Dinosaurs Flexed Muscles Before Hatching, Study Finds

The oldest collection of fossilized dinosaur eggshells and embryos, found in China, suggests the animals grew quickly and flexed their muscles before they hatched, making them ready for action at birth.

The eggs and embryos were from about 190 million to 197 million years ago, during the early Jurassic period, according to a study published today in Nature. More than 200 bones were extracted at the site, near Dawa in southern China’s Yunnan, near Myanmar.

The bones are from about 20 embryos called Lufengosaurus, creatures that grew to about 26 feet (8 meters) long. An analysis of femurs, the longest bones in the leg, suggested they doubled in size while the dinosaurs were still embryos, from half an inch (12 millimeters) to an inch. It’s not clear how fast this happened, though it suggests the embryos developed quickly, the researchers said.

“We are, for the first time, looking in great detail at how embryos in dinosaur eggs would have developed as they grew,” said Robert Reisz, a paleontologist at the University of Toronto, Mississauga, and leader of the international team that worked on the bones. “Others could only look at a single glimpse, when the death of the embryo was frozen.”

The findings were unusual because they came from more than one nest. The embryos weren’t found intact, so the group concentrated on the 24 femurs to track growth. The relatively rapid rate at which the femur size doubled suggested the animals had a quick growth rate and spent a short period of time in the egg, Reisz said.

Bone Thickness

The bones were of uneven thickness, which suggests the embryos were moving to develop bone strength, as chickens and mice do, the researchers wrote. The group also found evidence of ancient collagen fibers, which typically strengthen bones, around the fossils.

This is also the first time such thin fragments of eggshells have been found; they were 100 microns thick, or about the thickness of a human hair. They were found because the excavation was conducted using trowels and other delicate instruments, Reisz said.

Only a square foot of the site has been excavated, so more discoveries may be coming, Reisz said.

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