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the watertight egg, also known as amniotic eggs, is a feature only found in
turtles, lizards, dinosaurs (including birds), mammals, and their relatives
(American Museum of Natural History, n.d.). Being that these organisms have a
watertight egg tells us that they inherited this feature from a common
ancestor. Amniotes are the class of animals that express this characteristic.

The watertight contains a fluid-filled membrane that surrounds the embryo and
keeps it from drying out. This allowed amniotes to colonize on land, since they
no longer needed a wet environment to lay their eggs. Dinosaurs are divided in
Saurischians and Ornithischians based on hip structure. Then, the hip structure
of Ornithischians dinosaurs is advanced compared to Saurischians. The pubis
bone located at the front of the pelvis has an extension that points towards
the rear (American Museum of Natural History, n.d.). The hip structure is
thought to resemble a bird’s hip. Finally, after the amniotic egg, animals
started keeping their embryos within. The placenta’s blood vessels linked the
mother’s bloodstream to the embryo. This is known as being viviparous. The
embryo develops within the mother with the help of the placenta producing
chemicals that regulate the time of birth and soft tissues.

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