The petrosal and inner ear ofHerpetocetussp. (Mammalia: Cetacea) and their implications for the phylogeny and hearing of archaic mysticetes

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Abstract
This paper describes the petrosal (periotic) and the inner ear ofHerpetocetussp., an archaic mysticete whale (Mysticeti, Mammalia) from the Yorktown Formation (Pliocene) of North Carolina, USA. Parsimony analysis of 28 petrosal characters ofHerpetocetussp. and 11 other cetacean taxa supports the monophyly of mysticetes and the division of odontocetes and mysticetes. The in-group taxa of this analysis are:Herpetocetus, Parietobalaena, Pelocetus,Balaenidae,Eschrichtius,and Balaenopteridae. Odontocetes and the archaeoceteZygorhizawere used as successive outgroups to root phylogenetic trees and to establish character polarities. Among the modern mysticetes, the Balaenopteridae (rorquals) and the Eschrichtiidae (gray whales) are more closely related to each other than either is to the Balaenidae (bowhead and right whales). Several Miocene “cetotheriid” mysticetes and balaenids share some resemblance in the petrosal, suggesting their affinities. Quantitative information of the inner ear ofHerpetocetussp. was obtained by serial sectioning and computer graphic reconstruction.Herpetocetussp. is much less developed than odontocetes in the cochlear structures that are crucial for high frequency hearing. Some cochlear structures in this fossil mysticete resemble more closely the non-echolocating modern mysticetes than early fossil toothed whales, indicating a possible specialization in low frequency hearing. This suggests that the archaic mysticetes of the Miocene and Pliocene did not have high frequency hearing necessary for echolocation.Herpetocetussp. is similar to modern mysticetes but different from odontocetes in the spherical shape of the vestibule.