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Mass extinctions have repeatedly shaped global biodiversity. The Cretaceous-Paleogene (K-Pg) mass extinction caused the demise of numerous vertebrate groups, and its aftermath saw the rapid diversification of surviving mammals, birds, frogs, and teleost fishes. However, the effects of the K-Pg extinction on the evolution of snakes—a major clade of predators comprising over 3,700 living species—remains poorly understood. Here, we combine an extensive molecular dataset with phylogenetically and stratigraphically constrained fossil calibrations to infer an evolutionary timescale for Serpentes. We reveal a potential diversification among crown snakes associated with the K-Pg mass extinction, led by the successful colonisation of Asia by the major extant clade Afrophidia. Vertebral morphometrics suggest increasing morphological specialisation among marine snakes through the Paleogene. The dispersal patterns of snakes following the K-Pg underscore the importance of this mass extinction event in shaping Earth’s extant vertebrate faunas.

12 Fossil snakes are relatively well represented in the Upper Cretaceous of northern Africa, with 13 material known from Morocco, Sudan, Egypt, Libya, Algeria, and Niger. The Moroccan Kem 14 Kem beds yield a particularly diverse snake assemblage, with Simoliophiidae, Madtsoiidae, 15 ?Nigerophiidae and several unnamed taxa co-occurring. These fossils are important for our 16 understanding of the early evolutionary history of snakes, and may shed light on the ecology 17 and initial diversification of basal snakes. We describe a new taxon, Norisophis begaa gen. et 18 sp. nov., from the Kem Kem beds of Begaa, in southeast Morocco. It is characterised by a 19 marked interzygapophyseal constriction, parazygantral foramina, an incipient 20 prezygapophyseal process, and an anterio-posteriorly short centrum. Several characteristics 21 shared with Najash, Seismophis, Madtsoiidae, and Coniophis suggest that Norisophis is a stem 22 ophidian. N. begaa further increases the diversity and disparity of snakes within the Kem Kem 23 beds, supporting the hypothesis that Africa was a mid-Cretaceous hotspot for snakes. 24 25