First dinosaur remains from Serbia: Sauropod and theropod material from the uppermost Cretaceous (Maastrichtian) of Osmakovo

Diverse and abundant continental vertebrate assemblages are known from the Upper Cretaceous (Campanian–Maastrichtian) of several European countries. They formed on an island archipelago and are marked by their distinctive combination of unusual endemic groups and immigrants from North America, Asia and Gondwana. Although dinosaur fossils have been described from nearby Bulgaria, body fossils of continental vertebrates were previously unknown from the Mesozoic of Serbia. Here, we describe the dinosaurian component of a new vertebrate assemblage from the uppermost Cretaceous (Maastrichtian) of Osmakovo in southeastern Serbia. A single larger bone represents the partial left ulna of a sauropod dinosaur. Its anatomy is suggestive of titanosaurian affinities, although it appears distinct from the titanosaurs Magyarosaurus and Petrustitan from the Maastrichtian of Romania. The small size of this element may indicate its juvenile nature or that it represents an island dwarf similar to Magyarosaurus, but distinguishing these possibilities will require histological work. Sixteen isolated theropod teeth, some incomplete, were recovered by screenwashing and studied using comparative anatomy and machine learning analysis. Nine of these teeth are referred to Dromaeosauridae based on both approaches. Three teeth that are strongly recurved and lack serrations were identified as Aves by machine learning but might alternatively represent dromaeosaurid premaxillary teeth and are thus referred to Paraves. The remaining teeth comprise one specimen of the enigmatic Paronychodon and three too poorly preserved to identify beyond Theropoda. This assemblage shows similarities to contemporaneous assemblages from Romania and suggests the potential for further discoveries in the Mesozoic of Serbia.