What do the fossil skeletons tell us about the evolution of today's whales? Select all that apply)

A. Whales evolved from land-dwelling ancestors.

B. Whale ancestors were mammals, but modern-day whales are not.

C. The nostrils moved from the front to the top of head.

D. As whales became aquatic, the hind limbs became vestigial.

E. The skull became elongated over time.

F. To evolve into a marine animal, the bones became less dense.

A, C, D and E are correct.

Whales did evolve from hoofed land-dwelling mammal ancestor, Pakicetus.

The nostrils moved from the front to the top of head to help them breathe while most of their body is submerged in water.  As whales became aquatic, they used their tail more and more and the hind limbs became useless and vestigial. The skull became elongated over time because this design helped them be more aerodynamic in water and helped them evolve echolocation.

Explanation:

B. is incorrect because whales are mammals too.

F. is incorrect because their bones became more dense to help them stay submerged.