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How Did Dinosaurs Reproduce?

Crocodiles and birds, the closest living relatives of extinct dinosaurs, build nests and lay eggs. So it's not surprising that fossils provide evidence that non-avian dinosaurs did so, too. But direct fossil evidence for nests and eggs is unusual because these objects are fragile and easily eroded or destroyed.The Central Asiatic Expeditions, led by AMNH's Roy Chapman Andrews and Walter Granger, discovered some of the earliest, well-preserved dinosaur eggs in Mongolia during the 1920s. The oval-shaped eggs, about 20 cm long, were thought to belong to the most commonly found dinosaur at the Flaming Cliffs, Protoceratops. However, AMNH expeditions in the 1990s discovered identical eggs, one of which contained the embryo of an Oviraptor-like dinosaur, which altered our view of which dinosaur laid these eggs. Also, skeletons of Oviraptor were discovered squatting on top of clusters of eggs, with their arms folded back against their body, just like many living birds brood on their nests.Even some giant, long-necked sauropod dinosaurs laid eggs and built nests, as documented by discoveries during the 1990s by AMNH paleontologists in Patagonia. There, rock layers containing thousands of spherical fossil eggs, about 15 cm in diameter, were strewn across low ridges and flats over several square km. Careful collecting revealed that a few of these eggs contained fossilized embryos of a kind of sauropod called a titanosaur. When hatching, these tiniest giants would have been only 30-36 cm long. A few well-preserved nest structures showed that the titanosaurs intentionally excavated a basin about 1.5 m across in which to lay between 15 and 35 eggs at a time, and the presence of fossil eggs in at least six different rock layers showed that the titanosaurs returned to this nesting site many times.
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