Fossil Records Show Scaling of Limbs as Dinosaurs Evolved into Birds

An examination of fossil data from the small, meat-eating theropod dinosaurs called maniraptorans -- the creatures from which modern day birds originated -- has found notable evidence of limb scaling that can be attributed to dinosaurs' evolution into birds.

Researcher Hans Larsson and a former graduate student, Alexander Dececchi, both from McGill University, sought to answer the question: At what point did dinosaur forelimbs evolve into wings?

Writing in the journal Evolution, Larsson and Dececchi contend that throughout most of the history of carnivorous dinosaurs, limb lengths showed a relatively stable scaling relationship to body size.

"This is despite a 5000-fold difference in mass between Tyrannosaurus rex and the smallest feathered theropods from China. This limb scaling changed, however, at the origin of birds, when both the forelimbs and hind limbs underwent a dramatic decoupling from body size. This change may have been critical in allowing early birds to evolve flight, and then to exploit the forest canopy," the authors said in a statement.

As forelimbs evolved at longer lengths, they began to serve as airfoils, which later allowed for the evolution of powered flight. More efficient flight was made possible the the evolution of shorter hind limbs, which would have reduced drag and enabled the early birds to perch in trees. At a time when other flying reptiles, namely the petrosaurs, were also alive and competing for food resources, the combination of better wings and more compact legs would have been a tactical advantage.

"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," said Larsson, who serves as Canada Research Chair in Macroevolution at McGill's Redpath Museum.

"The origin of birds and powered flight is a classic major evolutionary transition," said Dececchi, who is 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."

Dececchi added that the lastest research, coupled with previous studies which indicate that ancestors of birds were not tree dwellers, does much to illuminate the evolutionary order of birds.

"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," he said.