A HORSE, AN OTTER, AND A BEAR WALK INTO A BAR AND GET CARDED: REFINING AGES FOR IMPORTANT FOSSIL LOCALITIES AT HAGERMAN FOSSIL BEDS NATIONAL MONUMENT, IDAHO, USA

A diverse Blancan fauna providing insight into Pliocene faunal evolution and paleoenvironmental conditions is preserved at Hagerman Fossil Beds National Monument (HAFO). Establishing ages and spatiotemporal relationships between HAFO’s fossil localities is critical to a comprehensive understanding of the monument's Pliocene history. Previously, volcanic ash deposits were mapped in several areas within and outside of HAFO. However, without geochemical correlations and consideration of faults, slope, and depositional heterogeneity, the accuracy of projecting tephra units across long distances is limited. Additionally, many fossil localities are in previously unmapped areas. New mapping and expanded geochemical correlations of tephra layers have improved our understanding of the spatiotemporal relationships of new and established fossil localities.

The Hagerman Horse Quarry is HAFO's youngest fossil locality. It preserves hundreds of single-toed horses (Equus simplicidens). At a similar elevation to the south, a paratype locality for the peccary Platygonus pearcei was long thought contemporaneous to the quarry; it is now known to be older. Our current work suggests that most HAFO localities are older than the quarry, leaving its context mostly unresolved.

Several older localities can be temporally linked however, including some valuable for understanding carnivoran evolution. Agriotherium, a bear otherwise restricted to the Hemphillian, is confirmed to extend into the Blancan at approximately 3.9-3.8 Ma. We also confirm that the holotype and paratype localities for the oldest New World river otter (Lontra weiri), while stratigraphically 10 m apart, are separated by a fault and contemporaneous. We refine their age to ca. 3.9 Ma which better aligns with molecular phylogenetic data for Lontra. Ferinestrix vorax is restricted to HAFO, but a 2020 discovery shows it persisted for several hundred thousand years. With an age range of 4.1-3.8 Ma, our oldest specimen predates Siberia’s Ferinestrix rapax, supporting an earlier branching event for this giant meline badger lineage.

Continuing tephrochronologic research will further refine the ages for these and other fossil localities at HAFO.