Image: Dino eggs
Julius T. Csotonyi
After analyzing a 77-million-year-old fossilized nest, scientists determined it was likely made by a member of a group of emu-like dinosaurs, or a dromaeosaurid, which zipped around on two legs.
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updated 11/13/2008 6:09:08 PM ET 2008-11-13T23:09:08

An animal that looked like a big bird, had feathers like a bird, laid eggs like a bird and behaved like a bird was, in fact, a dinosaur, according to paleontologists who just linked such a dinosaur with a nest full of neatly arranged eggs.

The 77-million-year-old findings could represent one of the last dinosaur species before certain dinosaurs fully evolved into birds.

The carnivorous, feathered egg layer was likely either a caenagnathid, meaning a member of a group of emu-like dinosaurs, or a dromaeosaurid, which zipped around on two legs.

"We now know that egg-laying traits required gradual evolutionary changes," study leader Darla Zelenitsky told Discovery News.

Zelenitsky, a University of Calgary paleontologist, explained that the nest, found at Two Medicine Formation, Mont., during the 1990's, resembled previously excavated dinosaur nests belonging to an oviraptor and a troodontid. The Montana nest, however, is also very bird-like.

She and colleague Francois Therrien of the Royal Tyrrell Museum of Paleontology first discovered the nest in a private collection housed at Canada Fossils Limited in Calgary. It was labeled as belonging to a hadrosaur, or an herbivorous duck-billed dinosaur.

Such plant specialist dinosaurs are probably not as closely related to birds, which tend to have varied diets that can include everything from seeds to insects to small animals.

The researchers analyzed the nest and its contents in greater detail. Based on its characteristics, they determined that a small meat-eating dinosaur had instead carefully constructed the nest.

The findings are published in the latest issue of the journal Palaeontology.

"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.

© 2012 Discovery Channel

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