Basilosaurus

Basilosaurus is an extinct genus of early whale from the late Eocene, about 41 to 33.9 million years ago. It is one of the first fully aquatic cetaceans known from abundant fossils, illustrating a key stage in whale evolution. The name, meaning "king lizard," reflects an early misclassification as a reptile.

Basilosaurus belongs to the family Basilosauridae within the infraorder Archaeoceti, a group of primitive whales. It had an elongated, serpentine body, a long skull with sharp teeth, and tiny hind limbs concealed within the body wall. Unlike modern whales, Basilosaurus likely did not possess a dorsal fin, and propulsion was primarily by a large tail fin.

The genus was named by Richard Owen in 1839 from fossils found in Alabama. The species Basilosaurus cetoides is the type species, with B. isis described from Egyptian material. For many years the animal was misinterpreted as a giant reptile, but subsequent study established its cetacean identity.

Size estimates vary, but Basilosaurus could reach about 15 to 20 meters in length, with some reports exceeding 20 meters. It was a carnivore, feeding on fish and other marine vertebrates. The presence of vestigial hind limbs reflects its transitional status between early archaeocetes and modern whales.

Fossils have been recovered in North America and Africa, indicating a wide distribution in warm, shallow seas during the late Eocene. Basilosaurus contributes to understanding whale evolution, particularly the transition to fully aquatic locomotion and the emergence of modern cetacean features.