Evolution and function of the jaw musculature and adductor chamber of archosaurs (crocodilians, dinosaurs, and birds)

Crocodilians, dinosaurs, and birds are part of successful group of reptiles known as archosaurs, little is known about the evolution of the adductor chamber, which includes the jaw musculature, trigeminal nerves, and particular blood vessels, hindering hypotheses of homology and feeding function in fossil taxa such as crocodyliforms and non-avian dinosaurs. First, I review the cephalic musculature of dinosaurs and identify problems and prospects involved in inferring feeding form and function in the clade. Second, using a flamingo as a case study, I introduce a new CT imaging/dissection methodology that enables 3D visualization of cephalic vasculature and differentiation of adductor chamber contents. Third, I develop a robust hypothesis of jaw muscle homology by analyzing the topological patterns of soft tissues in the adductor chambers of extant reptiles. Fourth, I identify major evolutionary changes in the orbitotemporal region (e.g., trigeminal nerve, braincase, palate) during the evolution of crocodilians. Fifth, I identify major evolutionary changes in the orbitotemporal region of dinosaurs with respect to the evolution of birds. Sixth, I discuss the evolution of cranial kinesis and its functional significance in dinosaurs and other reptiles. The general results were: (1) Flamingos have a novel vascular device associated with the hyolingual system. (2) Crocodylians have a novel soft-tissue topological pattern that violates the trigeminal topological paradigm. (3) Sensory branches of the trigeminal nerves are topologically conservative and represent evolutionarily stable dermatomes. (4) Despite the suturing of the palate to the braincase early in their evolution, the epipterygoid was a persistent structure in the skull of crocodyliforms, evolved several different morphotypes, and was not eliminated until recently along the lines to modern crocodylians. (5) The trigeminal nerve, protractor muscles, and epipterygoid exhibit mosaic evolution among dinosaurs and prove to be phylogenetically and functionally informative structures. (6) Dinosaurs do not exhibit the suite of morphological characters indicative of cranial kinesis which suggests that intracranial synovial joints may more likely be related to growth rather than feeding function. Hence, jaw musculature and its neighboring tissues in the adductor chamber are key cephalic structures that exhibit characteristic morphological, functional, and phylogenetic patterns among extant and fossil archosaurs.