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Ancient Reptiles Had Advanced Hearing, Fossils Suggest

Adrianne Appel
for National Geographic News
September 14, 2007
 
The earliest evidence yet of modern ears in terrestrial vertebrates has been discovered in reptile fossils from central Russia, paleontologists say.

The animals may have been among the first nocturnal vertebrates, using their advanced hearing and large eyes to communicate with each other and hunt insect prey at night.

This would have given them a niche during a time when Earth was crowded with animals.

The reptiles lived 260 million years ago, during the Permian period, which lasted from 299 to 251 million years ago.

That's about 50 million years earlier than modern ears were thought to have developed in terrestrial vertebrates.

"The fact that they evolved a modern ear emphasizes that the ecosystems in the Permian period were advanced," said lead author Johannes Müller of Humboldt University in Berlin.

The ears were found in six fossils of different species of para-reptiles, small, lizardlike animals about 1.5 feet long (0.45 meter long). "Think of a chubby lizard with a wide head," Müller said.

All the reptiles had a true middle ear, which allowed them to hear a range of frequencies. (Related news: "Big Dinosaurs Heard Only Low-Pitch Sounds, Experts Suggest" [June 8, 2007].)

"It is the oldest evidence of advanced hearing in any land animal that lays a hard-shelled egg," he said.

One Big Eardrum

The ancient reptiles' advanced hearing apparatus was very similar to that of modern reptiles, Müller said.

"It's like a hole in the cheek region, covered with a thin sheet," he said.

"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.

Unknown Fate

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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