News in Science

Sea animals are 'growing larger'

Marine explosion Marine animals are getting bigger, according to new research.

But whether this is evidence to support a long-standing rule about animal evolution is a subject for debate.

The findings come from a study reported today in Science that measured a gob-smacking 17,208 animals, most of which were recorded in a 50-volume Treatise on Invertebrate Palaeontology.

The treatise included photographs and illustrations of animals such as starfish, squid, snails, sharks, fish, whales, sea lions, crustaceans and clam-like brachiopods that live on the sea floor, dating back 540 million years

"It took seven to eight years to make all those measurements," says co-author palaeontologist Dr Noel Heim at Stanford University, adding some of the measuring work was outsourced to the younger generation.

"During the summer we have a large number of high school students working with us who can sit down and make measurements."

He and colleagues studied the five major groups of marine animals that have a fossil record.

The massive data set revealed that while some groups like crustaceans have gotten a little smaller, animals in the ocean as a whole are getting bigger.

"The mean size of animals today is 150 times that of animals in Cambrian," says Heim.

He and colleagues used a computer model to simulate evolution and found that natural selection must have been at play to result in the size increase they saw.

Cope's rule

The researchers say that the findings are support for a rule in science known as Cope's rule.

In the 1800s American palaeontologist Edward Drinker Cope noticed that there was a tendency for certain animals to get larger over time.

"The classic example is horses which started out at a miniature dog size," says Heim.

But, says Heim, until now, tests to see if marine animals follow Cope's rule have been quite limited.

The researchers suspect size offers marine animals advantages such as being able to move or hide faster or catch larger prey.

Palaeontologist Dr John Alroy of Australia's Macquarie University welcomes the study.

"It's a wonderful data set ... put together in a really interesting way by a lot of people. It represents an awful lot of work ... and I'm really impressed by that," says Alroy.

But, he is not convinced that Heim and colleagues have the data to confirm Cope's rule applies in the sea.

"The original idea of Cope's rule was that there are strong trends within lineages - meaning you would see changes in size from one species to its descendent species," says Alroy.

He says the increase in average size seen by Heim and colleagues could be explained by the greater success of groups of animals with larger species, such as whales or large molluscs.

While Alroy says Cope's rule has been shown to occur in some marine animals over short periods of time, most lineages in the latest study show clear limits to their size, which could reflect ecological pressures on evolution.

In other words, Cope's rule can't always win out.

"If big was always good you'd have dogs the size of blue whales within a few million years," says Alroy.

Alroy authored a paper in 1998 that showed that Cope's rule applied to North American mammals including horses, dogs and bears.

He says in this specific case, after the dinosaurs were wiped out, there was an opportunity for mammals, which were small at the time, to quickly expand in size and fill the empty niche left by the dinosaurs.