215 Both tooth shape and mode of mastication gained an enor- mous variability during the evolution of large herbivorous mammals, which allowed them to exploit diverse ecologi- cal niches. The variability in mastication includes specific adaptations to diet and is linked to the evolutionary history of a group. In addition, teeth are composed of the hardest tissue of the vertebrate body and thus are the most abun- dant vertebrate remains in the fossil record. Because of that and the resistance of the enamel to the wear process, teeth are preferred objects in paleontology. Teeth are worn and partly abraded by the continuous chewing processes and need to serve the entire lifespan of an animal. Wear creates facets typically appearing on the crown of molars forming light reflecting polished surfaces (Mills 1955, Butler 1973). These wear facets occur as matching pairs on upper and lower teeth; several numbering systems and nomenclatures were erected for descriptive and comparative purposes (see summary in Schultz et al. 2018, 2020, this volume). Attritional facets are gener- ated during mastication by tooth-tooth contact between antagonistic molars, abrasional facets occur during tooth- food interaction without a direct contact to the antagonistic tooth structure (Maier & Schneck 1981, Academy of Pros- thodontics 2005, Grippo et al. 2004). Both types of facets generally occur on the same occlusal surface. Erupting teeth usually do not show the fully functional shape right from the beginning. Many teeth need at least some initial use to develop precise functioning surfaces. Despite the continuous abrasion of the surface over time, such func- tional surfaces need to function with the same precision for a relatively long period, at least for most of the animal’s life. In this chapter, we concentrate on the functional surfac- es that last for a long period and which characterize various large herbivorous mammals. In the following we distinguish between functional surfaces being composed of primary and secondary surfaces. The two conditions were intro- duced by Fortelius (1985): (1) teeth with primary functional crown shape are shaped and functional as they erupt with full enamel cover, and (2) teeth with secondary functional crown shape are formed and maintained by wear. Follow- Evolutionary trends in the mastication patterns in some perissodactyls, cetartiodactyls, and proboscideans Julia A. Schultz, Sandra Engels, Leonie C. Schwermann, and Wighart v. Koenigswald Introduction distal mesial 0° 0° 45° 45° 90° 90° buccal lingual A B C D E F Primary enamel surfaces Blades symmetrical asymmetrical compression cutting shear-grinding Rasp Nipper Secondary enamel surfaces II G H I J K L I Fig. 11.1. Difference between primary and secondary functional enamel surfaces. A, B, all teeth that retain a primary func- tional surface mostly compress the food between the enamel coated (black layer) basins of the upper and lower teeth, or shear food along steep walls. C-F, secondary functional surfaces are characterized by enamel crests (black lines) and dentin basins (yellow), functional equivalent tools are blades (C, D), rasps (E) or nippers (F). G, the mastication compass is used to illustrate the direction of the lower jaw movements during the power stroke. The center of the circle depicts the maximal intercuspation between phase I and phase II. The arrows indicate the direction, the length of each arrow indicates the inclination during the jaw motion. In case phase II is descending the arrow is illustrated in gray. H-L, summary of tooth functions distinguished in this chapter: compression between primary occlusal surfaces (H, I), cutting along the enamel edges of a secondary occlusal surface (J), and shear-grinding along rasp-like secondary occlusal surfaces (K, L). T. Martin & W. v. Koenigswald (eds.): Mammalian Teeth – Form and Function. Pp. 215-230, 9 figs. © 2020 by Verlag Dr. Friedrich Pfeil, München, Germany – ISBN 978-3-89937-266-3 DOI: http://doi.org/10.23788/mammteeth.11 Published 22 December 2020