New Reconstruction of Cranial Musculature in Ceratopsian Dinosaurs: Implications for Jaw Mechanics and ‘Cheek’ Anatomy
Authors: Nabavizadeh Ali 
Published: November 2023

For over a century, researchers have attempted to reconstruct cranial musculature in the horned ceratopsian dinosaurs, like Triceratops, in order to elucidate feeding behavior and mechanics in this important group of large herbivores. Here I present a new interpretation of muscle architecture in ceratopsian crania redefining the jaw mechanics of these animals. Muscular anatomy is interpreted through in‐depth observation of muscle scarring and osteological correlates in taxa spanning the entire clade. M. adductor mandibulae externus (mAME), the main adductor muscle group originating within the supratemporal fenestra, has conservatively been reconstructed as inserting along the apex and caudal aspect of the tall coronoid process. The jugal flares out laterally, allowing communication between the caudal aspect of the maxilla and the endocranial adductor chamber. Accordingly, a large pocket is created allowing one enlarged muscular layer of mAME to exit the adductor chamber rostroventrally and extend much farther rostrally than previously proposed, inserting fibers all along the prominent lateral dentary ridge characteristic of ceratopsians. This lateral dentary ridge has traditionally been suspected as insertion of a novel, buccinator‐like ‘cheek’ muscle. This new mAME reconstruction discounts the necessity of this type of novel musculature and, in turn, repurposes mAME as both a major jaw adductor as well as a secondarily functional ‘cheek’ containing food within the oral cavity. MAME then also creates a direct line of action between the rostral extent of the lateral dentary ridge and the origin of the muscle within the frill, lining up directly through the jugal opening and creating a massive palinal feeding stroke of the mandible. This corroborates with previous microwear studies while cradling the entire lower jaw around the sides, ideal for withstanding a much larger bite force. M. pterygoideus ventralis (mPTV), originating ventrally on the pterygoid flange and traditionally wrapping ventral to and laterally on the retroarticular process of the mandible, is now reconstructed to insert more rostrally on the lateral aspect of the dentary. The lack of a prominent retroarticular process in ceratopsians and the presence of a large depression just ventral to the coronoid process indicate this architecture. This creates a vertical line of action of mPTV for a powerful, orthal feeding stroke compensating for mAME creating a much more palinal vector. M. pterygoideus dorsalis (mPTD), originating on the dorsum of the pterygoid flange and inserting medially on the retroarticular process, is now reconstructed as extending its origin rostrolaterally onto the lateral aspect of the maxilla resting in a clear depression. The laterally flaring jugals create a continuous surface between the dorsum of the pterygoid phalange and the caudal aspect of the lateral surface of the maxilla, which is indicative of this reconstruction. MPTD in this orientation creates a rostrally‐oriented vector, compensating for the vector of mPTV becoming more vertical. These newly reconstructed muscle configurations hold many important implications for the future of studies of feeding mechanics in these massive herbivores through computer modeling methodologies, among others.