The embryos, found in their still well-preserved eggs, date to the earlyÂ Jurassic Period 190 million years ago. The researchers say they are the oldest known embryos for any land-dwelling vertebrate.
They belong toÂ Massospondylus, a member of a group of dinosaurs called prosauropods that were ancestors to the giant, plant-eating sauropods.Â Sauropods are the iconic four-legged dinosaurs known for their long necks and long tails.
Professor Robert Reisz of the University of Toronto Mississauga and his colleagues made the discovery while analyzing the fossilized eggs, originally found in South Africa. Reiszâ€™s research assistant, Diane Scott, prepared the delicate fossils under high-powered microscopes and compiled the illustrations.
â€œI donâ€™t think anybody else could have done this job,â€ Reisz said.
The embryos are so remarkably well preserved that they permitted a complete reconstruction of the skeleton and detailed interpretations of the anatomy.
The embryos were close to hatching, revealing their ossification (how much of their skeletons had turned to bone). The fossils also show that the future hatchlings would have been “oddly-proportioned” and would have looked very different from the adults of the species, according to the researchers.
- close to 8 inches long
- relatively long front limbs
- disproportionately large heads
- about 16.5 feet long
- relatively tiny heads
- long necks
- most likely walked on two limbs
The above suggests that as the dinosaurs matured, their necks and hind limbs grew much faster than their forelimbs and head. Later dinosaurs in this group, the sauropods, had body proportions more similar to those of the Massospondylus embryos.
To some extent, these dinosaurs then developed as humans do today. The infancy is “awkward,” as the researchers put it, and a more erect stance and evenly-proportioned body only comes later. Additionally, the embryos lack teeth. With the awkward body proportions, it’s then likely that the hatchlings would have required parental care. If that’s the case, these fossils also document the oldest record of parental care, according to the paleontologists.
â€œThis project opens an exciting window into the early history and evolution of dinosaurs,â€ says Reisz. â€œProsauropods are the first dinosaurs to diversify extensively, and they quickly became the most widely spread group, so their biology is particularly interesting as they represent in many ways the dawn of the age of dinosaurs.â€