Crouching bird, hidden dinosaur

Birds have evolved an unusual way of standing, their legs tipped back into a distinctive crouching position. A new theory has emerged about how this happened and and not surprisingly the trait arises as so often from their distant relatives the dinosaurs.

Researchers in Britain, Germany, China and the US used advanced 3-d computer scanning to model the skeletons of dinosaurs to show how evolution changed the way they stood and moved. Importantly, the models also allowed the researchers to add flesh to the bones, showing how dinosaur posture changed as forelimbs and tails gained or lost weight.

Their controversial findings suggest that it was not loss of tail weight but increased forelimb weight that caused the crouched standing position to emerge. Details of their findings are released on line this evening (wed) by the journal Nature .

Our leg bones line up when we stand, with our centre of mass positioned directly above our feet. Birds too need a centre of mass above their feet, but their heavy wings mean the only way this can happen is if they tip back, lowering their thigh bones into a horizontal position.

This decided crouch was evident in the dinosaur species from which birds evolved, the two-legged theropods. Prof John Hutchinson of the Structure and Motion Laboratory at the Royal Veterinary College in Britain and colleagues scanned in the skeletons of 17 dinosaurs and then used digital technology to put meat on the bones. This could be modified to match the skeletal changes that occurred as dinosaurs developed their bird-like posture.

Palaeontologists had long assumed that the driver for this was a shrinking counterbalancing tail, bringing the theropod thigh into a crouch. The report’s authors found however that an increase in the weight and size of the forelimbs is what actually brought on the evolutionary change. “Our results surprised even us,” Prof Hutchinson said.

These enlarged forelimbs began to emerge in feathered dinosaurs Microraptor and Velociraptor and in the earliest bird, Archaeopteryx. But instead of a long tail serving as counterbalance, these species adjusted leg bone position to control centre of mass making the tail an unnecessary nuisance.

At this point the modified forelimbs were used for purposes other than flight, but the evolutionary scene was set and the dinosaur crouch became an ideal body form for the birds that followed.