The herbivores grew large first, perhaps because they had an advantage in eating the vegetation left flourishing after the plant-eating dinosaurs were gone.
Just like with today’s lions and elephants, the largest carnivores who came along a bit later remained an order of magnitude smaller than the biggest prehistoric herbivores.
Why did mammal size level off? Available land area and the earth’s temperature, Smith said.
Ninety percent of the food mammals eat goes to maintaining their core body temperature, and the amount of food is related to the amount of land supporting a population.
The biggest mammals evolved when a cooler climate meant lower sea levels and more land area. Also, bigger animals conserve heat better, a problem when temperatures rise.