An unusual theropod frontal from the Upper Cretaceous of north Patagonia

Abstract

We report an isolated left frontal (MCF-PVPH 320) corresponding to a medium-sized theropod dinosaur from the Portezuelo Formation (Coniacian)of northern Patagonia. It shows a unique combination of traits that are not present in any other known Cretaceous theropod from South America. MCF-PVPH 320 is robust and anteroposteriorly short, with a flat and smooth dorsal surface largely excavated by the supratemporal fossa. Endocranially, the olfactory bulb impression is elongate, and the olfactory tract impression is markedly shortened anteroposteriorly. MCF-PVPH 320 differs greatly from the frontals of Late Cretaceous theropods, such as abelisaurids, megaraptorines and carcharodontosaurids. In contrast, character states including the thickness of the bone, V-shaped frontoparietal suture, reduced participation on the orbital margin and markedly short olfactory tract impression suggest the presence of an unknown mid-sized to large allosauroid for the Portezuelo Formation.