I describe and figure petrosal bones for a sample of 35 (12 extinct, 23 extant) artiodactylans, perissodactylans, †mesonychids, and archaic ungulates. Detailed herein are the cladistic characters of the petrosal used in the recent combined analysis of molecular and morphological data by Spaulding et al. (2009). That analysis, the largest in taxa and characters for artiodactylans (including cetaceans) to date, showed that hippopotamids are the closest living relatives of cetaceans. It also showed that in the shortest trees †Indohyus is on the stem lineage to Cetacea and that †mesonychians are positioned outside Artiodactyla; however, these positions for fossils are highly unstable, as †mesonychians are more closely related to cetaceans than is †Indohyus in trees only two steps longer.

I show that in many ways the osteology of the hippopotamid ear resembles that of certain stem cetaceamorphans more than it resembles the ear regions of suines (pigs and peccaries). Previous studies have suggested that many artiodactylans lacked an inflated tegmen tympani of the petrosal; however, that generalization is not supported by data presented herein. Petrosal characters, such as the presence of the prefacial commissure fossa, presence of a convex and hyperinflated tegmen tympani, and the absence of a subarcuate fossa, are shown to be synapomorphies of hippopotamids and cetaceans. Some of these features were previously argued to represent a special similarity between †mesonychids and cetaceans, but these are here interpreted as homoplasies. Other features previously argued to be extremely similar between †mesonychians and cetaceans to the exclusion of other ungulates, such as the presence of the anterior process of the tegmen tympani, are shown to be more widely distributed among ungulates than previously recognized. A number of artiodactylans, including ruminants, are also shown to have transpromontorial sulci on the petrosal despite reports that the internal carotid artery is absent in the neck of ruminants. The petrosals of †anthracothere and †entelodont species exhibit varied morphology, with the †anthracothere †Bothriogenys having the greatest gross similarity to the hippopotamid and cetaceamorphan condition; however, shortest trees indicate that these similarities are convergent.