Analysis of fossilized remains from dinosaurs known as troodontids reveals how one species' hip and leg bones may have developed over its lifespan, from embryo to adult—with potential implications for resolving longstanding taxonomic challenges. David Varricchio of Montana State University, US, and Heath Caldwell of North Carolina State University, US, present their findings in the open access journal PLOS One on September 9, 2026.
Troodontids were a family of bird-like dinosaurs with long legs, large brains and eyes, and feathers. Fossils indicate they lived in North America in the late Cretaceous period. For decades, a limited amount of fossilized remains hampered understanding of troodontids, but recent new fossil discoveries are advancing the field.
To help fill in the blanks, Varricchio and Caldwell analyzed pelvic and hindlimb fossils collected in western Montana, US, in an area thought to have been a coastal plain when the dinosaurs lived there between about 75 to 78 million years ago. The fossils are thought to represent individuals of the species Troodon formosus at various stages of its lifespan, from embryo to adult, enabling the researchers to reconstruct how a typical individual's hip and leg bones may have developed over time.
The analysis suggests that, during its lifetime, T. formosus underwent specific changes in the texture, shape, and relative proportions of different parts of these bones. For instance, the ratio of femur to tibia length was higher in adult fossils than in juveniles. These changes appear to have occurred at consistent growth stages, suggesting a limited influence of environmental factors. The analysis also suggests that adults varied distinctly in size and, in line with prior research, suggests foot problems may have been common in the species.
The researchers say these findings support designation of a specific subset of the remains—a collection of pelvic and hindlimb fossils from juveniles and adults—as the primary representative, or "type specimen," of T. formosus. This would replace a single tooth that has been the type specimen for decades and could help resolve longstanding concerns about the validity of the Troodon genus. The authors note that this assignment would depend on a decision that is currently pending from the International Commission of Zoological Nomenclature.
Further research could help deepen understanding of the lifestyle of Troodons, such as how they walked or ran.
The authors add: "In this study, we investigated the morphology of the hindlimb and pelvic skeleton for the bird-like dinosaur Troodon formosus from a relatively large sample of embryonic to skeletally mature individuals. Analysis of these bones provided an unprecedented opportunity for understanding the biology of a historically enigmatic group of animals and will serve as a valuable resource for paleontologists for years to come."
Author interview: https://plos.io/4yCXHXB
In your coverage, please use this URL to provide access to the freely available article in PLOS One: https://plos.io/4wRUe65
Citation: Varricchio DJ, Caldwell HR (2026) Pelvic and hindlimb osteology and ontogeny of a troodontid from the upper member of the Two Medicine Formation (Cretaceous) of Montana. PLoS One 21(9): e0356249. https://doi.org/10.1371/journal.pone.0356249
Author countries: USA.
Funding: The author(s) received no specific funding for this work.
