Cranial osteology of a new specimen of Allosaurus Marsh, 1877 (Theropoda: Allosauridae) from the Upper Jurassic of Portugal and a specimen-level phylogenetic analysis of Allosaurus

Abstract Allosaurus fragilis was first described in the Upper Jurassic of Portugal in 1999 based on a set of postcranial remains collected at the Andrés fossil site in Pombal. The description of a second species, Allosaurus europaeus, based on a posterior part of an articulated skull and a sequence of cervical vertebrae (ML 415) collected at Praia de Vale Frades in Lourinhã, has been controversial, and its validity has been challenged. A detailed description of a new sample of cranial fossil material collected at Andrés that can be confidentially attributed to Allosaurus is presented. In addition, a comprehensive phylogenetic assessment of the Morrison Formation fossil record of Allosaurus is also performed. This analysis indicates that only two valid Allosaurus species were present in the Late Jurassic ecosystems of North America: Allosaurus fragilis and Allosaurus jimmadseni. An emended diagnosis for Allosaurus fragilis is here proposed. The Portuguese specimens MNHN/UL.AND.#, ML 415, and MG 27804 are here interpreted as belonging to Allosaurus fragilis. This interpretation has important implications to better understand the evolutionary history of the theropod fauna of the Lusitanian Basin, supporting the existence of close biogeographic relationships between the landmasses on both sides of the proto-North Atlantic Ocean during the Kimmeridgian–Tithonian.