We know some dinosaurs like Ichthyosaur, a marine reptile which lived between 187 and 178 million years ago, had live births. In the photo below you can see a fossil of a pregnant Ichthyosaur with skeletons of three unborn young inside and one just being born.
Other dinosaurs laid eggs. Below is just one of a batch of 22 dinosaur eggs with fossil embryo smuggled from China and returned to the Chinese consulate general in 2011.
Remember the scene in Jurassic Park when they witness a dinosaur hatching from an egg:
Well this month Robert Reisz from the University of Toronto and his fellow paleontologists working in the Yunnan province of China discovered a cluster of 200 tiny fossils bones left behind by ancient dinosaur embryos. Upon further inspection it turned out to be the oldest remnants of dinosaur embryos ever discovered by human beings.
The scientists discovered the embryos in a layer of sedimentary rock. They date them to the early Jurassic Era, making them approximately 190 million years old. The fossils’ importance extends beyond their age, though. They are probably unhatched embryos of Lufengosaurus at different developmental stages, providing a unique opportunity to investigate the embryonic development of a prehistoric species.
The research team focused their analysis on the most prevalent and best-preserved bones: femurs, or thigh bones. These little leg bones ranged from 0.5 to 0.9 inches in length, shorter than matchsticks. The bones were porous, filled with cavities that would have once allowed blood to flow to the growing tissue. The size of the cavities is determined by how fast the animal grows — which made researchers realize these embryos got big quickly. The researchers also found an asymmetrical thickening in the femurs associated with muscle action on the bone. The finding suggests the baby dines were kicking and twitching inside their eggs.
The fast growth rate makes sense, given that Lufengosaurus grew to gigantic size 20 feet (6 meters) in length.