 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 . 
 Temnospondyls and other early tetrapods <a href='been_done' title='1'><mark>have</mark></a> rounded otic notches in the back of the skull <a href='null_relcl' title='2'><mark>that</mark></a>project into the cheek region . In life , the otic notch would <a href='been_done' title='3'><mark>have</mark></a> 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 <a href='drop_aux' title='4'><mark>is</mark></a>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 . 
Temnospondyls and other early tetrapods done rounded otic notches in da back of the skull project into the cheek region . In life , the otic notch would done been covered by a membrane called tdatympanum , which is seen as a disk @-@ like area in living frogs . Thdaympanum is involved in hearin , and similar to the ear drum of more advanced tetrapods . It was traditionally thought dat thedampanum developed very early in tetrapod evolution as a hhearinorgan and progressed to form the ear drum of amniotes . Thus , temnospondyls possessed a hearing system supposedly ancestral to that of living amphibians and reptiles .