Abelisauridae

Abelisauridae, whose name means "Abel's lizards," is a family, or clade, of ceratosaurian theropod dinosaurs. These creatures were most active during the Cretaceous Period, primarily across the ancient southern landmass known as Gondwana. Their fossils have since been uncovered on the modern continents of Africa, South America, and western Europe, as well as on the Indian subcontinent and the island of Madagascar. The earliest possible member of this group is *Eoabelisaurus* from Patagonia, which may date back to the Early Jurassic, and some species persisted until the very end of the Mesozoic Era, roughly 66 million years ago.

Like most theropods, abelisaurids were two-legged meat-eaters. They had sturdy hind limbs and skulls covered in grooves and pits, giving them a heavily ornamented appearance. In many species, such as *Carnotaurus*, the arms were tiny and nearly useless, the skull was short, and bony crests sat above the eyes. Most known abelisaurids measured between 5 and 9 meters (17 to 30 feet) from nose to tail tip, though an unnamed specimen from northwestern Turkana in Kenya may have reached 11 to 12 meters (36 to 39 feet) in length. Before scientists understood them well, fragmentary abelisaurid bones were sometimes mistaken for South American tyrannosaurids.

**Description**

The hind limbs of abelisaurids were typical of ceratosaurs: the astragalus and calcaneum (upper ankle bones) were fused to each other and to the tibia, forming a structure called a tibiotarsus. The tibia was shorter than the femur, giving the leg a stocky build. The foot had three functional toes (the second, third, and fourth), while the first toe, or hallux, did not touch the ground.

**Skull**

Abelisaurid skulls varied in shape but were generally very tall and short in length. In *Carnotaurus*, for instance, the skull was nearly as tall as it was long. The premaxilla bone was very tall, making the front of the snout blunt rather than pointed, unlike many other theropods. Two skull bones—the lacrimal and postorbital—projected into the eye socket from the front and back, nearly splitting it into two parts. The eye would have sat in the upper compartment, which in *Carnotaurus* tilted slightly outward, possibly giving some binocular vision. These same bones also met above the eye socket, forming a ridge or brow. Many skull bones were sculpted with long grooves, pits, and bumps. As with other ceratosaurs, the frontal bones of the skull roof were fused together. Carnotaurines often had bony projections from the skull. *Carnotaurus* had two prominent horns sticking out above the eyes, while its relative *Aucasaurus* had smaller bumps in the same spot. *Majungasaurus* and *Rajasaurus* each had a single horn or dome rising from the top of the skull. These features, like the horns of modern animals, may have been used for species recognition or intimidation. In *Arcovenator*, the upper edge of the postorbital (and likely the lacrimal) was thickened and rough, forming a strong brow ridge that rose above the skull roof. This ridge might have supported a keratinous or scaly structure used for display.

**Forelimbs and Hands**

Information about abelisaurid forelimbs comes from *Eoabelisaurus* and the carnotaurines *Aucasaurus*, *Carnotaurus*, and *Majungasaurus*. All had small, seemingly useless forelimbs. The forearm bones (radius and ulna) were extremely short—only 25% of the upper arm length in *Carnotaurus* and 33% in *Aucasaurus*. The entire arm was held straight, and the elbow joint could not move. Like other ceratosaurs, the hand had four basic digits, but that is where the similarity ends. There were no wrist bones; the four palm bones (metacarpals) attached directly to the forearm. The first and fourth digits had no finger bones (phalanges), the second digit had one, and the third had two. These two outer fingers were very short and immobile. Claws were tiny in *Eoabelisaurus* and completely absent in carnotaurines. More primitive relatives like *Noasaurus* and *Ceratosaurus* had longer, movable arms with fingers and claws. Paleobiologist Alexander O. Vargas suggested that the evolution of these tiny forelimbs may have been caused by a genetic defect—the loss of function in two genes, HOXA11 and HOXD11, which normally regulate forelimb development.

**Distribution**

Abelisaurids are usually thought of as a Cretaceous group. The earliest possible abelisaurid is *Eoabelisaurus mefi* from the Jurassic of Argentina, though some researchers consider it a ceratosaurid, an abelisauroid, or a close relative outside the abelisaurid family. Indeterminate remains have also been found from the Jurassic of Madagascar and Tanzania. Most abelisaurid fossils come from the southern continents that once formed Gondwana. When first described in 1985, only *Carnotaurus* and *Abelisaurus* were known, both from the Late Cretaceous of South America. Later, abelisaurids were found in Late Cretaceous India (*Indosuchus* and *Rajasaurus*) and Madagascar (*Majungasaurus*), which were closely connected for much of the Cretaceous. It was once thought that the absence of abelisaurids from mainland Africa meant the group evolved after Africa split from Gondwana, around 100 million years ago. However, the discovery of *Rugops* and other abelisaurid material from the middle Cretaceous of northern Africa disproved that idea.