South American Mesozoic snake diversity is mostly represented by genera from the Cenomanian (Najash), Santonian–Campanian (Dinilysia), and Campanian–Maastrichtian (Alamitophis, Patagoniophis, Rionegrophis, and Australophis) of Patagonia, Argentina. In this paper, we describe a new snake genus and species, Seismophis septentrionalis, from the Cenomanian (early Late Cretaceous) of the Alcântara Formation, Maranhão, northeastern Brazil. The new snake comprises a posteriormost trunk vertebra and possibly a poorly preserved midtrunk vertebra. Both vertebrae share small size, zygosphene moderately thick with a rectilinear roof, absence of paracotylar foramina, presence of parazygantral foramina, and strongly marked parasagittal ridges of the neural arch. The new snake is here considered of uncertain systematic affinities, but probably close to the limbed snake Najash rionegrina. Although the material is very fragmentary and the systematic assignment is still unresolved, this snake represents the oldest, as well as probably the most primitive snake from Brazil.

Key words: Reptilia, Squamata, Ophidia, snakes, Cretaceous, Alcântara Formation, Brazil.

Anne. S. Hsiou [anniehsiou@ffclrp.usp.br], Departamento de Biologia, FFCLRP, Universidade de São Paulo, Av. Bandeirantes 3900, 14040-901, Ribeirão Preto, SP, Brasil; Adriana M. Albino [aalbino@mdp.edu.ar], CONICET, Departamento de Biología, Universidad Nacional de Mar del Plata, Funes 3250, 7600 Mar del Plata, Argentina; Manuel A. Medeiros [manuel.açfredo@ufma.br], Ronny A.B. Santos [rooneysmall@yahoo.com.br], Departamento de Biologia, Centro de Ciências Biológicas e da Saúde, Universidade Federal do Maranhão (UFMA), Avenida dos Portugueses, s/n0, Campus Universitário do Bacanga, CEP. 65085-580, São Luis, MA, Brasil.