Multituberculates were rodent-like mammals that existed for some 130 million years and survived the mass extinction event that decimated the non-avian dinosaurs (Cretaceous–Paleogene boundary; 66 million years ago). Despite this lengthy record, multituberculates remain one of the more poorly understood mammalian groups, a situation resulting in part from their fossil record consisting largely of isolated teeth and jaws, which makes confident identification difficult. Fortunately, the blade-like lower fourth premolar (p4) of many multituberculates is frequently preserved, and has significant diagnostic power, allowing researchers to distinguish multituberculates, sometimes to species level. Various methods have been used in examining the p4, ranging from qualitative assessments and basic measurements to more sophisticated statistical methods that quantify form (called morphometrics). A recent study comparing the effectiveness of qualitative and quantitative methods for distinguishing the p4s of species of the Late Cretaceous–early Paleocene multituberculate genus Mesodma concluded that size is the most important contributor to morphological variation among the included species. To test this hypothesis, we applied the study methods to a novel dataset that included additional species of Mesodma, and to a second dataset consisting of unidentified p4s of Mesodma. Our results suggest that rather than size being the most important variable in distinguishing species of Mesodma, that shape and size are more informative when analyzed together. Our results confirm previous hypotheses that shape and size are intricately linked, and that biological significance is sometimes difficult to maintain when attempting to isolate each variable. The use of quantitative methods such as those proposed in the original study, with appropriate caution, were found to be useful in distinguishing among the p4s of the various species of Mesodma and have potential for use in studies on other multituberculates more broadly.

Although knowledge of their fossil record continues to improve, multituberculates nonetheless remain one of the more poorly understood mammalian clades, which can be attributed to a record comprised of isolated teeth and fragmentary jaws. Fortunately, the p4 of multituberculates is the most common form of remains for this group and is a principal source of diagnostic characters in systematic studies, the p4 of cimolodontan multituberculates is both common and a source of diagnostic characters in systematic studies. The results of a recent morphometric study on the neoplagiaulacid Mesodma suggest that p4 size may be more useful than shape in diagnosing the various species referred to this genus. We tested this hypothesis by applying two different morphometric methods (2D geometric morphometrics and linear measurements) to two samples: (1) one including the p4s of four known species of Mesodma (M. ambigua, M. thompsoni, M. formosa, and M. pygmaea), and (2) a sample of unidentified p4s of Mesodma from the Bug Creek Anthills locality of northeastern Montana. Our results indicate that while form explains most of the morphological variation in p4s of the various species of Mesodma, linear-measurement data support differences in p4 morphology that are not recovered by form data alone. Depending on the methods used, we found evidence for the presence of one or more species of Mesodma in the Bug Creek Anthills fauna. Although shape and size both contribute to morphological variation in the p4 of Mesodma, our results suggest that the diagnostic power of each varies with the type of methodology employed.