Avoiding the lockdown: Morphological facilitation of transversal chewing movements in mammals

The evolution of mammals is characterized, amongst other developments, by an increasing relevance of effective food processing in form of an increasingly durable dentition, complex occlusal surfaces, and transverse chewing movements. Some factors have received increasing attention for the facilitation of the latter, such as the configuration of the jaw joint, the chewing muscle arrangement and lever arms, or the reduction of interlocking cusps on the cheek teeth occlusal surface. By contrast, the constraining effect of the anterior dentition (incisors and canines) on transverse chewing motions, though known, has received less comprehensive attention. Here, we give examples of this constraint in extant mammals and outline a variety of morphological solutions to this constraint, including a reduction of the anterior dentition, special arrangements of canines and incisors, the nesting of the mandibular cheek teeth within the maxillary ones, and the use of different jaw positions for different dental functions (cropping vs. grinding). We suggest that hypselodont anterior canines or incisors in some taxa might represent a compensatory mechanism for self-induced wear during a grinding chewing motion. We propose that the diversity in anterior dentition among mammalian herbivores, and the evolutionary trend towards a reduction of the anterior dentition in many taxa, indicates that the constraining effect of the anterior dentition, which is rigidly linked to the cheek teeth by the osseous jaws, represents a relevant selective pressure in mammalian evolution.