Animal lineages tend to evolve toward larger sizes over time, according to a new study which found that over the past 542 million years, the mean size of marine animals has increased 150-fold.
"We've known for some time now that the largest organisms alive today are larger than the largest organisms that were alive when life originated or even when animals first evolved," said Jonathan Payne, a paleobiologist at Stanford University's School of Earth, Energy & Environmental Sciences.
It remained unclear, however, whether the average size of animals has been changing over time and, if so, whether that reflects a trend, or directionality, in body size evolution.
The new study found that over the past 542 million years, the mean size 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.
"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," Payne said.
The study supports Cope's rule, researchers said.
Named after paleontologist Edward Cope, 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.
Scientists have tested Cope's rule in other animal groups, but conclusions have been mixed. Corals and dinosaurs seem to follow Cope's rule but birds and insects do not.
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.
To compile such a vast dataset, the team relied heavily on the Treatise on Invertebrate Paleontology, a 50-volume book set that includes detailed information about every invertebrate animal genus with a fossil record known to science.
Using photographs and detailed illustrations of fossils in the Treatise, the team was able to calculate and analyse body size and volume for 17,208 marine genera.
A pattern soon became apparent: not all classes - groups of related species and genera - of animals trended toward larger size, but those that were bigger tended to become more diverse over time.
The team suspects this is due to advantages associated with a larger size, such as the ability to move faster, burrow more deeply and efficiently in sediment, or capture larger prey.
The study is published in the journal Science.
