Reassessment of the enigmatic Late Cretaceous theropod dinosaur, Bagaraatan ostromi

Abstract The Late Cretaceous Bagaraatan ostromi, described by Osmólska in 1996, is one of the most enigmatic theropod dinosaurs. The holotype possesses a peculiar combination of features, which Osmólska suggested were indicative of a primitive position among theropods that could not be resolved further. Other researchers have pointed to affinities with either derived bird-like coelurosaurs or tyrannosaurids. Here, we reanalyse all material collected by Osmólska, which reveals it to be a chimaera of multiple theropod taxa. The femur, tibiotarsus, one pedal phalanx, and most of the bones undescribed in Osmólska’s paper are identified as Caenagnathidae indet. The mandible, cervical vertebrae, pelvis, tail, and one pedal phalanx, which we consider the holotype of B. ostromi, show tyrannosaurid affinities, which are here supported by phylogenetic analyses. We find only two potentially unique diagnostic features of the Bagaraatan holotype: double surangular foramina and a horizontal ridge on the lateral surface of the postacetabular process of the ilium. Both, however, may be ontogenetically or intraspecifically variable, and thus we conclude that the holotype of B. ostromi represents an indeterminate tyrannosaurid. The small size of the holotype and its possession of many features known to characterize juvenile Tyrannosaurus rex indicate that the skeleton belongs to a juvenile, which is one of the smallest juvenile tyrannosaurids currently known.