Austrocheirus: Upper Cretaceous Dinosaurs

Austroheir (lat.: Austrocheirus) a genus of theropod dinosaurs of the late Upper Cretaceous period from the superfamily Abelisauroids (Abelisauroidea), which lived on the territory of modern South America.

So far, only one highly fragmentary specimen of this dinosaur is known, found in Argentina in the Late Upper Cretaceous layer (Lower Maastrichtian). This genus, represented by a single species of Austrocheirus isasii, was first scientifically described in 2010 by a group of paleontologists led by Martin Ezcurra.

The generic name “Austrocheirus” (Austrocheir; from the Greek words austros “southern” and cheirus “brush”) can be literally translated as “southern brush”, which indicates that the hallmark of this genus of dinosaurs that lived in the south is precisely forelimb brush.

The specific name “isasii” was given in honor of Marcelo Isasi (Marcelo Isasi), who for many years was the preparator of paleontological finds, and also discovered the fossil remains of this theropod.

So far, the only specimen of the species Austrocheirus isasii was found in March 2002 in the southwestern part of the Argentine province of Santa Cruz in the Arroyo Seco Lowland (Hoyada Arroyo Seco), near the western bank of the Rio La Leona River. The fossils were found in the Pari Aike Formation (Lower Maastrichtian).

Other dinosaur remains found from this formation include the bones of the basal ornithopod Talenkauen, the titanosaurus Puertasaurus, and the basal tetanur Orkoraptor.

The fossil find (holotype, specimen number: MPM-PV 10003) consists of fragments of caudal vertebrae, the third metacarpal bone of the left hand, one bone of the third finger of the left cyst, the lower end of the tibia, and a fragment of the third metatarsal bone of the left foot. The bones were found in an area of ​​2 square meters. Since they are all commensurate with each other, and no remains of other terrestrial vertebrates have been found, all bones found can be attributed to one single individual.

Austrocheir was a medium-sized theropod. According to experts, the length of his body was 7-8 meters. While derived members of the Abelisauridae family, such as Majungasaurus, Aucasaurus, and Carnotaurus, had proportionately very short, reduced forelimbs, they were not degeneratively small in Austrocheir.

Thus, Austrocheir is so far the only known medium-sized Late Cretaceous abelisauroid with unshortened forelimbs. Based on this, the researchers conclude that the reduction of the forelimbs of derivatives of abelisaurids is not directly related to the larger size of their body, but is explained by apomorphy in representatives of this family.

The structure of the third metacarpal bone can be used to estimate the approximate age of an individual. So, the outer layer of this bone has several lines of reduced growth located very close to each other this sign shows that we are talking about an adult. However, since the outer layer of bone has not yet been completely transformed by secondary bone plates, this individual was probably not too old.

This genus of dinosaurs differs from others in a number of unique anatomical features: for example, the third metacarpal bone (lat.: Metacarpale III) has a dorsoventrally narrowed diaphysis (bone body), while there is a longitudinal ridge on the bones of the toes (phalanges).

Austrocheir is the basal member of the abelisauroids, a superfamily within the infraorder Ceratosauria. However, the phylogenetic analysis performed by Escurra and colleagues in 2010 does not provide precise answers to the question of relationships with other basal abelisauroids.

According to the results of this analysis, Austrocheir forms a polytomy with the genera Noasaurus, Ligabueino, Masiakasaurus, Elaphrosaurus, and the Abelisaurid family.

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