A new stem turtle from the Middle Jurassic of the Isle of Skye, Scotland, and a reassessment of basal turtle relationships

A recent phylogenetic analysis of turtle interrelationships suggests that the Middle Jurassic was a crucial time period for understanding the evolution of crown-group turtles. However, turtle material of this age is scarce worldwide. Here, a new stem turtle, Eileanchelys waldmani, from the Middle Jurassic (Bathonian) of the Isle of Skye, Scotland, is described and compared to other basal species. With cranial and postcranial remains of several individuals, this is the most comprehensive Middle Jurassic turtle material known to date. Eileanchelys waldmani documents an intermediate stage in the evolution of early turtles between the Early Jurassic Kayentachelys aprix and the crown-group. Whereas most stem turtles are interpreted as terrestrial forms, taphonomic evidence suggests that E. waldmani may have been aquatic (freshwater), indicating that basal turtles were ecologically more diverse than previously thought. This new material provides the basis for a reassessment of other Middle Jurassic turtle remains from the UK, especially revisions of material from Kirtlington and Stonesfield (Oxfordshire). The material from Kirtlington is attributed to Paracryptodira and represents the earliest occurrence of this group. The name 'Protochelys blakii', which had been proposed for the Stonesfield remains, is considered to be a nomen dubium as this material lacks diagnostic features. However, this material is remarkable as it consists of fossilised epidermal scales from the carapace. A new phylogeny is proposed, resulting from a cladistic analysis of a revised and updated version of a previously published data set. Nineteen new species are included in order to achieve a more thorough representation of basal turtle taxa. Heckerochelys romani and Condorchelys antiqua are found to be more basal than Eileanchelys waldmani. Chengyuchelyids (Middle Jurassic of China) may be stem turtles more basal than Kallokibotion bajazidi. Naomichelys speciosa is closely related to meiolaniids and Mongolochelys efremovi, while Siamochelys peninsularis is nested within xinjiangchelyids.