Carnotaurus

Carnotaurus /ˌkɑrnɵˈtɔrəs/ is a genus of large theropod dinosaur that lived in South America during the Late Cretaceous period, between about 72 and 70 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. Derived from the Latin carno [carnis] ("flesh") and taurus ("bull"), the name Carnotaurus means "meat-eating bull", alluding to its bull-like horns. 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. The phylogenetic relations of Carnotaurus are uncertain; it may have been closer to either Majungasaurus or Aucasaurus.

Carnotaurus was a lightly built, bipedal predator, measuring 8 to 9 m (26 to 30 ft) in length and weighing at least 1 metric ton (0.98 long ton; 1.1 short tons). As a theropod, Carnotaurus was highly specialized and distinctive. It had thick horns above the eyes, a 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 and slender hindlimbs. The skeleton is preserved with extensive skin impressions, showing a mosaic of small, non-overlapping scales measuring 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 conspecifics. 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 suggest the animal was able to hunt down very large prey such as [[Sauropoda|sauropods, while other studies find it preyed mainly on relatively small animals. Carnotaurus was well adapted for running and was possibly one of the fastest large theropods.

Description
Carnotaurus was a large but lightly built predator.[1] The only known individual was about 8 to 9 metres (26 to 30 ft) in length,[A] making Carnotaurus one of the largest abelisaurids.[B] Only Ekrixinatosaurus and possibly Abelisaurus may have been similar or larger in size, though the incomplete remains of these genera make size estimations imprecise.[C][D][E] Its mass is estimated to have been 1,350 kg (1.33 long tons; 1.49 short tons)[F] 1,500 kg (1.5 long tons; 1.7 short tons)[G] and 2,102 kg (2.069 long tons; 2.317 short tons)[H] 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.[I] The pelvis and hindlimbs, on the other hand, remained relatively conservative, resembling those of the more basal Ceratosaurus. Both the pelvis and hindlimb bones 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.[J]

Skull
The skull, measuring 59.6 cm (23.5 in) in length, was proportionally shorter and deeper than in any other large carnivorous dinosaur.[K][L] The snout was broad, unlike the tapering snout seen in more basal theropods like Ceratosaurus, and the jaws were curved upwards.[9] As in other abelisaurids, the facial bones, especially the nasal bones, were sculptured with numerous small holes and spikes.[M] In life, a wrinkled and possibly keratinous skin would have covered these bones.[N] A prominent pair of horns protruded obliquely above the eyes. These horns, formed by the frontal bones,[O] were thick, flattened on their upper sides, and measured 15 cm (5.9 in) in length.[10] In life, they would probably have formed the bony cores of much longer keratinous horns.[P] The proportionally small eyes[11] were situated in the upper part of the keyhole shaped orbita (eye sockets).[Q] The upper part was slightly rotated forward, probably permitting some degree of binocular vision.[R]

The teeth were long and slender,[11] as opposed to the usually very short teeth seen in other abelisaurids.[9] On each side of the upper jaws there were four premaxillary and twelve maxillary teeth,[S] while the lower jaws were equipped with 15 dentary teeth per side.[T] In contrast to the robust-looking skull, the lower jaw was shallow and weakly constructed, with the dentary (the foremost jaw bone) connected to the hindmost jaw bones by only two contact points.[11][U] The lower jaw was found with 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 not connected to other bones and therefore get lost easily.[V][12]

Vertebrae
The vertebral column consisted of 10 cervical (neck), 12 dorsal, 6 fused sacral[W] and an unknown number of caudal (tail) vertebrae.[5] The neck was nearly straight, rather than having the S-curve seen in other theropods, and also unusually wide, especially towards its base.[13] The top of the neck's spinal column featured a double row of enlarged, upwardly directed bony processes called epipophyses, making the neck flat on top. These processes were the highest points of the spine, towering above the unusually low spinous processes.[5][12] The epipophyses probably provided attachment areas for a markedly strong neck musculature.[X] A similar double row was also present in the tail, formed there by highly modified caudal ribs protruding in a V-shape, resulting in a flattened top of the tail. The end of each caudal rib was furnished with a forward projecting expansion that connected to the caudal rib of the preceding vertebra.[12][14]

Forelimbs
The forelimbs were proportionally shorter than in any other large carnivorous dinosaurs, including Tyrannosaurus.[Y] The forearm was only a quarter the size of the upper arm. There were no carpalia in the hand, so that the metacarpals articulated directly with the forearm.[15] The hand showed four basic digits,[5] though apparently only the middle two of these ended in finger bones, while the fourth consisted of a single splint-like metacarpal that may have represented an external 'spur'. The fingers themselves were fused and immobile, and may have lacked claws.[16] Carnotaurus differed from all other abelisaurids in having proportionally shorter and more robust forelimbs, and in having the fourth, splint-like metacarpal as the longest bone in the hand.[15] A 2009 study suggests that the arms were vestigial in abelisaurids, because nerve fibers responsible for stimulus transmission were reduced to an extent seen in today's emus and kiwis, which also have vestigial forelimbs.

Skin
Carnotaurus was the first theropod dinosaur discovered with comprehensive fossil skin impressions.[18] These impressions, found beneath the skeleton's right side, come from different body parts, including the lower jaw,[18] the front of the neck, the shoulder girdle, and the rib cage.[Z] The largest patch of skin corresponds to the anterior part of the tail.[AA] Originally, the right side of the skull also was covered with large patches of skin—this was not recognized when the skull was prepared, and these patches were accidentally destroyed. Still, the surface texture of much of the right side of the skull is very different from that of the left side, and probably shows some features of the scalation pattern of the head.[18]

The skin was built up of a mosaic of polygonal, non-overlapping scales measuring approximately 5 mm (0.20 in) in diameter. This mosaic was divided by thin, parallel grooves.[AB] Scalation was similar across different body parts with the exception of the head, which apparently showed a different, irregular pattern of scales.[AC][19] There is no evidence of feathers.[18] Uniquely for theropods, there were large knob-like bumps running along the sides of the neck, back and tail in irregular rows. Each bump showed a low ridge and measured 4 to 5 cm (1.6 to 2.0 in) in diameter. They were set 8 to 10 cm (3.1 to 3.9 in) apart from each other and became larger towards the animal's top. The bumps probably represent clusters of condensed scutes, similar to those seen on the soft frill running along the body midline in hadrosaurid ("duck-billed") dinosaurs.[AD][18] 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.[1] Several notable traits that evolved within this family, including shortening of the skull and arms as well as peculiarities in the cervical and caudal vertebrae, were more pronounced in Carnotaurus than in any other abelisaurid.[AE][AF][14]

Though relationships within the Abelisauridae are debated, Carnotaurus is consistently shown to be one of the most derived members of the family by cladistical analyses.[AG] Its nearest relative may have been either Aucasaurus[20][21][22] or Majungasaurus;[23][24][25] this ambiguity is largely due to the incompleteness of the Aucasaurus skull material.[AH][AI] A recent review suggests that Carnotaurus was not closely related with either Aucasaurus or Majungasaurus, and instead proposed Ilokelesia as its sister taxon.[AJ]

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.[26] However, a 2008 review suggested that Abelisaurus was a derived abelisaurid instead.[AK] Carnotaurini was proposed to name the clade formed by Carnotaurus and Aucasaurus;[21] only those paleontologists who consider Aucasaurus as the nearest relative of Carnotaurus use this group.[