Paleontologists have long assumed that giant dinosaurs called sauropods, like all other dinosaurs, evolved from smallish bipedal ancestors and dropped down on all fours only as their bodies grew too large to be carried on two feet. But a pair of embryos dug up about 30 years ago--the oldest fossilized dinosaur embryos so far discovered--reveal a surprise. The fossils suggest that sauropods were already quadrupedal even as smaller creatures.
The clues are indirect, because the embryos are not sauropods but members of their closest kin, a group of much smaller herbivores called the prosauropods. Paleontologists found them inside remarkably well preserved eggs of a 5-meter-long animal called Massospondylus, which 190 million years ago roamed the floodplains of what is now South Africa.
After carefully removing the rock surrounding the fossil embryos, a team led by Robert Reisz of the University of Toronto's Mississauga campus in Canada and colleagues, discovered unusual body proportions. The neck was long, the tail short, and the hind and forelimbs were all roughly the same length. "It was an awkward little animal," Reisz concludes. To Reisz, the large head, tiny pelvis, and limb proportions all suggest that the embryo would have walked quadrupedally after hatching.
If the earliest sauropods also developed from embryos with quadrupedal proportions, Reisz and his colleagues propose in the 29 July issue of Science, sauropods may have become quadrupedal adults by retaining their juvenile state into adulthood, a phenomenon called pedomorphosis. "This would be significant because it means we might have to re-evaluate the origin of many features in sauropod skeletons we assumed had to do with weight support," says Matthew Bonnan of Western Illinois University in Macomb.
Some paleontologists, however, are wary of trying to read too much of the history of sauropod evolution from two embryos. So little is known about dinosaur embryology, they say, that it's dicey to reconstruct the locomotion of hatchlings and extrapolate to other taxonomic groups. "It's a stunning find," says Anusuya Chinsamy-Turan of the University of Cape Town, South Africa, but "I have all these questions."