Two ancient organisms, Pakicetus and Aetiocetus, and the modern whale may share a common line of
evolution. The diagram shows a skull of each of these large
mammals that share similarities in their
body forms.
Three Skulls in Whale Evolution
Eye socket
Nostril
Bottom jaw
Pakicetus
Eye socket
Nostril
Baleen Nostril Eye socket
Bottom jaw
Bottom jaw
Aetiocetus
Modern whale
(50 million years ago)
(25 million years ago)
Some scientists hypothesize that each skull represents a step in the evolution of modern whales. They claim
that modern whales are descendants of land mammals that adapted, over time, from living on land to living
completely in water. The modern whale skull has a food-filtering system called baleen that hangs from its
upper jaw bone. The ancient mammals had teeth.
Identify two structures in the modern whale skull that support the claim that whales gradually adapted to a water
environment. Explain why they support the claim.