Mammals Supersized in a Hurry After Dino Die-Out

By Andrew Moseman | November 29, 2010 12:06 pm

MammalSupersizeThe 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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Image: IMPPS

CATEGORIZED UNDER: Living World
  • Jill

    It bugs me when people use the phrase “mammals took over the world”. As far as multicellular animals go, it’s all about bugs. As far as multicellular life goes, it’s all about plants. And as far as life itself goes, it’s all about bacteria. Mammals are only the big winners when it comes to vertibrates — a very small group. And even that is debatable. It might only be true if you measure size of individuals, rather than something like total biomass or overall diversity.

  • JMW

    I think explosion is a misnomer.

    Assume a rodent-like mammal weighing about 2 pounds, 65 million years ago. Assume that, as its descendants evolved larger size, the overall average generation lasts 10 years. So we’re talking 2 to 2.5 million generations.

    How much does each generation need to grow, in order to evolve from 2 pounds to 34,000 pounds (17 tons, close enough to 17 tonnes for jazz). Turns out each generation would have to increase its mass by
    0.00039 percent for 2.5 million generations
    0.000487 percent for 2.0 million generations

    Of course, these are average growth rates over 25 million years, which completely ignores any punctuated equilibrium considerations. And the growth per generation is small, but to keep up that growth rate for that many generations is pretty impressive.

  • http://clubneko.net nick

    “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.”

    When you’re 100 tons and endothermic, and you don’t change temperature very fast, you wouldn’t need to spend so much energy keeping your body temperature stable either. Right? I would have to guess it would be more to do with having to keep your body temperature stable during an ice age, when sh!t be hella cold all the time, than just keeping a stable temperature in a large beast. Had the world been warm as during the dinosaur’s rule, endotherms wouldn’t have any problem getting big because keeping warm in a warm climate be e-z. Not getting too hot would be a problem, but then you just become a swamp dweller. As we imagine those 100 ton dinosaurs were.

  • Ryan

    @JMW

    Those growth rates seem pretty small. How fast have humans grown since the middle ages just because of changes in diet or whatever has caused humans to get larger since that time? I’d bet the rate is much more than the ones you’ve provided.

    Couldn’t less competition for food (from dinosaurs) allow growth to happen independently of changes in genetics just because the mammals are able to attain more fuel for their growth than before? I’m not a biologist, so I really don’t know. Who knows? Maybe they started putting growth hormones into their food supply.

  • http://www.evolit.tk uyen

    “Nevertheless, even the largest mammals grew nowhere near as large as the largest dinosaurs.”
    Wait, isn’t the largest dinosaur grew nowhere near as large as the largest mammal, the whales?

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