| Aucasaurus | |
|---|---|
 | |
| Mounted skeleton reconstruction | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Abelisauridae |
| Tribe: | †Carnotaurini |
| Genus: | †Aucasaurus Coriaet al., 2002 |
| Type species | |
| †Aucasaurus garridoi Coriaet al., 2002 | |
| Synonyms | |
| |
Aucasaurus (meaning 'Auca Mahuevo lizard') is agenus of medium-sizedabelisauridtheropoddinosaur fromArgentina that lived during theLate Cretaceous (Santonian toCampanianstage) of theAnacleto Formation.[1] It was smaller than the relatedCarnotaurus, although more derived in some ways, such as its extremely reduced arms and almost total lack of fingers.[2]The type skeleton is complete to the thirteenthcaudal vertebra, and so is relatively well understood, and was the most completeabelisaurid known when described in 2002. However, the skull is damaged, causing somepaleontologists to speculate that it was involved in a fight prior to death.[3]
The holotype ofAucasaurus is known from finds in theRío Colorado Subgroup, aLate Cretaceous group comprising theAnacleto Formation in theNeuquén Basin ofArgentina that has yielded manydinosaurfossils. Numeroussauropodeggs are also known from this deposit.[4] The type specimen belongs to a mature individual of at least eleven years old.[5]
Aucasaurus was a medium-sized abelisaur, measuring 5.5–6.2 m (18–20 ft) long and weighing approximately 700 kg (1,500 lb).[6][7][8] Its skull was not as short or as deep-snouted as that ofCarnotaurus. Also, instead of horns, it had a pair of low ridges above each eye.[4]
The small arms ofAucasaurus 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 inCarnotaurus. Thehand ofAucasaurus was unusual: fourmetacarpals were present, but the first and fourth lacked fingers. The second and third had fingers, but they were quite short and had no claws.[4] Traits found in the caudal vertebrae suggests a significant caudal musculature and rigidity, possibly indicating that it could have been a fast runner.[9]
A study was done on the braincase ofAucasaurus 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 theUniversity of Alberta. A latex cranialendocast was also made. The forebrain, midbrain, and hindbrain resemble the morphology described for the abelisauridsMajungasaurus andIndosaurus. However,Aucasaurus exhibits a floccular process that is relatively larger than that ofMajungasaurus. InAucasaurus theflocculus is enclosed in an 8-shaped floccular recess, similar in shape and size to that observed inAbelisaurus, 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 ofMajungasaurus, although the lateral semicircular canal is shorter inAucasaurus.[10]
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The 5th and 6th holotype tail vertebrae ofAucasaurus provide evidence of failed developmental vertebral segmentation, so two of the vertebrae are fused. Baianoet al. (2024) interpret this as acongenital disorder (birth defect) calledblock vertebrae. This represents the earliest known occurrence of thispathology in the non-avian theropod fossil record.[11]
In 2009, Novas suggested thatAucasaurus garridoi might be ajunior synonym ofAbelisaurus comahuensis.[12] In 2010,Gregory S. Paul renamedAucasaurus garridoi intoAbelisaurus garridoi.[6] Despite their similarities, other researchers have placed both genera as separate genera, and subsequent studies suggest thatAucasaurus was more closely related to other taxa such asCarnotaurus,[9] with some including them in theCarnotaurini.[13][14][15]
Below is a cladogram by Canalleet al. in 2009.[14]
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