Whale evolution and Oligocene southern ocean environments
Abstract Many odontocetes [modern toothed whales, Early Oligocene (35 Ma) — Recent] utilise high-frequency echolocation to facilitate predation, while mysticetes (baleen whales, Early Oligocene—Recent) possess a filter-feeding system which allows cropping of plankton. These adaptations are reflected in the unique skull structure of each group. Skulls of archaeocetes [primitive toothed whales, the ancestors of odontocetes and mysticetes, Middle Eocene—Late Oligocene (45 to 25 Ma)] indicate that they possessed neither baleen nor the capacity for echolocation. Studies on Northern Hemisphere Cetacea (whales and dolphins) have suggested a paucity of Oligocene Cetacea but also have stressed that the Oligocene was an important time in cetacean evolution. The observation of some authors that cetacean diversity declined in the Oligocene has been difficult to reconcile with the origins then of odontocetes and mysticetes. Hypotheses have stressed that the “decline” reflects decreased oceanic thermal gradients, upwelling, plankton diversity and thus fewer cetacean niches. The Austral Oligocene fauna allows alternative explanations. The earliest known mysticetes and odontocetes are New Zealand Early Oligocene forms. Their appearance probably was triggered by the initiation of the psychrosphere and concomitant Austral increases in upwelling and productivity. Austral cetacean radiations in the Middle Oligocene are attributable to the influence of the Circum-Antarctic Current on regional productivity and the creation of more new niches. The decline of archaeocetes, further radiations of odontocetes and mysticetes, and the development of a cosmopolitan cetacean fauna in the Late Oligocene (25 Ma) foreshadowed the radiation of modern cetacean stocks with the establishment of major “modern” ocean patterns in the Miocene (22-5 Ma).