basilosaurids

Basilosaurids are an extinct family of early toothed whales (archaeocetes) that lived during the early to middle Eocene, roughly 41 to 34 million years ago. They are among the earliest cetaceans known to be fully aquatic, representing an important stage in whale evolution. The family includes genera such as Basilosaurus and Dorudon; Zeuglodon is a historical name that has been used for related species. Basilosaurus, one of the largest known archaeocetes, could reach lengths of about 15–20 meters, whereas Dorudon was much smaller, around 3–5 meters.

Physically, basilosaurs possessed elongated, serpentine bodies with long skulls and numerous sharp teeth adapted for grasping prey such as fishes and cephalopods. They had a reduced forelimb and hind limbs that were vestigial and located within the pelvis, reflecting their transition from land-dwelling ancestors to fully aquatic life. The tail was used for propulsion along with a flexible spine, and the overall body plan indicates an advanced aquatic lifestyle for the time.

Ecology and biology suggest that basilosaurs inhabited shallow, warm seas in tropical to subtropical regions. They were marine predators, and their anatomy implies viviparous reproduction with in-water birth, similar to modern whales. Their fossils provide crucial evidence for the transition from terrestrial to fully aquatic cetaceans, including progressive hind limb reduction and integration into an aquatic mode of life.

Discovery and significance: Basilosauridae is placed within Archaeoceti, the archaic cetaceans, and helps illuminate the early evolution of whales. The name Basilosaurus originated from early misclassification as a reptile, but these animals are mammals and among the earliest known whales.