| Summary: | We report the oldest known specimen of the metriorhynchid crocodylomorph genus Geosaurus, and the first specimen discovered from the Lower Kimmeridge Clay Formation of England (Kimmeridgian stage, Rasenia cymodoce Sub-Boreal ammonite Zone). This specimen, an isolated tooth, has the tri-facetted labial surface morphology that is unique to Late Jurassic species of Geosaurus. Its dimensions suggest that it is one of the posterior teeth. Macrowear on the labial surface (enamel spalling) shows evidence of tooth–food abrasion. The presence of Geosaurus near the base of the lower Kimmeridgian, along with the recently re-described middle Oxfordian specimen attributable to Torvoneustes, shows that the subclade Geosaurini evolved and diversified prior to the Kimmeridgian. Members Geosaurini evolved a diverse array of sophisticated feeding mechanisms, including durophagous forms (Torvoneustes), barracuda-like forms with ‘scissor-like’ occlusion (Geosaurus), species reminiscent of false killer whales/Type 1 North Atlantic killer whales (Dakosaurus) and a large genus similar to extant Type 2 North Atlantic killer whales (Plesiosuchus). With the description of the isolated Geosaurus tooth, all four Geosaurini genera are now known from the Lower Kimmeridgian Clay Formation. The craniodental plasticity of Geosaurini, and the astonishing range of feeding mechanisms they evolved, is unparalleled amongst thalattosuchians.
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