THE evolutionary success of a particular group of organisms is judged by the number of different species in that group. But that is not the complete story, say palaeontologists in North Carolina, USA. Calculating the abundance of each species can reveal remarkably different trends. Most studies of ancient ecosystems involve counting the number of different species in a fossil find. But determining the relative abundance of each species requires the painstaking examination of many fossils. "It's just damned slow," says Frank McKinney of Appalachian State University in North Caroline, USA.

However, McKinney realised that the relative abundance of species is significant when he was studying tiny invertebrates called bryozoans in the Adriatic Sea. Living colonies contained roughly equal numbers of species from two major bryozoan groups, but one group accounted for more than 95 per cent of the colonies' biomass.

McKinney and his colleagues then looked at bryozoan fossils from the past 150 million years, carefully weighing shells from two groups. They found that one group, called cyclostomes, accounted for nearly all the species until 100 million years ago, when a second group, the cheilostomes, became common.

During the great extinction that killed the dinosaurs at the end of the Cretaceous, numbers of both classes plummeted.

Counting the number of species suggests that the cheilostomes rebounded more quickly than the cyclostomes, but McKinney's mass measurements tell a different story. He found that the cyclostomes enjoyed a burst of abundance just after the extinction. "We didn't expect to see (that pattern) at all," he says.

"It's not a devastating blow to the counting of taxa to understand biodiversity," says David Jablonski of the University of Chicago. "It says there are additional parameters that have really interesting stories to tell," he says.