TRIGEMINAL NERVE BRANCHING PATTERNS REVEAL DIVERSITY AND EVOLUTION OF FACIAL SENSITIVITY AMONG ARCHOSAURS

Vertebrates evolved numerous types of integumentary sensory systems, many of which enhance facial somatosensation. Species with sensitive faces exhibit high densities of trigeminal nerve-innervated receptors at the ends of nerve branches that course through bony canals and foramina in the face and mandibles. These bony features of the trigeminal system are often used to infer facial sensation in extinct vertebrates. However, the form and function of the trigeminal system are diverse in extant reptiles, and its physiological significance and phylogenetic patterns are unclear. Extant reptiles display morphological diversity in proximal (i.e., trigeminal fossa contents and trigeminal division pathways), intermediate (i.e., inferior alveolar canal neurovasculature and its relation to teeth and integument), and distal structures (i.e., symphyseal neurovasculature and bill-tip-organs). These structural differences are reflected in the behavioral diversity (e.g., lingual vs. jaw prehension in squamates, tactile-feeding in birds) across Sauropsida. Using CT data, we performed morphometric analyses of the inferior alveolar canal of several extant sauropsids to build a robust phylogenetic bracket for extinct archosaurs. We employed a novel quantitative method to compare inferior alveolar canal branching patterns and found a more complex arrangement in crocodylian inferior alveolar canals in comparison to most other reptiles, suggesting reduced ability for mandibular sensation in non-crocodylian taxa. By employing the same techniques to investigate extinct taxa including pseudosuchians (Araripesuchus, Simosuchus, Junggarsuchus) and non-avian dinosaurs (Tyrannosaurus, Majungasaurus, Acristavus), we reveal significant diversity in trigeminal nerve distribution and thus facial sensation in extinct archosaurs. Comparisons show similar branching patterns between dinosaurs and terrestrial pseudosuchians indicating similarity in sensory behaviors in these taxa. Additionally, there is an increase in branching complexity along the pseudosuchian line, corresponding with a transition from terrestrial to semi-aquatic environments and the appearance of tactile-feeding in the clade. Overall, these findings assist in reconstruction of soft tissues from osteological correlates in fossil taxa and will help uncover patterns of reptilian somatosensory ecology and evolution.