The oldest fossilized dinosaur embryos yet discovered are revealing tantalizing clues about dinosaur evolution, scientists say. Findings published today in the journal Science indicate that some of the prehistoric creatures started out on four legs before growing into bipedal behemoths. They also support the notion that newly hatched dinos did not fend for themselves and instead relied on their parents for nourishment.

Robert Reisz of the University of Toronto at Mississauga and his colleagues studied five fossilized embryos inside dinosaur eggs recovered from Golden Gate Highlands National Park in South Africa. The eggs were first discovered nearly 20 years ago, but the painstaking removal of the rocky matrix and eggshells was only carried out over the past year, after a painstaking excavation. The embryos date to 190 million years ago, which was the beginning of the Jurassic period, and illustrate the development of Massospondylus, a creature that grew to five meters in length. "Most dinosaur embryos are from the Cretaceous period [146 to 65 million years ago]," Reisz says. "There are articulated embryos in the Late Cretaceous, like duck bill dinosaurs and theropods, but those are at least 100 million years younger."

Because well-studied Massospondylus skeletons of both juveniles and adults are available, the researchers could chart the growth of the animal. They suggest that Massospondylus's horizontal neck, heavy head and immature limb proportions would have led to it walking on all fours shortly after hatching. As it matured, the authors posit, its neck grew faster compared with its head and forelimbs, resulting in a body type more suited to bipedal locomotion. The fossils also lack teeth. "These embryos, which were clearly ready to hatch, had overall awkward body proportions and no mechanism for feeding themselves, which suggests they required parental care," Reisz comments. "If this interpretation is correct, we have here the oldest known indication of parental care in the fossil record."