Emended diagnosis
Medium-sized (1–2 m condylobasal length) archaic mysticetes, differing from all other Mysticeti in possessing extremely elongate nasals (> 65% bizygomatic width), zygomatic processes that are longitudinally twisted with dorsolaterally facing lateral surface, supramastoid crest that does not extend anterior to posterior margin of temporal fossa, secondary squamosal fossa, an anteroposteriorly more elongate and narrow intertemporal region with a well-developed sagittal crest, and a periotic with a low and discontinuous superior process with anterior and posterior apices. Eomysticetids differ from toothed mysticetes in lacking large emergent teeth, and in possessing a more extremely elongate rostrum and kinetic maxilla. Eomysticetids differ from all other Chaeomysticeti in retaining akinetic and rigid nasals and premaxillae, a dorsoventrally shallow palatal keel, occipital shield far posterior to postorbital process of the frontal, large and anteroposteriorly elongate temporal fossae, unfused and short posterior processes of the tympanoperiotic, highest point of skull formed by the nuchal crest and not the anterior apex of the occipital shield, anteroposteriorly thickened paroccipital processes, anteroposteriorly oriented zygomatic processes, axis without vertebrarterial canal, anteroposteriorly elongate cervical series, and an elongate humerus. Eomysticetids further differ from balaenids and neobalaenids in lacking rostral arching and primitively retaining unfused cervical vertebrae and an olecranon process of the ulna, and from Balaenopteridae, Cetotheriidae, Eschrichtiidae, and ‘cetotheres’ s.l. in primitively retaining a concave glenoid fossa.
Type species: Eomysticetus whitmorei.
Included genera: Eomysticetus, Micromysticetus, Tohoraata, Tokarahia, and Yamatocetus.