Description and Classification

Aucasaurus was a medium-sized carnivorous dinosaur, reaching estimated lengths of around 5 to 6 meters (16 to 20 feet) and weighing up to 1,500 kilograms (about 3,300 pounds). Like other members of the Abelisauridae family, it walked on two powerful hind legs and had a short, deep skull. Its head featured rough, bumpy areas of bone above its eyes, but it lacked the prominent horns seen in its close relative, Carnotaurus.

One of the most notable features of Aucasaurus was its incredibly tiny forelimbs. These arms were even smaller in proportion to its body than those of Tyrannosaurus rex or even other abelisaurids like Carnotaurus. The hands were so reduced that the finger bones were fused together, and the arms were likely useless for grasping prey. Skin impressions found with the fossil show that Aucasaurus was covered in small, non-overlapping scales.

Aucasaurus belongs to the theropod group Ceratosauria, and within that, to the family Abelisauridae. This family includes other South American predators such as Abelisaurus, Skorpiovenator, and the horned Carnotaurus. Aucasaurus is considered part of a subgroup called Carnotaurini, which are characterized by their very short snouts and reduced arms. Its nearly complete skeleton helps scientists understand the anatomy of these unique predators better.

Distinguishing Features

Aucasaurus can be identified by several unique characteristics that set it apart from other abelisaurid dinosaurs:

• Extremely reduced forelimbs that were proportionally smaller than those of most other abelisaurids, including Carnotaurus, and were likely non-functional for hunting or grasping.
• The skull, while deep, lacked the large, distinct horns found on Carnotaurus, instead possessing rough, thickened bone (rugosities) above the eyes.
• A relatively more slender build compared to some heavily built abelisaurids.
• Specific openings (fenestrae) in the skull in front of the eyes were proportionally larger compared to many related species.
• The bones of the hand (metacarpals) were fused, and it lacked wrist bones, indicating a very stiff and greatly reduced hand structure.

Paleoenvironment and Diet

Aucasaurus lived in what is now Patagonia, Argentina, during the Late Cretaceous. The environment, represented by the rocks of the Anacleto Member of the Rio Colorado Formation, was a semi-arid landscape with seasonal rivers and extensive floodplains. This region, known as Auca Mahuevo, is famous for being a massive nesting site for titanosaur sauropods, the giant long-necked plant-eating dinosaurs. Aucasaurus shared this environment with these sauropods, as well as other dinosaurs, crocodiles, turtles, and snakes.

As a carnivorous theropod, Aucasaurus was a predator. Its diet likely consisted of smaller dinosaurs, young sauropods (perhaps from the nearby nesting grounds), or other reptiles available in its habitat. While it was a capable hunter, its very small arms meant it relied entirely on its powerful jaws and strong hind legs to catch and kill prey. Like many predators, it might have also scavenged carcasses when opportunities arose.

Significance and Ongoing Research

The discovery of Aucasaurus is highly significant because its skeleton is one of the most complete ever found for an abelisaurid. This exceptional fossil provides paleontologists with detailed information about the anatomy of this group, particularly the extreme reduction of the forelimbs and the structure of the skull. It helps scientists understand the diversity and evolution of theropod dinosaurs in South America during the final chapter of the dinosaur age.

Ongoing research on Aucasaurus continues to refine its place within the abelisaurid family tree. Scientists study its bones to learn more about how it moved, hunted, and lived. Comparing Aucasaurus with other abelisaurids, such as Carnotaurus and newly discovered species, helps paint a clearer picture of how these fascinating predators adapted to their environments. The presence of Aucasaurus at a major sauropod nesting site also offers clues about the interactions between predators and their prey in ancient ecosystems.