Carnotaurus is a genus of theropod dinosaur that lived in South America during the Late Cretaceous period, between 69 and 66 million years ago. The only species is Carnotaurus sastrei. Known from a single well-preserved skeleton, it is one of the best-understood theropods from the Southern Hemisphere. The skeleton, found in 1984, was uncovered in the Chubut Province of Argentina from rocks of the La Colonia Formation. Carnotaurus is a derived member of the Abelisauridae, a group of large theropods that occupied the large predatorial niche in the southern landmasses of Gondwana during the late Cretaceous. Within the Abelisauridae, the genus is often considered a member of the Brachyrostra, a clade of short-snouted forms restricted to South America.
Carnotaurus was a lightly built, bipedal predator, measuring in length and weighing . As a theropod, Carnotaurus was highly specialized and distinctive. It had two thick horns above the eyes, a unique feature unseen in all other carnivorous dinosaurs, and a very deep skull sitting on a muscular neck. Carnotaurus was further characterized by small, vestigial forelimbs and long, slender hind limbs. The skeleton is preserved with extensive skin impressions, showing a mosaic of small, non-overlapping scales approximately 5 mm in diameter. The mosaic was interrupted by large bumps that lined the sides of the animal, and there are no hints of feathers.
The distinctive horns and the muscular neck may have been used in fighting others of its species. According to separate studies, rivaling individuals may have combated each other with quick head blows, by slow pushes with the upper sides of their skulls, or by ramming each other head-on, using their horns as shock absorbers. The feeding habits of Carnotaurus remain unclear: some studies suggested the animal was able to hunt down very large prey such as sauropods, while other studies found it preyed mainly on relatively small animals.
Discovery
The only skeleton (holotype MACN-CH 894) was unearthed in 1984 by an expedition led by Argentinian paleontologist José Bonaparte. This expedition also recovered the peculiar spiny sauropod Amargasaurus. It was the eighth expedition within the project named "Jurassic and Cretaceous Terrestrial Vertebrates of South America", which started in 1976 and was sponsored by the National Geographic Society. The skeleton is well-preserved and (still connected together), with only the posterior two thirds of the tail, much of the lower leg, and the hind feet being destroyed by weathering. The skeleton belonged to an adult individual, as indicated by the fused sutures in the . It was found lying on its right side, showing a typical death pose with the neck bent back over the torso. Unusually, it is preserved with extensive skin impressions. In view of the significance of these impressions, a second expedition was started to reinvestigate the original excavation site, leading to the recovery of several additional skin patches. The skull was deformed during fossilization, with the snout bones of the left side displaced forwards relative to the right side, the nasal bones pushed upwards, and the pushed backwards onto the . Deformation also exaggerated the upward curvature of the upper jaw. The snout was more strongly affected by deformation than the rear part of the skull, possibly due to the higher rigidity of the latter. In top or bottom view, the upper jaws were less U-shaped than the lower jaws, resulting in an apparent mismatch. This mismatch is the result of deformation acting from the sides, which affected the upper jaws but not the lower jaws, possibly due to the greater flexibility of the joints within the latter.
The skeleton was collected on a farm named "Pocho Sastre" near Bajada Moreno in the Telsen Department of Chubut Province, Argentina. Because it was embedded in a large hematite concretion, a very hard kind of rock, preparation was complicated and progressed slowly. In 1985, Bonaparte published a note presenting Carnotaurus sastrei as a new genus and species and briefly describing the skull and lower jaw. The generic name Carnotaurus is derived from the Latin carno [carnis] ("flesh") and taurus ("bull") and can be translated with "meat-eating bull", an allusion to the animal's bull-like horns. The specific name sastrei honors Angel Sastre, the owner of the ranch where the skeleton was found. A comprehensive description of the whole skeleton followed in 1990. After Abelisaurus, Carnotaurus was the second member of the family Abelisauridae that was discovered. For years, it was by far the best-understood member of its family, and also the best-understood theropod from the Southern Hemisphere. It was not until the 21st century that similar well-preserved abelisaurids were described, including Aucasaurus, Majungasaurus and Skorpiovenator, allowing scientists to re-evaluate certain aspects of the anatomy of Carnotaurus. The holotype skeleton is displayed in the Argentine Museum of Natural Sciences, Bernardino Rivadavia; replicas can be seen in this and other museums around the world. Sculptors Stephen and Sylvia Czerkas manufactured a life-sized sculpture of Carnotaurus that was previously on display at the Natural History Museum of Los Angeles County. This sculpture, ordered by the museum during the mid-1980s, is probably the first life restoration of a theropod showing accurate skin.
