From left to right: a neotheropod, a tetanuran, a coelurosaur, a paravian and Archaeopteryx (Image: Davide Bonnadonna)

It took 50 million years of continual shrinking to turn massive, lumbering dinosaurs into the first small flying birds.

“No other dinosaur group has undergone such a long and extended period of miniaturisation,” says Mike Lee of the South Australian Museum in Adelaide. “Statistically this trend was far stronger than by chance, analogous to flipping a coin a dozen times and getting all heads.”

Lee and his colleagues have performed the most comprehensive analysis yet of fossil theropods, the two-footed meat-eating dinosaurs, like Velociraptor, from which birds evolved. They have charted how 224-million-year-old dinosaurs weighing 238 kilograms evolved into proto-birds, including Archaeopteryx, that weighed just 0.8 kg.


The analysis reveals that the ancestors of birds shrank without interruption. “What was impressive was the consistency of the size change along the dinosaur-to-bird transition, with every descendant smaller than its ancestor,” says Lee. Getting smaller must have offered advantages at every turn.

Incredible shrinking raptor

Lee tracked how 1549 skeletal features changed in 120 species of theropod from all over the world, spanning the 50-million-year period over which theropods evolved into Archaeopteryx and modern birds.

He identified 12 major evolutionary steps when groups of theropods split to form new kinds of dinosaur.

At each of these break points, the theropods that ended up as birds shrank. They also changed four times as fast as other theropods that did not become birds.

“This study provides compelling evidence that the iconic small size of birds results from a chance but sustained pattern of selection for smaller body size spanning millions of years,” says Gregory Erickson of Florida State University in Tallahassee.

Good to be small

Lee says each wave of shrinkage added survival traits we now see in birds. “The gradual evolution of smaller and smaller body size would have allowed the bird predecessors to explore novel niches and body plans off limits to their larger relatives,” he says. “It would have permitted them to chase insects, climb trees, leap and glide, and eventually develop powered flight.”

One crucial change happened in theropods called Tetanurae, which include famous predators like Allosaurus. They evolved an obliquely angled thigh bone. This shifted their centre of gravity forward, pushing their bodies into a tilted posture like that of modern birds and ensuring that their wings were near the centre of gravity. “It paves the way for flight, and would not have been possible at a larger body size,” says Lee.

While their bodies got smaller, theropods’ skulls stayed relatively large. That meant they could carry larger brains relative to their body size. Smaller dinosaurs were also more likely than large ones to develop insulating feathers, enabling them to hunt at night.

“Size reduction, whatever processes drove it, certainly seems to have allowed the bird lineage to fill niches that small-bodied animals can, and to undergo a fairly extensive radiation into these,” says Bhart-Anjan Bhullar of Yale University.

Their small size may also have helped birds survive the mass extinction that wiped out all the other dinosaurs 65 million years ago, says Bhullar. “We have mounting evidence that the end-Cretaceous extinction simply took out all landlocked animals above a certain size, say a few kilograms,” he says. “Birds happened to be among those dinosaurs that were small, and were lucky to boot.”

Journal reference: Science, DOI: 10.1126/science.1252243