the way that longer-armed theropods could. Their precise function is unknown.

The hind limbs were relatively long and slender. At the front of the tail, tall transverse processes supported substantial caudofemoral muscles that could pull the thigh backwards. This anatomy may have helped acceleration. A comparatively stiff front tail could also have limited side-to-side flexibility. The skeleton does not decide whether Carnotaurus was an exceptional sprinter or provide a reliable speed in kilometres per hour.

Age, setting and other animals

The skeleton comes from the second facies association of the La Colonia Formation, in rocks assigned to the Maastrichtian, the final age of the Cretaceous. The formation is younger than about 71.7 million years and includes deposits near the end of the period, but the exact age of the horizon containing MACN-CH 894 has not been narrowed to a short interval.

La Colonia includes estuarine, tidal-flat and low coastal deposits where fresh water met marine influence. Fish, turtles, snakes, small mammals, birds, dinosaurs and aquatic reptiles are known from the region. This setting describes where the skeleton was buried. It does not mean Carnotaurus was aquatic or regularly hunted marine animals. Its individual profile can be viewed alongside other theropods in the dinosaur collection.

Diet and possible behaviour

Teeth, jaws and its overall anatomy identify Carnotaurus as a carnivore. The fossils do not reveal its exact prey. It may have taken available land vertebrates and scavenged carcasses, but claims that it hunted only small animals or regularly attacked sauropods are not supported by a direct feeding trace.

Researchers have proposed that the horns helped individuals display, recognise one another or engage in physical contests. A strong neck could have supported head movements, but comparison with modern cattle does not prove that Carnotaurus charged or butted like a bull. No series of injuries or pair of interacting skeletons records such encounters.

One specimen cannot show whether the species had pronounced differences between males and females or how horns changed with age. There is no direct evidence of pack hunting, parental care, colour, calls or a maximum running speed. Such details in films and artwork are reconstruction, not observation.

Older interpretations and popular claims

Early accounts assigned the skeleton to older deposits and described Carnotaurus as a Middle Cretaceous animal. Geological revision placed it in the much younger La Colonia Formation, in the Maastrichtian. The exact layer age remains uncertain, so a precise calendar date should not be attached to the individual.

Older illustrations often arranged the large skin bumps in regular rows. Re-examination of the impressions found a less ordered distribution. The fossil does not show armour plates, an exact colour pattern or a continuous mosaic of identical bumps across the body.

Carnotaurus is sometimes labelled the fastest large dinosaur. Its long hind limbs and tail musculature are relevant to movement, but bones do not retain speed. Estimates require assumptions about muscle volume, body mass and gait. It is safer to describe an athletic-looking anatomy than to repeat one disputed speed number.

The horns are also not proof of bull-like combat. Display, recognition and contest are plausible hypotheses, but none is directly established. Nor do tiny arms mean that the whole animal was weak: its head, neck, trunk and hind limbs were strongly developed even as the forelimbs became reduced.

What a reconstruction can show

The fossil directly establishes a compact, high skull with paired horn cores, extremely short forelimbs, strong hind limbs and local scale impressions. The skeleton constrains body proportions, but missing parts must be restored by comparison with relatives. The horn sheath, colour, unpreserved skin, gait, speed, prey and social behaviour remain unknown or debated.

Carnotaurus is therefore valuable not because every part of its life is known, but because one unusually complete individual preserves anatomical details rarely found together in an abelisaurid. A responsible reconstruction can make the skeleton legible while leaving uncertain behaviour and soft tissue visibly uncertain.