Extreme Muzzles of Mastication: Craniofacial Muscular Support Systems of Large Herbivores (Dinosaurs, Mammals, and More)
Authors: Nabavizadeh Ali 
Published: November 2023

Herbivory has evolved countless times independently throughout vertebrate evolution. Among extant amniotes, it is seen in various mammals as well as some reptiles and birds and the fossil record greatly extends this diversity to non‐mammalian synapsids (e.g., dicynodonts and pelycosaurs), non‐avian dinosaurs (e.g., horned ceratopsians, duck‐billed hadrosaurs, armored ankylosaurs and stegosaurs, and long‐necked sauropods), parareptiles, and others. This study explores (through dissections, osteological correlates, and literature review) the vast functional diversity of cranial musculature acting as massive functional support systems in the muzzles of the largest herbivores over millions of years. Trigeminal jaw muscle anatomy has changed drastically throughout amniote evolution and the anatomical constraints that have been placed on individual subclades has dictated how they accommodate for a transition to an herbivorous lifestyle. These anatomical variations have modified to adapt to withstanding feeding forces in various grinding feeding motions in both browsing and grazing. Jaw motions include orthal (i.e., vertical closure of oral cavity), transverse, wishboning, proal (i.e., rostrally oriented), and palinal (i.e., caudally oriented) feeding strokes. For instance, the diversity in dinosaur jaw muscle anatomy suggests how, in accordance with various joint morphologies, muscles adapt to different feeding strategies, such as orthal feeding with slight bilateral long‐axis rotation of the dentaries (e.g., in basal ornithischians, stegosaurs, and pachycephalosaurs), palinal feeding with more pronounced bilateral long‐axis rotation of the dentaries (e.g., in hadrosaurs and ankylosaurs), and strictly orthopalinal feeders (e.g., ceratopsians). These variations are seen, in large part, with the extent of rostral insertion of adductor musculature providing greater mechanical advantage as well as the stability and support needed for the more complex feeding motions observed in tooth wear. This rostral muscle elongation is also seen in herbivorous synapsids, such as dicynodonts and edaphosaurid pelycosaurs, also accommodating palinal motion. Conversely, large herbivorous mammals are adapted to perform various transverse feeding motions, with few exceptions, such as in elephants which are proal feeders. The masseter muscle is especially attached farther forward on the zygomatic arch in some large herbivorous mammals, lateral to the tooth row to a certain extent, also accommodating mechanical advantage and support for more complex feeding motions. Lastly, another major factor in the mammalian line of cranial musculature is the presence of muscles of facial expression, which have taken many forms for different feeding habits, such as the proboscis in elephants and the tendon‐bearing musculature seen in ungulates such as rhinoceroses, hippopotamuses, bovids, and horses. These cranial muscle support systems are ideal for withstanding the most powerful browsing and grazing habits and this study aims to show the remarkable diversity of these magnificent terrestrial beasts.