50 million years of dinosaur shrinking led to today’s birds

To give us birds as we know them today, theropods, the line of dinosaurs that evolved into birds, shrank in body size continuously for 50 million years, reports a study published today in the journal Science.

Michael S. Y. Lee, associate professor of ecology and evolutionary biology at the University of Adelaide in Australia, and a team of researchers from Italy, the United Kingdom, and Hungary, used advanced statistical techniques and a dataset of more than 1,500 body traits from 120 well-documented species of theropod and early birds to get a better understanding of changes in the animals’ body sizes.

Unlike in past studies, the statistical methods the scientists used looked at traits across all branches of the theropod tree — and across the entire dinosaur body.

Their approaches revealed that, throughout early history, theropod body size shrank 12 times, from an initial average mass of 163 kilograms (360 pounds) to 0.8 kilograms (1.8 pounds) in Archaeopteryx, the earliest-known bird. The changes occurred from about 210 to 160 million years ago.

Keeping up a trend like that for such a long time required the process of shrinking to be sustained, the researchers say. A long shrinking process was one of two key drivers of the dinosaur-to-bird transition, they believe. Because it lasted so long, it changed the way theropods developed, allowing them to evolve bird-specific traits — shorter snouts, smaller teeth, and insulating feathers.

The second driver of the transition was a rapidly evolving skeleton: Theropod bones evolved four times faster than in other dinosaurs, says Lee. The femur, or thighbone, is one of the most consistently preserved dinosaur fossils. Lee and his colleagues found clear evidence for shrinking femurs in the theropod lineage over millions of years.

In an accompanying article in Science that explains the significance of Lee’s findings, paleontologist Michael J. Benton of the University of Bristol says a crucial trigger of the evolution from theropods to decreasingly smaller birds “may have been a move to the trees, perhaps to escape from predation or to exploit new food resources.”

Tree living, he writes, requires small bodies, and reduced body size enables other characteristics that enhance success in the trees. Among them: enlarged eyes for three-dimensional vision (to avoid collisions when leaping from branch to branch); enlarged brains (to cope with diverse arboreal habitats); insulating feathers (to enable nocturnal activity, since many insects are nocturnal); and elongated forelimbs, distinct from the hindlimbs in their function and with expanded flight surfaces (to enable ever more daring leaps from tree to tree).

Lee agrees, noting that instead of one bird-specific trait driving the evolution of the rest, the traits jointly influenced one another to lead to today’s wide assortment of birds. — Matt Mendenhall, Managing Editor

Originally Published July 31, 2014