Climate did in giant Mongolian mammals.

Mammals romping around Mongolia 40 million years ago could put to shame the stars of modern zoos. During the Eocene epoch, early relatives of rhinoceroses reached four and a half times the weight of an adult elephant, placing them among the largest land mammals ever. These great creatures flourished in warm, wet woodlands but disappeared because the climate went sour, according to a comprehensive study of Mongolia's ancient inhabitants.

The new analysis, involving 454 fossil species, was carried out by Jin Meng of the University of Massachusetts at Amherst and Malcolm C. McKenna of the American Museum of Natural History in New York. The mammal community, they report in the July 23 NATURE, changed markedly 33.5 million years ago, just after the end of the Eocene epoch. A world dominated by rhinolike creatures and many medium-size mammals gave way to one ruled by rodents and rabbits. At the same time, the global climate cooled by 13 [degrees] C, and the Mongolian forests turned into dry, open grasslands where smaller mammals had the advantage, says Meng.

Meng and McKenna compare the major evolutionary shift, which they name the Mongolian remodeling, to a roughly contemporaneous event in Europe. In both places, cool grasslands replaced warm forests, large herbivores disappeared, and smaller plant-eaters rose to dominance.

Past studies have yielded some contradictory conclusions on whether climate shifts at the end of the Eocene influenced the extinction and evolution of mammals.

John Alroy of the National Museum of Natural History in Washington, D.C., is not convinced by the new report. "It's possible [that the cooling caused a major change in Mongolian communities], but I don't think their data are robust enough to tell." In his study of North American mammals, published in the May 1 Science, Alroy found that medium-size tree climbing mammals disappeared in the middle of the Eocene, not at the end of this epoch.