Pelvic and hindlimb osteology and ontogeny of a troodontid from the upper member of the Two Medicine Formation (Cretaceous) of Montana

Troodontid theropod dinosaurs from the Campanian of western North America have historically been known from fragmentary remains. This limited material has made understanding the biology and taxonomy of this clade challenging. Here we describe the pelvic and hindlimb osteology for a troodontid from the Flagg Butte Member, the uppermost unit, of the Two Medicine Formation of western Montana, USA. This sample of embryonic to skeletally mature material provides a nearly complete picture of this part of the skeleton. In addition to providing details of anatomy for this taxon, information on individual variation and pathology can also be assessed. This material has historically been referred to as Troodon formosus and we follow the recent proposal to formally establish this taxonomic assignment. As is frequently reported in troodontid literature, many of the pedal elements for T. formosus in this sample are potentially pathologic. Overall element morphology is relatively consistent through ontogeny, with a majority of major skeletal landmarks being observable across the growth series. This observed invariance through growth in Troodon further constrains our understanding of intraspecific variation for troodontids and justifies the taxonomic uniqueness of potentially immature specimens (e.g., Philovenator curriei ). The major ontogenetic changes that do occur (element robustness, skeletal fusion, surface texture) appear to happen at consistent growth stages, potentially reflecting low ontogenetic plasticity. The material described here provides additional opportunities for researching large-bodied troodontids in North America.