Abelisauridae

Abelisauridae (meaning "Abel's lizards") is a family (or clade) of ceratosaurian theropod dinosaurs. Abelisaurids thrived during the Cretaceous Period, on the ancient southern supercontinent of Gondwana, and today their fossil remains are found on the modern continents of Africa and South America, as well as on the Indian subcontinent and the island of Madagascar.

Like most theropods, abelisaurids were carnivorous bipeds. They were characterized by stocky hindlimbs and extensive ornamentation of the skull bones, with grooves and pits. In later abelisaurids, like Carnotaurus, the forelimbs became vestigial, the skull became shorter and bony crests developed above the eyes. Most of the known abelisaurids would have been between 5 to 9 meters (17 to 30 ft) in length, from snout to tip of tail. Before becoming well-known, fragmentary abelisaurid remains were occasionally misidentified as possible South American tyrannosaurids.

Anatomy
Complete skeletons have been described only for the most derived abelisaurids (Carnotaurus and Aucasaurus), making it difficult to establish defining features of the postcranial skeleton for the family as a whole. However, most are known from at least some skull material, so abelisaurid synapomorphies come mainly from the skull.

Although skull proportions varied, abelisaurid skulls were generally very tall and often very short. In Carnotaurus, for example, the skull was nearly as tall as it was long. The premaxilla in abelisaurids was very tall, so the front of the snout was blunt, not tapered as seen in many other theropods.

The lacrimal and postorbital bones projected into the eye socket from the front and back, nearly dividing it into two compartments. The eye would have been located in the upper compartment of the orbit, which was tilted slightly outwards in Carnotaurus, perhaps providing some degree of binocular vision. The lacrimal and postorbital also met above the orbit, to form a pronounced ridge or brow above the eye.

Extensive sculpturing is seen on many of the skull bones, in the form of long grooves, pits and protrusions. Like other ceratosaurs, the frontal bones of the skull roof were fused together. Carnotaurine abelisaurids commonly had bony projections from the frontals. Carnotaurus had two pronounced horns on the frontals, projecting laterally above the eyes, while its close relative Aucasaurus had smaller projections in the same area. Majungasaurus and Rajasaurus had a single bony horn or dome, projecting upwards from the fused frontals. These projections, like the horns of many modern animals, might have been displayed for species recognition or intimidation.

The forelimbs are known only in the carnotaurine abelisaurids, Aucasaurus and Carnotaurus, in which they were vestigial. The radius and ulna were extremely short, only 25% of the length of the humerus in Carnotaurus and 33% in Aucasaurus. Typically for ceratosaurs, the carnotaurine hand contained four digits. However, it is there that any similarity ends. No wrist bones existed, with the four metacarpals articulating directly with the radius and ulna. There were no phalanges on the first or fourth digits, one on the second digit and two on the third digit. The forelimbs do not appear to have borne claws.

It is unknown if this peculiar forelimb morphology applies to other abelisaurids, as their forelimbs have not been discovered. Abelisaurid hindlimbs were more typical of ceratosaurs, with the astragalus and calcaneum (upper ankle bones) fused to each other and to the tibia, forming a tibiotarsus. The tibia was shorter than the femur, giving the hindlimb stocky proportions. There were three functional digits on the foot (the second, third, and fourth), while the first digit, or hallux, did not contact the ground.

Taxonomy and systematics
Paleontologists Jose Bonaparte and Fernando Novas coined the name Abelisauridae in 1985 when they described the eponymous Abelisaurus. The name is formed from the family name of Roberto Abel, who discovered Abelisaurus, as well as from the Greek word σαυρος/sauros meaning 'lizard'. The very common suffix -idae is usually applied to zoological family names and is derived from the Greek suffix -ιδαι/-idai, which indicates a plural noun. Abelisauridae is a family in rank-based Linnaean taxonomy, within the infraorder Ceratosauria and the superfamily Abelisauroidea, which also contains the family Noasauridae. It has had several definitions in phylogenetic taxonomy. It was originally defined as a node-based taxon including Abelisaurus, Carnotaurus, their common ancestor and all of its descendants.

