Abstract
The Tetonius-Pseudotetonius (T-P) transition is an often-cited example of phyletic gradualism, but rates of evolution and the roles of neutral and adaptive processes across this lineage remain unclear. Linking Tetonius and Pseudotetonius, two omomyid primates, are a series of stratigraphic and morphologic intermediates revealing trends suggestive of a functional and developmental reorganization of the dentition. Notable changes involved the P3, which reduced size and became indistinguishable from the canine and I2, and the P4, which became a robust tall-cusped tooth comparable to a molar. We test two hypotheses: (1) neutral evolution can explain the observed phenotypic differences in the lineage, and (2) P4 lost developmental association with P3 and became integrated with the molars. First, we calculate the rate of evolutionary differentiation, based on the ratio between inter- and intra-species variation in length and width of P3, P4, M1, and M2 teeth, between lineage segments and over the entire lineage. We test the second hypothesis by comparing bivariate correlations between teeth within individual lineage segments. As the lineage evolved, correlations between P3 and the molars diminished, whereas correlations between P4 and the molars increased. We found evidence of varying degrees of stabilizing selection in the lengths and widths of all cheek teeth and evidence of neutral evolution in the width of P4. These results support a trend towards P4 becoming integrated into the molar morphogenetic field, and demonstrates that morphological rates of evolution, and consequently the degree of selective pressures, vary through time and between teeth.
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Acknowledgements
We would like to thank Kenneth Rose and Thomas Bown for their thorough work and publications on the Tetonius – Pseudotetonius lineage, which inspired this research. We thank Kenneth Rose, Johns Hopkins School of Medicine, Smithsonian National Museum of Natural History, and the University of Michigan Museum of Paleontology for allowing us to measure additional Tetonius – Pseudotetonius specimens housed in their collections. We also thank Will Clyde for his work and help in establishing sediment ages and correlating the sediments of the Clarks Fork and Bighorn Basins, which allowed us to estimate ages of individual Tetonius – Pseudotetonius specimens.
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Schottenstein, N.H., Hubbe, M. & Hunter, J. Modules and Mosaics in the Evolution of the Tetonius – Pseudotetonius Dentition. J Mammal Evol 27, 677–696 (2020). https://doi.org/10.1007/s10914-019-09488-3
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DOI: https://doi.org/10.1007/s10914-019-09488-3