dinosaurs shrank to become birds
August 1, 2014

The Incredible Shrinking Dinosaur: New Study Details How Theropods Became Birds

redOrbit Staff & Wire Reports - Your Universe Online

The grand mystery over how massive, carnivorous dinosaurs gave rise to flying birds has a simple solution, as it turns out – the meat-eaters simply kept shrinking and shrinking over a period of 50 million years, according to research appearing in Friday’s edition of the journal Science.

In their paper, an international team of scientists from the South Australian Museum, the University of Adelaide School of Earth and Environmental Sciences, the Museum of Geology and Paleontology Capellin, the University of Bologna, the University of Southampton and the University of Debrecen present a detailed family tree of dinosaurs and their avian descendants which maps out the lengthy and somewhat unlikely transformation.

According to the Associated Press (AP), the study authors compared the fossils of 120 different species and analyzed 1,500 different skeletal features such as thigh bones in compiling the genealogical record for a class of two-legged meat-eaters known as theropods, which gave rise to modern birds.

Author Michael S. Y. Lee of the University of Adelaide and his colleagues explained to the AP’s Seth Borenstein that their explanation of the transformation saw theropods beginning to grow smaller, with their chests becoming larger and the number of feathers on their bodies increasing until they morphed into the earliest version of birds.

“For a couple decades scientists have linked birds to this family of dinosaurs because they shared hollow bones, wishbones, feathers and other characteristics. But the Lee study gives the best picture of how steady and unusual theropod evolution was,” Borenstein said, noting that theropod skeletons changed four times more quickly than other dinosaurs, but that some members of the family (including the Tyrannosaurus rex) did not shrink like the others.

Lee said that he and his colleagues were surprised by the consistent shrinkage of the theropods over time. While other types of dinosaurs fluctuated in body size, theropods weighed about 600 pounds 220-230 million years ago, and then shrank to 360 pounds by around 200 million years ago.

After that point, they became smaller more quickly, slimming down to around 100 pounds in just 25 million years. By 167 million years ago, six-pound paravians, more direct ancestor of birds, came into existence. Four million years after that, the first birds (some weighing less than two pounds) likely arrived on the scene, the study said.

“Birds evolved through a unique phase of sustained miniaturization in dinosaurs,” Lee said in a statement. “Being smaller and lighter in the land of giants… provided these bird ancestors with new ecological opportunities, such as the ability to climb trees, glide and fly. Ultimately, this evolutionary flexibility helped birds survive the deadly meteorite impact which killed off all their dinosaurian cousins.”

“This study means we can't see the origin of birds as a sudden or dramatic event,” University of Bristol Professor Michael Benton added in an interview with BBC News. “The functions of each special feature of birds changed over time – feathers first for insulation, and later co-opted for flight; early reductions in body size perhaps for other reasons, and later they were small enough for powered flight; improvements in sense of sight and enlargement of brain – even a small improvement in these is advantageous.”

Image 2 (below): The feathered dinosaur Microraptor pounces on a nest of primitive birds (Sinornis). Both species lived during the Cretaceous Period (about 120 million years ago) in what is now northern China. Credit: Image by Brian Choo