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Cope’s Rule Rules: Marine Animals Have Gotten Bigger Over Time

Renowned 19th century American paleontologist Edward Drinker Cope proposed "Cope's Rule," hypothesizing that animal lineages tend to increase in body size over time. The dinosaur fossils he dug up in the American West seemed to bear this out. But Cope was not infallible. In a mistake famed in the annals of paleontology, he published a description of the wondrous long-necked marine reptile Elasmosaurus in 1868 with its head erroneously placed at the end of the tail. However, it appears Cope had his head on straight about body size. Scientists on Thursday said the most comprehensive test of "Cope's rule" ever conducted, involving 17,208 different marine animal groups spanning the past 542 million years, demonstrated a clear trend toward larger size over time. The analysis went back to Earth's dawn of animal life. It found that the average animal in the oceans today is 150 times larger in mass than the average one roughly half a billion years ago. In fact, the largest marine creature on record lives today: the roughly 100-foot-long (30 meters) blue whale.

So why is bigger better? The researchers have some ideas."It’s easier to eat other animals if you’re large. It’s also easier to avoid being eaten," said Stanford paleobiologist Noel Heim, whose study appears in the journal Science. "In water, larger animals can be more active because of the increased mass relative to their surface area. They feel less 'drag' than small animals. Larger animals also have a higher metabolic rate, which also contributes to a more active lifestyle," Heim added.

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