The massive, meat-eating, ground-shaking dinosaurs evolved into agile flying birds by “shrinking” that lasted for over 50 million years, a study said.

The theropod dinosaurs, which gave rise to present-day birds, were the only dinosaurs that kept getting inexorably smaller, an evolutionary process that proved to be the key to their survival.

“Birds evolved through a unique phase of sustained miniaturisation in dinosaurs,” said lead author Michael Lee, an associate professor at University of Adelaide in Australia.

The study examined over 1,500 anatomical traits of dinosaurs to reconstruct their family tree.

The researchers used sophisticated mathematical modelling to trace evolving adaptations and changing body size over time and across dinosaur branches.

“These bird ancestors also evolved new adaptations, such as feathers, wishbones and wings, four times faster than other dinosaurs,” co-author Darren Naish, a vertebrate palaeontologist at the University of Southampton in Britain noted.

“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,” Lee said.

“Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins,” Lee added.

The study that appeared in the journal Science concluded that dinosaur species that evolved into birds were more evolutionarily innovative than other dinosaurs.