What is the oldest sea mammal?

It's impossible to say for certain which sea mammal is the oldest in terms of evolutionary history, as the fossil record is incomplete. However, we can look at the oldest known fossils of different groups of sea mammals:

* Indohyus: A small, four-legged, semi-aquatic mammal that lived about 48 million years ago, considered a close relative of modern whales.

* Pakicetus: An early whale ancestor that lived about 50 million years ago, indicating an early divergence from the lineage that led to modern land mammals.

Based on these fossils, Pakicetus is generally considered to be the oldest known sea mammal, although it wasn't fully aquatic like modern whales.

However, it's worth noting that even older fossil remains of early whale ancestors might be discovered, potentially pushing back the date of the oldest sea mammal even further.