A geological analysis of the spot revealed that slow flooding probably smothered the eggs, which seem to have been laid in a colonial nesting site. After the flood, the embryos and eggs rotted and fell apart, leaving a mound of disarticulated bones. The bones date to the Lower Jurassic period, or between 199.6 million and 175.6 million years ago. That makes them just as ancient as the oldest known embryos ever found, which were discovered at a nesting site of long-necked Massospondylus dinosaurs in South Africa.
It was a boon for science that the dino embryos had fallen apart, instead of fossilizing inside their eggs, Reisz told LiveScience.
"People are extremely possessive and fond of their embryos inside their eggs — imagine us asking them to take pieces out and do the sections on them and cut them, and essentially do damage to them," he said. "These bones are completely disarticulated, and we have a lot of them — so it's not unreasonable to be able to take a few and cut them, and see what their internal anatomy is like."
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.
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.