''Aucasaurus'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of medium-sized

abelisaurid Abelisauridae (meaning "Abel's lizards") is a family (or clade) of ceratosaurian theropod dinosaurs. Abelisaurids thrived during the Cretaceous period, on the ancient southern supercontinent of Gondwana, and today their fossil remains are found ...

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

from

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

that lived during the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

(

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. The ...

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...

stage Stage or stages may refer to: Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly British theatre newspaper * Sta ...

) of the

Anacleto Formation The Anacleto Formation is a geologic formation with outcrops in the Argentina, Argentine Patagonia, Patagonian provinces of Mendoza Province, Mendoza, Río Negro Province, Río Negro, and Neuquén Province, Neuquén. It is the youngest formation w ...

. It was smaller than the related ''

Carnotaurus ''Carnotaurus'' (; ) is a genus of theropod dinosaur that lived in South America during the Late Cretaceous period, probably sometime between 71 and 69 million years ago. The only species is ''Carnotaurus sastrei''. Known from a single well-p ...

'', 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 The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...

, and so is relatively well understood, and is the most complete

yet described. However, the skull is damaged, causing some

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

s to speculate that it was involved in a fight prior to death. In 2009, Novas suggested that ''Aucasaurus garridoi'' might be a

junior synonym The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently. * In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Linna ...

of '' Abelisaurus comahuensis''. In 2010,

Gregory S. Paul Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dino ...

renamed ''Aucasaurus garridoi'' into ''Abelisaurus garridoi''. Despite their similarities, other researchers have placed both genera as separate taxa.

Discovery

''Aucasaurus'' is known from finds in the

Río Colorado Subgroup The Río Colorado Subgroup, formerly named as Río Colorado Formation, is a Late Cretaceous (Santonian to Early Campanian) geologic subgroup of the Neuquén Basin in northern Patagonia, Argentina. It belongs to the Neuquén Group and contains the ...

, a

group comprising the

in the

Neuquén Basin Neuquén Basin ( es, Cuenca Neuquina) is a sedimentary basin covering most of Neuquén Province in Argentina. The basin originated in the Jurassic and developed through alternating continental and marine conditions well into the Tertiary. The bas ...

that has yielded many

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s. Numerous

sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their bo ...

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...

s are also known from this deposit.

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 ''

''. Also, instead of horns, it had a pair of low ridges above each eye.

Forelimbs and hands

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 A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "h ...

of ''Aucasaurus'' was unusual: four

metacarpals In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeleton, skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpa ...

were present, but the first and fourth lacked fingers. The second and third had fingers, but they were quite short and had no claws.

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 The University of Alberta, also known as U of A or UAlberta, is a public research university located in Edmonton, Alberta, Canada. It was founded in 1908 by Alexander Cameron Rutherford,"A Gentleman of Strathcona – Alexander Cameron Rutherfor ...

. A latex cranial

endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...

was also made. The forebrain, midbrain, and hindbrain resemble the morphology described for the abelisaurids ''

Majungasaurus ''Majungasaurus'' (; ) is a genus of abelisaurid theropod dinosaur that lived in Madagascar from 70 to 66 million years ago, at the end of the Cretaceous Period (geology), Period, making it one of the last known non-avian dinosaurs that went ex ...

'' and ''

Indosaurus ''Indosaurus'' () is a genus of carnivorous theropod dinosaur that lived in what is now India, about 69 to 66 million years ago during the Maastrichtian division of the Late Cretaceous. The species I. matleyi weighed roughly 700 kg (1540&nbs ...

''. 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''.

Classification

''Aucasaurus'' was closely related to ''Carnotaurus'' and they are united in the

Carnotaurini Carnotaurini is a tribe of the theropod dinosaur family Abelisauridae from the Late Cretaceous period of Patagonia. It includes the dinosaurs ''Carnotaurus sastrei''; the type species, ''Aucasaurus garridoi'', and ''Abelisaurus comahuensis''. Thi ...

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

References