I report here on the discovery of the most complete megalosaurid skull known to date, from the Calcaire de Caen Formation of northwesten France. The specimen, which represents a new species is questionably referred to Poekilopleuron. It includes a nearly complete, unornamented skull, characterized by its low lateral profile. In addition to its historical significance, it provides additional characters that are crucial for reconstructing the phylogeny and biogeography of basal theropods. Poekilopleuron? valesdunensis new species is closely related to Eustreptospondylus and Afrovenator, and is united with Torvosaurus in the family of Megalosauridae. The Megalosauridae appear less derived than the Allosauroidea but resemble the Spinosauridae, with which they form the sister-group to the Neotetanurae. The proposed phylogenetic analysis hypothesizes two previously unrecognized major radiations of carnivorous dinosaurs.