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ABSTRACT
Two well-preserved specimens of a “cetothere” from the Lower Miocene Awa and Mizunami Groups, Japan, are described as Isanacetus laticephalus, gen. et sp. nov. It is distinguished by the following combination of characters: fossa for the stapedial muscle elongated anteroposteriorly, with a fully ossified floor; anteroposteriorly broad supraorbital process of frontal, with the orbital margin concave in dorsal view; posterior thrust of the medial rostral elements reaches the level of the center of the orbit; apex of the occipital shield reaches beyond the level of the anterior end of the zygomatic process of the squamosal; nasal slender, elongate, and partly located anterior to level of the preorbitai angle of the supraorbital process of the frontal; and zygomatic process of the squamosal slender, elongate, and directed forward and slightly outward.
A parsimony analysis of 16 cetaceans and 77 characters failed to identify any synapomorphies for the “Cetotheri-idae.” The analysis supports the following hypotheses: “Cetotheriidae” as commonly used is a paraphyletic grade; “cetotheres” are more closely related to Balaenopteridae + Eschrichtiidae than they are to Balaenidae; and “cetotheres” form two subgroups, one which includes Cetotheriun and another which includes Isanacetus, Parietobalaena, and Aglaocetus. The latter subgroup is more closely related to Balaenopteridae + Eschrichtiidae, but is also paraphyletic.
