How did birds emerge from a lineage of large dinosaurs whose clawed feet were planted firmly on the ground? Size really matters, according to a team of scientists that traced the incredibly fast shrinkage along 50 million years of ancient avian evolution.
The findings, published in the journal Science, show how the continuous miniaturization of this dinosaur lineage allowed for a whole host of physical changes that made powered flight possible.
Paleontologists have long known that birds evolved from dinosaurs known as theropods, a group that included the formidable Tyrannosaurus rex. But there's been considerable argument over whether the branch of theropods leading to birds really was quickly shrinking until the earliest birds emerged roughly 160 million years ago.
Previous analyses may not have been reliable because they focused on fast-evolving branches of the theropod family tree, or because they only looked at the evolution of a few traits, the study authors said.
So the international team of researchers analyzed more than 1,500 bones and body traits from 120 species of well-known theropods and early birds.
The scientists found that in the last 50 million years or so before Archaeopteryx (the first known bird) took flight, dinosaurs in the line leading to birds shrunk 12 times, from an average of 359 pounds to a mere 1.8 pounds.
This fast shrinkage that took place between about 210 million and 160 million years ago probably helped to trigger a host of different traits in shape, not just size. Miniaturization is linked to a shortened development period, which means that the increasingly tiny dinosaurs would hold onto more "juvenile" traits in adulthood – shorter snouts, smaller teeth, bigger eyes and bigger brains – some of which were uncannily bird-like traits.
"This prolonged miniaturization is consistent with many aspects of bird origins," the study authors wrote.
Why the push toward smaller and smaller animals? It could be that the dinosaurs were adapting to living in trees, which comes with a whole host of challenges ill-suited for bulky creatures, Michael Benton of the University of Bristol in England, who was not involved in the study, wrote in a commentary.
For arboreal life, you need a small body, big eyes for good 3-D vision so you don't ram yourself into a tree while you're hopping from branch to branch, big brains to deal with a complex environment, and insulating feathers so you can operate at night and munch on nocturnal insects, Benton said. That's not to mention longer forelimbs to go from tree to tree, which might eventually develop a wing-like surface area to help glide.
"The fossils showed that from about 170 to 120 million years ago, all these dinosaurs — the Paraves — were experimenting with flight in various modes, including parachuting, gliding, and leaping from tree to tree," Benton wrote. "One clade, represented by Archaeopteryx, made the crucial leap to powered flight, and so birds were born."
Size wasn't the only driver; the researchers also think that the theropod branch leading to birds was evolving certain skeletal characteristics at four times the normal evolutionary rate. These two drivers – miniaturization and skeletal changes – probably interacted to lead to birds as we know them today, the scientists said.