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When did the first cetaceans appear in the fossil record?

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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).

The first cetaceans, known as archaeocetes, appeared in the fossil record during the Early Eocene Epoch, approximately 55.8 million to 49 million years ago.1

The earliest known fossil cetacean is the genus Pakicetus, which was discovered in 48.5-million-year-old river delta deposits in present-day Pakistan. Unlike modern whales, Pakicetus was a terrestrial-dwelling mammal that possessed functional legs and an auditory system adapted for hearing in air. These early cetaceans primarily inhabited the warm, shallow Tethys Sea and its margins.21