A new species of Nanhsiungchelys (Testudines: Cryptodira: Nanhsiungchelyidae) from the Upper Cretaceous of Nanxiong Basin, China, and the role of anterolateral processes on the carapace in drag reduction

Abstract

Nanhsiungchelyidae are a group of large turtles that lived in Asia and North America during the Cretaceous. Here we report a new species of nanhsiungchelyid, Nanhsiungchelys yangi sp. nov., from the Upper Cretaceous of Nanxiong Basin, China. This is the second valid species of Nanhsiungchelys, and the holotype consists of a well-preserved skull and lower jaw, as well as the anterior parts of the carapace and plastron. The diagnostic features of Nanhsiungchelys include a huge estimated body size (~55.5 cm), a special network of sculptures on the surface of the skull and shell, weak cheek emargination and temporal emargination, deep nuchal emargination, and a pair of anterolateral processes on the carapace. However, Nanhsiungchelys yangi differs from the other species of Nanhsiungchelys in having a triangular-shaped snout and wide anterolateral processes. A phylogenetic analysis of nanhsiungchelyids places Nanhsiungchelys yangi and Nanhsiungchelys wuchingensis as sister taxa. Some nanhsiungchelyids bear special anterolateral processes on the carapace, which are unknown in extant turtles. Here we test the function of these processes in Nanhsiungchelys yangi using computational fluid dynamics, and the results suggest these processes could enhance locomotory performance by remarkably reducing drag force when the animal was swimming through water.