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How land mammals evolved to be so massive

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IndricotheriumAlthough today’s awe-inspiritng African Bush Elephant (Loxodonta africana) might seem a mighty beast, it’s a fraction of the size of ancient mammals that roamed the Earth 37 million to 2.7 million years ago.

The Eocene and Oligocene’s Indricotherium measured in at more than five meters tall, and the Miocene and Pleistocene’s Deinotherium likely weighted some 17,000 kilograms. But how did these mammals shoot up from their mostly miniature predecessors that lived during the Age of the Dinosaurs—and what kept them from getting even bigger?

A new analysis of fossils found across the globe has plotted the meteoric rise of the mammals, a class group that has been around for some 200 million years.

“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, says of our shared mammalian line. Early mammals that lived alongside the dinosaurs tended to be modest, mouse-sized types (with a ranged from about 3 grams to 15 kilograms).

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.

“There seemed to be remarkable consistency of body size across all different continents,” Smith says. And that similarity was especially striking, she notes, “because the continents are really different.” The findings were published online November 25 in Science.

She and her colleagues propose that the animals’ body size was determined in large part by global climate and land area. “The largest mammals evolved when Earth was cooler and terrestrial land area was greater,” Smith and her colleagues wrote in their paper. These two abiotic factors are not unrelated—with cooler climate translating into larger ice caps and thus more exposed land.

Another pattern they found (one “that kind of blew us away,” Smith says) was a consistent difference between herbivores and carnivores. “The largest [mammalian] carnivores have never been more than a ton,” Smith says, noting that this apparent size limit “might be more to do with how carnivores work” and often hunt in groups to bring down midsized rather than the largest herbivores.

So why did mammals never grow quite as massive as their former overlords the dinosaurs (the largest of which were nearly 10 times the size as of the largest mammal)?

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.

Although the hulking Deinotherium died out some 2.5 million years ago, many of North America’s megafauna persisted until close to 13,000 years ago. So though there might not been any wooly mammoths roaming the tundra these days, more research is helping scientists paint a better picture the evolutionary tale of mammals. Plus, Smith notes, “it’s just kind of cool to envision a planet with all of these big mammals.”

Image: Indricotherium, courtesy of Wikimedia Commons/Dmitry Bogdanova

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  1. 1. jtdwyer 4:28 pm 11/25/2010

    As I understand, the dinosaurs generally benefited from significantly higher atmospheric oxygen levels and their highly efficient air-sac respiratory system, shared with modern birds. While the size of modern birds are generally restricted by flight requirements the large terror-birds did fill the large predator niche after the dinosaurs’ extinction.

    Efficient respiration may have allow dinosaurs to produce energy more effectively in their high oxygen environment, again providing more energy for growth.

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  2. 2. reflectogenesis 5:37 pm 11/25/2010

    Maybe mammals developed efficient fractal patterns of arterial and neurological structure. Would this cause a phase change in biological tissue, making them more efficient at scavenging toxic gaseous decomposition products resulting in bigger brains,greater social skills greater herding instincts ranging over wider territories.

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  3. 3. JamesDavis 8:03 am 11/26/2010

    "jtdwyer", I agree with you. The bigger the trees get, the more oxygen they produce, thus giving cleaner richer oxygen to the animals. The bigger the tree gets, the bigger the herbivore has to get to reach its food. The bigger the herbivore gets the bigger the carnivore has to get to reach its food. The smaller animals acted as vacuum cleaners, cleaning up the forest’s floors. The bigger the animals got, the more CO2 they produced – the more the trees benefited.

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  4. 4. rocket7777 6:39 am 11/27/2010

    Glacier causes erosion. After it start to melt, it create super growth. Similarly, whale can feed on plankton growing in rich cold ocean.

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  5. 5. Shortie 12:38 am 11/29/2010

    Interesting hypotheses.
    1. Do we have any accurate composite data on global plant type/distribution?
    2. What do we know about the relative efficiencies of flora predating current deciduous forests by 100 to 500 mya?

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  6. 6. jtdwyer 5:02 am 11/29/2010

    For some general ideas you can refer to the non-authoritative source:

    (or other geologic periods). There are links to en tries for more specific types of period flora, for example, angiosperms, Mesozoic gymnosperms like Conifers, Gymnosperm taxa like Bennettitales…

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  7. 7. Earthling 5:27 pm 11/30/2010

    I don’t believe oxygen levels affected the max. size of dinosaurs or mammals. Earth’s surface gravity most likely changed, not due to Earth expansion though. A new theory posits that the consolidation of the continents to form Pangea caused a shift of the Earth’s cores to "balance" the rotating globe. Being off-center, the cores would exert lower gravitational force on Pangea.
    Based on this theory, as Pangea broke apart, surface gravity would have rapidly changed at the end of the Cretaceous and then less rapidly to today’s level. This would explain why the largest mammals didn’t continue to maintain their maximum size.

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  8. 8. Grasshopper1 4:10 pm 12/5/2010

    Nice theory. Just one question: instead of growing, couldn’t the herbivores adapt to eat a different plant that wasn’t so tall?

    On a separate note, in the article it says "… that lived during the age Age of the Ddinosaurs …" Shouldn’t that say "that lived during the Age of the Dinosaurs"?

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  9. 9. gjh42 1:19 pm 12/7/2010

    "A new theory posits that the consolidation of the continents to form Pangea caused a shift of the Earth’s cores to "balance" the rotating globe. Being off-center, the cores would exert lower gravitational force on Pangea."

    Wow. It’s hard to know where to start with this. If the "cores" did shift, it would not be to lower the surface gravity on the major landmass. Rather, the center of gravity would move minutely closer to the center of the landmass as it accumulated (resulting in the core being a hair in the other direction from the center), thus maintaining surface gravity at a constant average worldwide. The ocean would shift to follow the center of gravity, making the landmass closer to sea level. Any lowered gravity at high elevations would be totally inadequate to produce a change in animals’ general body size Pangea-wide.

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  10. 10. Earthling 3:10 pm 12/7/2010

    The Earth’s cores make up about 35% of the Earth’s mass; the denser lower mantle makes up about 50%. If they shifted away from Pangea to offset the rotational imbalance caused by the consolidation of the continents, the gravitational impact on Pangea would be significant based on the inverse distance-squared rule.
    The center of mass would shift away, not toward Pangea. Think of a spinning top: While it’s spinning, add a tiny mass at the widest point on the circumference. The only way it can remain in balance, i.e., not wobble out of control, is to add mass directly opposite from the mass that was originally added on the opposite side of center along the radial line drawn from the initial mass through the center.

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