Evolution: Theropod dinosaurs put their best foot forward

New data on the shape of toe pads and foot scales of early flying theropods, a group of three-toed dinosaurs that includes Tyrannosaurus Rex, Velociraptor and birds, are presented in a Nature Communications paper this week. The findings may help to improve our understanding of the behaviour and lifestyles of extinct relatives of living birds, including their grasping and hunting abilities.

The shape and size of the feet of living birds are known to correspond to their jumping, perching, wading, swimming, climbing, and grasping capabilities. The shape of claws, bones, and joints can be used to infer the function of these same characteristics in fossils of early flying theropods. Although the function of soft tissues from the feet, such as foot pads and scales, is understood in living birds, they are not often preserved in the fossil record, making it difficult to infer their purpose in extinct species.

Michael Pittman and colleagues examined the soft tissue details from the feet of 12 fossil specimens related to 8 living bird relatives — including small, crow-sized early birds such as Anchiornis and Confuciusornis as well as the close bird relative Microraptor. They then combined these data with measurements of the fossilised claws and bones. The authors suggest that fossils with similar foot adaptations, such as well-developed convex, rounded foot pads found in Microraptor, used their feet for hunting prey in a similar manner to modern raptors, such as hawks. Fossils with flattened foot pads, such as Anchiornis, used their feet for a more ground-based lifestyle, and some species may even have used them for food manipulation as observed in living parrots. However, not all of the examined fossils were an exact match to examples from living birds, suggesting that extinct species such as Sapeornis may have exhibited some behaviours and lifestyles that we cannot identify in living birds today.

The findings suggest that there was unexpected diversity in adaptations related to the behaviour and lifestyle of theropod flyers as their flight capability developed, as well as complex adaptations in non-bird flyers that are similar to living predatory birds.