AMHERST, Mass. - The evolution of mammals appears to be related to global climate changes, according to a new study by University of Massachusetts biologist Jin Meng. The research, conducted with Malcolm C. McKenna of the American Museum of Natural History in New York City, will be published in the July 23 issue of the journal Nature. This is Meng’s fourth article in the prestigious journal.
"The most significant finding is that the study shows a link between mammalian evolution and major climate changes," said Meng.
According to Meng, the scientists based their work on the study of hundreds of thousands of fossil specimens found throughout the Mongolian Plateau, an area larger than a million square kilometers, located along northern China and southern Mongolia. Fossils from 454 different mammal species, including the ancestors of modern-day horses, rhinos, rabbits, and hedgehogs, were examined as part of the study. Other species were essentially relatives of modern-day rabbits and rodents, Meng said. The study is based on information accumulated since the 1920s, and brings together a large body of scientific work, written mainly in English, Chinese, and Russian.
The team focused on the Paleogene – a geological period that stretched from 65 million years ago to 25 million years ago, Meng said. (Dinosaurs are believed to have lived from 245 to 65 million years ago, during the Mesozoic time, he said.)
The changes in animal life were reflected by animal size differences, species appearances and extinctions, and the evolution of rodent teeth, Meng said. Major evolutionary changes occurred at short intervals that coincided with major climate shifts. These major changes were followed by long intervals of stabilization, with fewer evolutionary changes.
Researchers rely on an intriguing method to help determine the ages of the fossils: they conduct tests on the breakdown of radioactive isotopes in the volcanic ashes and lavas in the same rocks that contain the specimens; such isotopes essentially serve as an eons-long stopwatch. Meng notes that the volcanic ashes do not preserve the fossils; in most cases, the fossils were preserved in stream or lake deposits, he said. The scientists also estimated the ages of some fossils by correlating them with those found on other continents, where the ages of similar fossils have been established.
There were two major global climate changes during the Paleogene, according to Meng. The first, which occurred approximately 55 million years ago, was a warming period called the Eocene, when the mean annual temperature was 30 degrees Celsius, or about 85 degrees Fahrenheit. Humid conditions prevailed, and the planet was heavily forested and inhabited by some of the largest land mammals that have ever existed, Meng said, "including one species four-and-a-half times as heavy as an elephant." Other typical mammals of the time were the ancestors of modern-day horses and rhinos, he said.
A subsequent cooling-off period called the Oligocene occurred approximately 34 million years ago, when the average temperature fell significantly. The cooler era, which is similar to the climate we live in today, resulted in a more arid environment and open country. This proved to be more hospitable to smaller animals such as rodents, rabbits, and hedgehog-like animals, Meng said.
Although researchers examined various skeletal remains in the study, including bones, skulls, and some complete skeletons, they relied greatly on fossilized teeth in order to determine the sizes of animals that lived millions of years ago. The teeth vary in size, from one approximately 10,000 square millimeters (about four inches square), to four tiny teeth, each 1 square millimeter (slightly larger than a dot made by a pencil), stuck in a jawbone fragment that fits inside a medicine capsule. The shape of the teeth is significant as well: the animals that lived during the warmer Eocene period had cuspid, or sharp, pointed teeth, while the small Oligocene animals, which lived during the cooler time, had higher-crowned, ridged teeth which are useful in grinding tough, fibrous grasses, Meng said. "All the data are consistent and correlate with major climate changes," he said.
Meng did his undergraduate work at Beijing University. He earned his master’s degree at the Graduate School of Sciences and Technology in Beijing, and the Institute of Vertebrate Paleontology and Paleoanthropology, also in Beijing. He received his doctorate at Columbia University. Meng conducted postdoctoral research at the National Museum of Natural History at the Smithsonian Institution; the University of Alberta, Canada; and the American Museum of Natural History in New York City before joining the UMass faculty in 1996.