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Odontometric Patterns in the Radiation of Extant Ground-dwelling Squirrels within Marmotini (Sciuridae: Xerini)

Abstract I document odontometric variation across ground-dwelling squirrels of the Holarctic tribe Marmotini. Dental size, which correlates well with published average body mass values across species, accounts for most odontometric variation across the clade. Dental shape variation primarily reflects relative size of P3 (upper cheek teeth) and relative width of p4–m1 + length of m3 (lower cheek teeth). Shape variables and relative tooth crown height covary significantly across species, suggesting a common functional complex or shared genetic control. When dental morphology is mapped on published DNA-based phylogenies, Sciurotamias (Chinese rock squirrels), Ammospermophilus (antelope squirrels), and basal subgenera within Spermophilus (ground squirrels [Callospermophilus and Otospermophilus]) are shown to retain inferred primitive dental morphology—small to moderate dental size, relatively small P3, relatively narrow p4–m1 + shortened m3, and relatively low tooth-crown height. Other clades depart from this morphotype in size (very small in Tamias [chipmunks] and very large in Marmota [marmots]), tooth shape (especially 2 clades representing Eurasian and North American subgenus Spermophilus), or in both attributes (notably Cynomys [prairie dogs]), with frequent homoplasy. A plot of odontometric distance against published estimates of divergence time between sister clades suggests a roughly “clocklike” accumulation of odontometric change through time but highlights episodes of rapid odontometric evolution during the origins of Marmota, Cynomys, and Spermophilus parryii (arctic ground squirrel).