68 - Adaptation or Exaptation, an Analysis of Sensory Evolution in a Marine Neosuchian Adaptive Radiation

Poster Board Number: 68
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Abstract Body : Sensory systems evolve to facilitate organismal interaction with the environment. All extant crocodylians possess a complex system of integumentary sensory organs (ISOs), innervated by the trigeminal nerve, that they utilize in prey capture. The trigeminal nerve is an ancient structure, responsible for somatosensation in the face. However, in contemporary crocodylians, the trigeminal-ISO system has hypertrophied to enable heightened sensitivity to changes in water pressure and, potentially, pH and temperature. This pressure sensitivity allows crocodylians to capture prey. These modern semiaquatic ambush predators evolved from terrestrial crocodylomorph precursors, which presumably benefited little from a water-based sensory system. It is often thought that such complex sensory systems arise as adaptations to new environments, but they could alternatively be exaptations, coincident with new environments. In this study, we aim to demonstrate whether the trigeminal somatosensory system of ISOs increases robusticity before or after the evolution of fully pelagic crocodylomorphs, and, therefore, whether its emergence is consistent with adaptation for, or exaptation prior to, transitioning to marine environments. Because it is a cranial nerve, the trigeminal nerve is tightly packed alongside other cranial features like the skull. Its evolutionary enlargement leaves impressions in the surrounding bones. For example, the prootic, laterosphenoid, quadrate, and pterygoid bones form a pocket, the trigeminal fossa, within which sits the trigeminal ganglion. This and other osteological correlates, such as the size of the maxillomandibular foramen and the number of ISO foramina signal the potential for facial somatosensation. To study the trigeminal nerve in crocodylomorphs, we use traditional computed tomography (CT) and diffusible iodine-based contrast-enhanced computed tomography (diceCT) scans of extinct and extant crocodylomorphs. We characterize changes in trigeminal ganglion size—a proxy for facial sensitivity—within tethysuchia, a group of Late Jurassic to Early Paleogene neosuchid crocodylomorphs, and their ancestors. Tethysuchia was selected because of the primarily aquatic function of the trigeminal system, and because tethysuchians are a group that diversified following an ecological niche transition from terrestrial to aquatic lifestyles. Notably, the hydrodynamics of freshwater and saltwater are not especially different relative to body mass for crocodylians. Therefore, we hypothesize that if the trigeminal somatosensory system became more robust in pelagic, as compared to semiaquatic, tethysuchian taxa, then it suggests a gain of sensory ability consistent with an adaptive mode of evolution, which may emphasize sensitivity to features that vary greatly across pelagic environments, including pH and temperature. Alternatively, if the trigeminal system is hypertrophied in semiaquatic taxa, and remains so in the pelagic forms, it suggests that the emergence of the trigeminal-ISO system is consistent with exaptation in pelagic lineages for mechanoreception, achieved by semi-aquatic and fresh-water taxa prior to their transition to marine environments. The outcomes of our project will increase the phylogenetic and temporal resolution of trigeminal nerve diversification and highlight its potential, or lack thereof, as a model of sensory system evolution in a marine neosuchian adaptive radiation.