109-2 Assessing Intraspecific Variation in Late Paleocene-Early Eocene Ectypodus (Multituberculata) and the Taxonomic Utility of Multituberculate Lower First Molars

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives (Posters)

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Abstract:

Multituberculates (Order Multituberculata) are one of the longest-surviving clades of mammals on Earth, persisting from the mid-Jurassic to the late Eocene (~130 Ma). While broad patterns of diversity change in this order are relatively well-documented, species-level differentiation remains difficult—particularly during the late Paleocene, when multituberculate taxic diversity is thought to have been at its lowest. Accurately assessing species diversity during this interval is especially challenging due to the rarity of specimens from this period and, in turn, a limited understanding of intraspecific morphological variation, which impacts our ability to distinguish between true biological species. However, recent screenwashing efforts have recovered hundreds of isolated multituberculate teeth from latest Paleocene through earliest Eocene sediments of the Bighorn Basin, Wyoming, enabling more detailed morphological study. Here we assess morphological variation in the lower first molars (M₁) of the genus Ectypodus to re-evaluate species diversity within this interval, as the two currently recognized species, E. powelli and E. tardus, are based on only a handful of specimens.

Each M₁ was µCT-scanned, segmented into 3D surfaces, and landmarked. Measurements such as centroid size, tooth length, cusp size, and footprint area were derived from the landmarks, and cusp count was also recorded. These data were then visualized using principal component analysis (PCA). Any observed clusters were tested for significant differences in mean shape and size using Procrustes ANOVA and t-tests, respectively. Preliminary results suggest that Ectypodus specimens from this interval show no clear separation in overall size or shape. Increased tooth length was significantly associated with the addition of cusps in both buccal and lingual cusp rows, and in teeth with more cusps, each cusp was proportionally smaller relative to footprint area. Other variation, especially in the distobuccal angle of the crown outline, may relate to genus-level identity. Based on comparisons to previously published specimens, it was found that teeth with a smaller, sharper angle likely belong to Ectypodus, whereas those with a larger, shallower angle likely belong to other contemporaneous genera (Parectypodus and possibly Microcosmodon). These results suggest that multituberculate M₁s can be taxonomically informative even in isolation, and the absence of distinct morphological groupings across biozones may indicate that only a single species of Ectypodus inhabited the Basin during this interval, remaining relatively unchanged despite significant environmental perturbation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10364

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