Description and Classification

Austrocheirus was a medium-sized carnivorous dinosaur, likely reaching lengths of around 5 to 6 meters (approximately 16 to 20 feet). As a theropod, it would have walked on two legs and possessed sharp claws and teeth, typical of predatory dinosaurs. The known fossil material, though limited, provides some clues about its build. The hand bones (metacarpals) of Austrocheirus are relatively long and slender compared to many other theropods from the same time and region, such as the robust-handed abelisaurids like Carnotaurus.

The classification of Austrocheirus has been a subject of discussion among paleontologists due to the fragmentary nature of its remains. When first described, it was not assigned to any specific major theropod group with certainty. The scientists who named it suggested it might be a basal (meaning it branched off early in the group’s history) member of the Tetanurae, a large group that includes most theropods like allosaurs, tyrannosaurs, and even birds. However, Austrocheirus was considered distinct from groups like the abelisaurids, which were common in South America. Some features of its hand and foot are unique, making it difficult to place. It is not considered a close relative of the giant carcharodontosaurids like Giganotosaurus, nor clearly an abelisaurid. Further fossil discoveries are needed to clarify its exact position within the theropod family tree.

Distinguishing Features

Austrocheirus is set apart by a few features, primarily observed in its hand and foot bones:

• Relatively long and slender metacarpals (hand bones) compared to the more robust metacarpals seen in contemporary South American abelisaurid theropods.
• The third metacarpal (one of the bones in the palm of the hand) is notably long, being more than 75% the length of the second metacarpal.
• Specific characteristics of its metatarsals (foot bones leading to the toes) and pedal phalanges (toe bones) also contribute to its distinctiveness, though these are more technical details for scientists.
• Overall, its hand structure suggests it was different from the typically short-armed and reduced-fingered abelisaurids that dominated the southern continents during the Late Cretaceous.

Paleoenvironment and Diet

Austrocheirus lived in Patagonia, Argentina, during the Maastrichtian age, the very end of the Late Cretaceous period. The fossils were discovered in rocks of the Cerro Fortaleza Formation. This region, during that time, would have consisted of plains with river systems, supporting a diverse ecosystem of plants and animals. The climate was likely temperate to warm.

As a theropod dinosaur, Austrocheirus was a carnivore, meaning it ate meat. Its diet would have consisted of other animals living in its environment. Given its medium size, it might have preyed upon smaller herbivorous dinosaurs, young individuals of larger dinosaur species, or other vertebrates. The specific animals it hunted are unknown, but South America at this time was home to a variety of plant-eating dinosaurs like ornithopods and titanosaurs, as well as other reptiles.

Significance and Ongoing Research

The discovery of Austrocheirus is significant because it adds to the known diversity of theropod dinosaurs in South America during the final stages of the Cretaceous period. It represents a theropod lineage that is not clearly an abelisaurid, which were the dominant large predators in Gondwana (the ancient southern supercontinent) at the time. This suggests that other types of theropods, perhaps representing earlier evolutionary branches or distinct groups, persisted alongside them.

Ongoing research primarily revolves around trying to better understand its evolutionary relationships. The limited nature of the known fossils means that its placement within the Theropoda group is still uncertain. Paleontologists continue to compare its features with newly discovered theropods and re-evaluate existing ideas about theropod evolution. The discovery of more complete specimens of Austrocheirus would be crucial to definitively classify it and understand its anatomy, behavior, and role in the Late Cretaceous ecosystems of Patagonia. It highlights how much there is still to learn about the diversity of dinosaurs just before the mass extinction event that ended the Mesozoic Era.