Abelisauridae

Lore & Background

Abelisaurids were characterized by stocky hind limbs and extensive ornamentation of the skull bones, with grooves and pits. In many abelisaurids, such as Carnotaurus, the forelimbs are vestigial, the skull is shorter, and bony crests grow above the eyes. The hind limb had the astragalus and calcaneum fused to the tibia, though this structure is not called a tibiotarsus (a term reserved for birds), and the tibia was shorter than the femur. Three functional digits were on the foot, while the first digit did not contact the ground. The skull was generally very tall and very short in length; in Carnotaurus the skull was nearly as tall as it was long. The premaxilla was very tall, making the front of the snout blunt. Two skull bones, the lacrimal and postorbital, projected into the eye socket from front and back, nearly dividing it into two compartments. Sculpturing in the form of long grooves, pits, and protrusions is seen on many skull bones. Carnotaurines commonly had bony projections: Carnotaurus had two pronounced horns above the eyes, while Majungasaurus and Rajasaurus had a single bony horn or dome projecting upward. Forelimbs were small and vestigial, with the forearm bones extremely short (25% of humerus length in Carnotaurus). The hand had four basic digits but reduced wrist bones (carpals); the first and fourth digits lacked phalanges, and the second and third digits had only one or two extremely short, immobile finger bones. Manual claws were very small in Eoabelisaurus and totally absent in carnotaurines.

Reader's Guide

Abelisauridae is significant as a major group of predatory dinosaurs that dominated the southern continents during the Cretaceous. Their fossils help reconstruct the biogeography of Gondwana: initially thought to be absent from Africa, the discovery of Rugops and other material from mid-Cretaceous northern Africa showed the group existed before the breakup of Gondwana. Before becoming well known, fragmentary abelisaurid remains were occasionally misidentified as possible South American tyrannosaurids. Their shared skull features with carcharodontosaurids led to suggestions of a relationship, but no cladistic analysis has supported this, and the two groups are otherwise very different. The evolution of vestigial forelimbs in abelisaurids has been attributed to loss of function in the HOXA11 and HOXD11 genes.

Did You Know?

• Abelisaurid forelimbs were so reduced that in carnotaurines, manual claws were totally absent.
• The skull of Carnotaurus was nearly as tall as it was long.
• Fragmentary abelisaurid remains were once misidentified as possible South American tyrannosaurids.
• The earliest possible abelisaurid is Eoabelisaurus from the Middle Jurassic of Argentina, though some researchers consider it a ceratosaurid or abelisauroid outside abelisaurids.