Main » 2008 » July » 2 » A REASSESSMENT OF SOME POORLY KNOWN TURTLES FROM THE MIDDLE JURASSIC OF CHINA, WITH COMMENTS ON THE ANTIQUITY OF EXTANT TURTLES
A REASSESSMENT OF SOME POORLY KNOWN TURTLES FROM THE MIDDLE JURASSIC OF CHINA, WITH COMMENTS ON THE ANTIQUITY OF EXTANT TURTLES
Recent studies have shown that the Middle Jurassic is a key time for understanding the origin and early evolution of crown group turtles, but few turtle-bearing localities from this epoch are known. This study adds to our knowledge by providing a detailed description of two poorly characterized specimens (IVPP-V6507 and IVPP-V8805) from the Middle Jurassic of China that previously were both assigned to the poorly known species Chengyuchelys baenoides. This reappraisal allows us to present new observations, images, and taxonomic conclusions about these specimens and others from the same locality (Dashanpu, Zigong Prefecture, Sichuan Province, China). With at least four species, Dashanpu is the most diverse locality from the Middle Jurassic; all other turtle localities from this time have only one species each. We place Chengyuchelys into a cladistic analysis of turtle relationships for the first time. Our analysis places Chengyuchelys as sister to Xinjiangchelys latimarginalis and solidly on the stem of Cryptodira. This result is surprising because Chengyuchelys retains mesoplastra. Nevertheless, derived osteological and scalation characters of this taxon drive its phylogenetic position as a relatively advanced stem cryptodire. This finding, combined with a reappraisal of other Middle Jurassic turtles from Asia, shows that the crown group Testudines had evolved by the Middle Jurassic. Unfortunately, many key specimens from Sichuan Province are still poorly characterized morphologically and stratigraphically. This lack of phylogenetic and temporal resolution hinders our understanding of the origins of extant turtle clades, but also highlights the importance of Jurassic Asian turtles for ultimately resolving these issues.
Diagram illustrating changing ideas about the origins of Testudines and Cryptodira: A, traditional hypothesis (e.g., Gaffney and Meylan, 1988; Shaffer et al., 1997); B, hypothesis of Joyce (2004, 2007); C, hypothesis of Danilov and Parham (2006); D, preferred hypothesis of this study. E, pattern if Sinaspideretes wimani is a Middle Jurassic Cryptodire. Taxa are either stem based (open semi-circle) or node based (circle).