Climatic effects on dental development of Theropithecus oswaldi from Koobi Fora and Olorgesailie

Eleven isolated molars of Theropithecus oswaldi from the Upper Burgi Member at Koobi Fora (ca. 1.89 Ma), the Okote Member at Koobi Fora (ca. 1.5 Ma) and from Olorgesailie (ca. 0.7 Ma) were sectioned in order to determine whether individuals from different strata/sites experienced similar amounts of stress during odontogenesis. Although none of the specimens exhibited hypoplasias, accentuated striae of Retzius were found in all sections. Overall, Theropithecus oswaldi from the Upper Burgi Member at Koobi Fora showed the most disruptions. A recurrent pattern of accentuated striae of around 366 days, or one calendar year, was found in all specimens, at all chronological levels. Each year was divided by accentuated striae into three intervals, comprising two longer periods (132-138 days) and one shorter period (90-102 days). These findings of episodic stress recurring within an annual cycle are believed to reflect seasonality of food availability. The onset of each rainy season may have constituted a stressful time for individuals resulting in accentuated striae, whereas the height of the dry season may have similarly resulted in accentuated striae in Theropithecus oswaldi molars. It is noteworthy that these stressful events affecting individual animals maintained a similar annual periodicity over approximately 1.2 million years. If these microstructural findings can be confirmed on a larger sample, and using different species, the usefulness of dental studies can be extended from life history reconstructions to reconstruction of palaeoenvironment.