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Measuring Molarization: Change Through Time in Premolar Function in An Extinct Stem Primate Lineage

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Abstract

The dentition can be thought of as comprising a series of functional modules, which may include pre-ingestive food preparation, ingestion, sexual signalling, and mastication. Changes in diet may be manifested in shifts in the boundaries between these modules along the toothrow. In particular, because of their position in the mouth, premolars may act in ingestion and/or mastication of food. With shifts toward masticatory function, premolars may become “molarized”. Although it is common in descriptions of fossil taxa to use terms like “premolarization” and “molarization”, these are rarely clearly defined or linked to quantitative measures. Here, we make use of dental topographic analysis (DTA) of the lower fourth premolar (p4) and lower second molar (m2) to quantify and contextualize molarization in a lineage previously identified as exhibiting increasing p4 molarization through time: the microsyopine microsyopids from the early Eocene of the Bighorn Basin, Wyoming. Increases in premolar size have been cited as evidence of molarization in other groups, so we also measured size of the premolars and molars. Our results are consistent with observations about increasing premolar molarization in microsyopines through time, but we observed no associated change in the topography or function of the molars, nor did we observe an increase in the size of the premolars. Later microsyopid taxa may have relied more heavily than earlier species on fibrous foods such as leaves, a conclusion not evident from the molars alone. In general, claims of “molarization” should consider shifting function alongside observations of changes in size or shape.

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Acknowledgements

We thank Justin Gladman and Sergi López-Torres for help micro-CT scanning the material, Derrick Lin for help in prepping the micro-CT scans for analysis, and Jade Abdul-Sater for tirelessly uploading scans to MorphoSource. We also thank the curators and collections staff of the University of Wyoming, the Yale Peabody Museum, the Smithsonian, the Duke Fossil Primate Center, and the United States Geological survey for access to materials, and to T.M. Bown for access to unpublished stratigraphic data. We thank the NSF and National Geographic Society for field work support to Ken Rose (NSF grants BSR-8500732, BSR-8918755, IBN-9419776, EAR-0000941, and EAR-0616376) and Amy Chew (NSF grants, SGP-0739718 and SGP-0616430, and National Geographic Society Waitt Program 172 grant W315-14). Funding for this project was provided by a Pilot Research Grant to KRS from the Department of Anthropology, University of Toronto, the Richard Gilder Graduate School at the American Museum of Natural History, and a University of Toronto Scarborough Collaboration Fund grant and an NSERC Discovery Grant to MTS.

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  1. Department of Evolutionary Anthropology, Duke University, Durham, NC, USA

    Keegan R. Selig

  2. Division of Paleontology, American Museum of Natural History, New York, NY, USA

    Keegan R. Selig

  3. New York Consortium in Evolutionary Primatology, New York, NY, USA

    Keegan R. Selig

  4. Department of Anthropology, University of Toronto Scarborough, Scarborough, ON, Canada

    Mary T. Silcox

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Selig, K.R., Silcox, M.T. Measuring Molarization: Change Through Time in Premolar Function in An Extinct Stem Primate Lineage. J Mammal Evol 29, 947–956 (2022). https://doi.org/10.1007/s10914-022-09623-7

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