The earliest dated Basilemys (Testudines, Nanhsiungchelyidae) in North America informs polar dispersal during middle Cretaceous Hothouse conditions

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, a large, terrestrial turtle common in the Upper Cretaceous 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 Formation 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.

2021年夏季，在蒙大拿州利马（Lima）附近的上白垩统边疆组（Upper Cretaceous Frontier Formation）中采集到碎块状龟类化石材料。该龟壳表面饰有成行的锥形结节（pyramidal tubercules），结节间以凹坑分隔，部分特征足以将其归入Basilemys属——这是北美上白垩统中常见的大型陆生龟类。该标本产自边疆组中部地层单元底部附近的厚层钙质古土壤（calcareous paleosol）中。我们采用激光剥蚀电感耦合等离子体质谱法（laser-ablation inductively coupled plasma mass-spectrometry, LA-ICP-MS）开展碎屑锆石U-Pb年代学（detrital zircon U–Pb geochronology）分析，以限定该地层段的沉积年代。该古土壤层的最大沉积年龄跨土仑期（Turonian）-康尼亚克期（Coniacian）边界（89.89 ± 1.9 Ma），确立了该属已知最早的测年记录。Basilemys属在桑托阶（Santonian）之前的化石记录极为匮乏，仅有的报道为来自犹他州直崖组（Straight Cliffs Formation）的碎块状Basilemys未定种（Basilemys sp.）化石材料，其产出地层大致被归为康尼亚克期至上桑托阶。Basilemys是北美发现的南雄龟科（Nanhsiungchelyidae）唯一成员，此次新发现的边疆组标本将南雄龟科向北美扩散的时间限定在白垩纪中期温室期（middle Cretaceous Hothouse）——一段全球快速变暖的时期，并佐证了这些鳖类干群（stem-trionychians）仅通过白垩纪高纬度白令陆桥单次扩散进入北美西部。