Quantification of molar sub-regions suggests increasing herbivory during primate origins.

Abstract

We investigated patterns of evolutionary changes to primate molar morphology, focused on the front-most (mesial) region. Methodological constraints have prevented objectively quantifying anatomical sub-regions to conduct tests of the evolutionary processes that shaped them. Previously, sub-regions across a dental surface were qualitatively scored or subjectively described. We overcame this methodological constraint with Hecate, an algorithm incorporating computer vision and machine learning. Hecate segmented and quantified sub-regions (e.g., cusps and basins) across 500 mandibular second molars that represent major primate clades, outgroups, and key fossils. Combining ancestral state reconstruction, evolutionary model comparison, and other phylogenetic comparative methods, we investigated regional patterns of evolutionary change in primate molar teeth. Here, we show an initial reduction of curvature (associated with increased herbivory) at the front of the molar at the base of Euprimates. This shift marks the start of a trend that continued in parallel among most descendant radiations and suggests widespread herbivory throughout primate evolution.