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