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Birds Evolved from Maniraptoran Dinosaurs: Study

First Posted: Sep 18, 2013 08:03 AM EDT

Researchers from McGill University say that birds have evolved from a group of small, meat-eating theropod dinosaurs called maniraptorans that existed about 150 million years ago.

Many maniraptorans were bird-like as they had small body sizes along with hollow bones, feathers, and high metabolic rates, according to the findings of the researchers.

The research was led by Professor Hans Larsson and a former graduate student, Alexander Dececchi  from McGill university.

The researchers examined fossil data from the period marking the origin of birds. In a study published in the September issue of Evolution, the researchers write that the lengths of the limb showed a comparatively constant scaling relationship to body size through the history of carnivorous dinosaurs.

"Our findings suggest that birds underwent an abrupt change in their developmental mechanisms, such that their forelimbs and hind limbs became subject to different length controls," Larsson, Canada Research Chair in Macroevolution at McGill's Redpath Museum, said in a press release.

"This decoupling may be fundamental to the success of birds, the most diverse class of land vertebrates on Earth today," Larsson added.

Also a 5000-fold difference in the mass between Tyrannosaurus rex and the smallest feathered theropods from China was found. This limb scaling occurred at the origin of birds, when both the forelimbs and hind limbs went through a dramatic separation from the body size. This transformation may have been crucial in allowing early birds to evolve flight, and then to exploit the forest cover, the authors conclude.

"The origin of birds and powered flight is a classic major evolutionary transition," said Dececchi, now a postdoctoral researcher at the University of South Dakota. "Our findings suggest that the limb lengths of birds had to be dissociated from general body size before they could radiate so successfully. It may be that this fact is what allowed them to become more than just another lineage of maniraptorans and led them to expand to the wide range of limb shapes and sizes present in today's birds."

The elongation of the forelimbs, made them long enough to serve as an airfoil, allowing for the evolution of powered flight. Flight control became more improved and efficient in early birds after the hind limbs shrunk.

 "This work, coupled with our previous findings that  the ancestors of birds were not tree dwellers, does much to illuminate the ecology of bird antecedents." said Dr. Dececchi. "Knowing where birds came from, and how they got to where they are now, is crucial for understanding how the modern world came to look the way it is."

Another group of flying reptiles called the pterosaurs existed along with the early birds and dominated the skies and competed for food. The scientists believe that if the birds had shorter legs it would have helped them fly better by reducing the drag during the flight and it would also have played a crucial role in their survival. Modern birds tuck their legs as they fly, even while moving and perching.

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