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

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.