As the key physical interface between mammals and their environment, teeth are extremely important biological structures and a great deal is known about the biology of tooth eruption. Nonetheless, there remains significant disagreement on whether life history, phylogeny, or possibly other factors, are driving the evolution of dental eruption sequence in mammals. Schultz’s Rule, which states that eruption sequence is adaptive and associated with life history variables like postnatal growth and longevity, has also been used in dozens of studies of extant and fossil mammals since it was proposed by Smith in 2000. However, none of these studies account for phylogeny in their analyses.

To address this, I assessed postcanine dental eruption sequence across a large sample of primates and artiodactyls in order to test two hypotheses: 1) dental eruption sequence is associated with life history variables like post-natal growth and longevity in mammals (Schultz’s Rule), and as such could be adaptive, and 2) eruption sequence is a good phylogenetic character for primates and artiodactyls. I examined the dentition of more than 8,000 animals and assessed postcanine eruption sequence in 31 primate and 81 terrestrial artiodactyl genera, spanning 10 families in each clade. With the inclusion of 30 additional primate genera from the literature, this is the most comprehensive compilation of dental eruption sequence in mammals to date.

Tests of phylogenetic signal show that dental eruption sequence is phylogenetically conserved in primates and artiodactyls as are almost all of the life history and body size traits tested here. However, only body size in primates is significantly associated with dental eruption sequence. Ancestral state reconstruction supports that the third molar erupted before one or more of the premolars in the ancestor of primates, while the third molar erupted after the premolars in the ancestor of artiodactyls. More derived clades within primates and artiodactyls have a different eruption sequence, where the third molar erupts after the premolars in anthropoids (Primates), and the third molar erupts before one or more of the premolars in Ruminantia (Artiodactyla). This change likely occurred during the Paleogene in both primates and artiodactyls. Within primates and artiodactyls, several taxa exhibit a dental eruption sequence that is either secondarily derived or a reversal to the ancestral state, and several taxa exhibit intra- and intraspecific variation in dental eruption sequence. Many of these secondary changes likely occurred in the Miocene and Pliocene. Dental eruption sequence is phylogenetically conserved in primates and artiodactyls, and changes in eruption sequence may be associated with changes in body size. Variation in dental eruption sequence may also be related to the biomechanics of chewing. In both primates and artiodactyls, a fused mandible is associated with taxa that erupt the third molar after the premolars, while an unfused mandible is associated with taxa that erupt the third molar before one or more of the premolars. The evolution of symphyseal fusion of the mandible is convergent in several mammalian taxa, and in primates, mandibular fusion is thought to be an adaptation to the increasing mechanical stress associated with increasing body size in herbivorous animals.

The data in this study suggest that dental eruption sequence is likely driven by factors that include body size, fusion of the mandibular symphysis, and the biomechanics of chewing. It also seems likely that limited resource availability, as is seen in the island fossil caprine Myotragus and other extant high elevation caprines, can further affect the timing and sequence of dental eruption. Overall, Schultz’s Rule, as it is currently written and applied, is not supported by this extended data set. Dental eruption sequence is phylogenetically conserved and is not associated with age at eruption of the first molar, longevity, or many other life history and size traits, with the exception of body size. Phylogeny, not life history, explains dental eruption sequence in mammals. My research demonstrates that dental eruption sequence is a far better predictor of phylogeny, and it will likely prove useful in phylogenetic hypotheses about relationships between extinct and extant mammalian taxa.