Snakes are the most diverse group of lizards1, 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 marine2,3,4 or terrestrial5,6 environment and how their unique feeding mechanism evolved1,7,8. The Cretaceous Coniophis precedens was among the first Mesozoic snakes discovered9, but until now only an isolated vertebra has been described9,10 and it has therefore been overlooked in discussions of snake evolution. Here we report on previously undescribed material11 from this ancient snake, including the maxilla, dentary and additional vertebrae. Coniophis is not an anilioid as previously thought11; 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 serpentine body and carnivory, snakes evolved a highly specialized, kinetic skull, which was followed by a major adaptive radiation in the Early Cretaceous period. This pattern suggests that the kinetic skull was a key innovation that permitted the diversification of snakes.
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We thank the curators and staff of the American Museum of Natural History, the University of California Museum of Paleontology and the Yale Peabody Museum of Natural History, and the staff of Digimorph at the University of Texas at Austin, for specimen access. We are especially grateful to J. Head for discussions about snake vertebrae, which prevented a major error in the assignment of material to Coniophis. This research was funded in part by US NSF grant DEB-0132227 to J.A.G. N.R.L. was funded by the Yale Institute for Biospheric Studies.
The authors declare no competing financial interests.
This file contains Supplementary Text and Data 1-2, Supplementary Figures 1-2 and additional references. (PDF 554 kb)
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This file contains the Supplementary Character illustrations 155-228. (PDF 3157 kb)
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Longrich, N., Bhullar, B. & Gauthier, J. A transitional snake from the Late Cretaceous period of North America. Nature 488, 205–208 (2012) doi:10.1038/nature11227
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