Cheek Teeth

The heterodont dentition of mammals comprises four functional tooth types arranged from anterior to posterior: incisors, canines, premolars, and molars. The premolars and molars are collectively termed cheek teeth because of their position along the inner surface of the cheeks, behind the canines. The distinction between premolars and molars is developmental rather than strictly morphological: premolars appear in both the deciduous (milk) and permanent dentitions, whereas molars are monophyodont—they erupt only once, as permanent teeth without deciduous precursors.

The occlusal surfaces of cheek teeth bear complex arrangements of cusps, crests, and basins that are critical for food processing. The ancestral mammalian molar—the tribosphenic molar—featured three primary cusps arranged to create both shearing and crushing actions during occlusion, an adaptation that proved highly effective for processing insect chitin in early mammals. From this tribosphenic foundation, mammalian cheek teeth have diversified into a range of ecomorphological types. Bunodont teeth, with low, rounded cusps suited for crushing and pounding, characterise omnivores such as bears, pigs, and primates including humans. Selenodont teeth, in which cusps are elongated into crescent-shaped crests oriented mesiodistally, are hallmarks of artiodactyl herbivores (deer, cattle, camels) and facilitate side-to-side grinding of fibrous plant matter. Lophodont teeth, bearing transverse ridges (lophs) connecting cusps, are found in perissodactyls such as rhinoceroses and horses, as well as many rodents. Secodont teeth with dominant shearing crests characterise carnivorans, culminating in the specialised carnassial pair (upper P4 and lower m1) of felids and canids.

Crown height is another key variable in cheek tooth morphology. Brachydont (low-crowned) teeth are typical of animals consuming soft foods, while hypsodont (high-crowned) teeth are adaptive responses to the abrasive demands of a grass- or grit-rich diet. Some lineages have evolved hypselodont (ever-growing) cheek teeth—notably horses and many rodents—in which the crown grows continuously to compensate for constant wear. A diastema (gap) often separates the anterior teeth from the cheek teeth in herbivorous mammals, facilitating the posterior transfer of cropped food to the grinding surfaces.

Among dinosaurs, the most elaborate cheek teeth evolved in the Ornithischia—the herbivorous clade that includes ornithopods, ceratopsians, thyreophorans, and pachycephalosaurs. Peter Galton's landmark 1973 paper in Lethaia drew attention to the medial inset of ornithischian cheek teeth relative to the outer jaw surface. He argued that this inset created a lateral space roofed by the overhanging maxilla and floored by the dentary, which in life would have been enclosed by fleshy cheeks. These cheeks would have functioned to retain food within the oral cavity during chewing, analogous to the muscular cheeks of mammals.

The 'cheek hypothesis' has been debated extensively. Nabavizadeh (2020) reconstructed the cranial musculature of ornithischian dinosaurs and concluded that while some form of buccal soft tissue was likely present, it was not homologous to mammalian muscular cheeks. No living sauropsid possesses cheeks or lips analogous to those of mammals. The ornithischian buccal tissue was probably a uniquely evolved structure—perhaps a keratinised or collagenous covering rather than a muscular cheek—but its presence in some form is supported by the consistent medial inset of cheek teeth across diverse ornithischian lineages.

The earliest ornithischians, such as the fabrosaurids, had simple leaf-shaped cheek teeth arranged in a single row that did not interlock during occlusion. In heterodontosaurs, the cheek teeth became more tightly packed and developed a self-sharpening mechanism: enamel was deposited asymmetrically (on the outer face of upper teeth and the inner face of lower teeth), so that differential wear maintained chisel-like cutting edges. This basic pattern was progressively elaborated through ornithopod evolution, with the hypsilophodontids developing closely packed, directly occluding cheek tooth rows, and iguanodontids evolving larger jaw muscles housed in expanded temporal regions of the skull.

Hadrosaurid dinosaurs (duck-billed dinosaurs) evolved what is arguably the most complex dental system in vertebrate history. According to LeBlanc et al. (2016), who conducted the first comprehensive tissue-level ontogenetic study of hadrosaurid dental batteries, each jaw ramus could contain up to approximately 300 teeth arranged in about 60 tooth positions, with up to six generations of teeth stacked vertically at each position. The functional surface at any given time consisted of multiple teeth at different stages of wear, creating a large, constantly replenished grinding platform.

The key innovation underlying the hadrosaurid dental battery was heterochrony—specifically, an acceleration in the timing and rate of dental tissue formation. Dentine and cementum were deposited so precociously that the pulp cavity of each tooth was completely infilled before eruption. This meant that erupted teeth were already non-vital (lacking living pulp tissue), allowing them to be ground down entirely—root and all—without causing pain or infection. Teeth were never shed as in typical reptiles; instead, they were worn away completely through use.

