If rodents in Spain are any guide, periodic changes in Earth's orbit may account for the apparent regularity with which new species of mammals emerge and then go extinct, scientists reported. It so happens, the paleontologists say, that variations in the course that Earth travels around the Sun and in the tilt of its axis are associated with episodes of global cooling. Their new research on the fossil record shows that the cyclical pattern of these phenomena corresponds to species turnover in rodents and probably other mammal groups as well.