Ocean animals have been getting bigger over the last half-billion years. Not a little bigger. Not even a lot bigger. They have _ gigantically, scientists now conclude. Their new finding lends support for something known as "Cope's rule." It holds that animals tend to evolve into species that are much larger than their distant ancestors. This hypothesis takes its name from the 19thcentury paleontologist Edward Drinker Cope. While studying fossils , he was the first to notice this trend. Noel Heim is a paleontologist at Stanford University in Palo Alto, Calif. He also is a co-author of the new study. His team compared the body size of animals between the Cambrian Period and modern times. This was a span of 542 million years. The animals studied included species from more than 17,000 genera. They ranged from ancient trilobites, plesiosaurs (extinct reptiles with a long neck and flippers) and many less familiar creatures to today's whales and clams. Ocean animals today are an average of 150 times larger than they were during the Cambrian, Heim's group reports. The smallest animals alive today -- tiny crustaceans called ostracods -- are only about one-tenth the size of the Cambrian's tiniest animals. But today's largest ocean animals -- whales -- are more than 100,000 times bigger than the biggest in the Cambrian. "Classes of animals that were already big ... tended to live longer," Heim says. They also tended to change more than classes of animals that were small did. The size gains in ocean animals are much larger than would be expected by chance, saysJonathan Payne. He's a co-author who also works at Stanford. The scientists don't know what drives the trend. One possibility is an arms race between predators and prey. The idea here is that larger animals are less likely to become some other animals' meal. Another possibility has to do with oxygen. Land animals evolved from species that started in the ocean. Some of these land animals eventually returned to the ocean. And they kept the ability to breathe oxygen-rich air. That may have made it easier for them to outgrow animals that had to filter their oxygen out of the water. The ocean animals' change in size   _  .
is determined by environment cannot be predicted by any factor is fully explained by the new study relates to the size of their ancestors