Were dinosaurs destined to be big? Testing Cope's rule

Nov 02, 2012
A stylized scale of relative dinosaur sizes. Image by Matt Martyniuk, from Wikimedia Commons.

In the evolutionary long run, small critters tend to evolve into bigger beasts—at least according to the idea attributed to paleontologist Edward Cope, now known as Cope's Rule. Using the latest advanced statistical modeling methods, a new test of this rule as it applies dinosaurs shows that Cope was right—sometimes.

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

To see if Cope's rule really applies to dinosaurs, Hunt and colleagues Richard FitzJohn of the University of British Columbia and Matthew Carrano of the NMNH used dinosaur thigh bones (aka femurs) as proxies for animal size. They then used that femur data in their to look for two things: 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 looked across the "family tree" of dinosaurs and found that some groups, or clades, of dinosaurs do indeed trend larger over time, following Cope's Rule. Ceratopsids and , for instance, show more increases in size than decreases over time, according to Hunt. Although birds evolved from , the team excluded them from the study because of the evolutionary pressure birds faced to lighten up and get smaller so they could fly better.

As for the upper limits to size, the results were sometimes yes, sometimes no. The four-legged (i.e., long-necked, small-headed herbivores) and ornithopod (i.e., iguanodons, ceratopsids) clades showed no indication of upper limits to how large they could evolve. And indeed, these groups contain the largest that ever lived.

Theropods, which include the famous Tyrannosaurus rex, on the other hand, did show what appears to be an upper limit on body size. This may not be particularly surprising, says Hunt, because were bipedal, and there are physical limits to how massive you can get while still being able to move around on two legs.

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

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. The traditional idea that somehow "bigger is better" because a bigger animal is less likely to be preyed upon is naïve, Hunt says. After all, even the biggest animals start out small enough to be preyed upon and spend a long, vulnerable, time getting gigantic.

Explore further: T. rex gets new home in Smithsonian dinosaur hall

More information: Testing Cope's Rule and the Existence of an Upper Bound for Body Size in Non-Avian Dinosaurs, gsa.confex.com/gsa/2012AM/webprogram/Paper211594.html

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User comments : 6

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tadchem
5 / 5 (2) Nov 02, 2012
"Everything should be made as simple as possible, but not simpler." would seem to apply to Cope's Rule as well.
baudrunner
1 / 5 (10) Nov 02, 2012
A case can be made for intelligent design, where the creation of animals on this world is concerned, but that case must allow for evolution, and for the intervention of visitors to our world who are not of this world. My take overall on the long-term presence of the dinosaurs has everything to do with the fertilization of the earth's crust and the rising temperatures of the climate caused by the release of waste and methane by these giant creatures.
Parsec
4.3 / 5 (6) Nov 02, 2012
A case can be made for intelligent design, where the creation of animals on this world is concerned, but that case must allow for evolution, and for the intervention of visitors to our world who are not of this world. My take overall on the long-term presence of the dinosaurs has everything to do with the fertilization of the earth's crust and the rising temperatures of the climate caused by the release of waste and methane by these giant creatures.

Isn't there some kind of crackpot/neo-religious forum that you can post on where you won't be ridiculed?
LagomorphZero
3 / 5 (1) Nov 02, 2012
-Although birds evolved from theropod dinosaurs, the team excluded them from the study because of the evolutionary pressure birds faced to lighten up and get smaller so they could fly better.

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

It doesn't seem like this is really an useful rule then, any one can be right sometimes after they exclude outliers
PhotonX
4.2 / 5 (5) Nov 02, 2012
A case can be made for intelligent design, where the creation of animals on this world is concerned, but that case must allow for evolution, and for the intervention of visitors to our world who are not of this world. My take overall on the long-term presence of the dinosaurs has everything to do with the fertilization of the earth's crust and the rising temperatures of the climate caused by the release of waste and methane by these giant creatures.
You should work little harder on this. With just a little more effort, you should be able to work in the 9/11 and Kennedy assassination conspiracy theories. A case can be made for those, too, as long as the hijackers were able to miracle themselves out of the planes before the collisions, and Oswald had special EUT enhanced magical bullets provided by the intervention of visitors to our world. Sheesh.
Ojorf
5 / 5 (1) Nov 04, 2012
Doesn't evolution tend towards larger animals because it had to start small and then fill available niches? Also every extinction event tended to hit larger animals on land harder than smaller ones. Afterwards smaller ones got bigger again to fill vacant niches.

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