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 30-metre-long blue whale.

Renowned 19th-century American palaeontologist Edward Drinker Cope had proposed ‘Cope’s Rule,’ hypothesising that animal lineages tend to increase in body size over time.

The dinosaur fossils he dug up in the American West seemed to bear this out.

In water, larger animals can be more active because of increased mass relative to their surface area

But Cope was not infallible. In a mistake famed in the annals of palaeontology, 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 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 towards larger size over time.

The research covered five major marine animal categories:

• Chordates including fish, sharks, marine mammals like whales, and marine reptiles like plesiosaurs, mosasaurs and ichthyosaurs;

• Molluscs including squid, clams and snails;

• Echinoderms, including starfish and sea urchins;

• Arthropods, including crabs, shrimp, lobsters and the once ubiquitous trilobites; and

• Brachiopods that look 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 palaeobiologist Noel Heim, whose study appears in the journal Science.

“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.”

The scientists said the trend towards 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 palaeobiologist Jonathan Payne.

Instead, it indicates “an active evolutionary process that favoured animals with larger sizes,” Payne said.