A transitional snake from the Late Cretaceous period of North America

Previously undescribed material from the maxilla, dentary and spine of the Cretaceous Coniophis precedens shows that it is the most primitive known snake. Snakes brought down to earth Fossils of snakes are extremely rare, and the resulting shortage of information has fuelled a heated debate over the origins of this distinctive group. Did they evolve in the sea? Or on land, as suggested by remains of the primitive Cretaceous snake Najash ? A study of the long-neglected remains of the snake Coniophis , originally described by famous dinosaur-hunter Othniel C. Marsh in 1892, has come up with some facts that favour a land-based genesis for snakes. Coniophis was a terrestrial species, with a snake-like body but a primitive, lizard-like head. This suggests that early snakes were burrowers that achieved their elongate form before evolving the characteristic highly mobile skull of modern snakes. Snakes are the most diverse group of lizards 1 , but their origins and early evolution remain poorly understood owing to a lack of transitional forms. Several major issues remain outstanding, such as whether snakes originated in a marine 2 , 3 , 4 or terrestrial 5 , 6 environment and how their unique feeding mechanism evolved 1 , 7 , 8 . The Cretaceous Coniophis precedens was among the first Mesozoic snakes discovered 9 , but until now only an isolated vertebra has been described 9 , 10 and it has therefore been overlooked in discussions of snake evolution. Here we report on previously undescribed material 11 from this ancient snake, including the maxilla, dentary and additional vertebrae. Coniophis is not an anilioid as previously thought 11 ; a revised phylogenetic analysis of Ophidia shows that it instead represents the most primitive known snake. Accordingly, its morphology and ecology are critical to understanding snake evolution. Coniophis occurs in a continental floodplain environment, consistent with a terrestrial rather than a marine origin; furthermore, its small size and reduced neural spines indicate fossorial habits, suggesting that snakes evolved from burrowing lizards. The skull is intermediate between that of lizards and snakes. Hooked teeth and an intramandibular joint indicate that Coniophis fed on relatively large, soft-bodied prey. However, the maxilla is firmly united with the skull, indicating an akinetic rostrum. Coniophis therefore represents a transitional snake, combining a snake-like body and a lizard-like head. Subsequent to the evolution of a s