Evolution Of Smaller Dinosaurs Helped Their Lineage Survive As Birds
May 7, 2014

Evolution Of Smaller Dinosaurs Helped Their Lineage Survive As Birds

Brett Smith for redOrbit.com - Your Universe Online

While dinosaurs may have disappeared from the face of the Earth, their lineage has survived in the form of birds and new research published in the journal PLOS Biology has found that both dinosaurs and birds evolved into smaller and smaller sizes – potentially contributing to their success.

“Dinosaurs aren't extinct; there are about 10,000 species alive today in the form of birds,” said study author Roger Benson, a paleobiologist at Oxford University. “We wanted to understand the evolutionary links between this exceptional living group, and their Mesozoic relatives, including well-known extinct species like T. rexTriceratops, and Stegosaurus.”

The study team reached their conclusion by calculating the overall mass of more than 420 dinosaur species using the size of their leg bones. The team discovered that dinosaurs exhibited relatively quick rates of body size progression following their origins, about 220 million years ago. Eventually, these rates slowed: only the evolutionary path bringing about birds continued to change size at this rate, and did so for 170 million years, resulting in ecological variety not observed in other dinosaurs.

“We found exceptional body mass variation in the dinosaur line leading to birds, especially in the feathered dinosaurs called maniraptorans,” Benson said. “These include Jurassic Park's Velociraptor, birds, and a huge range of other forms, weighing anything from 15 grams to 3 tonnes (3.3 tons), and eating meat, plants, and more omnivorous diets.”

The team said that small bodily proportions might have been crucial to sustaining evolutionary potential in birds, which shattered the smaller body size limit of around 1 kilogram (2.2 pounds) observed in other dinosaurs.

“How do you weigh a dinosaur? You can do it by measuring the thickness of its leg bones, like the femur. This is quite reliable,” said study author Nicolás Campione, a postdoctoral researcher at Uppsala University in Sweden. “This shows that the biggest dinosaur Argentinosaurus, at 90 tonnes (99 tons), was 6 million times the weight of the smallest Mesozoic dinosaur, a sparrow-sized bird called Qiliania, weighing 15 grams. Clearly, the dinosaur body plan was extremely versatile.”

The team reviewed rates of body size progression on the whole family tree of dinosaurs, sampled during their first 160 million years. If close relatives are relatively comparable in size, then development was probably rather slow. However, if they are very distinct in size, then progression should have been rapid, the study team said.

“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 team member David Evans, a paleontologist at the Royal Ontario Museum. “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.”

The evolutionary sequence leading to birds kept testing distinct, often dramatically smaller, sizes – permitting adaptations to arise faster than among larger dinosaurs, the researchers said. Other dinosaur families got stuck in slim ecological niches, and eventually became extinct. This shows that essential modern groups such as birds might derive from sustained, speedy evolutionary rates over the span of hundreds of millions of years, only seen through the fossil record.

“The fact that dinosaurs evolved to huge sizes is iconic,” said study author Matthew Carrano of 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.”