Frogs and all other living tetrapods have a rod @-@ like bone called the stapes that aids in hearing by transferring vibrations from the ear drum — or homologous tympanum — to the inner ear . Temnospondyls also have a stapes , which projects into the otic cavity . The stapes likely evolved from the hyomandibula of lobe @-@ finned fishes . The positioning of the stapes and the shape of the otic region suggests that the tympani of temnospondyls and frogs are homologous , but the tympani of these amphibians are no longer considered homologous with the hearing systems of reptiles , birds , and mammals . Therefore , ear structures in temnospondyls were not ancestral to those of all other tetrapods .