Dinosaur Fossil Showing Early Signs Of Miniaturization Necessary For Flight Found By Amnh Paleontologists



Artist's reconstruction of Mahakala omnogovae, a two-foot-long dinosaur unearthed in the Gobi Desert.

© Frank Ippolito

An 80-million-year-old dinosaur fossil unearthed in the Gobi Desert of Mongolia demonstrates that miniaturization, long thought to be a hallmark of bird origins and a necessary precursor of flight, occurred progressively in primitive dinosaurs. The specimen was found in the southern Gobi Desert region of Mongolia, an area made famous by spectacular dinosaur discoveries by paleontologists from the American Museum of Natural History. The most recent find, described in the September 7 issue of Science, comprises the fossilized bones of a new dinosaur the researchers have named Mahakala, and includes portions of its skull, forelimb, and hindlimb, as well as much of the vertebral column.

"Paleontologists have long thought that miniaturization occurred in the earliest birds, which then facilitated the origin of flight," said Alan Turner, lead author on the study and a graduate student at the American Museum of Natural History and Columbia University in New York. "Now, the evidence shows that this decrease in body size occurred well before the origin of birds and that the dinosaurian ancestors of birds were, in a sense, pre-adapted for flight."

Because most dinosaurs were too massive to fly, miniaturization has long been considered crucial to the origin of flight. Although paleontologists have shown that birds evolved from bipedal carnivorous dinosaurs known as theropods, fossil evidence of miniaturization and other characteristics leading to flight have been sparse. Now Mahakala is providing the first signs of some of these early evolutionary steps. In particular, while other dinosaurs of the Cretaceous Period were evolving in favor of increased body size, Mahakala represented a progressive step towards miniaturization of body forms that would be necessary for feathered dinosaurs to eventually take flight. 

"Flight isn't an easy thing, because you are, in effect, countering the force of gravity," said Turner. "Being really small appears to be a necessary first step. Other groups that evolved flight, such as pterosaurs and bats, all evolved from small ancestors. Traditionally it's been thought that the earliest birds were the first theropods to become really small. With the discovery of Mahakala we were able to show that this miniaturization occurred much earlier." 

Mahakala is an early evolutionary offshoot of the group of carnivorous dinosaurs known as dromaeosaurids that also includes the agile, sickle-clawed Velociraptor made famous in the 1993 movie Jurassic Park. As such, it serves an important role in understanding how the group and its close relatives, the birds, evolved. Unfortunately, due to their fragility, skeletons of such small, primitive animals are among the rarest of fossils.

After a microscopic examination of the fossilized bones, the researchers concluded that the animal was a nearly full-grown young adult when it died, measuring less than two feet in length and weighing about 24 ounces. When integrated into the broader context of the dinosaur family tree,Mahakala shows that dinosaur size decreased progressively as they evolved toward birds. While this is something that has long been expected, Mahakala provides the first empirical evidence of this phenomenon. 

"The last decade has seen massive amounts of data collected indicating that many of the characteristics of modern birds have antecedents in nonavian dinosaurs," said Mark Norell, a coauthor on the paper and a Curator in the Division of Paleontology at the American Museum of Natural History. "Many of the animals that were thought to look like giant lizards only a few years ago are now known to have been feathered, to have brooded their nests, to have been active, and to have had many other defining bird characteristics, like wishbones and three forward-facing toes. To this list we can now add that the precursors of birds were also small, primitive members of a lineage that later grew much larger-long after their divergence from the evolutionary stem leading to birds."

In addition to Turner and Dr. Norell, other members of the team included Diego Pol from the Museo Paleontologico Egidio Feruglio in Argentina; Julia Clarke, from North Carolina State University; and Gregory Erickson from Florida State University. Pol, Clarke, and Erickson are also Research Associates at the American Museum of Natural History.

The study was supported by the National Science Foundation, the American Museum of Natural History, and Columbia University.

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