A softshell turtle (Testudines: Trionychidae: Plastomeninae) from the uppermost Cretaceous (Maastrichtian) Hell Creek Formation, North Dakota, USA, with implications for the evolutionary relationships of plastomenines and other trionychids

Plastomeninae, a clade of fossil turtles that has recently undergone significant revision, are currently known from the Late Cretaceous to the Eocene, with some genera known to survive the end-Cretaceous mass extinction (e.g., Hutchemys). Only one taxon survives past the Paleocene into the Eocene (Plastomenus thomasii). Despite the majority of Hutchemys being known from the Paleocene, only a single species is known from the Cretaceous. Here we describe Hutchemys walkerorum sp. nov. from the Upper Cretaceous of North Dakota. This new species can be assigned to Hutchemys by fading surface sculpturing of the carapace and distally split costals and distinguished from other Hutchemys species by several features, including aspects of the nuchal, costals, and placement of a carapacial constriction. The new species represents the second species of Hutchemys known from the Cretaceous, and potentially provides a direct anagenetic evolutionary lineage to H. tetanetron. It represents a third plastomenine evolutionary lineage present in the Hell Creek fauna of North Dakota, with Gilmoremeys lancensis and Helopanoplia distincta. A phylogenetic analysis recovers H. walkerorum among other Hutchemys species in a monophyletic group of derived plastomenines (Plastomenini clade nov.) and recovers a basal clade of trionychids (Kuhnemydinae subfam. nov.), along with Chitrainae subfam. nov., Cyclanorbinae, and Trionychinae. Trionychids likely originated in Asia during the middle-late Early Cretaceous, while plastomenines, an exclusively North American clade, evolved in, or prior to, the Campanian. Plastomenines were at their peak diversity in the Maastrichtian through the Paleocene, with complete extinction of the clade occurring by the Lutetian (Eocene).