This fossilized embryo, 190 million years old, belonged to Massospondylus, a member of a group of dinosaurs known as prosauropods. (University of Toronto)

The oldest-known embryos of any land-dwelling animal ever found show the infancy and growth of early dinosaurs had something in common with humans, says a group of Toronto researchers.

The researchers, including University of Toronto Mississauga paleontologist Robert Reisz and David Evans, a curator in vertebrate paleontology at the Royal Ontario Museum, examined delicate fossilized eggs dating from the early part of the Jurassic period, 190 million years ago.

The fossils, found in 1976 in South Africa, belong to Massospondylus,   a member of a group of dinosaurs known as prosauropods. These dinosaurs are the ancestors of sauropods — large, four-legged dinosaurs with long necks such as brontosaurus and diplodocus.

The embryos are unusually well-preserved, allowing for a complete reconstruction of the skeleton and detailed interpretations of their anatomy.

They show that infancy for the Massospondylus was an awkward period, during which the young were oddly proportioned compared to adults, as is the case among human infants, which have disproportionately large heads. And just like us, Massospondylus babies started out on all fours, until they were able to walk on two legs.

The findings appear in the latest issue of the Journal of Vertebrate Paleontology.

The researchers say the 20-centimetre-long embryos had relatively long front limbs and big heads. In contrast, the five-metre-long adults had relatively tiny heads and long necks and mostly likely were bipedal, because their forelimbs are much shorter than their hind limbs. The researchers say that as the dinosaurs matured, their necks and hind limbs likely grew much faster than their forelimbs and head.

Such insights would not have been possible without some detailed setup work by Diane Scott, Reisz's research assistant. Back in 2004, Scott spent six months peering through a high-powered microscope as she prepared the tiny skeletons of the unhatched dinosaurs within the eggs using dental tools to separate rock from fossilized bone.

"It's very similar to how you have your teeth scaled. We use a similar kind of instrument, and the way it vibrates the plaque off your teeth, it vibrates the rock off the bone. You still have to be very careful, otherwise you can go right through it. It's embryonic material, so it is not hard to begin with," says Scott, who also created detailed illustrations of the embryos.

Preliminary insights into the embryos' anatomy were published in the journal Science in 2005. But in 2007, Scott was able to examine newly discovered fossilized embryos of the same dinosaur discovered in Cape Town. That extra material allowed her to complete her illustrations of the dinosaur's skull and forearm, giving Reisz and Evans more material to examine.