Why did some reptiles return to water during the process of evolution?

Millions of years ago, reptiles left water for dry land, but they did not leave water for good. Many reptiles went back to the water, not once, but many times. Reptiles left water in the first place to increase their chances for survival. They went back to water for the same reason. Most of the animal body is made of water. The temperature of lakes, rivers, and seas do not change much. This is an advantage for reptiles as they cannot keep up their body temperature by themselves. Land animals need big bones to support their weight. On the other hand, water can hold animals up. That is why the whale is bigger than the elephant.

However, big water animals find it difficult to move quickly. Water reptiles like the long dead ichthyosaurs solved this problem by swimming like fish. Snakes, crocodilians, and lizards moved their body up and down in waves, or they paddled. Breathing is also a problem for reptiles. The oxygen for breathing is dissolved in the water. Fish breathe by means of gills. Reptiles have lungs which are not suited for making use of the dissolved oxygen. Therefore, some reptiles have modified their body surfaces to make use of small amounts of oxygen dissolved in the water.