Mammals exploded in size after dinosaurs went extinct and continued to grow for about 25 million years, according to a new study that appears in the journal Science. In the first 140 million years of their existence, most mammals stayed small, most the size of today’s rodents. Then, after the end of the Cretaceous epoch, when dinosaurs disappeared, mammals began to grow, and grow and grow, for about 25 million years.

About 34 million years ago, during the early Oligocene epoch, the largest known land mammal ever, a rhinoceroslike creature known as Indricotherium transouralicum, lived in Eurasia. The species grew to be about 40,000 pounds.

“Basically what happened was the biggest dinosaurs were the herbivores, and when you remove the biggest herbivores there’s nobody eating those plant resources,” said Jessica Theodor, a paleontologist at the University of Calgary in Alberta and one of the authors of the study. “It basically left open space for mammals.”

Dr. Theodor and her colleagues studied fossil records to determine how mammal size grew over time. In most cases, they used teeth to predict body mass.

“Mammals have higher metabolic rates, and we think they are kind of capped at a lower size,” Dr. Theodor said. “It takes more plant material to sustain a higher metabolic rate, so there isn’t enough food to grow bigger.”

Today’s largest land-based mammals, elephants, rhinoceroses and hippopotamuses, are all mainly herbivorous, just as the largest dinosaurs were.