Dino weigh-in sheds light on evolution

Rapidly shrinking body sizes allowed feathered dinosaurs to exploit ecological niches throughout their evolution and become hugely successful suggests research led by the University of Oxford. An international team – jointly led by Oxford and the Royal Ontario Museum – ‘weighed’ 426 dinosaurs and found rapid rates of body size evolution shortly after their origins around 220 million years ago. These rates then slowed and only the evolutionary line leading to birds continued to change at the same rate for 170 million years, producing new ecological diversity unseen in other dinosaurs. “How do you weigh a dinosaur?” said Dr Nicolás Campione from Uppsala University. “You can do it by measuring the thickness of its leg bones, like the femur. This is quite reliable.” The weigh-in showed that the biggest dinosaur, the Argentinosaurus, weighed 90 tonnes, and was six million times larger than the smallest dinosaur, the sparrow sized Qiliania, which weighed just 15g. The team studied rates of body size evolution of the entire family tree of dinosaurs throughout their first 160 million years on Earth. They found that if close relatives were fairly similar in size then evolution was probably quite slow, but if different in size then evolution was faster. “What we found was striking. Dinosaur body size evolved very rapidly in early forms, likely associated with the invasion of new ecological niches. In general, rates slowed down as these lineages continued to diversify,” said Dr David Evans from the Royal Ontario Museum. “But it is the sustained high rates of evolution in the feathered maniraptoran dinosaur lineage that led to birds, the second great evolutionary radiation of dinosaurs.” This evolutionary line continued to experiment with different, often radically smaller body sizes, allowing new body designs and adaptions to emerge more rapidly than in other dinosaurs. Those groups failing to adapt became locked in narrow ecological niches and ultimately went extinct. This suggests that birds might result from sustained, rapid evolutionary rates over hundreds of millions of years. “The fact that dinosaurs evolved to such huge sizes is iconic,” said Dr Matthew Carrano from the Smithsonian Institution’s National Museum of Natural History. “And yet we’ve understood very little about how size was related to their overall evolutionary history. This makes it clear that evolving different sizes was important to the success of dinosaurs.” Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage