Deep evolutionary diversification of semicircular canals in archosaurs

Area of Science:

• Paleontology
• Evolutionary Biology
• Biomechanics

Background:

• Living archosaurs (birds and crocodylians) exhibit diverse locomotion, but early sensory system evolution is poorly understood.
• Semicircular canal (SCC) morphology is hypothesized to relate to locomotor strategies.
• Fossil sampling of early archosaur lineages is limited, hindering evolutionary studies.

Purpose of the Study:

• Analyze SCC shape and size in extinct and extant archosaurs with varied locomotion.
• Test form-function hypotheses regarding SCCs and locomotion.
• Chronicle SCC evolution across major archosaur divergences.

Main Methods:

• Comparative analysis of SCC morphology (shape and size) in a diverse sample of fossil and living archosaurs.
• Statistical testing of correlations between SCC morphology and locomotor habits (bipedal, semi-aquatic, flying).
• Phylogenetic analysis to reconstruct SCC evolution on the archosaur family tree.

Main Results:

• SCC shape shows statistical association with flight and bipedalism, but variation is minor and potentially linked to braincase geometry.
• High disparity in SCC shape and size observed in stem-archosaurs, indicating early sensory diversity.
• Deep divergence in SCC morphology occurred at the bird-crocodylian split; extant crocodylian SCCs are not a primitive reptilian condition.
• Key bird SCC traits (large size, enhanced sensitivity) evolved early in non-flying dinosaur precursors.

Conclusions:

• Archosaur SCC evolution is characterized by deep divergence and high sensory diversity from early radiation.
• Locomotor strategies are not the sole drivers of SCC morphology evolution.
• Understanding SCC evolution provides insights into sensory adaptations preceding major locomotor innovations in birds and crocodylians.