Cetaceans are related to the hoofed mammals, or ungulates, and their ancestry is linked more or less closely to that of cows, horses, and hippopotamuses. Current thinking is that they are highly derived artiodactyls, with a particularly close evolutionary relationship to the hippos. The fossil record of cetacean ancestry dates back more than 50 million years to the early Eocene epoch.
Most paleontologists agree that cetaceans arose from the Mesonychidae, an extinct family of primitive terrestrial mammals that inhabited North America, Europe, and Asia. Mesonychids can generally be described as cursorial (adapted for running) carrion feeders with large heads, powerful jaws, and five-
Most of the fossil evidence for this initial radiation of the stem or basal Cetacea, the extinct suborder Archaeoceti, has come from Eocene Tethys sediments in India, Pakistan, and Egypt, although some archaeocete material has also been found in Nigeria and Alabama (United States). The archaeocetes diversified between 45 and 53 million years ago (mya), and the group had spread into midtemperate waters by 40 mya, toward the end of the middle Eocene. More than 35 different species have been identified for the interval 35–53 mya, during which time archaic cetaceans had expanded from riverine and near-
Presumably, they had also begun to move their tails in an up-
Five families of Archaeoceti are recognized: Pakicetidae, the amphibious earliest cetaceans; Ambulocetidae, the walking whales; Remingtonocetidae, the gavial-
Probably the best-
The archaeocetes are replaced in the fossil record by odontocetes and mysticetes beginning in the Oligocene, about 38 mya. By approximately the middle of that epoch, the archaeocetes appear to have died out completely. The oldest known cetacean in the mysticete clade is Llanocetus denticrenatus, found in late Eocene rocks on the Antarctic Peninsula. This species’ most characteristic feature was its series of lobed, widely spaced teeth, which were somewhat reminiscent of the teeth of the crabeater seal (Lobodon carcinophagus). Like the crabeater seal, L. denticrenatus was probably a filter feeder on krill-
Odontocetes also radiated rapidly and widely during the Oligocene, by the end of which there were more than 13 families and 50 species of cetaceans in the world’s oceans. This diversity was probably driven by changes in foraging opportunities related to breakup of the southern supercontinent of Gondwana, opening of the Southern Ocean, and the consequent polar cooling and sharpening of latitudinal temperature gradients. Several of the early odontocete lineages failed to survive beyond the Miocene (5–23 mya). The shark-
The cetotheres radiated further during the Miocene (5–23 mya), with more than 20 genera in which the blowholes were positioned about as far back on the top of the head as they are in living mysticetes. Also, by the early Miocene, the two main branches of cetotheres were evident, one leading to the modern right whales (Balaenidae) and the other to the rorquals (Balaenopteridae) and gray whale (Eschrichtiidae). Gray whales do not appear in the fossil record until only about 100,000 years ago, and their ancestry is therefore particularly problematic. For their part, the odontocetes also experienced a major Miocene radiation. Beaked whale (Ziphiidae) fossils are common in marine sediments worldwide by 5–10 mya, and these include animals belonging to the modern genus Mesoplodon. Sperm whales in the family Physeteridae, similar in some important ways to the living species, were present by 22 mya.
Dolphins and porpoises as cetologists know them today also emerged in the Miocene, perhaps about 12 mya. The large, speciose odontocete family Delphinidae is one of the least resolved of the 14 extant cetacean families. In spite of fairly blatant external morphological differences among genera within the family, such as the globe-
One of the more high-