Evolution cetacea - mammals.worldmy.info

Search
Go to content

Main menu

Evolution cetacea

Cetacea > General of Cetacea

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.

The Atlantic spotted dolphin

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-toed feet with hoof-like claws. The transition from a wholly terrestrial to an amphibious existence is believed to have taken place initially in the Tethys Sea, a large, shallow, near-tropical seaway that extended from the present-day Mediterranean eastward to beyond the South Asian subcontinent.

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-shore habitats and become adapted to occupy oceanic settings as well. Their eyes and kidneys had probably become capable of tolerating different salt balances, they may have lost much of their hair and begun to acquire blubber for insulation and fat storage, their underwater hearing capability had become enhanced, and they had probably developed nasal plugs to close the nostrils when diving.

Presumably, they had also begun to move their tails in an up-and-down, rather than side-to-side, fashion for more efficient swimming. Archaeocetes exhibited many features typical of living cetaceans, including an elongate upper jaw with bony nostrils set back from the tip, a broad shelf of bone above the eye, anteroposteriorly aligned incisors, and an enlarged mandibular canal on the inner side of the lower jaw. They had a dense outer ear bone, or tympanic bulla, and later forms had an expanded basicranial air sinus similar to that of modern cetaceans. A major difference between archaeocetes and the more derived cetaceans is that the archaeocete skull was not telescoped; that is, it did not have overlapping bony elements. Most, and possibly all, archaeocetes had external hind limbs. In some instances at least, they probably used all four limbs for locomotion both in water and on land. Although they are often depicted as having sinuous, almost eel-like bodies, the basic skeletal structures of most archaeocetes would have supported bodies not much different in overall design to those of living cetaceans.

Five families of Archaeoceti are recognized: Pakicetidae, the amphibious earliest cetaceans; Ambulocetidae, the walking whales; Remingtonocetidae, the gavial-convergent cetaceans (the gavial is a long-snouted, freshwater, fish-eating crocodilian of the south Asian subcontinent); Protocetidae, the first pelagic cetaceans; and Basilosauridae, the so-called zeuglodonts, referring to their complex, many-cusped teeth (the Greek zugotos means yoked or joined, and odous, of course, tooth). The most primitive archaeocete identified to date was Nalacetus, known mainly from isolated teeth. Pakicetus, another small, very early archaeocete, had eyes on top of its head, drank only fresh water (confirmed from oxygen isotope ratios in its tooth enamel), and was predominantly wolf- or hyena-like in appearance. The other families of archaeocetes had been largely supplanted by the zeuglodonts during the late Eocene.

Probably the best-known zeuglodont was Basilosaurus, or the “king lizard” (from the Greek basileus for king and sauros for lizard). This animal could be almost 70 ft (21 m) long and weighed at least 11,000 lb (5,000 kg). Its small head in relation to the long body made it appear truly serpentine. The front appendages had been modified into short, broad paddles, but were still hinged at the elbow; and the rear appendages had atrophied to nothing more than stumps. Basilosaurids may have had dorsal fins and horizontal tail flukes, and they were likely hairless, or nearly so. In short, Basilosaurus was well along the path to becoming what cetologists now think of as a whale. 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-like invertebrates or possibly small schooling fish. At least four families of tooth-bearing mysticetes have been described from the Oligocene (24–38 mya). The transition leading to rudimentary baleen plates in the spaces between teeth probably occurred about 30 mya with the emergence of the Cetotheriidae, or primitive baleen-bearing mysticetes. It is a slight misconception to say that the presence of teeth is a diagnostic feature of Odontoceti, the so-called toothed whales, because all archaeocetes and some of the primitive fossil mysticetes also had teeth. Further, all of the modern baleenbearing mysticetes have teeth in the early fetal stages of their development.

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-like invertebrates or possibly small schooling fish. At least four families of tooth-bearing mysticetes have been described from the Oligocene (24–38 mya). The transition leading to rudimentary baleen plates in the spaces between teeth probably occurred about 30 mya with the emergence of the Cetotheriidae, or primitive baleen-bearing mysticetes. It is a slight misconception to say that the presence of teeth is a diagnostic feature of Odontoceti, the so-called toothed whales, because all archaeocetes and some of the primitive fossil mysticetes also had teeth. Further, all of the modern baleenbearing mysticetes have teeth in the early fetal stages of their development.

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-toothed dolphins (Squalodontidae), with their sharp, triangular, serrated teeth, were likely active carnivores, while the very longbeaked Eurhinodelphinidae, with their overhanging upper jaws and many small, conical teeth, were more like the dolphins that cetologists know today. Both of these groups had vanished from the fossil record, and others had dwindled to mere remnants, by the end of the Miocene.

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-headed (pilot whales) versus long-beaked (common dolphins) dichotomy, the family’s validity is supported by several lines of evidence. For example, intergeneric hybrids have been observed for many delphinids both in captivity and in the wild, and all 17 included genera share the same basic skull architecture. Most of the morphological diversification within the family is related to body size and foraging structures such as rostral length and width, and the number, size, and form of the teeth. A recent phylogenetic analysis of the delphinids based on full cytochrome b gene sequences has revealed that certain of the genera may represent artificial assemblages of species and that extensive revision is needed at both the genus and subfamily levels.

One of the more high-profile and controversial issues in cetacean systematics that has arisen in recent years is the contention by some molecular biologists that sperm whales are more closely related to the baleen whales than to other odontocetes. However, this view has been refuted, contradicting as it does a host of morphological, paleontological, and even some other molecular evidence confirming that the odontocetes are a monophyletic group. As one expert summarized it, the proposed split linking sperm whales with mysticetes “would require morphological convergences and reversals of a magnitude that defies credibility.”

Back to content | Back to main menu