Meat-eating dinosaurs are believed to have evolved into birds by shrinking over the course of a million years.
Researchers discovered theropods were the only dinosaurs that got continuously smaller, the BBC reported. Their skeletons were believed to have changed between four times faster than other dinosaurs'. They shrunk about 12 times before being what scientists consider to be a modern bird. The finding was published in a recent edition of the journal Science.
The research team set out to determine if this type of evolutionary shrinking tied to the development of birds was unique to these types of dinosaurs. The shrinking occurred about two million years ago.
"Birds evolved through a unique phase of sustained [miniaturization] in dinosaurs," lead author Associate Professor Michael Lee, from the University of Adelaide's School of Earth and Environmental Sciences and the South Australian Museum, said in a statement.
"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. Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins," he said.
The researchers looked at 1,500 anatomical traits of dinosaurs to make their findings. The team used sophisticated mathematical modeling techniques to trace how the ancient creatures developed and adapted to their changing surroundings.
"Studies of bipedal carnivorous dinosaurs - such as Tyrannosaurus rex and Velociraptor - keep finding more and more bird-like traits, such as feathers, wishbones, hollow skeletons and a three-fingered hand," associate Professor Lee said.
The findings suggest the branch of dinosaurs leading to modern day birds was more "evolutionarily innovative" than other family trees.
"Birds out-shrank and out-evolved their dinosaurian ancestors, surviving where their larger, less evolvable relatives could not," Lee said.
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