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Fast-growing dinosaurs kicked inside eggs, say scientists

Researchers used new ancient fossil finds to learn about dinosaurs' early development. The evidence suggests dinosaurs wiggled inside their eggs and grew faster than any birds or mammals living today. 

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How baby dinos grew

The bone bed contained spinal bones, limb bones, shoulder blades and even a few fragments of skull, but Reisz and his team focused their analysis on the most prevalent and best-preserved bones: femurs, or thigh bones. These little leg bones ranged from 0.5 to 0.9 inches (12 to 22 millimeters) in length, shorter than matchsticks.

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The bones were porous, filled with cavities that would have once allowed blood to flow to the growing tissue. The size of the cavities is determined by how fast the animal grows — which made researchers realize these embryos got big quickly.

"They grow very fast — faster than we expected, and faster than most other dinosaurs that have been studied this way," Reisz said.

The fast growth rate makes sense, given that Lufengosaurus grew to 20 feet (6 meters) in length.

The researchers also found an asymmetrical thickening in the femurs associated with muscle action on the bone. The finding suggests the little dinos were kicking and twitching inside their eggs.

The team also discovered evidence of organic material — probably collagen, part of the connective tissue that forms ligaments and tendons. If the material isn't too deteriorated, it could be compared to collagen in living animals, thus providing a new way to look at relationships between modern creatures and the extinct dinosaurs, Reisz said.

"We're setting a new benchmark as to what can be done in dinosaur embryology," Reisz said. He and his colleagues report their findings Thursday (April 11) in the journal Nature. An upcoming goal, Reisz said, is to give the embryos their first dental exam.

"One of the things we may try in the near future is to look at the embryonic teeth themselves," he said. "They're very cool."

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