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Early in the Oligocene Epoch, Cetacea of the primitive suborder Archaeoceti had already declined sharply from their apparent abundance in Eocene seas. By the beginning of the Miocene, archaeocetes are known to have survived only in the northeast Atlantic and southwest Pacific Oceans. Concurrently with this decline, the first members of the suborders Odontoceti and Mysticeti appeared. They are known from only a few specimens, mostly in upper Oligocene deposits, on both coasts of North America, in Germany, Austria, Italy, the Caucasus, Azerbaijan, Australia, and New Zealand. Two important odontocete genera, Agorophius and Xenorophus, come from beds that are probably no older than late Oligocene and that are certainly not as old as Eocene, where these genera have previously been placed. The wide distribution of known Oligocene Cetacea, especially their presence in Australia and New Zealand, indicates the probable existence of a cosmopolitan cetacean fauna by the end of Oligocene time. The Oligocene Odontoceti are represented by Agorophius and related forms and by the Squalodontidae. Several types of skull telescoping are shown by contemporary members of these groups. Recently collected squalodont skulls from the Oligocene of South Carolina show differences in the pattern of cranial bones that may be ontogenetic. Some toothed whales have morphologic features that have led to their being assigned variously to Archaeoceti and Mysticeti because they are regarded as representing a transitional stage between the two suborders. These forms are all of late Oligocene age; they cannot be mysticete ancestors because true Mysticeti are known from middle Oligocene deposits.