DIAGNOSTIC TOOTH MORPHOLOY OF MESOHIPPUS AND MIOHIPPUS: A CASE FOR SYNONOMIZATION

Distinguishing the Oligocene equids Mesohippus and Miohippus has challenged paleontologists for years as many of the original diagnostic traits are not quantifiable. These genera coexisted geographically and temporally for five million years, further complicating matters. Due to unclear diagnoses and geographic and temporal overlap, modern studies have focused on tooth morphologies such as tooth dimensions and hypostyle shape to distinguish the genera. These researchers have argued for the synonymy of Mesohippus and Miohippus even though these studies have been limited geographically and temporally. This study addresses the validity of size comparisons of upper premolar 4 (P4) and molar 1 (M1) occlusal surface areas and hypostyle shapes as diagnostic traits. I focused on 4 sites for this analysis; John Day Fossil Beds National Monument in Oregon (John Day Fm., 30-23.8 Ma), Badlands National Park in South Dakota (Chadron & Brule Fm., 37-30 Ma), Toadstool Geologic Park in Nebraska (Chadron & Brule Fm., 37-30 Ma), and the Renova Formation in Montana (30-23 Ma). Using the comparison of P4 to M1 occlusal surface areas, the correct genus identification rate of fossil teeth in collections was 42%. A Mann-Whitney U test further supported the lack of distinction between these genera as no statistical significance (p = 0.33) was reported between P4 and M1 occlusal surface areas. The results of two ordinal logistic regressions indicated hypostyle shapes are statistically linked to tooth wear (approximated by height) (p = 5.78 × 10^-5 and 0.0014), not genus as previously thought. These teeth morphologic traits have not shown to be statistically significant, making them a poor diagnostic metric. Based on these findings, Mesohippus and Miohippus should not be considered distinct genera and be synonymized to Miohippus. This changes the previously defined fossil record of Equidae and emphasizes the importance of utilizing quantifiable diagnostic characters.