Phylogeny of a new gigantic paravian (Theropoda; Coelurosauria; Maniraptora) from the Upper Cretaceous of James Ross Island, Antarctica

Abstract

A description with phylogenetic analyses is provided for Imperobator antarcticus, gen. et sp. nov., an early Maastrichtian, basal paravian (Theropoda; Maniraptora) from the Naze Peninsula, James Ross Island, Antarctica. In 2003, researchers uncovered the remains of a theropod later referred to Dromaeosauridae. Dromaeosaurids are a clade of maniraptorans including well-known members such as Velociraptor and Deinonychus. The specimen displays a case of gigantism in paravians, a condition best documented in the dromaeosaurids Achillobator, Austroraptor, Dakotaraptor, and Utahraptor. In addition to certain morphological traits that differ from the dromaeosaurid norm, the smooth surface of the distal metatarsal II prevents referral of the ‘Naze Theropod’ to dromaeosaurids (ginglymoidy of the distal surface of metatarsal II being considered an unambiguous synapomorphy of dromaeosaurids). The specimen also lacks a hypertrophied ungual of the second pedal digit, and is surprisingly small in comparison with those of equivalently sized dromaeosaurids such as Utahraptor. A heuristic search and Bayesian phylogenetic analysis were performed, providing the first phylogenetic analyses of this enigmatic theropod. Both analyses support a placement of this taxon within Paraves, a clade which includes Dromaeosauridae, Troodontidae, and Avialae (birds). Despite previous referral to ‘Deinonychosauria,’ placement near or within any of the three major paravian subclades could not be retained. We also offer the first biostratigraphic placement of the Campanian–Maastrichtian, non-avian Antarctic dinosaurs and can, with confidence, determine the contemporaneous nature of the latest Cretaceous, dinosaur fauna in Antarctica.