Temnospondyls and other early tetrapods have rounded otic notches in the back of the skull that project into the cheek region . In life , the otic notch would have been covered by a membrane called the tympanum , which is seen as a disk @-@ like area in living frogs . The tympanum is involved in hearing , and is similar to the ear drum of more advanced tetrapods . It was traditionally thought that the tympanum developed very early in tetrapod evolution as a hearing organ and progressed to form the ear drum of amniotes . Thus , temnospondyls possessed a hearing system supposedly ancestral to that of living amphibians and reptiles .