Dinosaurs were reptiles that dominated the Earth during the Mesozoic Era (250 to 65 mya, or million years ago). The initial dinosaurs were small (less than a meter to few meters in size) during the Triassic period (upto 202 mya), but they grew much larger during the Jurassic and Cretaceous periods, when the fearsome T. Rex roamed the Earth.

Massospondylus carinatus (Embryo and Adult) (Courtesy: Sciam)
Scientists have discovered the oldest fossilized dinosaur embryos yet, which reveal tantalizing clues about dinosaur evolution. The embryos 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 are about 190 million years old, which was the beginning of the Jurassic period, and illustrate the development of Massospondylus, a creature that grew to five meters in length.
Scientists were able to compare the fossils of fully grown Massospondylus with the embryos, and 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, its neck grew faster compared with its head and forelimbs, resulting in a body type more suited to bipedal locomotion:):).