Birds are a group of warm-blooded vertebrates that have adapted the ability to fly. Some defining characteristics of birds are feathers, hollow bones, and a beak in place of teeth. Birds evolved from Theropod dinosaurs some time during the Late Triassic Period or the Jurassic Period. The earliest known bird, Archaeopteryx, shows charactersitics of both birds and dinosaurs. Archaeopteryx fossils are dated at around 150 million years ago. By the end of the Cretaceous Period, birds had become a highly successful and diverse group of animals.