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What evidence shows that toothed whales developed echolocation by the Late Oligocene Epoch?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

Evidence that toothed whales (odontocetes) developed echolocation by the late Oligocene Epoch is primarily derived from the fossilized skull of Simocetus, a dolphinlike whale that lived approximately 28 to 23 million years ago.1

Analysis of the Simocetus skull reveals several key anatomical features associated with the echolocation systems found in modern whales and dolphins:1

Melon Structure: The shape of the skull suggests the presence of a melon, a fatty tissue organ used to project sound.1
Musculature: The skull exhibits specific attachment points for the muscles required to produce the high-frequency clicks used in echolocation.21
Acoustic Reception: The fossilized remains indicate the presence of necessary air spaces that would have allowed the animal to detect returning sound waves after they reflected off objects in the environment.1

These findings indicate that echolocation evolved early in the history of toothed whales, enabling them to occupy a broad range of ecological niches by the late Oligocene.1