Dinosaurs shrank continually into birds

Dinosaurs were the original incredible shrinking animals.

More than 50 million years of steady miniaturization and rapidly changing bodies transformed the massive animals into relatively tiny ones: birds.

A new analysis of bones, feathers and teeth from 120 dinosaur species validates what some scientists had suspected: Modern birds’ ancient ancestors evolved quickly and shrank continually, researchers report in the Aug. 1 Science. 

“Other studies have been sniffing around this area,” says paleontologist Roger Benson of the University of Oxford, “but the scale of this study is unprecedented. They looked across the whole skeleton.”

Though scientists knew that all birds evolved from dinosaurs, no one quite understood the details of how and when the shrinking occurred — or how fast these dinosaurs evolved, says study coauthor Michael Lee, a paleontologist at the South Australian Museum and the University of Adelaide.

Other studies trying to pin down evolution rates focused on just one or two fossil features, such as the relative lengths of legs or wings, Lee says. His team cataloged 1,549 items, including the shapes of teeth, bones and skulls as well as feather type and distribution. The analysis encompassed hundreds of other fossil details of theropods, the meat-eating, walking-on-two-legs dinosaurs that led to birds.

Borrowing statistical tools typically used in genetics, the researchers built a detailed theropod family tree. They used the fossil record to date branches of the tree and work out just how long it took for dinosaurs’ bodies to change.

The dinosaur lineage leading to birds evolved skeletal adaptations roughly four times as fast as other dinosaurs, such as T. rex, Velociraptor and Allosaurus, the scientists found.

“Birds and their ancestors were out-evolving all the other dinosaurs that were running around at the same time that they were,” Lee says.

And every set of avian ancestors were smaller than their predecessors, the team found. Lee thinks small body size let some theropod dinosaurs explore and adapt to all sorts of new niches and lifestyles.

“If you’re large, there are only a few things you can do,” he says. “You basically just lumber around.” But once the ancestors of birds started shrinking, he says, “they could start climbing trees, they could start experimenting with gliding and leaping — and ultimately they started flying.”