Abelisauridae

Abelisauridae is a diverse family of carnivorous theropod dinosaurs. Their name, which means "Abel's lizards," honors Roberto Abel, who discovered the genus *Abelisaurus*.

These predators were a major part of the dinosaur ecosystems during the Cretaceous period. They were primarily found on the ancient southern supercontinent known as Gondwana. Today, paleontologists find their fossilized remains across several modern landmasses. These include Africa, South America, and Europe, as well as the Indian subcontinent and Madagascar.

The physical structure of an abelisaurid was highly specialized for its environment. Most were bipeds, meaning they walked on two legs, and they possessed stocky hind limbs. Their ankle bones, the astragalus and calcaneum, were fused to each other and to the tibia. This fused structure is called a tibiotarsus. The tibia bone was shorter than the femur, which contributed to their thick, sturdy leg proportions.

Their feet typically featured three functional digits that made contact with the ground. The first digit, or hallux, did not touch the floor while they walked.

One of the most striking features of this family was the shape of the skull. Abelisaurid skulls were generally very tall and remarkably short in length. For example, the skull of *Carnotaurus* was nearly as tall as it was long.

The front of the snout, known as the premaxilla, was very tall, resulting in a blunt appearance rather than a tapered one. Two specific bones, the lacrimal and postorbital, projected into the eye socket. These bones nearly divided the socket into two separate compartments. In *Carnotaurus*, the upper compartment was tilted outward, which may have allowed for some binocular vision.

Many species within this group displayed elaborate ornamentation on their heads. The skull bones often featured textures like long grooves, pits, and protrusions. Some abelisaurids had bony crests or horns growing above their eyes. *Carnotaurus* possessed two pronounced horns, while its relative *Aucasaurus* had smaller projections.

Other genera, such as *Majungasaurus* and *Rajasaurus*, featured a single bony horn or dome.

Scientists believe these structures served purposes like species recognition or intimidation. In *Arcovenator*, a thickened brow ridge may have even supported scaly or keratinous structures for display.

In many advanced abelisaurids, the forelimbs became vestigial, meaning they were greatly reduced in size and function. In the genus *Carnotaurus*, the arms were extremely short and the elbow joint was immobile. The forearm bones, the radius and ulna, were only 25% of the length of the upper arm bone, or humerus. In *Aucasaurus*, these bones were 33% of the humerus length.

These animals lacked wrist bones, so the metacarpals attached directly to the forearm. They also lacked finger bones, or phalanges, on the first and fourth digits. Paleobiologist Alexander O. Vargas suggested this reduction might have been caused by a genetic defect involving the HOXA11 and HOXD11 genes.

The history of our understanding of these dinosaurs has changed as new fossils are discovered. In 1985, paleontologists Jose Bonaparte and Fernando Novas first described the family. At that time, only *Carnotaurus* and *Abelisaurus* were known from South America. Initially, researchers thought abelisaurids lived in Africa only after Africa separated from Gondwana 100 million years ago. However, the discovery of *Rugops* in the middle Cretaceous of Africa disproved this idea.

More recent finds, such as *Arcovenator* in France, have even provided evidence of their presence in Europe.

Abelisaurids varied significantly in size and evolutionary relationships. Most known members were between 5 and 9 meters long from snout to tail. However, an unnamed specimen from Kenya may have reached lengths of 11 to 12 meters.

The family is divided into various groups, such as the Carnotaurinae. Some scientists have even identified a specific clade of South American forms called Brachyrostra. These complex relationships show how abelisaurids successfully occupied different niches across the globe throughout the Mesozoic era.