Without a single living specimen to observe, how do you figure out how an extinct species moved? Penn State paleontologist Alan Walker and his team found a clue to mammals’ motion in an unlikely place: their ears.
Walker zeroed in on the three semicircular canals of the inner ear that, like a built-in gyroscope, play a role in balance. The canals serve as sensors for the muscles of the eyeballs and the neck, providing reflexes that keep images steady as an animal moves. “They’re angular accelerometers, measuring spin,” Walker says, “and naturally if you move slowly, the sensitivity of the system doesn’t need to be as great as when you move acrobatically.” When they examined the canal structures of more than 200 mammal species, the research team found that fast, agile movers—like gibbons and leaping tarsiers—have exceptionally large canals for their size, while slow, deliberate animals like sloths have small ones.
Now that they’ve established the relationship between ear structure and movement in living animals, the researchers can make predictions about the motion of extinct creatures by measuring the bony canal remnants in fossils. “When you have a skeleton, you can use biomechanics and analogy to try to reconstruct locomotion. With arboreal animals that don’t leave footprints, you can’t tell whether you’re right or not. This provides an independent way to assess agility and jerkiness of locomotion,” says Walker.