Graduate Research Assistant University of Missouri Columbia Columbia, Missouri, United States
Abstract Body : Cranial kinesis, or movement of cranial bones beyond those of the mandible, is widespread in reptiles and birds and has been associated with a number of functions. An extreme example of this is found in snakes, which have evolved a unique system of prey manipulation during swallowing, controlled by movements of the palatal bones. Using alternating movements of the palate, the jaws and braincase move to engulf the prey item. This “pterygoid walk” is powered by the muscles of the palate known as the dorsal constrictor musculature, including the protractor pterygoideus (mPPT), levator pterygoideus (mLPT), and retractor pterygoideus (mRPT). These small muscles, deep to several layers of jaw muscles and bone, have been difficult to study with traditional dissection techniques. Their morphology, however, is critical to the functional anatomy of one of the most diverse clades of reptiles. Here we use DiceCT (Diffusible Iodine Contrast-Enhanced Computed Tomography) and Avizo Xfiber muscle fascicle tracking software to reconstruct the architecture of the dorsal constrictor musculature of a sample of three snakes. Our snake sample included Pantherophis obsoletus (black rat snake), Agkistrodon contortrix (copperhead), and Morelia spilota (carpet python). Each muscle was segmented in Avizo to calculate data on muscle volume, PCSA, attachment surfaces, fiber architecture, and pinnation. Segmented muscle volumes and muscle attachment surfaces were used to calculate the force and direction of each muscle and were normalized by intercondylar width. We found that muscle morphology and architecture were highly conserved across these three snakes. The mPPT is parallel fibered and entirely rostrocaudaully oriented. The mLPT and mRPT also have low pinnation, with the mLPT’s fibers running posteroventrally, and the mRPT’s fibers running anteroventrally from the braincase. Agkistrodon has a relatively stronger mPPT than the other species, though the reason for this is not clear from this small sample. Overall, these three muscles are relatively conservative across this sample of snakes, though a clearer picture will result from future work increasing the sample size of snake diversity and including the protractor quadrati and retractor vomeris muscles, where present. This future work will clarify the functional anatomy of snake feeding mechanisms, and open new questions about the evolutionary history of snake muscle architecture and feeding kinematics.
