HEARING

A 50 million year old earbone (the anvil) of Pakicetus lies between poppy seeds that show its tiny size. Photo taken with a scanning electron microscope.

The ear of modern whales and dolphins is specialized to listen to sounds underwater. Unlike nearly all other mammals, whales lack the airfilled duct that goes from the outside of the head to the eardrum (the external auditory meatus). The eardrum itself is not a flat translucent sheet of tissue as it is in other mammals, but instead a thick opaque cord-like structure. All modern and fossil cetaceans still do have three little earbones (hammer, anvil, and stirrup), just like their land mammal ancestors, but these bones are greatly modified in shape.

Modern odontocetes are specialized to hear the high-frequency sounds that are the reflections of their echolocating calls. They produce these calls with several specialized organs in the forehead and around the blowhole. Mysticetes are mainly sensitive to low-frequency sounds that carry for hundreds of miles through the ocean. The actual way in which sounds in whales are transmitted is poorly understood, even in the modern species.

The hearing organ of Eocene whales was not specialized as that of modern cetaceans. Instead it represents a compromise of adaptations relating to underwater sound reception and hold-overs of a hearing system used for listening to sounds in air. The eardrum of these cetaceans is more flat than that of their modern relatives, and the external auditory meatus is still present.

Scientists have found a lot of fossils that show what the ear was like in Eocene whales. Some of these bones consist of heavy, dense fossil bone and preserve well as fossils. Others include the earossicles, tiny bones that are delicately suspended in the ear. One of these, the anvil, is known for Pakicetus.

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