The average size of marine animals has increased by a factor of 150 since the Cambrian Period began about 542 million years ago, a US study has said.
The findings, published Thursday in the US journal Science, provided fresh support for Cope's rule, a theory in biology that suggests that animal lineages tend to evolve toward larger sizes over time.
Named after paleontologist Edward Cope, the 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.
But it is still not clear if Cope's rule applies to marine animals. Some scientists have wondered whether the pattern observed in land mammals was a real evolutionary phenomenon or merely a statistical one resulting from random, non-selective evolution, also known as neutral drift.
In the new study, Noel Heim of the Stanford University and his colleagues compiled a dataset including more than 17,000 groups, or genera, of marine animals spanning five major phyla -- arthropods, brachiopods, chordates, echinoderms, and molluscs - over the past 542 million years.
"Our study is the most comprehensive test of Cope's rule ever conducted," Xinhua news agency quoted Heim as saying. "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."
They found that the minimum size of marine life has decreased by a factor of less than 10 since the Cambrian, while the maximum size of life in the sea has ballooned by more than a factor of 100, 000 in the same amount of time.
The researchers compared their analysis to a number of popular models and found that no random processes could explain these significant differences.
"The degree of increase in both mean and maximum body size just aren't well explained by neutral drift," Heim said. "It appears that you actually need some active evolutionary process that promotes larger sizes."
Heim suspected such active factors associated with a larger size include the ability to move faster, burrow more deeply and efficiently in sediment, or capture larger prey.
The findings could prompt scientists to investigate if there was a trend in the evolution of other traits, the researchers said.
"The discovery that body size often does evolve in a directional way makes it at least worth asking whether we're going to find directionality in other traits if we measure them carefully and systematically," co-author Jonathan Payne, a paleobiologist at Stanford's School of Earth, Energy and Environmental Sciences, concluded.
