Gardiner's frogs from the Seychelles islands, one of the smallest frogs in the world, do not possess a middle ear with an eardrum yet can croak themselves, and hear other frogs.
The illustration shows how a Gardiner's frog can hear with its mouth: Top left: The skin of the animal reflects 99.9% of an incoming sound wave hiting the body close to the inner ear. Without a middle ear, sound waves cannot be transported to the inner ear. Bottom left: the mouth acts as a resonating cavity for the frequencies of the frogs' song, amplifying the amplitude of the sound in the mouth. The body tissue between the buccal cavity and the inner ear is adapted to transport these sound waves to the inner ear.
The way sound is heard is common to many lineages of animals and appeared during the Triassic age (200-250 million years ago). Although the auditory systems of the four-legged animals have undergone many changes since, they have in common the middle ear with eardrum and ossicles, which emerged independently in the major lineages. On the other hand, some animals notably most frogs, do not possess an outer ear like humans, but a middle ear with an eardrum located directly on the surface of the head.
Incoming sound waves make the eardrum vibrate, and the eardrum delivers these vibrations using the ossicles to the inner ear where hair cells translate them into electric signals sent to the brain. Is it possible to detect sound in the brain without a middle ear? The answer is no because 99.9% of a sound wave reaching an animal is reflected at the surface of its skin.
"However, we know of frog species that croak like other frogs but do no have tympanic middle ears to listen to each other. This seems to be a contradiction," says Renaud Boistel from the IPHEP of the University of Poitiers and CNRS. "These small animals, Gardiner's frogs, have been living isolated in the rainforest of the Seychelles for 47 to 65 million years, since these islands split away from the main continent. If they can hear, their auditory system must be a survivor of life forms on the ancient supercontinent Gondwana."