The nasal opening, which will later evolve to become blowhole in modern cetaceans, is located near the tip of the long snout.

Increasingly lateral-facing eyes might be used to observe underwater prey, and are similar to the eyes of modern cetaceans.

They had small brains; this suggests they were solitary and did not have the complex social structure of some modern cetaceans.

Unlike modern cetaceans, basilosaurids retain a large external auditory meatus.

Unlike modern cetaceans, it had a neck with seven vertebrae.

Their dentition varied, but started to evolve towards the non-masticating teeth of modern cetaceans, and they were probably active hunters.

It is unclear at present whether protocetids had flukes (the horizontal tail fin of modern cetaceans).

In modern cetaceans, the semicircular canals are much smaller relative body size than in other mammals.

Similar to modern cetaceans, such as whales and dolphins, they were air-breathing.

This suggests that it is a transitional species between extinct land mammals and modern cetaceans.