New theropod vertebral materials from the Upper Cretaceous Iren Dabasu Formation, Inner Mongolia, China

The Upper Cretaceous Iren Dabasu Formation is best known for its abundance of large-bodied herbivorous dinosaurs, whereas small-to medium-sized vertebrates remain comparatively underrepresented. Here we describe eight newly recovered isolated vertebrae that provide additional evidence of taxonomic diversity within this assemblage. One specimen is confidently referred to Troodontidae, providing new evidence for the presence of this clade in the formation. Another is tentatively identified as an ornithomimosaurian axis; although it cannot be confidently referred to any previously reported ornithomimosaurian material from the Iren Dabasu Formation, its relatively small size suggests that it may represent a juvenile individual. The remaining vertebrae are too fragmentary or morphologically ambiguous to permit secure taxonomic assignment, but they nevertheless expand the known range of vertebral morphologies present in the assemblage. Together, these specimens refine current knowledge of the Iren Dabasu vertebrate fauna and highlight the importance of isolated elements for reconstructing faunal diversity in assemblages dominated by large-bodied taxa.