Citters Tend To Grow Bigger In Size

The cope rule suggests that small citters grow into bigger beasts. The cope rule that was suggested by Edward Cope a paleontologist is proved right by a new test that was conducted using advanced statistical modeling methods.

"For a long time, dinosaurs were thought to be the example of Cope's Rule," says Gene Hunt, curator in the Department of Paleobiology at the National Museum of Natural History (NMNH) in Washington, D.C. Other groups, particularly mammals, also provide plenty of classic examples of the rule, Hunt says.

In order to check whether the rule was applicable to dinosaurs, Hunt and colleagues Richard FitzJohn of the University of British Columbia and Matthew Carrano of the NMNH used dinosaur thigh bones (which is also known as femurs), as proxies for animal size.

Later this femur data was used in statistical model in order to check two things, the directional trends in size over time and whether there were any detectable upper limits for body size.

"What we did then was explore how constant a rule is this Cope's Rule trend within dinosaurs," said Hunt. 

They later checked with the "family tree" of dinosaurs and found that some groups, or clades, of dinosaurs do indeed trend larger over time, following Cope's Rule. The researchers stated the example of Ceratopsids and hadrosaurs who display an increase in size than decreases over time.

As for the upper limits to size, the results were sometimes yes, sometimes no. The four-legged sauropods and clades showed no indication of upper limits to how large they could evolve. 

Whereas, Tyrannosaurus rex did not show what appears to be an upper limit on body size. Because they are bipedal, and there are physical limits to how massive you can get while still being able to move around on two legs.

As for why Cope's Rule works at all, that is not very well understood, says Hunt. "It does happen sometimes, but not always," he added.

Hunt, FitzJohn, and Carrano will be presenting the results of their study on Nov. 4, at the annual meeting of The Geological Society of America in Charlotte, North Carolina, USA.