Description
Carnotaurus was a large but lightly built predator. The only known individual was about in length, making Carnotaurus one of the largest abelisaurids. Ekrixinatosaurus and possibly Abelisaurus, which are highly incomplete, might have been similar or larger in size. A 2016 study found that only Pycnonemosaurus, at , was longer than Carnotaurus; it was estimated at . Its mass is estimated to have been , , , , and in separate studies that used different estimation methods. Carnotaurus was a highly specialized theropod, as seen especially in characteristics of the skull, the vertebrae and the forelimbs. The pelvis and hind limbs, on the other hand, remained relatively conservative, resembling those of the more basal Ceratosaurus. Both the pelvis and hind limb were long and slender. The left (thigh bone) of the individual measures 103 cm in length, but shows an average diameter of only 11 cm.
Skull
The skull, measuring in length, was proportionally shorter and deeper than in any other large carnivorous dinosaur. The snout was moderately broad, not as tapering as seen in more basal theropods like Ceratosaurus, and the jaws were curved upwards. A prominent pair of horns protruded obliquely above the eyes. These horns, formed by the frontal bones, were thick and cone-shaped, internally solid, somewhat vertically flattened in cross-section, and measured in length. Bonaparte, in 1990, suggested that these horns would probably have formed the bony cores of much longer keratinous sheaths. Mauricio Cerroni and colleagues, in 2020, agreed that the horns supported keratinous sheaths, but argued that these sheaths would not have been greatly longer than the bony cores.
As in other dinosaurs, the skull was perforated by six major skull openings on each side. The frontmost of these openings, the (bony nostril), was subrectangular and directed sidewards and forwards, but was not sloping in side view as in some other ceratosaurs such as Ceratosaurus. This opening was formed by the nasal and premaxilla only, while in some related ceratosaurs the maxilla also contributed to this opening. Between the bony nostril and the orbit (eye opening) was the antorbital fenestra. In Carnotaurus, this opening was higher than long, while it was longer than high in related forms such as Skorpiovenator and Majungasaurus. The antorbital fenestra was bounded by a larger depression, the , which was formed by recessed parts of the maxilla in front and the behind. As in all abelisaurids, this depression was small in Carnotaurus. The lower front corner of the antorbital fossa contained a smaller opening, the , which led into an air-filled cavity within the maxilla. The eye was situated in the upper part of the keyhole-shaped orbit. This upper part was proportionally small and subcircular, and separated from the lower part of the orbit by the forward-projecting . It was slightly tilted forward, probably permitting some degree of binocular vision. The keyhole-like shape of the orbit was possibly related to the marked skull shortening, and is also found in related short-snouted abelisaurids. As in all abelisaurids, the (on the skull roof between the eyes) was excluded from the orbit. Behind the orbit were two openings, the on the side and the on the top of the skull. The infratemporal fenestra was tall, short, and kidney-shaped, while the supratemporal fenestra was short and square-shaped. Another opening, the , was located in the lower jaw – in Carnotaurus, this opening was comparatively large.
On each side of the upper jaws there were four premaxillary and twelve maxillary teeth, while the lower jaws were equipped with fifteen dentary teeth per side. The teeth had been described as being long and slender, as opposed to the very short teeth seen in other abelisaurids. However, Cerroni and colleagues, in their 2020 description of the skull, stated that all erupted teeth have been severely damaged during excavation and were later reconstructed with plaster (Bonaparte, in 1990, only noted that some lower jaw teeth had been fragmented). Reliable information on the shape of the teeth is therefore limited to replacement teeth and tooth roots that are still enclosed by the jaw, and can be studied using CT imaging. The replacement teeth had low, flattened crowns, were closely spaced, and inclined forwards at approximately 45°. In his 1990 description, Bonaparte noted that the lower jaw was shallow and weakly constructed, with the (the foremost jaw bone) connected to the hindmost jaw bones by only two contact points; this contrasts to the robust-looking skull. Cerroni and colleagues instead found multiple but loose connections between the dentary and the hindmost jaw bones. This articulation, therefore, was very flexible but not necessarily weak. The bottom margin of the dentary was convex, while it was straight in Majungasaurus.
The lower jaw was found with ossified hyoid bones, in the position they would be in if the animal was alive. These slender bones, supporting the tongue musculature and several other muscles, are rarely found in dinosaurs because they are often cartilaginous and not connected to other bones and therefore get lost easily. In Carnotaurus, three hyoid bones are preserved: a pair of curved, rod-like ceratobranchials that articulate with a single, trapezoidal element, the basihyal. Carnotaurus is the only known non-avian theropod from which a basihyal is known. The back of the skull had well-developed, air-filled chambers surrounding the braincase, as in other abelisaurids. Two separate chamber systems were present, the paratympanic system, which was connected to the middle ear cavity, as well as chambers resulting from outgrowths of the air sacs of the neck.
A number of autapomorphies (distinguishing features) can be found in the skull, including the pair of horns and the very short and deep skull. The maxilla had excavations above the promaxillary fenestra, which would have been excavated by the antorbital air sinus (air passages in the snout). The nasolacrimal duct, which transported eye fluid, exited on the medial (inner) surface of the lacrimal through a canal of uncertain function. Other proposed autapomorphies include a deep and long, air-filled excavation in the and an elongated depression on the of the .
Vertebrae