Cope's Rule - which states that small animals tend to evolve into larger ones - is sometimes true, sometimes not, a new statistical model shows.
Gene Hunt, curator in the Department of Paleobiology at the National Museum of Natural History (NMNH) in Washington, DC, used dinosaur femurs as proxies for animal size. They then used that data in their model to look for two things: trends in size over time and whether there were any upper limits for body size.
"What we did then was explore how constant a rule is this Cope's Rule trend within dinosaurs," he says.
The team found that some groups of dinosaurs do indeed tend to getlarger over time, following Cope's Rule. Ceratopsids and hadrosaurs, for instance, show more increases in size than decreases over time.
As for the upper limits to size, the results were sometimes yes, sometimes no. The four-legged sauropods - long-necked, small-headed herbivores - and ornithopods such as iguanodons and ceratopsids seemed to have no upper limits on how large they could evolve. And indeed, these groups contain the largest land animals that ever lived.
However, theropods, which include Tyrannosaurus rex, did show what appears to be an upper limit on body size. This may not be particularly surprising, says Hunt, because theropods were bipedal, and there are physical limits to how big you can get while still being able to move around on two legs.
In fact, it's not really understood why Cope's Rule works at all, says Hunt. The traditional idea that somehow 'bigger is better' because a bigger animal is less likely to be preyed upon won't really do, he says. After all, even the biggest animals start out small enough to be preyed upon and spend a long, vulnerable, time getting that big.