Brontosaurus Babies Jurassic Period Newborn brontosaurus, member of the plant-eating sauropod family, see light for the first time. Little is known of the time required to complete the process of birth, nor is it known how long it took to achieve adulthood. The dinosaur was a reptile of a species now extinct; no contemporary reptile can trace his ancestry directly to the dinosaur of 135 million to 200 million years ago. The two are remote cousins at best.