A dinosaur replica of the Iguanodon is seen during the exhibition "Dinosaurs of Peru" at the Kennedy Park in Miraflores (Photo : Reuters)

Paleontologist Edward Cope formulated a theory known as the Cope Rule. His theory states that species have a tendency to increase their body size over the process of evolution.

A team of researchers wanted to test if Cope's Rule was applicable to a group of animals that had become extinct. Curator Gene Hunt of the National Museum of Natural History (NMNH) and his colleagues performed a new test to find out if the rule applies to dinosaurs, as dinosaurs have been considered a good example of Cope's Rule.

Using advanced statistical methods, researchers have found that the rule applies to dinosaurs, but not always. For their study, experts used dinosaur thigh bones (also known as femurs) as proxies to detect animal size.

They used femur data in their statistical model to observe an increasing trend in size over time and also to find whether there are any noticeable upper limits for body size.

"What we did then was explore how constant a rule is this Cope's Rule trend within dinosaurs," Hunt said in a press release from The Geological Society of America.

He and his colleagues studied the family tree of dinosaurs and noticed some group of dinosaurs have become larger over time. They did not consider birds, which evolved from theropod dinosaurs, for their study because evolutionary pressures forced the birds to reduce their size over time so that they could fly better.

Various groups of dinosaurs including ceratopsids and hadrosaurs have shown an increase in their size over the process of evolution, which shows that Cope's Rule was right in their cases. Also, long-necked and small-headed sauropods and another group called as ornithopods did not show any indication of upper limits to their size, and these groups had the largest land animals.

However, experts noticed that Cope's Rule did not apply to all groups of dinosaurs. For example, when researchers tested theropods, the group of dinosaurs to which the popular tyrannosaurus rex belongs to, they found that the species indeed showed indication of upper limits to how long they can grow over evolutionary time.

It is not yet clear how the Cope rule actually works. "It does happen sometimes, but not always," Hunt said. He pointed out that the idea of saying "bigger is better," as bigger animals will not be preyed upon, does not work. And this is because, he noted, even the bigger animals would be small enough to be preyed upon before they become large over their lifespan.

The findings of the study were presented at the annual meeting of The Geological Society of America in Charlotte, North Carolina, USA.