Eomysticetids are united by the following combination of primitive and derived characters relative to more advanced chaeomysticetes (Balaenomorpha): zygomatic process without a supramastoid crest; reduction of the superior process of the periotic into a low ridge with anterior and posterior apices in medial or lateral view; blowholes situated ahead of the eyes; an elongated intertemporal region with long parietal and frontal exposures on the cranial vertex; elongated nasals; large coronoid processes of the mandibles; flat rostrum; laterally bowed mandibles; absence of functional teeth; and large mandibular foramina.

[2] There are eight genera of Eomysticetidae: Echericetus, Eomysticetus, Matapanui, Micromysticetus, Tohoraata, Tokarahia, Waharoa and Yamatocetus.

[5][6][7][8] As members of Chaeomysticeti, eomysticetids used their baleen plates to filter krill and other planktonic organisms.

Although superficially similar to thalassotherian chaeomysticetes, their large mandibular canal indicates that they were incapable of lunge-feeding as in modern-day balaenopterids.

A fatty pad on the mandibular canal suggests that eomysticetids could hear underwater.