"Based on features of the nest, we know that the mother dug in freshly deposited sand, possibly the shore of a river, to build a mound against which she laid her eggs and on which she sat to brood the eggs," Therrien said. Analysis of the substrate under the nest shows the mother disrupted the rock underneath, proving that she put out substantial digging effort when crafting her mini nursery. Zelenitsky said eggshell fragments from at least five eggs indicate that approximately one dozen asymmetrical, elongated eggs were laid against the sides of the mound, creating a shape somewhat like a sunflower. The raised center, devoid of eggs, was flat, providing a comfortable spot for the dinosaur mom to rest during the brooding. High-powered magnification of the outer surface of the eggshells revealed their unique structure. Ridge patterns, arranged like wire in a chain link fence, gave strength to the eggs, which were around five inches thick at their widest point. The eggs additionally appear to have been laid two at a time, a significant determination since more primitive dinosaurs and today's crocodiles and alligators lay their eggs all at once. Birds, in contrast, lay eggs one at a time. "Here we have an intermediate stage where this dinosaur laid eggs two at a time," Zelenitsky said, explaining that modern birds have lighter skeletons that promote the single egg method. While the mother of the nest she studied was a small dinosaur, she estimates that it weighed around 88 pounds. It was close -- but not close enough -- to being more like a big, flightless bird. Jack Horner, curator of paleontology at the Museum of the Rockies, and his colleagues previously discovered another rare North American carnivorous dinosaur nest full of eggs at, appropriately enough, a site called Egg Mountain in Montana. He attributed the nest to a Troodon, which also laid its eggs in pairs. He said this suggests each oviduct of the mother harbored just one egg at a time, which implies that she had to remain in or near the nest until at least the last pair of eggs had been laid, further supporting that dinosaurs brooded their clutches. Zelenitsky and Therrien aren't sure what happened to their Cretaceous dinosaur's brood, but they think it's possible that a flood occurred, prompting the mother dinosaur to flee to safety and abandon her unhatched offspring. Related Links: |
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