The early fishes did not hear airborne sounds, and their ears were used mostly to maintain body balance. (dinosaur rides)Ears for hearing on land require a taut membrane in the skull to pick up airborne vibrations. Living species of frog have such a membrane shaped like a tiny drumhead, constructed of special skin. A deep notch in the frog’s skull holds the eardrum (known technically as a tympanum), and between it and the brain stands an air-filled chamber: the middle ear. (dinosaur factory)To transmit sound to the brain, a slender ear bone runs from the eardrum to the canals of the ear in the side of the brain-case. If it could be discovered when this type of ear first evolved, it would constitute an important clue about when the sexual chorus first began.
The eardrum doesn’t preserve in fossils, but the notch for it in the skull does. (dinosaur costume)Earliest of all amphibian fossils is the famous Ichtbyostega from the lake beds of Greenland (its name means “fish with a roof,” a reference to its primitive fishlike structure and the thick roof of its skull). This Ur-amphibian has no definite notch for an ear, and couldn’t have possessed any special auditory adaptations. Therefore, when Ichthyostega and its kind waddled over the primeval land, they must have marched into a silent world where the humid stillness was broken only by the rustling of ancient rushes in the wind and the near-silent footsteps of Ur-spiders hunting in the leaf mold.