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    Dinosaur Descendants Shrank to Survive

    Tyrannosaurus rex went extinct in a relatively short compared to the dinosaur line that led to birds. (Credit: Royal Ontario Museum)
    Tyrannosaurus rex went extinct in a relatively short compared to the dinosaur line that led to birds. (Credit: Royal Ontario Museum)
    Rosanne Skirble
    Not all dinosaurs went extinct. Some are alive today in the form of birds. A new study finds that shrinking helped these birds continue to thrive and evolve.  

    “If we really want to know how birds came about, then we need to study the line leading to birds, which includes this big diversity of animals like triceratops and stegosaurus and T. Rex,” said Roger Benson, associate professor of paleontology at Oxford University and lead author of the study reported PLOS Biology.

    His team followed the evolution more than 400 dinosaurs, noting their size and what they weighed.  

    “The largest dinosaur in our study we believed weighed 90 tons, and the smallest dinosaur was a bird called Qiliania and that weighed 15 grams," Benson said. "So you could fit Qiliania six million times inside Argentinosaurus, the largest dinosaur.”
     
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    Since these species are extinct, Benson's team calculated that weight with an analysis of fossil limbs which, like pillars, would hold up the weight of the dinosaur.

    “And there’s a welcome strained relationship between the robustness of those pillars and the mass of the animal that is observed among modern mammals and reptiles, and so it’s fairly reliable for knowing the masses of extinct animals,” he said.
    The lightweight Archaeopteryx is considered the first bird. Its shrinking size might have been key to the evolutionary success of birds. (Credit: Roger Benson)The lightweight Archaeopteryx is considered the first bird. Its shrinking size might have been key to the evolutionary success of birds. (Credit: Roger Benson)
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    The lightweight Archaeopteryx is considered the first bird. Its shrinking size might have been key to the evolutionary success of birds. (Credit: Roger Benson)
    The lightweight Archaeopteryx is considered the first bird. Its shrinking size might have been key to the evolutionary success of birds. (Credit: Roger Benson)


    The study examined how fast body size changed on the entire family tree of dinosaurs, based on the thickness of their thigh bones. It found dinosaurs quickly evolved big bodies, soon after their origins about 220 million years ago.

    Then the rates of growth slowed and these giants went extinct after 20 million years. Benson says the one exception was the line leading to birds, which kept evolving a range of body mass, including radically smaller body sizes.

    “We find that fast rates of evolution are maintained for the whole study period, so for nearly 200 million years," Benson said. "So this is a very evolvable lineage. There’s a long ancient background to the modern radiation [spread] of birds through which their ancestors were constantly finding new ecological opportunities and inventing new ways of being an organism.”

    He says the long journey evident in the fossil record that connects the 10,000 bird species alive today underscores a bigger evolutionary question.

    “We're no longer asking how the radiation of living birds proceeded over, let's say, 100 million years," he said. "Instead we're asking where does biological diversity come from over very long time scales and ultimately those time scales might be relevant in establishing how life evolved in general, not just birds or just even vertebrates, but just organisms or animals in general?”  

    Benson says what is clear from this study is that evolving different sizes was important to the success of the feathered dinosaur lineage. His next step is to look at other patterns in dinosaur evolution to see how other features changed over time.

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