The velar chord and dynamic integration of the gular valve in crocodylians

Crocodylians evolved a unique gular valve that is capable of creating a water-tight seal between the oral and pharyngeal cavities, allowing the animal to safely submerge with an open mouth. The gular valve has traditionally been described as consisting of two separate parts: an active mobile ventral portion (consisting of the tongue and portions of the hyolingual apparatus) and a dorsal portion, which is a static fold on the hard palate (often termed the palatal velum). The results of the present study argue that the two portions of the gular valve are functionally integrated, not separate, and that the dorsal portion (herein the dorsal gular fold) is a dynamic element the shape and tension of which are influenced by active and passive forces. Using gross dissection, histology, and DiceCT, the present study documents a previously underscribed component of the gular valve, the velar chord, which links the hyolingual apparatus to the dorsal gular fold, functionally integrating the two halves of the gular valve. Through endoscopic videography and a variety of manipulations on living crocodylians, this study demonstrates that changes in the tension on the velar chord directly alter the shape and tension of the dorsal gular fold. The shape changes observed in the dorsal gular fold could be accommodated by a shallow depression in the ventral surface of the palatine bones, herein termed the velar fossa. The velar fossa is a prominent feature of Alligator mississippiensis and was observed in other crocodilians; however, a survey of living and fossil crocodylians demonstrated that the velar fossa is not a universal feature in this clade. Understanding the functional linkage between the dorsal and ventral portions of the gular valve has implications beyond the dive reflex of crocodylians, since active manipulation of the dorsal gular fold likely plays a role in a variety of behavioral and physiological processes such as deglutition and vocalization.