Dinosaurs underwent a 50-million-year-long evolutionary "shrink cycle" on their way to becoming today's bird species, researchers say.

Meat-eating, grounded dinosaurs known as theropods, which included Tyrannosaurus rex and Velociraptors, went through at least 12 periods of downsizing before evolving into modern birds, they say.

Beginning around 225 million years ago, some species went from an average weight of 350 pounds down to less than 2 pounds around 163 million years ago when the first true birds appeared, says study author Michael S. Y. Lee of the University of Adelaide in Australia.

"They just kept on shrinking and shrinking and shrinking for about 50 million years," says Lee, characterizing the theropods as "shape-shifters."

The researchers created a detailed evolutionary family tree by comparing fossils from 120 different species of theropods and of early birds, looking at 1,500 particular skeletal features.

They utilized sophisticated mathematical modeling to recreate evolving adaptions and changed body size over evolutionary time in a number of dinosaur branches.

Modern birds have been linked to theropod species because they share characteristics such as hollow bones, feathers and wishbones.

Theropod was the only dinosaur species to get continuously and inexorably smaller, and its skeleton evolved four times as fast as any other dinosaur type, the study confirmed.

And the evolutionary shrinking was consistent, the researchers said, unlike other dinosaur types that experienced ups and downs in their body sizes over millions of years.

Not all theropods shrank, the researchers acknowledge; T. Rex, for one, remained about the same size, suggesting it was more of a distant cousin of birds rather than on a direct branch of the family tree.

When the shrinking of theropods began, there were few small species offering much evolutionary competition, the researchers say.

By becoming smaller in a unique period of "sustained miniaturization," dinosaur ancestors of modern birds were able to inhabit a new evolutionary niche and develop a new way of life, Lee says.

"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 says.

That evolutionary path to new shapes and capabilities likely helped the early birds species survive the "killer" meteorite impact 65 million years ago that wiped out the larger dinosaurs, the researchers say.

The fact that modern birds are with us is down to their being the descendants of the fastest-evolving species of dinosaurs, they conclude.

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