‘Dinosaurs Among Us’ at American Museum of Natural History, Selected by Mark Dion

These Khaan mckennai dinosaurs are similar to birds in many ways. While these animals are nearly identical, scientists suspect the specimen on the left is a male, based on the presence of large structures beneath its tail that have a triangular, spearheaded shape. Those structures are smaller in the animal on the right, and lack the triangular, or “chevron,” shape, suggesting that the larger structures could have supported the muscles used in a tail-feather display, much like those still put on by birds, including the sage grouse and peacock.

©AMNH/M. ELLISON