Aucasaurus

Aucasaurus is a genus of medium-sized theropod dinosaur from Argentina that lived during the Late Cretaceous (Santonian to Campanian stage) of the Anacleto Formation.[1] 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.[2] 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
Temporal range: Santonian-Campanian
~85–80 Ma
↓
Mounted skeleton reconstruction
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: †Abelisauridae
Tribe: †Carnotaurini
Genus: †Aucasaurus
Coria et al., 2002
Type species
Aucasaurus garridoi
Coria et al., 2002
Synonyms

In 2009, Novas suggested that Aucasaurus garridoi might be a junior synonym of Abelisaurus comahuensis.[3] In 2010, Gregory S. Paul renamed Aucasaurus garridoi into Abelisaurus garridoi.[4] This has found no acceptance.

Description
Reconstruction
Size compared to a human

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.[5]

In 2010, Gregory S. Paul estimated its body length at 5.5 metres, its weight at 700 kilograms.[6] In 2016, its length was estimated to be 6.1 metres (20 ft) and at least 1.5 tons in weight (comparable to Skorpiovenator) but heavier than the other genus Ilokelesia, which was estimated to be 200-240 kilograms in weight. in a comprehensive analysis of abelisaur size.[7]

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 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.[5]

Braincase
3D scan of the 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.[8]

Discovery

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.[5]

Classification

Aucasaurus was closely related to Carnotaurus and they are united in the Carnotaurini.[5]

Below is a cladogram by Canalle et al. in 2009.[9]

Carnotaurinae
 

Majungasaurus

 
Brachyrostra
Carnotaurini
 

Aucasaurus

 
 

Carnotaurus

 
 
 
 

Ilokelesia

 
 
 

Skorpiovenator

 
 

Ekrixinatosaurus

 
 
 
 
 

See also

References
  1. The Theropod Database on Aucasaurus
  2. Coria, R. A.; Chiappe, L. M.; Dingus, L. (2002). "A new close relative of Carnotaurus sastrei Bonaparte 1985 (Theropoda: Abelisauridae) from the Late Cretaceous of Patagonia". Journal of Vertebrate Paleontology. 22 (2): 460. doi:10.1671/0272-4634(2002)022[0460:ANCROC]2.0.CO;2.
  3. Novas, F.E. (2009). The Age of Dinosaurs in South America. Indiana University Press. p. 281. ISBN 9780253352897.
  4. Paul, G.S. (2010). The Princeton Field Guide to Dinosaurs. Princeton University Press. ISBN 9781400836154.
  5. Benton, Michael J. (2012). Prehistoric Life. Edinburgh, Scotland: Dorling Kindersley. p. 320. ISBN 978-0-7566-9910-9.
  6. Paul, G.S., 2010. The Princeton Field Guide to Dinosaurs. Princeton University Press. p. 79
  7. Grillo, O. N.; Delcourt, R. (2016). "Allometry and body length of abelisauroid theropods: Pycnonemosaurus nevesi is the new king". Cretaceous Research. 69: 71–89. doi:10.1016/j.cretres.2016.09.001.
  8. Paulina-Carabajal, A.; Succar, C. (2015). "The endocranial morphology and inner ear of the abelisaurid theropod Aucasaurus garridoi". Acta Palaeontologica Polonica. doi:10.4202/app.2013.0037.
  9. Canale, J. I.; Scanferla, C. A.; Agnolin, F. L.; Novas, F. E. (2008). "New carnivorous dinosaur from the Late Cretaceous of NW Patagonia and the evolution of abelisaurid theropods" (PDF). Naturwissenschaften. 96 (3): 409–414. Bibcode:2009NW.....96..409C. doi:10.1007/s00114-008-0487-4. hdl:11336/52024. PMID 19057888.
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