Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

During major evolutionary transitions, groups acquire a new body plan that allows them to colonize new habitats and behave in new ways. The evolution of swimming cetaceans from land-living mammals is a prime example. We document changes to the inner ear sensory system, involved in balance and equilibrium, as extinct crocodile relatives called thalattosuchians underwent a similar transition in the Mesozoic (ca. 182-125 mya). We find that open-ocean thalattosuchians developed strikingly compact and thickened bony labyrinth after a long semiaquatic phase and after modifying their skeleton to become better swimmers. This differs from cetaceans, which miniaturized their bony labyrinths soon after entering the water. Therefore, thalattosuchians and cetaceans took different evolutionary paths from land to water.