Aucasaurus was a genus of medium sized theropod dinosaur from Argentina that lived during the Santonian – Campanian stage of the Anacleto Formation. It was smaller than the related Carnotaurus, although more derived in some ways, such as its extremely reduced arms and almost total lack of fingers. The type skeleton is complete to the thirteenth caudal vertebra, and so is relatively well understood, and is the most complete abelisaurid yet described. However, the skull is damaged, causing some paleontologists to speculate that it was involved in a fight prior to death.
Aucasaurus short, deep-snouted skull was not as short or as deep-snouted as that of Carnotaurus. Also, instead of horns, it had a pair of low ridges above each eye.
In 2010, Gregory S. Paul estimated its body length at 5.5 metres, its weight at 700 kilograms. In 2016, its length was estimated to be 6.1 metres (20 ft) in a comprehensive analysis of abelisaur size.
Aucasaurus small arms were also alike that of its horned relative, but were proportionally longer due to its small size, and the bones lacked the bony processes and some unusual proportions present in Carnotaurus. The hand of Aucasaurus was unusual: four metacarpals were present, but the first and fourth lacked fingers. The second and third had fingers, but they were quite short and had no claws.
Aucasaurus is known from finds in the Rio Colorado Formation, a Late Cretaceous formation in Argentina that has yielded many dinosaur fossils. Numerous sauropod eggs are also known from this deposit.