Reassessment of the phylogenetic interrelationships of basal turtles (Testudinata)

Recent discoveries from the Late Triassic and Middle Jurassic have significantly improved the fossil record of early turtles. These new forms offer a unique opportunity to test the interrelationships of basal turtles. Nineteen fossil species are added to the taxon sample of the most comprehensive morphological phylogenetic analysis of the turtle clade. Among these additional species are recently discovered forms (e.g., Odontochelys semitestacea, Eileanchelys waldmani, Condorchelys antiqua), taxa generally omitted from previous analyses (e.g., chengyuchelyids, Sichuanchelys chowi) and species included in a phylogenetic analysis for the first time (Naomichelys speciosa and Siamochelys peninsularis). The coding of several characters is reassessed in light of recent observations, but also in order to reduce unwarranted assumptions on character and character state homologies. Additional characters from previous analyses, as well as five new ones, are also included, resulting in a data matrix of 178 characters scored for 86 turtle species and 7 fossil outgroups.The data set resolves the relationships of most newly included taxa, with the exception of Sichuanchelys chowi and ‘Chengyuchelys’ dashanpuensis. The phylogenetic placement of Heckerochelys romani, Condorchelys antiqua and Eileanchelys waldmani as stem turtles more derived than Kayentachelys aprix but more basal than Meiolania platyceps and Mongolochelys efremovi is corroborated. The relationships of chengyuchelyids remain unclear and more investigation is needed regarding these forms, yet interestingly they are unstable with respect to stem turtles. In contrast to previous analyses, Arundelemys dardeni is placed within pleurosternids. Siamochelys peninsularis falls within xinjiangchelyids. Perhaps the most salient conclusion of the present study is the placement of Naomichelys speciosa as a basal member of a clade uniting meiolaniids, Mongolochelys efremovi and Otwayemys cunicularius. This clade of rather large stem turtles was spread worldwide during the Mesozoic at least and persisted up until the Pleistocene with meiolaniids.