Dinosaur embryos: the story they tell.

Dinosaur embryos: The story they tell

More than 60 years after scientists in the Gobi Desert made the first discovery of dinosaur eggs, paleontologists have completed the first study of dinosaur embryos preserved inside their eggs. These embryos, found in Montana four years ago, are revealing the diverse kinds of behavior that marked the early years of dinosaur life. While some dinosaurs appear to have been early developers that could walk about immediately after hatching, others remaining in the nest and relied on doting parents for sustenance, report John R. Horner of the Museum of the Rockies in Bozeman, Mont., and David B. Weishampel of Johns Hopkins University in Baltimore.

The scientists, whose findings appear in the March 17 NATURE, studied 19 embryos of a previously undiscovered and unnamed type of hypsolophodontid dinosaur, along with seven hadrosaur embryos. The hypsolophodontids, Orodromeus malekai, were herbivorous dinosaurs slightly larger than human beings, while the herbivorous hadrosaurs, Maisaura peeblesorum, reached 8 meters in length.

According to Horner, the hypsolophodontid embryos, found together in a clutch of unhatched eggs, had bones and joints that were almost fully developed. In contrast, the long bones of the hadrosaur embryos were capped by incomplete joints, even though they were closer to hatching than the hypsolophodontid specimens.

These observations support earlier evidence that newly hatched hadrosaurs remained nest-bound until they grew to sufficient length and developed fully formed joints. Horner had previously proposed this kind of behavior because he had found the bones of baby hadrosaurs resting in a nest-like structure. In the hypsolophodontid nests, however, he had found the unbroken bottom halves of eggshells. With their fully developed joints, these animals probably left the nest right after hatching, Horner says. Otherwise, they would have crushed their shells.