EVOLUTIONARY RATES IN INDIVIDUAL TOOTH LOCI IN THE TETONIUS AND PSEUDOTETONIUS LINEAGE OF ANAPTOMORPHINE PRIMATES

Previous research has suggested significant dental adaptation in the Tetonius-Pseudotetonius lineage of anaptomorphine primates resulting in a compact dentition in a short jaw with reduction of the teeth between the prominent central incisors and the cheek teeth. This adaptive trend has been argued to be a result of gradual diversifying evolution, with the anterior dentition evolving rapidly when compared to molars and the last premolar (p4). Here, we calculated the rate of evolutionary differentiation based on the ratio between inter and intra species variation in the size (L x W) of individual tooth loci (p3, p4, m1, and m2) in the Tetonius-Pseudotetonius lineage, to test the hypothesis that neutral selection can explain the phenotypic differences observed in tooth size across this lineage. The Tetonius-Pseudotetonius lineage was chosen because of its fine temporal and stratigraphic resolution within the Big Horn and Clark’s Fork Basins, with defined species at the beginning and end of the lineage and a detailed record of various intermediate stages. When evolutionary rates were calculated over the entire lineage the null hypothesis of neutral evolution could not be refuted for the p3 and m2; however, for p4 and m1 the rate is lower than expected by stochastic processes alone and support a long-term process of stabilizing selection. This scenario, however, is not constant across time: when rates were calculated between the defined species and intermediate forms, evidence of diversifying selection was observed in p3 between certain horizons while stabilizing selection was detected in p4, m1, and m2. These results support prior studies that inferred different rates of evolution occurred at individual tooth loci in the Tetonius-Pseudotetonius lineage, producing a heterochronic evolution of tooth size in the dentition. Additionally, this analysis demonstrates that evolutionary rates are strongly dependent on the amount of generational time being observed and that a combination of diverging and stabilizing selection across time can mimic the appearance of gradual diversifying selection if intermediate forms are unknown in the fossil record. Our results, therefore, illustrate how misleading evolutionary interpretations can be in a less complete record.