Mystacodon is the oldest known baleen whale, the holotype specimen dating to 36.4 million year ago to the Priabonian of the latest Eocene.

This flattening shows reduced function of the incisor teeth and grasping capabilities, and the increased length of the head allowed a greater mouth volume.

It is uncertain if the whale had any baleen in its mouth, and, unlike archaeocetes, it lacks a sagittal crest, indicating a reduction of the temporalis muscle which is used in biting.

The humerus in the arm is about as long as the shoulder blade and much longer than the radius and ulna in the forearm, a condition seen in modern slow-moving whales.

A Myliobatis eagle ray stinger and clupeiform forage fish scales–a group that includes herring, sardines, and anchovies–were found around the holotype.