Deep evolutionary diversification of semicircular canals in archosaurs

Living archosaurs (birds and crocodylians) have disparate locomotor strategies that evolved since their divergence-250 mya. Little is known about the early evolution of the sensory structures that are coupled with these changes, mostly due to limited sampling of early fossils on key stem lineages. In particular, the morphology of the semicircular canals (SCCs) of the endosseous labyrinth has a long-hypothesized relationship with locomotion. Here, we analyze SCC shapes and sizes of living and extinct archosaurs encompassing diverse locomotor habits, including bipedal, semi-aquatic, and flying taxa. We test form-function hypotheses of the SCCs and chronicle their evolution during deep archosaurian divergences. We find that SCC shape is statistically associated with both flight and bipedalism. However, this shape variation is small and is more likely explained by changes in braincase geometry than by locomotor changes. We demonstrate high disparity of both shape and size among stem-archosaurs and a deep divergence of SCC morphologies at the bird-crocodylian split. Stem-crocodylians exhibit diverse morphologies, including aspects also present in birds and distinct from other reptiles. Therefore, extant crocodylian SCC morphologies do not reflect retention of a primitivereptilian condition. Key aspects of bird SCC morphology that hitherto were interpreted as flight related, including large SCC size and enhanced sensitivity, appeared early on the bird stem-lineage in non-flying dinosaur precursors. Taken together, our results indicate a deep divergence of SCC traits at the bird-crocodylian split and that living archosaurs evolved from an early radiation with high sensory diversity.

Automated summary

Living archosaurs have evolved disparate locomotor strategies since their divergence 250 million years ago, with little known about the early evolution of sensory structures associated with these changes. By analyzing semicircular canal shapes and sizes in living and extinct archosaurs, researchers found that SCC shape is statistically associated with both flight and bipedalism, but may be more influenced by changes in braincase geometry than locomotor changes. The high disparity of both shape and size among stem-archosaurs indicate a deep divergence of SCC morphologies at the bird-crocodylian split, suggesting living archosaurs evolved from an early radiation with high sensory diversity.