Growth increments in Gomphotherium tusks and implications for late Miocene climate change in North America

Changes in mammalian faunas in North America during the late Miocene are thought to have been caused by the replacement of woodland habitats with grassland or steppe. The proposed cause of this habitat shift is the transition from the relatively low-seasonality early Miocene climate to a more highly seasonal climate regime by the end of the Miocene. Tusks, which are highly modified incisor teeth, of late Miocene Gomphotherium (Mammalia, Proboscidea) were sectioned to document the nature of tusk growth in this genus and to test for patterns of seasonal growth. Both the dentin and enamel of Gomphotherium tusks preserve incremental growth lines. Dentin has increments on three scales, inferred to represent annual (first-order), weekly (second-order), and daily (third-order) periodicities. Luminance and second-order increment thickness profiles were measured on transverse thin sections of tusk dentin viewed at low magnification, and the data were examined with bivariate plots and autocorrelation. Autocorrelograms of luminance data support the consistent identification and measurement of second-order increments within and among the tusks sampled. Profiles of second-order increment thickness in three tusks from the Barstovian North America Land Mammal Age (NALMA) show no indication of seasonal patterns of growth. One tusk from the late Clarendonian and one from the early Hemphillian NALMA have patterns of growth consistent with an increase in seasonality during the late Miocene, an interpretation supported by autocorrelation of the increment thickness data from these tusks. The growth patterns are consistent with the hypothesized changes in North American climate during the late Miocene and are suggestive of an increase in aridity and development of a wet season. Tests of this hypothesis are suggested.