Archaeocetes

Archaeocetes are an extinct group of early whales that represent the earliest stages in the evolution of cetaceans, bridging terrestrial mammals and modern whales. They lived from the early to late Eocene, approximately 50 to 34 million years ago, and are known from fossil remains found in Africa, Europe, Asia, and North America. Archaeocetes are considered a stem or transitional group within Cetacea, preceding the split between the two modern suborders, Odontoceti (toothed whales) and Mysticeti (baleen whales).

The archaeocete fossil record documents a progression from semi-aquatic to fully aquatic lifestyles. Early forms such as pakicetids and ambulocetids are interpreted as capable of both terrestrial and aquatic locomotion, while later families became increasingly specialized for life in water. By the time of protocetids and basilosaurids, most members show strong aquatic adaptations, with elongated bodies, limbs modified into flippers, and tail-driven propulsion. Basilosaurids in particular are among the most derived archaeocetes, possessing elongated serpentine bodies and reduced hind limbs; they are typically interpreted as fully aquatic.

Anatomically, archaeocetes retained and modified features from their terrestrial ancestors, including differentiated teeth and skulls suited to an aquatic predatory lifestyle, along with adaptations in the ear region for underwater hearing. The group is composed of several families, including Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, and Basilosauridae, among others. The archaeocetes ultimately gave rise to the two modern cetacean suborders, marking a major transition in mammalian evolution from land to sea.