The demise of the dinosaurs 65 million years ago opened the door of opportunity for mammals to take over the Earth—that much is clear. What's coming into focus, thanks to a study out in Science, is just how fast mammals maxed out their size once the terrible lizards were out of the way.
"For the first 140 million years of our evolutionary history we really did nothing—we were really kind of boring," Felisa Smith, an associate professor of biology at the University of New Mexico and coauthor of the new study. ... But across all of the major continents, during the first 25 million years after the dinosaurs were wiped out, mammals underwent an explosive growth spurt. By 42 million years ago, however, the researchers found, the intense growth had leveled off. [Scientific American]
Smith's team surveyed fossils from around the world, including 32 different mammalian orders. No matter where they looked, she says, they saw the same pattern. Mammals that survived the extinction event were small, mostly rodent-sized. Then all over the planet they exploded in size during that period of 20 to 25 million years.
"We had a giant Earth with nothing big on it anymore; and so I think that ecological opportunity allowed mammals to just go nuts." "Going nuts" meant land mammals diverging in shape and size. Some mammals attained weights of 15-17 tonnes, including Indricotherium, a mammal related to horses, and Deinotherium, a member of the elephant family. [BBC News]
Another factor that helped mammals put the "mega" in megafauna
was the Earth's climate, Smith argues. It was cooler then, which translates into larger ice caps and more exposed land area, giving mammals room to grow. Nevertheless, even the largest mammals grew nowhere near as large as the largest dinosaurs. The reason, Smith says, is bound up in physiology.
It likely has to do with thermoregulation, Smith says. As endothermic mammals, we spend the majority of our energy keeping body temperature stable. And if at least some of the largest dinosaurs were exotherms, they could use more of their energy to grow—and "when you're 100 tons, you don't change temperature very fast," Smith points out. [Scientific American]
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