Like other abelisaurids, Majungasaurus was a bipedal predator with a short snout. Although the forelimbs are not completely known, they were very short, while the hindlimbs were longer and very stocky. It can be distinguished from other abelisaurids by its wider skull, the very rough texture and thickened bone on the top of its snout, and the single rounded horn on the roof of its skull, which was originally mistaken for the dome of a pachycephalosaur. It also had more teeth in both upper and lower jaws than most abelisaurids.
Known from several well-preserved skulls and abundant skeletal material, Majungasaurus has recently become one of the best-studied theropod dinosaurs from the Southern Hemisphere. It appears to be most closely related to abelisaurids from India rather than South America or continental Africa, a fact which has important biogeographical implications. Majungasaurus was the apex predator in its ecosystem, mainly preying on sauropods like Rapetosaurus, and is also one of the few dinosaurs for which there is direct evidence of cannibalism.
Majungasaurus was a medium-sized theropod that typically measured 6–7 meters (20–23 ft) in length, including its tail. Fragmentary remains of larger individuals indicate that some adults reached lengths of more than 8 meters (26 ft). Sampson and Witmer estimated an average weight for an adult Majungasaurus of 1130 kilograms (2400 lb). The specimen they based it on (FMNH PR 2100) was not the largest one discovered. Larger specimen suggest that Majungasaurus crenatissimus could have reached similar sizes to its relative Carnotaurus, which has been estimated to weigh 1500 kilograms (3300 lb).
The postcranial skeleton of Majungasaurus closely resembles those of Carnotaurus and Aucasaurus, the only other abelisaurid genera for which complete skeletal material is known. Majungasaurus was bipedal, with a long tail to balance out the head and torso, putting the center of gravity over the hips. Although the cervical (neck) vertebrae had numerous cavities and excavations (pleurocoels) to reduce their weight, they were robust, with exaggerated muscle attachment sites and ribs that interlocked for strength. Ossified tendons attached to the cervical ribs, giving them a forked appearance, as seen in Carnotaurus. All of these features resulted in a very strong and muscular neck. Uniquely, the cervical ribs of Majungasaurus had long depressions along the sides for weight reduction. The humerus (upper arm bone) was short and curved, closely resembling those of Aucasaurus and Carnotaurus. Also like related dinosaurs, Majungasaurus had very short forelimbs with four extremely reduced digits, with only two very short external fingers and no claws. The hand and finger bones of Majungasaurus, like other carnotaurines, lacked the characteristic pits and grooves where claws and tendons would normally attach, and its finger bones were fused together, indicating that the hand was immobile.
Like other abelisaurids, the hindlimbs were stocky and short compared to body length. The tibia (lower leg bone) of Majungasaurus was even stockier than that of its relative Carnotaurus, with a prominent crest on the knee. The astragalus and calcaneum (ankle bones) were fused together, and the feet bore three functional digits, with a smaller first digit that did not contact the ground.
The skull of Majungasaurus is exceptionally well-known compared to most theropods and generally similar to that of other abelisaurids. Like other abelisaurid skulls, its length was proportionally short for its height, although not as short as in Carnotaurus. The skulls of large individuals measured 60–70 centimeters (24–28 in) long. The tall premaxilla (frontmost upper jaw bone), which made the tip of the snout very blunt, was also typical of the family. However, the skull of Majungasaurus was markedly wider than in other abelisaurids. All abelisaurids had a rough, sculptured texture on the outside faces of the skull bones, and Majungasaurus was no exception. This was carried to an extreme on the nasal bones of Majungasaurus, which were extremely thick and fused together, with a low central ridge running along the half of the bone closest to the nostrils. A distinctive dome-like horn protruded from the fused frontal bones on top of the skull as well. In life, these structures would have been covered with some sort of integument, possibly made of keratin. Computed tomography (CT scanning) of the skull shows that both the nasal structure and the frontal horn contained hollow sinus cavities, perhaps to reduce weight. The teeth were typical of abelisaurids in having short crowns, although Majungasaurus bore 17 teeth in both the maxilla of the upper jaw and the dentary of the lower jaw, more than in any other abelisaurid except Rugops.
Classification and systematicsEdit
Majungasaurus is classified as a member of the theropod clade Abelisauridae, which is considered a family in Linnaean taxonomy. Along with the family Noasauridae, abelisaurids are included in the superfamily Abelisauroidea, which is in turn a subdivision of the infraorder Ceratosauria. Abelisaurids are known for their tall skulls with blunt snouts, extensive sculpturing on the outer surfaces of the facial bones (convergent with carcharodontosaurids), very reduced (atrophied) forelimbs (convergent with tyrannosaurids), and stocky hindlimb proportions, among other features.
As with many dinosaur families, the systematics (evolutionary relationships) within the family Abelisauridae are confused. Several cladistic studies have indicated that Majungasaurus shares a close relationship with Carnotaurus from South America, while others were unable to firmly place it in the phylogeny. The most recent analysis, using the most complete information, instead recovered Majungasaurus in a clade with Rajasaurus and Indosaurus from India, but excluding South American genera like Carnotaurus, Ilokelesia, Ekrixinatosaurus, Aucasaurus and Abelisaurus, as well as Rugops from mainland Africa. This leaves open the possibility of separate clades of abelisaurids in western and eastern Gondwana. Detailed description of known abelisaurids like Aucasaurus as well as future discoveries and analyses may help to resolve the phylogenetic picture.