Carnotaurus

and Carnotaurus'', both discovered by the same 1984 expedition in Argentina, Natural History Museum of the University of Pisa 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. making Carnotaurus one of the largest abelisaurids. Its mass is estimated to have been , , , 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. The bottom margin of the dentary was convex, while it was straight in Majungasaurus. Stephen Czerkas (1997) suggested that these structures may have protected the animal's sides while fighting members of the same species (conspecifics) and other theropods, arguing that similar structures can be found on the neck of the modern iguana where they provide limited protection in combat. ==Classification==

Carnotaurus is one of the best-understood genera of the Abelisauridae, a family of large theropods restricted to the ancient southern supercontinent Gondwana. Abelisaurids were the dominant predators in the Late Cretaceous of Gondwana, replacing the carcharodontosaurids and occupying the ecological niche filled by the tyrannosaurids in the northern continents. or Majungasaurus. Carnotaurus is eponymous for two subgroups of the Abelisauridae: the Carnotaurinae and the Carnotaurini. Paleontologists do not universally accept these groups. The Carnotaurinae was defined to include all derived abelisaurids with the exclusion of Abelisaurus, which is considered a basal member in most studies. However, a 2008 review suggested that Abelisaurus was a derived abelisaurid instead. Carnotaurini was proposed to name the clade formed by Carnotaurus and Aucasaurus; only those paleontologists who consider Aucasaurus as the nearest relative of Carnotaurus use this group. A 2024 study recovered Carnotaurini as a valid clade consisting of Carnotaurus, Aucasaurus, Niebla and Koleken. Below is a cladogram published by Canale and colleagues in 2009. }} ==Paleobiology==

Function of the horns Carnotaurus is the only known carnivorous bipedal animal with a pair of horns on the frontal bone. Gerardo Mazzetta and colleagues (1998) propose that the horns might also have been used to injure or kill small prey. Though horn cores are blunt, they may have had a similar form to modern bovid horns if there was a keratinous covering. However, this would be the only reported example of horns being used as hunting weapons in animals. This interpretation was questioned by François Therrien and colleagues (2005), who found that the biting force of Carnotaurus was twice that of the American alligator, which may have the strongest bite of any living tetrapod. These researchers also noted analogies with modern Komodo dragons: the flexural strength of the lower jaw decreases towards the tip linearly, indicating that the jaws were not suited for high precision catching of small prey but for delivering slashing wounds to weaken big prey. As a consequence, according to this study, Carnotaurus must have mainly preyed upon large animals, possibly by ambush. In dinosaurs, the most important locomotor muscle was located in the tail. This muscle, called the caudofemoralis, attaches to the fourth trochanter, a prominent ridge on the thigh bone, and pulls the thigh bone backwards when contracted. Scott Persons and Phil Currie (2011) argued that in the tail vertebrae of Carnotaurus, the caudal ribs did not protrude horizontally ("T-shaped"), but were angled against the vertical axis of the vertebrae, forming a "V". This would have provided additional space for a caudofemoralis muscle larger than in any other theropod—the muscle mass was calculated at per leg. Therefore, Carnotaurus could have been one of the fastest large theropods. While the caudofemoralis muscle was enlarged, the epaxial muscles situated above the caudal ribs would have been proportionally smaller. These muscles, called the longissimus and spinalis muscle, were responsible for tail movement and stability. To maintain tail stability in spite of reduction of these muscles, the caudal ribs bear forward projecting processes interlocking the vertebrae with each other and with the pelvis, stiffening the tail. As a consequence, the ability to make tight turns would have been diminished, because the hip and tail had to be turned simultaneously, unlike in other theropods. Brain and senses Cerroni and Paulina-Carabajal, in 2019, used a CT scan to study the endocranial cavity that contained the brain. The volume of the endocranial cavity was 168.8 cm3, although the brain would only have filled a fraction of this space. The authors used two different brain size estimates, assuming a brain size of 50% and 37% of the endocranial cavity, respectively. This results in a reptile encephalization quotient (a measure of intelligence) larger than that of the related Majungasaurus but smaller than in tyrannosaurids. The pineal gland, which produces hormones, might have been smaller than in other abelisaurids, as indicated by a low dural expansion – a space on top of the forebrain in which the pineal gland is thought to have been located. The olfactory bulbs, which housed the sense of smell, were large, while the optic lobes, which were responsible for sight, were relatively small. This indicates that the sense of smell might have been better developed than the sense of sight, while the opposite is the case in modern birds. The front end of the olfactory tracts and bulbs were curved downwards, a feature only shared by Indosaurus; in other abelisaurids, these structures were oriented horizontally. As hypothesized by Cerroni and Paulina-Carabajal, this downward-curvature, together with the large size of the bulbs, might indicate that Carnotaurus relied more on the sense of smell than other abelisaurids. The flocculus, a brain lobe thought to be correlated with gaze stabilization (coordination between eyes and body), was large in Carnotaurus and other South American abelisaurids. This could indicate that these forms frequently used quick movements of the head and body. Hearing might have been poorly developed in Carnotaurus and other abelisaurids, as indicated by the short lagena of the inner ear. The hearing range was estimated to be below 3 kHz. ==Age and paleoenvironment==

Originally, the rocks in which Carnotaurus was found were assigned to the upper part of the Gorro Frigio Formation, which was considered to be approximately 100 million years old (Albian or Cenomanian stage). the saltasauroid titanosaur Titanomachya gimenezi; an unnamed ankylosaur; and an unnamed hadrosauroid, among others. Some of the snakes that have been found belong to the families Boidae and Madtsoiidae, such as Alamitophis argentinus. Mammals are represented by Reigitherium bunodontum and Coloniatherium cilinskii, the former of which was considered the first record of a South American docodont, and the possible gondwanatherians or multituberculates Argentodites coloniensis and Ferugliotherium windhauseni. Remains of an enantiornithine and, possibly, of a neornithine bird have been discovered. ==See also==