To unravel the link between past climates and animal faunas requires an exceptional fossil record. Chew, an associate professor at Western University of Health Sciences, California, used fossils from the Bighorn Basin of Wyoming, a nearly complete record of around 5 million years of mammalian evolution, to study responses of mammal communities through time.

"The Bighorn Basin fossil record, particularly from this part of the basin, is one of the best early Cenozoic terrestrial records in the world." remarks Chew.

"My colleagues have been assembling the fossil samples on which this work is based for more than 30 years. Their efforts have produced a superb, highly resolved, thoroughly studied record that is unparalleled. This record allows us to examine more sophisticated questions about faunal response to climate and environmental change than was previously possible."

Information on past climate in the Bighorn basin comes from the structure of carbon atoms, known as isotopes, preserved in the rock. This technique revealed three global warming events.

The first occurred 55 million years ago, and has been previously linked to decreasing body-size. However, data from the next two events, occurring two million years later, is required to test if this forms part of a larger evolutionary pattern.

"No other terrestrial record exists with the density of fossils necessary to test faunal response to the later hyperthermals [climatic warming]. The central Bighorn Basin record essentially documents a set of repeated, natural experiments in climate warming." Chew explains.

Chew examined the size of over 7500 fossil teeth from over one hundred types of mammals, and compared them before, during and after the climatic warming events. On average, the Bighorn Basin teeth were 10-20% smaller during the warm periods.

Some lineages of mammals became smaller themselves, in a process known as dwarfing. However, mostly size change was driven by an increase in the abundance of small species relative to large ones in the basin during warm periods.

These findings add to the increasing evidence for the strong link between climatic change and animal populations. "The ability to compare faunal response between events is critical for establishing mechanisms of change and predicting the consequences of future warming."

Different Mechanisms Of Body Size Change During The Hyperthermals Of The Early Eocene; CHEW, Amy, Western University of Health Sciences, Pomona, CA, USA - Journal of Vertebrate Paleontology.