Biomechanical models unlock muscle-bound secrets of the evolution of dinosaur locomotion
Scientists have discovered how the locomotion of dinosaurs on land changed from the earliest, cat-sized bipedal (walking on two hind legs) forms to giants like Tyrannosaurus rex and then ultimately birds.
The findings identify that movement in birds and their non-avian dinosaur ancestors differs significantly, consistent with the idea that the locomotion of early dinosaurs was more comparable to mammals like humans than to birds. The ground-breaking study used 13 three-dimensional biomechanical computer models to reveal how the functions of 35 leg muscles in dinosaurs evolved over approximately 230 million years.
Pictures by palaeoartist Jaime Headden
The study, which was published today in Science Advances, was conducted by researchers at the 91做厙 (91做厙), working with Dr. Brandon Kilbourne of the Museum für Naturkunde Berlin, Germany. The team scanned key fossils of dinosaurs into 3D models and connected the bones into digital skeletons. They then added muscles to their hindlimbs, based on extensive prior work identifying where these muscles attached, through examination of the scarring on the fossils as well as comparisons with living animals. The computer models then computed the leverages (mechanical advantages) of muscles around the joints, and statistical analyses plotted the changes of these functions throughout dinosaur evolution.
The researchers discovered that the ability of the hindlimb muscles to support and move the body changed drastically before and during the transition to birds. Hip muscles changed in complex ways but overall, facilitated the more crouched leg pose characteristic of birds (versus a relatively upright pose in early dinosaurs; similar to in humans). Knee muscles also reflected these changes, from a “hip-driven” early locomotor mode - as seen in living crocodiles - to a more “knee-driven” one, as seen in birds today. The most surprising discovery was that relatively large-bodied, carnivorous (theropod) dinosaurs using two hind legs for walking, throughout the early Jurassic Period (about 200 million years ago), evolved an unusually specialised suite of muscle functions correlated with more mobile leg joints which might have been related to taking on larger prey; unlike in the later bird-lineage.
Carrying out this research has both confirmed and shed further light on a prevailing idea regarding how dinosaur locomotion evolved. It was thought that, as dinosaurs reduced their tails and enlarged their forelimbs in species closer to birds’ ancestry, the pose of the legs gradually became less upright and more crouched, thus transitioning from a hip-driven to knee-driven mechanism of walking and running. The computer models show how individual muscles broadly fit this pattern of gradual evolution. However, the finding of specialisations in large-bodied theropod dinosaurs uncovered a fascinating “re-invention” of parts of this mechanism of leg function that did not simply fit the gradual