Diversity and evolution of the sauropsid trigeminal sensory system

Abstract

Understanding the evolution of sensory perception is essential in explaining organismal behavior. Organisms using environmental stimuli to navigate their environments, avoid predation, capture prey, and find a mate, have a selective advantage. One player in organismal fitness is how and how well organisms perceive, interpret, and react to signals from their environments. Perceptions are the organized properties of physical stimuli from the environment and are modulated by the nervous system. Several key innovations in early vertebrate evolution, such as ectodermal placodes and neural crest cells, enhanced the nervous system and increased the range and types of stimuli organisms could perceive. The new body plan resulting from these innovations included a cranium capable of respiration, feeding, and information acquisition via numerous sensory organs and a segmental series of pharyngeal arches. The first pharyngeal arch includes the fifth cranial nerve, the trigeminal nerve (CN V), and its associated vasculature and is the primary somatosensory nerve of the vertebrate head. The trigeminal nerve mediates sensory perception in the first pharyngeal arch as the arch engages in feeding behavior. The trigeminal system, though conserved in general form, is diverse across vertebrates, particularly sauropsids, which have evolved extreme forms of cranial somatosensation as in probe-feeding birds, infrared-sensing pit vipers, and touch-sensitive crocodilians. It remains to be understood how these adaptations for cranial sensation evolved among different clades of sauropsids, including specific lineages of lepidosaurs, crocodylians, and birds. Previous research has used osteological structures to predict nervous tissue anatomy in small ranges of extinct dinosaur and crocodylian species. However, these hypotheses require thorough testing using modern imaging, morphometrics, and phylogenetic comparative methods. Overall, we still lack consistent anatomical means of comparing relative sensation across lineages of reptiles, and the origins of extreme forms of sensation in the clade remain largely unexplored. This research identifies patterns in form, function, and evolution of the sauropsid trigeminal system. Osteological and soft tissue structures of the trigeminal system are compared in both embryonic and adult sauropsids to understand their relationship, physiology, and morphology. These data were compared to behavioral and ecological data from the literature and form-function relationships hypothesized across environments. From this knowledge, I draw conclusions on the presence of soft tissues in extinct taxa as well as their behaviors and ecologies and evolution of the system. This research will clarify the relationships between structure, function, behavior, and ecology of the extant reptilian trigeminal system to better understand evolutionary patterns of sensation.