Aucasaurus Garridoi By Paleocolour

''Aucasaurus'' is a genus of medium-sized abelisaurid theropod dinosaur from Argentina that lived during the Late Cretaceous (Santonian to 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 was the most complete abelisaurid known when described in 2002. However, the skull is damaged, causing some paleontologists to speculate that it was involved in a fight prior to death.

Discovery

The holotype of ''Aucasaurus'' is known from finds in the Río Colorado Subgroup, a Late Cretaceous group comprising the Anacleto Formation in the Neuquén Basin of Argentina that has yielded many dinosaur fossils. Numerous sauropod eggs are also known from this deposit. The type specimen belongs to a mature individual of at least eleven years old.

Description

''Aucasaurus'' was a medium-sized abelisaur, measuring long and weighing approximately . Its 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.

Postcranial skeleton

The small arms of ''Aucasaurus'' were also like 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. Traits found in the caudal vertebrae suggests a significant caudal musculature and rigidity, possibly indicating that it could have been a fast runner.

Braincase

A study was done on the braincase of ''Aucasaurus'' in 2015 by Ariana Paulina-Carabajal and Cecilia Succar, in which the skull material was scanned using a medical CT machine. Virtual three-dimensional inner ear and cranial endocasts were obtained and visualized using the imagine software at the University of Alberta. A latex cranial endocast was also made. The forebrain, midbrain, and hindbrain resemble the morphology described for the abelisaurids ''Majungasaurus'' and ''Indosaurus''. However, ''Aucasaurus'' exhibits a floccular process that is relatively larger than that of ''Majungasaurus''. In ''Aucasaurus'' the flocculus is enclosed in an 8-shaped floccular recess, similar in shape and size to that observed in ''Abelisaurus'', suggesting that the two Patagonian taxa were capable of a slightly wider range of movements of the head. The labyrinth of the inner ear is similar in shape and size to the semicircular canals of ''Majungasaurus'', although the lateral semicircular canal is shorter in ''Aucasaurus''.

Pathology

The 5th and 6th holotype tail vertebrae of ''Aucasaurus'' provide evidence of failed developmental vertebral segmentation, so two of the vertebrae are fused. Baiano ''et al''. (2024) interpret this as a congenital disorder (birth defect) called block vertebrae. This represents the earliest known occurrence of this pathology in the non-avian theropod fossil record.

Classification

In 2009, Novas suggested that ''Aucasaurus garridoi'' might be a junior synonym of ''Abelisaurus comahuensis''. In 2010, Gregory S. Paul renamed ''Aucasaurus garridoi'' into ''Abelisaurus garridoi''. Despite their similarities, other researchers have placed both genera as separate genera, and subsequent studies suggest that ''Aucasaurus'' was more closely related to other taxa such as ''Carnotaurus'', with some including them in the Carnotaurini.

Below is a cladogram by Canalle ''et al.'' in 2009.

See also

* Timeline of ceratosaur research

References

{{Portal bar, Dinosaurs

Abelisauridae
Dinosaur genera
Santonian dinosaurs
Anacleto Formation
Fossil taxa described in 2002
Taxa named by Rodolfo Coria
Taxa named by Luis M. Chiappe
Dinosaurs of Argentina