Glossary

Cheek teeth are the teeth located posterior to the canines in the dental arcade, positioned along the inner surface of the cheeks. In mammals, the term collectively refers to the premolars and molars—teeth characterised by complex occlusal surfaces bearing cusps, ridges, and basins specialised for grinding, shearing, and crushing food. Premolars are distinguished from molars ontogenetically: premolars are replaced once during diphyodont development (having deciduous precursors), whereas molars erupt only as permanent teeth. In non-mammalian vertebrates, the term is applied more broadly to any posterior jaw teeth that perform analogous food-processing functions. Cheek teeth attained their most elaborate development in herbivorous ornithischian dinosaurs. Hadrosaurids (duck-billed dinosaurs) evolved dental batteries containing up to approximately 300 teeth per jaw ramus stacked in 60 tooth positions, forming a constantly replenished grinding surface for processing tough plant fibre. Ceratopsians such as Triceratops independently evolved dental batteries with a distinct slicing function, their cheek teeth composed of five different dental tissue layers that self-sharpened through differential wear to create blade-like cutting edges. The morphology of cheek teeth is tightly correlated with diet across vertebrate lineages. In palaeontology, cheek tooth form, occlusal wear patterns, and dental microwear provide primary evidence for reconstructing the dietary ecology and feeding behaviour of extinct animals, making cheek teeth among the most informative anatomical structures in the vertebrate fossil record.

A **toothless beak** is a cranial feeding structure in which the jaw bones are entirely devoid of teeth (edentulous) and are instead covered by a keratinous sheath known as a rhamphotheca. The rhamphotheca envelops both the outer (rostral) and part of the inner (oral) surfaces of the jawbones, functionally replacing teeth for food acquisition and manipulation. Within theropod dinosaurs alone, fully edentulous beaks evolved independently at least seven times, appearing in lineages such as Oviraptorosauria, Ornithomimosauria, Therizinosauria, Ceratosauria (notably Limusaurus), and multiple clades of Mesozoic birds. Ornithischian dinosaurs, including ceratopsians and hadrosaurs, also possessed beaks, though typically in combination with posterior dentition. Biomechanical analyses using finite element modeling have demonstrated that keratinous beaks reduce stress and strain in the rostral skull, enhancing structural stability during feeding. The repeated convergent evolution of toothless beaks across Dinosauria reflects a complex interplay of selective pressures, including dietary shifts toward herbivory or omnivory, weight reduction, enhanced cranial stability, and possibly shorter incubation periods linked to the elimination of embryonic tooth development.