WASHINGTON - Large, meat-eating dinosaurs shrank over a period of 50 million years of evolution to become today's modern birds, a study showed Thursday.
Fossilized bones of a gigantic theropod dinosaur, Gigantorraptor erlianensis, are displayed on June 13, 2007 in Beijing
"Birds out-shrank and out-evolved their dinosaurian ancestors, surviving where their larger, less evolvable relatives could not," said lead author Michael Lee, a professor at the University of Adelaide's School of Earth and Environmental Sciences.
The bipedal, carnivorous dinosaurs dubbed theropods -- a category that included Tyrannosaurus Rex and Velociraptor -- were the only branch of dinosaurs to keep getting smaller and smaller, researchers said.
"Being smaller and lighter in the land of giants, with rapidly evolving anatomical adaptations, provided these bird ancestors with new ecological opportunities, such as the ability to climb trees, glide and fly," he explained.
"Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins."
These bird ancestors developed new adaptations, like feathers and wings, as much as four times more rapidly than other dinosaurs, said co-author Darren Naish, a paleontologist at Britain's University of Southampton.
The study examined more than 1,500 anatomical traits of dinosaurs as a way to reconstruct an evolutionary family tree.
The researchers used sophisticated mathematical models to trace the evolution of the adaptations and the changing size of the dinosaur group.