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Eggs tell story of baby dinosaurs' first steps

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Scientists have discovered fossilized eggs containing developing dinosaurs that probably started out moving around on all four limbs before learning to walk only on only two legs -- kind of like people.

The eggs contain some of the oldest dinosaur embryos ever discovered. These dinosaurs are called "prosauropods," and they are the ancestors to the famous, giant, plant-munching dinosaurs called sauropods. Brachiosaurus, Apatosaurus and Diplodocus are a few examples of sauropods.

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The funny thing is that these massive sauropods walked on four legs all their lives while adult prosauropods probably walked on two legs, at least some of the time. The newly discovered developing prosauropods provide some interesting clues about this transition from two legs to four legs.

Robert Reisz of the University of Toronto and his colleagues discovered six fossilized eggs from a prosauropod dinosaur called Massospondylus. They found the eggs in South Africa and estimate that they are about 190 million years old. The researchers describe their findings in the 29 July issue of the journal Science.

From studying the bones of the embryos curled up in some of the eggs, the authors concluded that the dinosaurs were probably four-legged walkers as babies and then began balancing on two legs later in life. The embryos appeared to have relatively large front limbs and heads, necks that stretched horizontally, and short tails, which all fit well with the idea that they walked on four legs at first.

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Reisz and his colleague propose that as the sauropods evolved later, they may have continued to walk on four legs, like they did as babies, even when they developed into adults.

The researchers also say that the embryos' body proportions, including their large heads, suggest that it would have been difficult for the hatchlings to move around efficiently. And, the embryos showed very little evidence of teeth. Both these findings suggest that the baby dinosaurs may not have been very independent at first and that their parents may have had to look after them for some time.

Prosauropods lived during the Triassic period, about 250-200 million years ago, and then during the first part of the Jurassic period, which was approximately 200 to 150 million years ago. The sauropods lived during later part of the Jurassic and during the Cretaceous period, about 150 to 65 million years ago. Many of the dinosaur embryos found before this discovery came from the Cretaceous period.