Scientists at the University of Liverpool and the Royal Veterinary College developed computer models of the skeletons of dinosaurs to show how body shape changed during dinosaur evolution and affected the way birds stand today.
The study reveals for the first time that, contrary to popular opinion, it was the enlargement of the forelimbs over time, rather than the shortening and lightening of the tail, that led to two-legged dinosaurs gradually adopting an unusually crouched posture, with the thigh held nearly horizontally – a trait inherited by their descendants: birds.
The research group used digitising technology to create 3D images of the skeletons of 17 archosaurs – land animals including living crocodiles and birds as well as extinct dinosaurs. They then digitally added ‘flesh’ around the skeletons to estimate the overall shape of the body as well as the individual body segments such as the head, forelimbs and tail.
Dr Karl Bates, from the University’s Institute of Ageing and Chronic Disease, said: “The evolution of birds from their dinosaurian ancestors is historically important not only to dinosaur research but also to the development of the theory of evolution itself.
“Way back in the 1860’s, Thomas Huxley used Mesozoic dinosaurs and modern birds as key evidence in promoting Darwin’s theory of evolution. In this study, modern digital technologies have allowed us to quantify the ‘descent with modification’ observed by Huxley all those years ago.
“This quantifiable evidence, derived from fossils, helps make evolution more apparent to a general audience, and helps demonstrate exactly how scientists understand what they do know about evolution.”
Dr Vivian Allen, from the Royal Veterinary College, said: “We started from a simple digital ‘shrink-wrap’ of the whole skeleton. From this, we expanded the ‘shrink-wrap’ to match how much flesh we think existed around the different parts of the skeleton. This was based on both detailed reconstruction of the muscular anatomy of each animal, and on what we have measured from CT scans of their living relatives.”
Prior research had shown that the first archosaurs, around 245 million years ago, were superficially like modern crocodiles – four-legged animals with long, heavy tails, although with longer limbs for living and moving on land. However, early in the evolution of the dinosaur lineage, about 235 million years ago, dinosaurs became bipedal, a trait inherited by their descendants – birds. Birds stand and walk in an unusually crouched posture, with the thigh held nearly horizontally.
Palaeontologists had agreed for years that this strange way of moving evolved gradually as the tail became shorter, shifting the centre of mass of certain dinosaurs progressively forward as those dinosaurs became more “bird-like”, and thereby requiring the legs to become less vertical and more crouched to keep the centre of mass balanced over the feet.
A major, unexpected implication of the team’s discovery is that, due to the effects on centre of mass position on leg posture, forelimb size and leg function are biomechanically linked. So, these changes in the forelimb anatomy of dinosaurs, both before and after flight, also altered the way they stood, walked and ran.
The study was supported by funds from the Natural Environment Research Council and the Royal Society.