Dinosaur skulls are a rare find, dinosaur embryos rarer still. But embryos of giant dinosaurs with intact skulls were unknown until now. In the middle of an ancient Argentinean nesting ground crammed with thousands of dinosaur eggs, scientists recently found six exquisitely preserved embryos, their articulated jaws, peg-shaped teeth, and even nostrils intact. The foot-long remains—still curled up inside their cantaloupe-sized eggs—offer unique insights into the early life of a group of long-necked plant-eaters called sauropods, which included the largest animals ever to walk the Earth.
Vertebrate paleontologist Luis Chiappe of the Natural History Museum of Los Angeles County, working with colleagues in Argentina, discovered the huge hatchery in 1997 at Auca Mahuevo in Patagonia. The identity of the nesting dinos remained elusive, however, until the current discovery. The contours of the embryos' skulls and the shape of their jaws and teeth quickly identified them as titanosaurs. In adulthood these creatures attained lengths of 30 to 100 feet, yet we now know they emerged from eggs hardly larger than those of an ostrich. How the titanosaurs grew so large while dining on a meager diet of leaves and seeds remains a mystery.
Unhatched titanosaur eggs, laid 80 million years ago, preserve the fine details of anatomy and skin texture shown in this illustration.Photograph courtesy of Steve Melendrez/The Natural History Museum of Los Angeles County
But the fossil embryos—which apparently were close to hatching—are filling in other gaps in our understanding of these huge beasts. Adult titanosaurs had nostrils high on their heads, but the embryos' nostrils were near the tips of their snouts, revealing that the creatures' entire facial structure changed as they aged. And the density and abundance of eggs at Auca Mahuevo show that the parents returned time after time to the colony, perhaps patrolling its periphery to protect their young from predators. "They gathered in great numbers, scooping out sand to lay the eggs in nests that they built," says Chiappe. "We're assembling a picture of sauropod reproduction that was not known before."