These Cretaceous predators evolved to lose their arms through a specific genetic defect.

Nature · Published 3 October 2026 Swipe more like this →

In short: Abelisaurids were formidable carnivorous bipeds that dominated the southern supercontinent of Gondwana. While their skulls were heavily ornamented with pits and ridges, their forelimbs were remarkably vestigial. Paleontologists suggest this dramatic reduction in arm size may have been driven by the loss of function in two specific developmental genes.

Abelisauridae, or Abel's lizards, were a group of ceratosaurian theropods that thrived during the Cretaceous period. First described in 1985 by Jose Bonaparte and Fernando Novas, the family name honors Roberto Abel, the discoverer of the genus Abelisaurus. These dinosaurs were primarily found across the ancient supercontinent of Gondwana, with fossil evidence now spanning Africa, South America, India, Madagascar, and western Europe. While they possibly appeared as early as the Jurassic, they persisted until the end of the Mesozoic era, roughly 66 million years ago.

The anatomy of these predators was distinct. They possessed stocky hind limbs and a blunt, tall snout. Their skulls were often covered in complex grooves, pits, and bony projections, such as the horns seen on Carnotaurus or the domes found on Majungasaurus. These features likely served as tools for intimidation or species recognition. Their forelimbs, however, were extremely reduced. In advanced carnotaurines, the radius and ulna were only a fraction of the length of the upper arm, and the elbow joint was entirely immobile. Paleobiologist Alexander O. Vargas proposed that this vestigiality resulted from a genetic defect involving the HOXA11 and HOXD11 genes, which regulate limb development.

Defining the family has proven complex due to varying taxonomic approaches. Under a node-based definition, the group includes Abelisaurus, Carnotaurus, and their descendants. A stem-based definition, however, is more inclusive, capturing basal forms like Rugops. The group's evolutionary history was once thought to be limited by continental drift, but the discovery of mid-Cretaceous fossils in Africa and the 2014 identification of Arcovenator in France confirmed a much wider distribution. Today, researchers continue to refine these relationships, with recent studies suggesting that some abelisaurids, such as Majungasaurus, grew significantly slower than other theropods, taking nearly two decades to reach full maturity.

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