This 100 million-year-old snake had hind legs and a lost bone that changes evolution

A nearly 100 million-year-old fossil of Najash rionegrina, an ancient rear-limbed snake from Argentina, has provided new insights into snake evolution, challenging long-held assumptions about their origins. Unlike modern snakes, this species retained hind legs and a cheekbone known as the jugal bone, which has nearly disappeared in contemporary snakes. The fossil’s well-preserved skull and skeleton suggest that early snakes were large, wide-mouthed predators rather than small, burrowing creatures as previously believed. The discovery emerged from a collaboration between paleontologists in Argentina and the University of Alberta, who used micro-computed tomography (micro-CT) scanning to examine the fossil in unprecedented detail. This technology allowed researchers to visualize internal skull structures, including nerves and blood vessels, without damaging the specimen. The detailed imagery resolved longstanding anatomical debates, particularly regarding the presence and role of the jugal bone in early snakes and their relatives. Najash rionegrina belonged to a lineage that lived in Northern Patagonia during the Cretaceous period, connected to ancient snake groups distributed across the southern supercontinent Gondwana. The fossil evidence indicates that these early snakes retained their hindlimbs for an extended evolutionary period before the emergence of the mostly limbless modern snakes. This challenges the traditional view that snakes evolved from small, burrowing ancestors, instead supporting the idea that their forebears were large-bodied predators with wide mouths. These findings not only reshape the narrative of snake evolution but also clarify the sequence of bone loss and skull specialization that led to the highly adapted forms seen today. By filling gaps in the fossil record, the study provides a clearer understanding of the anatomical transformations snakes underwent over millions of years, offering a more nuanced perspective on the evolutionary history of one of the most distinctive reptile groups.