古生物学：アルゼンチンの恐竜たちはどのようにして首を長く伸ばしたのか

A new example of a dinosaur known as a sauropodomorph, found in the Andes mountains of Argentina, sheds light on the evolution of early dinosaurs. The dinosaur, dating to around 231 million years ago, shows signs of extensions in the neck bones that may have resulted in the longer-necked sauropodomorphs found in the region from later periods. The findings are reported in Nature this week.

During the Late Triassic (around 237 to 201 million years ago), drastic ecosystem changes took place that led to the rise and diversification of animal groups such as dinosaurs, early relatives of mammals, and the group that includes crocodiles. Much of the evidence of early evolution of dinosaurs is found in South America, with most key discoveries limited to two basin regions in Argentina and Brazil.  

E. Martín Hechenleitner and colleagues report the discovery of an early sauropodomorph in a distinct region: the Santo Domingo Formation in Quebrada Santo Domingo, at an elevation of around 3,000 metres in the Andes of La Rioja, northwestern Argentina. The finding expands the known diversity and geographic range of early dinosaurs and offers new insights into sauropodomorph evolution, the authors claim.

The earliest known sauropodomorphs were around 10 kilograms and had relatively short necks. Neck elongation and increased body mass are two key traits in their early evolution. The new sauropodomorph described by the authors, named Huayracursor, is estimated to have been more than 1.5 metres long and weighed around 18 kilograms. It had a shorter neck relative to later sauropodomorphs, but the neck bones (cervical vertebra) showed signs of elongation. The findings provide one of the oldest pieces of evidence of increased body mass and neck elongation in early sauropodomorphs, the authors conclude.