A reassessment of the latest Cretaceous (Maastrichtian) pan-trionychid turtle Helopanoplia beecheri comb. nov.

The pan-trionychids Aspideretes beecheri and Helopanoplia distincta were named at the beginning of the 20 th century based on fossils collected from the Late Cretaceous (Maastrichtian) Lance Formation of Wyoming. Interestingly, even though numerous additional pan-trionychid remains from Late Cretaceous formations of the Upper Great Plains have since been referred to Helopanoplia distincta based on its unique surface texture, no additional specimens have been referred to Aspideretes beecheri . We figure and describe all material previously referred to Aspideretes beecheri and select specimens referred to Helopanoplia distincta . Interspecific and intraspecific variation is assessed by reference to extant trionychids as a basis for taxonomic revision. We re-assign all specimens historically referred to Aspideretes beecheri to Axestemys splendidus and Axestemys infernalis . An unusual plastron previously thought to represent Helopanoplia distincta is referred to Hutchemys walkerorum . Although the type of Helopanoplia distincta is characterized by a distinct, tuberculate shell surface texture, material previously referred to this taxon suggests that this texture is only occasionally found in some parts of the shell. The holotype of Aspideretes beecheri otherwise shows characteristics consistent with being the subadult of material previously referred to Helopanoplia distincta . We thus conclude that the two taxa are synonymous and form the new combination Helopanoplia beecheri . We urge caution when identifying fragmentary pan-trionychid remains, as a tuberculate surface texture pattern is found in at least two distinct taxa. We support the conclusion, nonetheless, that Helopanoplia beecheri is one of only a few turtles to have gone extinct at the Cretaceous–Paleogene boundary.