"We figure these guys looked like a lizard with a much larger tympanum"— the round area on a lizard's cheek that acts as an eardrum, Müller said.
But compared with modern reptiles, the ancient animals' tympanum was huge.
The eardrum, which covered two-thirds of the cheek area, is among the largest ever recorded for any land vertebrates with four legs or appendages.
Like modern animals, the ancient reptiles' middle ear connected to a small bony part, called a hearing ossicle, that connects to the inner ear and brain.
The ossicle transmits airborne sounds into the inner ear. Humans have three ossicles, and modern lizards have one.
Robert Reisz is the biology chair at the University of Toronto.
"The most interesting aspect here is that this is the earliest, clear evidence of a highly evolved hearing system," Reisz said.
The fossils were found along the Mezen' River in central Russia in the 1990s.
Müller originally examined the fossils in an effort to classify the animals. In the midst of his analysis, he discovered the ears. Müller and co-author Linda Tsuji report their findings in this week's journal Public Library of Science.
The para-reptiles' fate is controversial. Many paleontologists believe they became extinct by about 206 million years ago during a massive extinction event that wiped out many land and marine animals.
Other scientists believe the reptiles lived on as turtles.
But lead author Müller believes para-reptiles went extinct and that modern ears evolved independently in mammals, birds, lizards, and frogs.
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