See text 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 (50 to 23 million years ago).

[2] This initial diversification occurred in the shallow waters that separated India and Asia 53 to 45 mya, resulting in some 30 species adapted to a fully oceanic life.

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.

Sediments indicate that they lived in coastal areas and their compact bones suggest that they were ambush rather than fast-pursuit predators.

The size of a male sea lion, it had a large head with a long snout and robust, strongly worn teeth.

Their long snout, tiny eyes, and ear morphology suggest their vision was poor and that hearing was their dominant sense.

The fragmentary remains of remingtonocetid postcrania suggest that they had a long neck and large hind limbs that were probably able to support the body weight on land.

[2] The remaining families and later crown cetaceans form a clade united by six synapomorphies:[9] 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.

[10] 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.

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.