Clark et al. Supplementary Data

In the summer of 2021 near Lima, Montana, fragmentary turtle material was recovered from the Upper Cretaceous Frontier Formation. The shell is sculptured with rows of pyramidal tubercules separated by pits, in part warranting assignment to Basilemys (Hay, 1902), a large, terrestrial turtle common in the Upper Cretaceous stratigraphy of North America. This specimen comes from a thick, calcareous paleosol near the base of the middle unit of the Frontier Formation. We applied detrital zircon U-Pb geochronology using laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) to constrain the depositional age of the stratigraphic interval. The maximum depositional age for the paleosol straddles the Turonian-Coniacian boundary (89.89 ± 1.9 Ma) and establishes the earliest dated occurrence of the genus. The pre-Santonian fossil record of Basilemys is scarce and consists of fragmentary Basilemys sp. material reported from the Straight Cliffs Formation of Utah, broadly assigned to Coniacian–upper Santonian strata. Basilemys is the only member of Nanhsiungchelyidae found in North America, and this new Frontier specimen constrains nanhsiungchelyid dispersal into North America to the middle Cretaceous Hothouse, a period of rapid global warming, and reinforces a single dispersal of these stem-trionychians into western North America through high-latitude Cretaceous Beringia.