Archaeoceti

Paraphyletic group of primitive cetaceans from Early Eocene to Late Oligocene

Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene Representing the earliest cetacean radiation, they include the initial amphibious stages in cetacean evolution, thus are the ancestors of both modern cetacean suborders, Mysticeti and Odontoceti. This initial diversification occurred in the shallow waters that separated India and Asia , resulting in some 30 species adapted to a fully oceanic life. Echolocation and filter feeding evolved during a second radiation
All archaeocetes from the Ypresian (56–47.8 mya) and most from the Lutetian (47.8–41.3 mya) are known exclusively from Indo Pakistan, but Bartonian (41.3–38.0 mya) and Priabonian (38.0–33.9 mya) genera are known from across Earth, including North America, Egypt, New Zealand, and Europe. Although no consensus exists regarding the mode of locomotion of which cetaceans were capable during the late Lutetian, they were very unlikely to be nearly as well adapted to the open ocean as living cetaceans. They probably reached as far as North America along coastal waters, either around Africa and over to South America, or more likely, over the Tethys Sea (between Eurasia and Africa) and along the coasts of Europe, Greenland, and North America. The archaeocetes are paraphyletic in relation to their extant modern descendants, the Neoceti (neocetes). Neocetes consist of two subgroups, the toothed whales (odontocetes) and the baleen whales (mysticetes).

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. Also known exclusively from Pakistan and India, the ambulocetids include the oldest known whale, Himalayacetus, which is believed to be , some 4 million years older than the rest of its family. 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.

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.

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.