Contrary to earlier interpretations that hadrosaurid teeth were fused together by hard tissue (ankylosis), LeBlanc et al. demonstrated that individual teeth within the battery were suspended by periodontal ligaments—both to the walls of the jaw and, uniquely among vertebrates, to neighbouring teeth. This ligamentous network provided mechanical flexibility, allowing the battery to function as an integrated shock-absorbing unit during the powerful grinding motions required to process tough, fibrous Cretaceous vegetation. The sophisticated ligament system also explains taphonomic observations: isolated hadrosaurid teeth found abundantly at Late Cretaceous microsites represent dissociated dental batteries, not individually shed teeth.

The hadrosaurid dental battery began forming in the embryo and was probably functional immediately after hatching, as evidenced by embryonic and neonatal specimens showing multiple generations of interconnected teeth with plugged pulp cavities and ligamentous attachments.

Ceratopsian dinosaurs independently evolved dental batteries, but with a fundamentally different functional design from hadrosaurids. Whereas hadrosaurid batteries operated primarily as grinding surfaces, advanced ceratopsian (ceratopsid) batteries were specialised for slicing. Erickson et al. (2015), published in Science Advances, revealed that Triceratops cheek teeth were composed of five distinct dental tissue types: enamel, orthodentine, secondary dentine, coronal cementum, and a fifth tissue (giant tubule dentine). This represents a level of tissue complexity rivalling that of horse teeth (which have six tissue types) and exceeding that of any other known reptile.

These five tissues wear at different rates during feeding, producing a self-sharpening effect. As the tooth surface erodes, a characteristic fuller—a recessed central groove analogous to the blood groove on a sword blade—forms naturally on each tooth. The resulting cutting edges allowed ceratopsian cheek teeth to slice through dense, tough plant material with scissor-like efficiency. This wear-mediated self-sharpening mechanism is convergent with, but structurally distinct from, the self-sharpening enamel-dentine interface seen in rodent incisors and ornithopod cheek teeth.

Primitive ceratopsians employed relatively simple orthal (vertical) jaw movements, but dental microwear analysis by Varriale (2016) demonstrated that at least some derived forms, such as Leptoceratops, employed a more complex masticatory cycle including a rostral (forward) rotational component—a type of jaw movement previously thought to be restricted to mammals.

Sauropod dinosaurs present a striking contrast to ornithischians in their approach to herbivory. Sauropods did not evolve complex cheek teeth. Their dentitions were effectively homodont (uniform in shape), with teeth specialised for cropping rather than chewing. Diplodocids bore slender, pencil-like teeth restricted to the front of the jaw; camarasaurids had broader, spatulate teeth. In neither case were the teeth set medially inward from the jaw margin, and there is no evidence for cheek-like soft tissue structures.

Without cheek teeth for oral processing, sauropods are widely interpreted to have swallowed food with minimal or no mastication, relying instead on gut fermentation for digestion. Some researchers have proposed that gastroliths (stomach stones) aided mechanical breakdown of food in the digestive tract, though Wing et al. (2011) found that the evidence for a functional gastric mill in most sauropods is weak. The absence of cheek teeth in sauropods conferred a different evolutionary advantage: a small, lightweight head that could be supported at the end of an extremely long neck, enabling access to vegetation at heights and lateral reaches unavailable to other herbivores.

Cheek tooth morphology is one of the most powerful tools in vertebrate palaeontology for inferring the dietary ecology of extinct animals. Gross morphology—tooth shape, cusp pattern, crown height, occlusal area—provides first-order dietary indicators. Dental microwear analysis, which examines microscopic scratches and pits on worn tooth surfaces, adds higher-resolution dietary information. Heavy, parallel scratches indicate processing of tough, fibrous vegetation, while pitting suggests consumption of hard objects such as seeds or bone.

Dental microwear texture analysis (DMTA) has been applied to hadrosaurid and ceratopsian cheek teeth, revealing that hadrosaurids consumed a wider range of plant textures than ceratopsians, consistent with their grinding-type battery versus the slicing-type battery of ceratopsians. These analyses, combined with tooth replacement rates estimated from incremental lines of von Ebner in dentine, provide detailed reconstructions of feeding ecology across deep time.

Cheek teeth thus occupy a central position in comparative anatomy, functional morphology, and palaeobiology. Their study illuminates not only the mechanics of feeding but also the co-evolutionary dynamics between herbivores and the plants they consumed, the evolutionary pressures driving dental complexity, and the remarkable convergence between mammalian and dinosaurian solutions to the challenge of processing a plant-based diet.