Enamel Extension Rates Research Articles

Life‐History Correlates of Enamel Microstructure in Cebidae (Platyrrhini, Primates)

Previous studies have examined tooth eruption as it relates intrinsically to body mass, brain mass, and other life history variables, and extrinsically to ecological factors (e.g., age at foraging independence, environmental risk aversion, and maternal investment). Different models have been explored wherein each of these variables impacts ontogeny. However, anthropoid and strepsirhine primates exhibit interesting differences in the relationships of these ecological and life history variables with tooth eruption. Moreover, interactions between ecological variables and dental tissue growth have only been explored in the lemurs. This study examines dental microstructure of the New World monkey family, Cebidae, to provide further insight into forces influencing the evolution of primate dental ontogeny. The Cebidae were chosen because they are a diverse group which is distinct in ecology and phylogeny from the better known catarrhines of the Old World. Using phylogenetic generalized least squares analyses (PGLS), we test whether brain mass, body mass, or the three above-mentioned ecological variables have stronger correlations with enamel growth. Results show that ecological factors have stronger relationships with cebid dental growth rates than brain or body mass. Foraging independence has the most impact on overall enamel growth as it has the strongest correlation with enamel extension rates. However, another estimate of enamel growth, rate of secretion, has the highest correlation with maternal investment. Our results suggest that an overarching ecological model encompassing the three current ecological hypotheses is needed to further understand the evolution of dental ontogeny within primates.

Life history, enamel formation, and linear enamel hypoplasia in the Ceboidea

Linear enamel hypoplasia (LEH), a developmental defect of enamel, increases in frequency from prosimian to monkey to lesser ape to great ape grades (Guatelli-Steinberg 2000 Am. J. Phys. Anthropol. 112:395-410, [2001] Evol. Anthropol. 10:138-151; Newell 1998 Ph.D. dissertation, Temple University). This taxonomic pattern in the distribution of LEH is closely related to maturation length across the primate order (Newell 1998 Ph.D. dissertation, Temple University, 2000 Am. J. Phys. Anthropol. [Suppl.] 30:236). Longer maturation periods are associated with higher LEH frequencies; they appear to provide greater opportunity for defects to form. The present study explores the relationship between maturation length and LEH frequency within the Ceboidea. Because of its prolonged period of growth, Cebus is predicted to manifest LEH at a higher frequency than the more rapidly maturing ceboid genera. To test this hypothesis, two separate researchers (E.A.N. and D.G.-S.) examined LEH in nonoverlapping museum series of ceboids. The results support the hypothesis: in 13 genera (n = 1,276), E.A.N. found that LEH frequencies ranged from 0% in Callicebus, Cebuella, and Saimiri to 20% in Cebus. D.G.-S. found similar frequencies among five genera (n = 107), from 0% in Saimiri to 32% in Cebus. Thus, the broad pattern of LEH distribution evident across major taxonomic groups of primates is repeated within the Ceboidea. We also examined a related hypothesis linking the spacing of perikymata, which is influenced by enamel extension rates (Shellis 1998 J. Hum. Evol. 35:387-400), to LEH. The most likely areas of tooth crowns to exhibit LEH in human teeth are those in which perikymata are most closely spaced (Hillson and Bond 1997 Am. J. Phys. Anthropol. 104:89-103). We hypothesized that the longer-maturing Cebus, with its elevated LEH frequency, will also exhibit more closely spaced perikymata than other ceboids. Analysis of a small microscopic subsample (n = 8) lends limited support to this second hypothesis.

Surprisingly rapid growth in Neanderthals.

Life-history traits correlate closely with dental growth, so differences in dental growth within Homo can enable us to determine how somatic development has evolved and to identify developmental shifts that warrant species-level distinctions. Dental growth can be determined from the speed of enamel formation (or extension rate). We analysed the enamel extension rate in Homo antecessor (8 teeth analysed), Homo heidelbergensis (106), Homo neanderthalensis ('Neanderthals'; 146) and Upper Palaeolithic-Mesolithic Homo sapiens (100). Here we report that Upper Palaeolithic-Mesolithic H. sapiens shared an identical dental development pattern with modern humans, but that H. antecessor and H. heidelbergensis had shorter periods of dental growth. Surprisingly, Neanderthals were characterized by having the shortest period of dental growth. Because dental growth is an excellent indicator of somatic development, our results suggest that Neanderthals developed faster even than their immediate ancestor, H. heidelbergensis. Dental growth became longer and brain size increased from the Plio-Pleistocene in hominid evolution. Neanderthals, despite having a large brain, were characterized by a short period of development. This autapomorphy in growth is an evolutionary reversal, and points strongly to a specific distinction between H. sapiens and H. neanderthalensis.

Preliminary investigation of dental microstructure in the Yuanmou hominoid (Lufengpithecus hudienensis), Yunnan Province, China

Fieldwork in the Yuanmou Basin of southern China has uncovered a large assemblage of late Miocene hominoid fossils assigned to Lufengpithecus hudienensis. Two mandibular first molars from this species were made available for histological analysis as part of a larger ongoing study on the ontogeny of dental development in Miocene to Recent hominoids. Results are compared with published and unpublished data on tooth growth in a wide range of extant and extinct hominoids. The Yuanmou molars are smaller than those of Lufengpithecus lufengensis and have markedly shorter crown formation times, overlapping slightly with Pan, but most similar to Proconsul and Dryopithecus. In other aspects of molar development (including enamel extension rates and enamel thickness), L. hudienensis shows similarities with all extant hominoids, in particular, Pongo. Ultimately, charting the ontogeny of molar crown formation may help shed light on the relationship of Lufengpithecus hudienensis to orangutans, and other Miocene to Recent hominoids.

Variations in growth of the enamel crown in human teeth and a possible relationship between growth and enamel structure

A method for estimating, from measurements made in ground sections of mature teeth, the rate at which enamel formation extends over the crown during tooth development is described and used to examine enamel formation in the human dentition. The enamel extension rate varied with distance from the cusp along the enamel-dentine junction in permanent teeth, falling from a high level near the cusp to a constant minimum value. The early rate was higher, and the distance over which the rate fell to the minimum greater, on tall cusps. In deciduous teeth, limited data suggested that the rate may be fairly constant over the crown surface. The mean extension rate was about five times greater in deciduous than in permanent teeth. A function describing a stretching effect on newly-differentiated ameloblasts, arising from the geometry of the advancing margin of forming enamel, and dependent in part on the extension rate, was derived. This effect might influence ameloblast secretory area and hence prism size. The stretching effect was significantly smaller in deciduous than in permanent teeth. In conformity with this, prism domain area, a quantity measured in mature enamel and based on the geometrical relationship between ameloblasts and prisms, was significantly smaller in deciduous than in permanent enamel.