mong the Paleocene condylarths was a family, the Mesonychidae, that was exceptional in being carnivorous (Figure 7.39). Based on the features of the teeth and ear bones, mesonychids include the ancestors of whales (Cetacea). Recently, discoveries of spectacular mid-Eocene (52-46 Mya) fossils have documented some of the anatomical changes that this lineage underwent in its adaptation to marine life. Modern cetaceans have lost the hindlimbs (except for vestigial bones). They propel themselves by vertical flexure of the tail, which bears horizontal flukes that lack an internal skeleton. The skull is highly modified, with dorsally situated nostrils. The ear bones are highly modified for hearing under water.
Among the important recent discoveries (Thewissen and Hussain 1993; Thewissen et al. 1994; Gingerich et al. 1994) is Pakicetus, known from a skull found in riverine deposit along with terrestrial animals. It is very similar to a mesonychid, but the ear bones are intermediate between those of mesonychids and those of cetaceans. Ambulocetus, a slightly younger form, is an exquisite intermediate. The fingers were separate and terminated in small hooves, like those of mesonychids, but the structure of the lower back vertebrae, the joint between the hind leg and the body, and the large hind feet show that it swam by undulating the rear part of the body, as whales do. This form of locomotion was accentuated in the somewhat younger Rodhocetus. Younger still, by about 10 My, is Basilosaurus, which has long been recognized as a primitive cetacean. A specimen of Basilosaurus with complete hind legs has recently been found. The legs are complete in structure, but are far too small to have supported the weight of this large (25 feet), fully aquatic mammal. Thus the evolutionary origin of this highly modified order of mammals is becoming increasingly well documented.
We have noted that, according to anatomical evidence, both the Cetacea and Artiodactyla evolved from condylarth ancestors. Recent studies of DNA sequences show that, indeed, Cetacea are more closely related to Artiodactyla than to any other order of mammals. Some DNA data, in fact, suggest that cetaceans may be more closely related to ruminant artiodactyls (dear, antelopes, etc.) than to nonruminants (pigs, camels). That is, in a phylogenetic sense, cetaceans may be artiodactyls, not just related to artiodactyls (Graur and Higgins 1994).
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