Date: 01 Sep 2012

Turtles from the Jurassic Shishugou Formation of the Junggar Basin, People’s Republic of China, with Comments on the Basicranial Region of Basal Eucryptodires

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Abstract

Five fossil turtle species (?Sichuanchelys sp., three species of Xinjiangchelys, and an indeterminate species of Annemys) are present in the Shishugou Formation (late Middle to early Late Jurassic) of the Junggar Basin, northwestern China. Two of these, X. radiplicatoides sp. nov. and Annemys sp., are each represented by an associated skull and shell. These demonstrate that the Xinjiangchelyidae, as currently defined, encompasses different grades of evolution. X. radiplicatoides is primitive in features of the basicranial region, lack of emargination of the skull roof, and presence of an inflated postorbital region. Annemys sp., which has a low skull with deeply emarginated temporal and cheek regions and large foramina palatinum posterius, is similar to basal eucryptodires from the Early Cretaceous of Asia, particularly Hangaiemys. Early stages in the evolution of the basicranial region in eucryptodires are documented by the well-preserved basicranial region of X. radiplicatoides and Annemys sp. The slit-like structure of the foramen palatinum posterius in X. radiplicatoides is consistent with the interpretation that this opening developed by closure of the interpterygoid vacuity around the palatine artery. Processes of the basisphenoid that extend laterally into the pterygoid, identified here as basipterygoid processes, are well developed in Xinjiangchelys and most Early Cretaceous sinemydids/macrobaenids. Although a high taxonomic diversity of turtles is present in the Shishugou Formation, diversity at individual localities is low, often with a single taxon being present or overwhelmingly dominant, and most localities differ in the kinds of turtles that are dominant at that locality. This pattern of high alpha diversity (total diversity within a unit), low diversity within individual localities, and high beta diversity (between-locality diversity within a unit) is unusual in turtle assemblages, and suggests that the paleoecology of the Shishugou Formation has unusual aspects compared to similarly diverse turtle assemblages, where diversity at a locality typically reflects total diversity within the unit.