Evolution and development of the mammalian multicuspid teeth.

Background: Mammalian premolars and molars (cheek teeth) are teeth with multiple cusps, which are important characteristics of mammals. Endothermic mammals have high basal metabolic rates and must take in much energy by efficient mastication of food using their multicuspid cheek teeth. From the phylogenetic (evolutionary) perspective, the mammalian multicuspid teeth are derived from the reptilian unicuspid teeth with a single cone by adding new cusps around the original cone. Nearly 100 million years of long geological time were required for the unicuspid tooth to transform into a tribosphenic molar, which is the prototype of all molars in modern mammals. From the ontogenetic (developmental) perspective, the shape of the tooth germ becomes complex by adding the secondary enamel knots repeatedly in a short embryonic period. The secondary enamel knots are signaling centers that determine the future cusp positions.

Highlight: Here we first reviewed the evolutionary process of the tribosphenic molar in the Mesozoic Era (the age of dinosaurs), and cusp homologies in the fossil record. Next, we reviewed the developmental mechanisms controlling the patterning of secondary enamel knots, which determine the final cusp patterns of molars in modern mammals. Finally, we discussed the possible relationship between the two processes from the extremely different time scales.

Conclusion: A parallel relationship between ontogeny and phylogeny of the mammalian multicuspid teeth was expected.