New Insights into the Origin and Phylogeny of Cryptodiran Turtles Based on Key Fossil Taxa from the Mesozoic of Asia

Abstract

Most turtles from the Jurassic to Early Cretaceous of Asia are referred to the poorly circumscribed taxa Xinjiangchelyidae, Sinemydidae, and Macrobaenidae, groups that mostly include shell-based, generalized, small to mid-sized aquatic forms that are widely considered to represent the stem lineage of Cryptodira. These groups are critical for reconstructing the plesiomorphic anatomy of crown-cryptodires, the most diverse group of living turtles, and they are therefore particularly relevant for understanding the origin and early divergence of the primary clades of extant turtles. The complete description of excellent xinjiangchelyid and sinemydid material from the Upper Jurassic of Mongolia and the Middle Jurassic and Early Cretaceous of China (referable to Annemys levensis, A. latiens, Xinjiangchelys wusu nov. sp. and Manchurochelys manchoukuoensis) provides new insights into the anatomy of these otherwise poorly known groups. The overall similarity of the shells of the two Annemys species combined with significant differences in the skull indicate that these turtles probably partitioned the aquatic niche by exploring different feeding strategies. Among xinjiangchelyids, at least three different skull morphotypes can be differentiated, which implies a moderate level of ecological diversification among Late Jurassic Asian turtles. Phylogenetic definitions of Xinjiangchelyidae, Sinemydidae, and Macrobaenidae are provided for nomenclatural clarity and precision. Phylogenetic analysis weakly supports the inclusion of Xinjiangchelys wusu n. sp., A. levensis, and A. latiens in a monophyletic polytomy with other xinjiangchelyids, including Xinjiangchelys junggarensis and X. radiplicatoides. However, the analysis supports the unorthodox, though tentative placement of xinjiangchelyids and sinemydids outside of crown-group Testudines. A particularly interesting new observation is that the skull of xinjiangchelyids retains such primitive features as a reduced interpterygoid vacuity and basipterygoid processes. The homology of the basipterygoid processes is confidently demonstrated based on a comprehensive review of the basicranial anatomy of Mesozoic turtles and a new nomenclatural system is introduced for the carotid canal system of turtles. The loss of the basipterygoid process and the bony enclosure of the carotid circulation system occurred a number of times independently during turtle evolution suggesting that the reinforcement of the basicranial region was essential for developing a rigid skull, thus paralleling the evolution of other amniote groups with massive skulls. A thorough revision of the phylogeny of Macrobaenidae, Sinemydidae, and closely allied forms yielded two main competing hypotheses: in the first, these taxa form a paraphyletic grade, whereas in the second they form a monophyletic clade. The inclusion of problematic tree changing taxa, such as Panpleurodires (stem + crown side-neck turtles) has a major influence on the phylogenetic relationships of Sinemydidae and closely allied forms. Manchurochelys manchoukuoensis nests within Sinemydidae together with Sinemys spp. and Dracochelys bicuspis in the majority of the analyses.