Many dinosaurs, like T. rex, had scrawny arms, but paleontologists have discovered that as dinosaurs gradually evolved bigger arms, they began to stand and move more like birds.
The change, documented in the journal Nature, passed on to the descendants of dinosaurs -- birds themselves.
"Our study shows how mass was allocated to the forelimbs, starting in non-flying dinosaurs, to turn them into longer, heavier, more muscular wings that became more and more effective for flapping during flight," co-author John Hutchinson of the Royal Veterinary College's Structure and Motion Lab told Discovery News.
Hutchinson and his colleagues used digitizing technology to create 3D images of the skeletons of 17 archosaurs, a group that included living crocodiles and birds as well as extinct dinosaurs. The researchers then digitally added flesh around the skeletons to estimate the overall shape of the body as well as the individual body parts, such as the head, forelimbs and tail.
The scientists found that as the arms got bigger, eventually turning into wings in some species, the hind limb posture got progressively more crouched as the center of mass moved forward. Before then, dinosaurs that stood on two legs had a fairly straight posture, similar to that of humans. Maniraptoran dinosaurs, such as Velociraptor, which were in the lineage that evolved into birds, dramatically developed this crouching trait.
Posture varies among today's birds, with larger species like ostriches tending to have straighter legs to more economically support their own weight. But even these birds still have fairly crouched limbs overall.
The discovery calls into question an earlier theory that held dinosaurs became more bird-like when their tails grew shorter and lighter. Hutchinson and his team acknowledge that happened, but only after the other changes took place.
He explained that "the shortening of the tail coincided with a reduction of a once-large tail muscle that connected to the thigh and moved the leg back and forth during locomotion. Birds only have a tiny remnant of that muscle."