A new baenid turtle, Saxochelys gilberti, gen. et sp. nov., from the uppermost cretaceous (Maastrichtian) Hell Creek Formation: sexual dimorphism and spatial niche partitioning within the most speciose group of Late Cretaceous turtles

The uppermost Cretaceous (Maastrichtian) Hell Creek Formation in North America has yielded a diverse assemblage of fossil turtles dominated by baenids. A population of over 30 individual skeletons from the Hell Creek Formation of North Dakota comprises a new baenid turtle, Saxochelys gilberti, increasing the number of recognized Hell Creek turtles to at least 26, 11 of which are baenids. Saxochelys gilberti is differentiated from all other baenids based on the presence of a nuchal scale, finely sculptured shell, absence of an omega-shaped femoral-anal sulcus, presence of an omega-shaped extragular-humeral sulcus, a cavum tympani that is relatively smaller than the diameter of the orbit, presence of a robust single scale on the posteromedial portion of the parietals, and jugal exclusion from the orbital margin. Two distinct plastral morphologies, concave (N = 8) and flat (N = 9), are interpreted as representing male and female individuals, respectively. A ca. 10% size difference between the sexes indicates that S. gilberti is a sexually size-dimorphic turtle with larger females. This suggests female mate choice in this baenid turtle. A referred S. gilberti skeleton from above the Cretaceous-Paleogene boundary provides evidence for the survival of yet another species of turtle across this extinction event. The type locality of S. gilberti is close both stratigraphically and geographically to another locality consisting of a completely different assortment of baenid species, suggesting a fine degree of spatial niche partitioning in baenid turtles that may help explain the extremely high diversity of the group within the Hell Creek Formation.