Mosaic evolution in first lower molars of Pliocene Ogmodontomys (Rodentia, Arvicolidae) from the Meade Basin of Southwestern Kansas

Thirteen characters and four indices from nine stratigraphically superposed samples of Ogmodontomys first lower molars (m1s) from the Meade Basin of southwestern Kansas, spanning an interval of approximately 2.0 million years of the Early and Middle Pliocene, were measured and analyzed by a variety of statistical techniques. The proportion of enamel atolls on the m1 were also examined. There were no statistically significant changes in eight measurements and one index. Although statistically significant differences were found for five of the variables and three indices during the sampling period, two of these variables and the three indices were interpreted as displaying an overall pattern of stasis within which statistically significant changes occurred. Three occlusal measurements displayed directional trends. Two of these represented changes in widths of dentine fields between triangle pairs (T1-T2, T3-T4) and the third was the deepening of a buccal re-entrant angle (BRA3), initially as the result of loss of the enamel atoll in the anteroconid between Fallen Angel B and Fox Canyon, presumably as the result of a speciation event from O. sawrockensis to O. poaphagus. A second significant deepening of BRA3 occurred during the late middle Blancan (Middle Pliocene), within the species O. poaphagus. Significant widening of the dentine field between T1-T2 also occurred at this time, whereas significant decrease of the field between T3-T4 occurred twice; once at the O. sawrockensis – O. poaphagus speciation event and once again from Deer Park to Paloma in O. poaphagus. When we include data from the Schmelzmuster (microhistological enamel banding pattern), the summary indicates a punctuated pattern of change in occlusal morphology and Schmelzmuster pattern, concentrated at the O. sawrockensis – O. poaphagus speciation event and again within O. poaphagus at various points in the Pliocene. The linea sinuosa (enamel-dentine junction), on the other hand, displays a more varied evolutionary pattern, with significant (though sometimes temporary) changes occurring throughout the history of both species. The most significant changes in m1 shape occur at the O. sawrockensis – O. poaphagus transition, but both phyletic change and “punctuated gradualism” (stairstep evolution) are also recorded in O. poaphagus.