Pakicetidae First identified as cetaceans by , the
pakicetids, the most archaic of whales, had long, slender legs and a long, narrow tail, and could reach the size of a modern wolf. They have only been found in sediments from freshwater streams in northwestern India and northern Pakistan, and were probably waders rather than swimmers. Dozens of fossils are known, but only of skulls, teeth, and jaw fragments; no complete skeletons have been found. The dentition varied; the smallest species had teeth like modern fish eaters, and the largest were more like modern hyenas. The pakicetids may have been predators or carrion feeders. Neither the skull nor the dentition of pakicetids resembles those of modern whales, but the
sigmoid process,
involucrum,
pachyostotic (compact) and rotated
ossicles of their ears still reveal their cetacean nature. and crocodile-like with large feet and a strong tail. Sediments indicate that they lived in coastal areas and their
compact bones suggest that they were ambush rather than fast-pursuit predators. Ambulocetids are also known exclusively from Pakistan and India. The anterior margin of external nares is located above or behind the third upper incisor, the rostrum is wide, the supraorbital processes are present but short, the anterior edge of the orbit is located above the second or third upper molar, the postorbital process forms a 90° angle with the sagittal crest, and the cervical vertebrae are short.
Protocetidae The
Protocetidae, known from both Africa and America, were a diversified family with hind limbs and a strong tail, indicating that they were strong swimmers that colonized shallow and warm oceans, such as reefs. They greatly affected cetacean evolution , because they spread across Earth's oceans. In other genera (
Georgiacetus and
Aegicetus), the pelvis was not connected to the vertebral column, suggesting the hind limbs could not have supported the body weight.
Basilosauridae Basilosaurids, which had tiny hind limbs and flipper-shaped fore limbs, were obligatorily aquatic and came to dominate the oceans. They still lacked the echolocation and
baleen of modern odontocetes and mysticeti. In the vertebral column, the neck vertebrae are short, the thoracic and lumbar vertebrae are of similar length, the sacral vertebrae are unfused, the sacroiliac joints are absent, and the short tail has a
ball vertebra (indicating the presence of a fluke). The scapulae are broad and fan-shaped with anterior acromions and small supraspinous fossae. The ulnae are large and have transversely flat olecranons, the wrists and distal forearms are flattened in the plane of the hands, and the hind limbs are tiny. ==Taxonomy==