Bony Inner Ears Offer Clues to Lizard Evolutionary Relationships, But Not Ancient Habitats

All land animals with backbones, including humans, snakes, and lizards, have an inner ear organ inherited from a common ancestor. The inner ear plays a crucial role in balance, hearing, and sensing body position. Enclosed by protective bones, it allows animals to know whether they are upright, upside down, or in motion, even with their eyes closed.

The bones surrounding the inner ear are especially important to paleontologists because unlike the organ itself, they readily fossilize, making them one of the few parts of the sensory system that can be studied in extinct species.

Scientists have long hypothesized that animals living in similar environments—whether underwater, in trees, or underground—might evolve inner ears with similar shapes to help them balance or hear more effectively. If true, the fossilized bones around the inner ear could offer a window into the lifestyles of long-extinct animals, such as their habitat preferences.

To test this idea, researchers from the Museum, Columbia University, Stony Brook University, New York Institute of Technology, North Carolina Museum of Natural Sciences, Roanoke College, and Yale University measured the bony encasements of the inner ear in living species of lizards and snakes and used machine learning to determine whether these measurements could predict two things: the habitat in which an animal lived and its evolutionary lineage.

This CT scan shows the position of the inner ear in the skull (pink structure) of a fossil specimen of Peltosaurus, part of an extinct lineage of armored lizards that was included in the new study.
© AMNH

The results, which were published recently in the Zoological Journal of the Linnean Society, were mixed, but revealing.

The researchers found that the shape of the bones surrounding the inner ear does a poor job of predicting habitat. Even in living species with known habitats, bony inner ear measurements could not reliably distinguish one species’ preferred habitat from another. However, measurements of the bony inner ear did allow the researchers to accurately predict the evolutionary classification of living lizards and snakes.

“The anatomy of the bony inner ear appears to preserve a strong signal of common ancestry, even if it does not accurately predict the habitat of the animals we sampled,” said Meghan Forcellati, the lead author of the study and a comparative biology Ph.D. student in the Museum’s Richard Gilder Graduate School.

This finding could help solve a major problem in reptile evolution. Lizards often evolve similar-looking skeletons independently, a process known as convergent evolution. That makes it difficult to determine which living species are the closest relatives of extinct fossil groups. Bony inner ear anatomy, which appears less prone to convergent change, may offer a more reliable way to place fossils on the reptile family tree.

The research may have particular implications for understanding mosasaurs—giant marine lizards that lived during the age of dinosaurs. While evidence suggests mosasaurs are related to living toxicoferans, the focal group of the study that includes snakes and certain lizards like monitor lizards and iguanas, their exact evolutionary position remains debated. Bony inner ear fossils could help clarify where these dramatic marine predators fit into reptile evolution.

“While bones enclosing the inner ear may not reveal how extinct reptiles lived, this study highlights their promise for answering an even bigger question: how today’s snakes and lizards are connected to their long-lost extinct relatives,” Forcellati said. “If models like the ones we built are trained on more fossil species, in addition to living ones, deep time evolutionary questions about the relationships of extinct fossil lizards including mosasaurs could potentially be addressed.”