Modern day whale species, have very small pelvic bones in comparison to their body size, and also they are useless. This is because this bones in the whale body are remnants from the time when their ancestors were land dwelling creatures, thus they had legs as the typical land mammal has. But during their evolution, as their ancestors were starting to adapt more to the water, the legs started to disappear, but the bones inside of the body remained, even though they were of not use. Even though millions of years had passed since this bone is useless, it still exists, and the reason for that is that there hasn't been any evolutionary pressure for it to not exist, since it is not causing any disadvantage into the movement and lifestyle of the whales.
