Objectives: Variation in primate masticatory form and function has been extensively researched through both morphological and experimental studies. As a result, symphyseal fusion in different primate clades has been linked to either the recruitment of vertically directed balancing-side muscle force, the timing and recruitment of transversely directed forces, or both. This study investigates the relationship between jaw muscle activity patterns and morphology in extant primates to make inferences about masticatory function in extinct primates, with implications for understanding the evolution of symphyseal fusion. Materials and methods: Three-dimensional mandibular landmark data were collected for 31 extant primates and nine fossil anthropoids and subfossil lemur species. Published electromyography (EMG) data were available for nine of the extant primate species. Partial least squares analysis and phylogenetic partial least squares analysis were used to identify relationships between EMG and jaw shape data and evaluate variation in jaw morphology. Results: Primates with partial and complete symphyseal fusion exhibit shape-function patterns associated with the wishboning motor pattern and loading regime, in contrast to shape-function patterns of primates with unfused jaws. All fossil primates examined (except Apidium) exhibit jaw morphologies suggestive of the wishboning motor pattern demonstrated in living anthropoids and indriids. Discussion: Partial fusion in Catopithecus, similar to indriids and some subfossil lemurs, may be sufficient to resist, or transfer, some amounts of transversely directed balancing-side muscle force at the symphysis, representing a transition to greater reliance on transverse jaw movement during mastication. Furthermore, possible functional convergences in physiological patterns during chewing (i.e., Archaeolemur) are identified.
Bibliographical noteFunding Information:
This research was made possible by support from the University of Minnesota Graduate School Doctoral Dissertation Fellowship and the Thesis Research Travel Grant. EMG data were collected with support from the National Science Foundation to CJV (BCS-0552285, BCS-0412153, BCS-0138565, BCS-0094666). Curatorial assistance and museum access provided by William Stanley and Lawrence Heaney (FMNH), Darrin Lunde and Esther Langan (NMNH), Eileen Westwig and Judith Galkin (AMNH), and Greg Gunnell (DLC-Division of Fossil Primates) are gratefully acknowledged. We thank the Smithsonian's Division of Mammals (Kristofer Helgen) and Human Origins Program (Matt Tocheri) for the scans of USNM specimens used in this research (http://humanorigins.si.edu/evidence/3d-collection/primate). Eric Delson graciously provided the cast of Epipliopithecus vindobonensis. The scan of Apidium phiomense was downloaded from www.MorphoSource.org, Duke University. Gregg Gunnell provided access to these data, the collection of which was funded by NSF BCS 1231288 and NSF DBI 1458192. This is Duke Lemur Center publication #1452.
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- geometric morphometrics