Most people can stand comfortably under the jawline of a mounted Tyrannosaurus rex or walk under the rib cage of a Brachiosaurus without bumping their heads. T. rex is as big as the largest known African elephant, and Brachiosaurus, like other great sauropods, was much larger than any land animal alive today. We are so used to the enormous size of dinosaurs that we almost forget to think about how they grew to be so large. How long did it take them, and how long did they live? And does the way they grew tell us about the way their bodies worked?

Until recently, we had no way to measure age in a dinosaur. Paleontologists had generally assumed that because dinosaurs were reptiles, they probably grew much as reptiles do today--that is, rather slowly. Thus, the thinking went, large dinosaurs must have reached very old ages indeed, but no one knew how old, because no living reptiles attain anything near the size of a dinosaur.

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