The study reveals that over the past 542 million years, the mean sized of marine animals has increased 150-fold.The research also found that the increase in body size that has occurred since animals first appeared in the fossil record around 550 million years ago is not due to all animal lineages steadily growing bigger, but rather to the diversification of groups of organisms that were already larger than other groups early in the history of animal evolution.
"That's also something we didn't know before," Payne said. "For reasons that we don't completely understand, the classes with large body size appear to be the ones that over time have become differentially more diverse."
Cope's rule was formulated in the late 19th century after paleontologists noticed that the body sizes of terrestrial mammals such as horses generally increased over time.
To test whether Cope's rule applies to marine animals as a whole, Payne and a team that included undergraduates and high school interns compiled a dataset including more than 17,000 groups, or genera, of marine animals spanning five major phyla-Arthropods, Brachiopods, Chordates, Echinoderms, and Mollusks-and the past 542 million years.
"Our study is the most comprehensive test of Cope's rule ever conducted," Heim said. "Nearly 75 percent of all of marine genera in the fossil record and nearly 60 percent of all the animal genera that ever lived are included in our dataset." PR
