Identifying tooth position of isolated teeth of sparassodonts (Mammalia: Metatheria) using geometric morphometrics

Isolated teeth make up much of the mammalian fossil record. The scientific value of these specimens is maximized when their specific locus can be identified, which permits precise comparisons with more complete specimens. However, identifying tooth locus can be challenging, particularly in metatherians, as these animals have multiple molar loci (M1-3/m1-4) that typically differ only in relatively subtle aspects. In this study, we use geometric morphometrics to analyze the first three upper molars (M1-3) of the metatherian clade Sparassodonta to determine whether it is possible to classify isolated teeth to locus using linear discriminant analysis. Discriminant analyses return high cross-validation reclassification rates of 72-83% and classify several specimens of unknown locus with high posterior probabilities, suggesting that they can be used to confidently identify tooth locus in metatherians. The morphological features that best distinguish tooth loci from each other are the shape of the stylar shelf and development of the ectoflexus and parastylar lobe. Specifically, the parastylar lobe is labiolingually narrower than the width of the stylar shelf at the midpoint of the tooth on M1, equal in width on M2, and greater than width on M3, forming an ectoflexus. Our study provides an open-access morphometric dataset that other researchers can use to determine tooth loci of isolated sparassodont teeth, such as those collected by screen-washing or surface collecting. Russell K. Engelman. Department of Biology, Case Western Reserve University, 10900 Euclid Ave., Cleveland, Ohio 44106, U.S.A. neovenatoridae@gmail.com Darin A. Croft. Department of Anatomy, Case Western Reserve University School of Medicine, 10900 Euclid Ave., Cleveland, Ohio 44106-4930, U.S.A. dcroft@case.edu