It has commonly been thought that snakes underwent progressive loss of their limbs by gradual diminution of their use1. However, recent developmental and palaeontological discoveries suggest a more complex scenario of limb reduction, still poorly documented in the fossil record2,3,4,5. Here we report a fossil snake with a sacrum supporting a pelvic girdle and robust, functional legs outside the ribcage. The new fossil, from the Upper Cretaceous period of Patagonia, fills an important gap in the evolutionary progression towards limblessness because other known fossil snakes with developed hindlimbs, the marine Haasiophis, Pachyrhachis and Eupodophis, lack a sacral region. Phylogenetic analysis shows that the new fossil is the most primitive (basal) snake known and that all other limbed fossil snakes are closer to the more advanced macrostomatan snakes, a group including boas, pythons and colubroids. The new fossil retains several features associated with a subterranean or surface dwelling life that are also present in primitive extant snake lineages, supporting the hypothesis of a terrestrial rather than marine origin of snakes.
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We thank O. Rieppel, H. W. Greene, J. C. Rage, F. Novas and A. Scanferla for discussion and review of earlier drafts; M. Reguero, S. Bargo, J. Bonaparte and A. Kramarz for access to material; P. Gallina for finding the holotype; A. Scanferla, A. B. Carvalho and M. Isasi for preparation of the specimens; and L. Lobo for the illustrations. This research was supported by the Jurassic Foundation (to S.A.) and the Fundação de Amparo à Pesquisa do Estado de São Paulo BIOTA/FAPESP (to H.Z.). The Agencia Cultura of Río Negro Province provided the exploration permits. Author Contributions S.A. was Chief Investigator and Head of the excavation campaigns in La Buitrera. H.Z. is responsible for the elaboration of the data matrix and phylogenetic analysis.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
This file includes a list of characters, the data matrix in Nexus format, and a list of apomorphies for the strict consensus of two equally parsimonious trees resulting from the phylogenetic analysis. (PDF 267 kb)
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Apesteguía, S., Zaher, H. A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature 440, 1037–1040 (2006) doi:10.1038/nature04413
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