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A recipe for birds: 50 million years of dinosaur shrinking

A flock of early birds (Longirostravis) preen one of their large dinosaurian relatives (Yutyrannus). Both species lived during the Cretaceous Period (~120 million years ago) in what is now northern China.
[Credit: Brian Choo]

To give us birds as we know them today, the line of dinosaurs that evolved into birds shrank in body size continuously for 50 million years, a new study in the 1 August issue of Science reports.

Several previous studies have looked at the massive size change dinosaurs underwent to become birds. It involved giant, ground-dwelling theropod dinosaurs (the dinosaurs associated with the first bird) evolving into small, feathered creatures capable of hopping from tree branch to branch.

Even though previous studies have explained this process, questions remain.

Some researchers have proposed that continuous shrinking of theropods is what allowed birds to evolve. But Michael S. Y. Lee, Associate Professor of Ecology and Evolutionary Biology at the University of Adelaide, argues that studies that say this might not have been accurate because their methods for understanding how theropods shrank over time were not accurate.

Now, using advanced statistical techniques and an exceptional dataset of more than 1,500 body traits coded from 120 well-documented species of theropod and early birds, Lee and his team get a better understanding of body size changes across theropods.

Unlike in past studies, the statistical methods Lee et al. 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 shrunk 12 times, from an initial average mass of 163 kilograms to 0.8 kilograms in Archaeopteryx, the earliest-known bird.

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. They suggest that miniaturization of theropods actually started 50 million years before Archaeopteryx, the earliest bird. Because the shrinking process was so uniquely long-lasting, it changed the way theropods developed. Specifically, it allowed them to develop bird-specific traits shorter snouts, smaller teeth, and insulating feathers.

The second driver the dinosaur to bird transition was very fast evolution of changes to the skeleton in the lines of theropod that would give way to birds; in these lines, adaptations evolved four times faster than in other theropods.

Lee et al. suggest that instead of one bird-specific trait driving the evolution of the rest, the traits jointly influenced one another to lead to birds as we know them today.