Later it was redefined as a stem-based taxon, including all animals more closely related to Abelisaurus (or the more complete Carnotaurus) than to Noasaurus. The node-based definition would not include animals like Rugops or Ilokelesia, which are thought to be more basal than Abelisaurus and would be included by a stem-based definition. Within Abelisauridae is the subgroup Carnotaurinae, and among carnotaurines, Aucasaurus and Carnotaurus are united in Carnotaurini.

Many abelisaurid skull features are shared with carcharodontosaurids. These shared features, along with the fact that abelisaurids seem to have replaced carcharodontosaurids in South America, have led to suggestions that the two groups were related. However, no cladistic analysis has ever found such a relationship and, aside from the skull, abelisaurids and carcharodontosaurids are very different, more similar to ceratosaurs and allosauroids, respectively.

Classification
Following is a list of abelisaurid genera by classification and location, assuming a stem-based definition:

FAMILY ABELISAURIDAE
 * Abelisaurus (Argentina)
 * Compsosuchus (India)
 * Indosaurus (India)
 * Indosuchus (India)
 * Kryptops (Niger)
 * Rugops (Niger)
 * Xenotarsosaurus (Argentina)
 * Subfamily Carnotaurinae
 * Ekrixinatosaurus (Argentina)
 * Ilokelesia (Argentina)
 * Majungasaurus (Madagascar)
 * Rajasaurus (India)
 * Skorpiovenator (Argentina)
 * Tribe Carnotaurini
 * Aucasaurus (Argentina)
 * Carnotaurus (Argentina)

Indosaurus may be a junior synonym of Indosuchus. Some scientists include Xenotarsosaurus from Argentina and Compsosuchus from India as basal abelisaurids, while others consider them to be outside Abelisauroidea. The French Genusaurus and Tarascosaurus have also been called abelisaurids but both are fragmentary and may be more basal ceratosaurians.

Phylogeny
A 2004 phylogenetic analysis, performed by Paul Sereno of the University of Chicago and several colleagues, obtained the following results:

Ilokelesia was originally described as a sister group to Abelisauroidea. However, Sereno tentatively places it closer to Abelisaurus than to noasaurids, a result which agrees with several other recent analyses. If a stem-based definition is used, Ilokelesia and Rugops are therefore basal abelisaurids. However, as they are more basal than Abelisaurus, they are outside of Abelisauridae if the node-based definition is adopted. Ekrixinatosaurus was also published in 2004, so it was not included in Sereno's analysis. However, an independent analysis, performed by Jorge Calvo and colleagues, shows it to be an abelisaurid.

With the description of Skorpiovenator in 2008, Canale et al. published another phylogenetic analysis focusing on the South American abelisaurids. In their results, they found that all South American forms (including Ilokelesia) grouped together as a sub-clade of carnotaurines, which they named Brachyrostra.

Distribution
Abelisauroids are typically regarded as a Cretaceous group, although possible abelisauroid remains are known from the Middle Jurassic of Madagascar. Abelisaurid remains have only been found in the southern continents, which once made up the supercontinent of Gondwana. When first described in 1985, only Carnotaurus and Abelisaurus were known, both from the Late Cretaceous of South America. Abelisaurids were then located in Late Cretaceous India (Indosuchus and Rajasaurus) and Madagascar (Majungasaurus), which were closely connected for much of the Cretaceous. It was thought that the absence of abelisaurids from continental Africa indicated that the group evolved after the separation of Africa from Gondwana, around 100 million years ago. However, the discovery of Rugops and other abelisaurid material from the middle of the Cretaceous in northern Africa disproved this hypothesis. Mid-Cretaceous abelisaurids are now known from South America as well, showing that the group existed prior to the breakup of Gondwana.