We provide the first detailed endocranial description of the petrosal bone of the ear region of the anthracotheriid artiodactyl Bothriogenys, based on two new specimens from the early Oligocene of the Jebel Qatrani Formation, Fayum, Egypt. The new fossils have petrosals with a hyperinflated (pachyostotic) tegmen tympani (the roof over the middle ear), resembling the condition in certain cetancodontans such as hippopotamids and basal cetaceamorphans (fossil stem taxa to extant Cetacea).

The morphology of the petrosal, particularly its relative size and density, has been considered an important indicator of the ability of a marine mammal to localize sound transmitted in water, yet petrosal size (pachyostosis) and density (osteosclerosis) have not previously been quantified independent of each other. We examine the new fossils in the context of a preliminary CT-based study of petrosal density (extant taxa only) and petrosal volume (extant and extinct taxa) in a sample of artiodactyls and outgroups. In our extant comparative sample, the petrosals of cetaceans are both dense and voluminous as has been previously stated. We find, however, that the tegmen tympani is relatively voluminous (pachyostotic) without being particularly dense (osteosclerotic) in Hippopotamus amphibius, an extant taxon that has been documented to show some aquatic hearing behaviors, albeit less derived ones than those seen in cetaceans. A voluminous tegmen tympani, which is present in Bothriogenys, may have specific implications for behavior that are distinct from increases in petrosal density.

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