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 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.
The research covered five major marine animal categories:
- Chordates including fish, sharks, marine mammals like whales, and
marine reptiles like plesiosaurs, mosasaurs and ichthyosaurs;
- Mollusks including squid, clams and snails;
- Echinoderms including starfish and sea urchins;
- Arthropods including crabs, shrimp, lobsters and the once
ubiquitous trilobites;
- and Brachiopods like lamp shells.
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.
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"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.
The scientists said the trend toward greater size is driven
primarily by the increased diversification and survival of larger
classes of animals compared with smaller ones. They found that new
species tend to be larger than their ancestors, not that a single
species gets larger over time.
"We found that this size increase cannot be explained by simple,
random evolution from a small starting size," added Stanford
paleobiologist Jonathan Payne.
Instead, it indicates "an active evolutionary process that favored
animals with larger sizes," Payne said.
(Reporting by Will Dunham; Editing by Mohammad Zargham)
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