Answer:
1. When ichthyosauri started aquatic life and could not make use of the sun warmth to hatch their eggs, they also switched back to viviparity, i.e., their eggs hatched inside mother's body. However, in some mammals, such as monotremes, reproduction remains oviparous. (They lay eggs, which hatch in the environment.)
2. Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is an outmoded term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals have the embryos develop inside eggs that remain in the mother's body until they are ready to hatch.
3. Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the mother.
