Firstthe watertight egg, also known as amniotic eggs, is a feature only found inturtles, lizards, dinosaurs (including birds), mammals, and their relatives(American Museum of Natural History, n.d.). Being that these organisms have awatertight egg tells us that they inherited this feature from a commonancestor. Amniotes are the class of animals that express this characteristic.The watertight contains a fluid-filled membrane that surrounds the embryo andkeeps it from drying out. This allowed amniotes to colonize on land, since theyno longer needed a wet environment to lay their eggs.
Dinosaurs are divided inSaurischians and Ornithischians based on hip structure. Then, the hip structureof Ornithischians dinosaurs is advanced compared to Saurischians. The pubisbone located at the front of the pelvis has an extension that points towardsthe rear (American Museum of Natural History, n.d.).
The hip structure isthought to resemble a bird’s hip. Finally, after the amniotic egg, animalsstarted keeping their embryos within. The placenta’s blood vessels linked themother’s bloodstream to the embryo. This is known as being viviparous. Theembryo develops within the mother with the help of the placenta producingchemicals that regulate the time of birth and soft tissues.