A new megaraptoran dinosaur from the Lower Cretaceous Wessex Formation, UK, and its implications for theropod evolution and biogeography

Abstract

Abstract Theropod dinosaur remains from the Barremian (Lower Cretaceous) Wessex Formation of the Isle of Wight, UK, are rare and often fragmentary. Here, we present a previously undescribed braincase from this formation that represents a new coelurosaurian, Invictibellatrix amberina gen. et sp. nov. Phylogenetic analysis and comparative anatomy indicate that Invictibellatrix is distantly related to all previously named theropods from this stratigraphic unit. Theropod braincases are often regarded as morphologically and phylogenetically conservative, but we demonstrate that neurocranial characters provide essential data that can help clarify the phylogenetic relationships of theropods. The sub-circular occipital condyle, sub-oval foramen magnum and presence of a sagittal nuchal crest indicate that Invictibellatrix is referable to Megaraptora, with phylogenetic analyses recovering it as an early-diverging member of this clade, within a paraphyletic array of Eurasian taxa. Incorporation of Invictibellatrix into a biogeographical analysis suggests that non-megaraptorid megaraptorans likely had a cosmopolitan distribution in the Early Cretaceous, and that the absence of megaraptorans in the Late Cretaceous of Laurasia is best explained by regional extinction. The recognition of this new genus increases our knowledge of Early Cretaceous European coelurosaurian diversity, which now includes tyrannosauroids, maniraptoriforms and megaraptorans.