Tanya M. Smith and colleagues (2003) measured the enamel of two Afropithecus molars, examining both their thickness and the periodicity of enamel formation. This was of interest because Afropithecus was thought to be the earliest thick-enameled ape.
Enamel thickness
Enamel thickness is not a simple value. The morphology of the tooth crowns are convoluted, and the enamel varies in thickness across the crown. Likewise, larger teeth might be expected to have thicker enamel than smaller teeth, just because of their size. A full study of the thickness of the enamel involves sectioning the tooth and taking observations of the area of the section taken up by enamel. In this study, relative enamel thickness was assessed as follows:
Relative enamel thickness was calculated by dividing the area of the enamel cap by the length of the enamel dentine junction, and this quantity was then divided by the square root of the area of the dentine and finally multiplied by 100. This provides a dimensionless index of enamel thickness that is suitable for comparisons across taxa (287).
In other words, enamel area (a square measure) is divided by the length of the enamel junction (a linear measure corresponding to the tooth topography and tooth size) and the square root of the dentine area defined under the enamel cap (a linear measure corresponding to the tooth size minus enamel). This isn't the only way one might measure relative enamel thickness, but scaling is inevitably a problem in structures with complex shapes.
The results list Afropithecus along with a number of other hominoids (which is why I found the paper). I reproduce the data here from the table on page 291, adding the value estimated for Gigantopithecus by Dean and Schrenk (2003):
| Taxon | RET | Range | Category |
| Proconsul africanus | 8.5 | thin | |
| Gorilla gorilla | 10.0 | 6.8 -- 13.4 | thin |
| Pan troglodytes | 10.1 | 7.0 -- 13.3 | thin |
| Hylobates lar | 11.0 | thin | |
| Dryopithecus laietanus | 12.7 | intermediate thin | |
| Oreopithecus bambolii | 13.0 | intermediate thin | |
| Pan paniscus | 13.6 | intermediate thin | |
| Proconsul major | 13.7 | intermediate thin | |
| Lufengpithecus hudeniensis | 14.1 | intermediate thin | |
| Rangwapithecus gordoni | 14.9 | intermediate thick | |
| Pongo pygmaeus | 15.9 | 11.3 -- 20.5 | intermediate thick |
| Proconsul heseloni | 17.0 | intermediate thick | |
| Sivapithecus sivalensis | 19.2 | 16.3 -- 20.9 | thick |
| Griphopithecus sp. | 19.3 | 16.5 -- 23.0 | thick |
| Afropithecus turkanensis | 21.4 | 19.9 -- 22.9 | thick |
| Australopithecus africanus | 21.4 | 21.3 -- 21.6 | thick |
| Homo sapiens | 22.4 | 13.8 -- 32.3 | thick |
| Proconsul nyanzae | 22.4 | thick | |
| Gigantopithecus blacki | 23 | thick | |
| Lufengpithecus lufengensis | 24.1 | thick | |
| Gracopithecus freybergi | 25.9 | thick | |
| Paranthropus robustus | 29.6 | thick |
The RET is relative enamel thickness, and the ranges given vary in sample sizes. Looking over the extant species, it is clear that the ranges of relative enamel thickness are pretty great. It is not clear from this tabulation if there are any patterns -- for example, if enamel thickness was relatively constant but tooth size varied, that would create some variation in relative enamel thickness. In any event, the small differences among many of the fossil species probably do not signify significant differences. Perhaps the broad categories of thin, thick and intermediate are the best one can do for the fossils.
Development rate
The enamel in teeth is secreted during development by cells called ameloblasts. The ameloblasts begin at the enamel-dentine junction and migrate outward toward the eventual crown surface. The completed enamel has a prismatic crystal structure, with prisms oriented more or less perpendicularly from the enamel-dentine junction. The ameloblasts alternately speed and slow down enamel deposition in accordance with circadian and other cyclic processes. This cyclicity results in undulations of the enamel prisms as they radiate toward the tooth surface (described further in Aiello and Dean 1990). The cyclicity also causes visible striations in the enamel that are visible in cross section.
One type of striation is generated by the growing field of ameloblasts at approximately weekly intervals. These are called striae of Retzius, and each one represents the external enamel surface at a one stage of crown development. The striae of Retzius on the sides of the tooth intersect with the enamel surface, forming raised lines called perikymata Between the striae of Retzius are a series of smaller cross striations that represent daily enamel deposition along hte enamel prisms. The number of these between each pair of Retzius lines is referred to as the periodicity of the enamel development. Together, the periodicity and the count of striae of Retzius allow an estimate of the time of enamel formation, which may be informative about the developmental rate of the teeth.
The estimates for enamel formation time in Afropithecus from tooth sections indicate a time of between 2.43 and 3.10 years (Smith et al. 2003:293). According to the study, this is similar to crown formation times in living hominoids. In the abstract, they put the conclusion as:
Although thick enamel may be formed through several developmental pathways, most Miocene hominoids and fossil hominids with relatively thick enamel are characterized by a relatively long period of cuspal enamel formation and a rapid rate of enamel secretion throughout the whole cusp, but a shorter total crown formation time than thinner-enameled extant apes. (283)
References:
Aiello L and Dean C. 1990. An Introduction to Human Evolutionary Anatomy. Academic Press, Oxford, UK.
Dean MC and Schrenk F. 2003. Enamel thickness and development in a third permanent molar of Gigantopithecus blacki. J Hum Evol 45:381-387.
Smith TM, Martin LB, and Leakey MG. 2003. Enamel thickness, microstructure and development in Afropithecus turkanensis. J Hum Evol 44:283-306.
