A cache of seven fossilized dinosaur eggs that was discovered in South Africa almost 30 years ago but only recently studied in depth reveals that the animals, as youngsters, must have needed help. The finding also suggests how related species later could have evolved their immense statures.
Skeletal remains showed up in six of the 190-million-year-old eggs, each about the size of a chicken egg. The eggs rank as the oldest fossils of dinosaur embryos yet discovered, says Robert R. Reisz, a paleontologist at the University of Toronto at Mississauga in Ontario.
The research team has identified the creatures as a common plant eater called Massospondylus carinatus. Adults of the species, one of a group called prosauropods, measured about 5 meters long.
The degree of bone development in the fossils, as well as the fact that the embryos were large enough to fill their shells, suggests that the creatures were ready to hatch. The embryos have tooth sockets, all of which are empty except for one containing what may be a partially erupted tooth. That suggests that M. carinatus hatchlings had no teeth or teeth too soft to be preserved by fossilization. In either case, new hatchlings probably couldn’t have nipped vegetation from plants and would have required parental care, says Reisz.
Although adults of this species were bipedal and had relatively small forelimbs, limb proportions of the embryos were more balanced, suggesting that hatchlings would have walked on all fours, says Reisz. The large head, relatively stiff neck, and puny pelvic bones of the embryos would have made the youngsters awkward—another clue supporting the notion of parental care. Reisz and his colleagues report their findings in the July 29 Science.
Body proportions of the M. carinatus embryos may provide clues to how evolution shaped subsequent dinosaur species, including a group of relatives known as sauropods. The juvenile M. carinatus body shape reflects the proportions of later sauropod adults.
If youngsters of these species retained the proportions of M. carinatus hatchlings and extended their adolescent growth spurt, then the creatures could have grown to gigantic proportions. Some later sauropod species were the largest land animals that ever lived, attaining weights between 80 and 100 tons (SN: 6/23/01, p. 397: Available to subscribers at Sahara yields second-largest dinosaur).
That a gradually extended juvenile-growth phase in prosauropods led to the immensity of some later sauropods is “an interesting idea,” says Paul Barrett of the Natural History Museum in London. Although the notion isn’t new, Reisz’ study provides the first skeletal evidence.