Fossils showing the development of a bird-like dinosaur called Troodon formosus from embryo to adult have given scientists new clues about how this species may have grown. Troodon was a small, agile dinosaur with long legs, a large brain, and feathers—features that made it closely related to modern birds. Researchers David Varricchio from Montana State University and Heath Caldwell from North Carolina State University analyzed a set of fossils found in western Montana, which are believed to represent Troodon formosus individuals at various stages of life. Their findings were published in the journal PLOS One on September 9, 2026. The study looked at changes in the hip and leg bones of Troodon as it matured. For example, the ratio of the femur (thigh bone) to the tibia (shin bone) was found to be higher in adult fossils compared to those of juveniles. These changes occurred consistently across different growth stages, indicating that the development of these bones was largely determined by biological factors rather than environmental ones. The research also noted that adult Troodon individuals varied in size, and the species may have commonly suffered from foot-related issues, as suggested by previous studies. The researchers suggest that a specific group of fossils—comprising pelvic and hindlimb remains from both juvenile and adult Troodon—should be designated as the primary representative, or type specimen, of the species. This would replace a single tooth that has been the official type specimen for Troodon for many years. Doing so could help clarify uncertainties about the validity of the Troodon genus, which has been a topic of debate among scientists. However, this change depends on a decision from the International Commission of Zoological Nomenclature, which oversees the formal naming of species. Further research on Troodon fossils could provide more insight into its behavior and movement, such as how it walked or ran. The researchers emphasized that their study of the hindlimb and pelvic bones of Troodon formosus offers a rare opportunity to understand the biology of a group of animals that has long been mysterious. These findings are expected to be a valuable resource for paleontologists in the years ahead.