Dinosaurs went extinct 65 million years ago… well, most of them. Some species went on to eventually become birds, and a group of researchers from Oxford University and the Royal Ontario Museum believe that shrinking played a large role in their success. The research was led by Oxfords’s Roger Benson and the results were published in PLOS Biology.
“Dinosaurs aren’t extinct; there are about 10,000 species alive today in the form of birds. We wanted to understand the evolutionary links between this exceptional living group and their Mesozoic relatives, including well-known extinct species like T. rex, Triceratops, and Stegosaurus,” Roger Benson said in a press release. “We found exceptional body mass variation in the dinosaur line leading to birds, especially in the feathered dinosaurs called maniraptorans. These include Jurassic Park’s Velociraptor, birds, and a huge range of other forms, weighing anything from 15 grams to 3 tonnes, and eating meat, plants, and more omnivorous diets.”
The team calculated the weight of over 400 different species of dinosaur that emerged around 220 million years ago. Estimating the weight is accomplished by measuring the bones in its legs, particularly the femur. As legs are responsible for holding up all of the body’s weight, this is a reliable and accurate method for determining size.
“This [method] shows that the biggest dinosaur Argentinosaurus, at 90 tonnes, was 6 million times the weight of the smallest Mesozoic dinosaur, a sparrow-sized bird called Qiliania, weighing 15 grams,” co-author Nicolás Campione stated. “Clearly, the dinosaur body plan was extremely versatile.”
They found that many species changed size rapidly, though not all of them maintained that rate. Some did, however, and those species were the ones that went on to give rise to the birds we see today. The ability to change size more quickly allowed those species to take advantage of ecological niches, while the others that stalled out were unable to adapt to changing environmental conditions and ultimately went extinct.
“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,” co-author David Evans explained. “But it’s the sustained high rates of evolution in the feathered maniraptoran dinosaur lineage that led to birds – the second great evolutionary radiation of dinosaurs.”
As the animals got smaller and began to fill new ecological roles, they had increased opportunities for new adaptations, leading to the incredible breadth of features we see in living birds today.
“The fact that dinosaurs evolved to huge sizes is iconic,” stated co-author Matthew Carrano. “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.”