Reassessment of the oldest British turtle: Protochelys from the Middle Jurassic Stonesfield Slate of Stonesfi eld, Oxfordshire, UK

Protochelys Lydekker, 1889 from the Stonesfield Slate (middle Bathonian) is the oldest British turtle and the only record to date of fossil epidermal shell scales preserved isolated from underlying bone. Although known since the 1840s, these remains have never been properly described, figured or compared with other taxa. Here, we provide a thorough reassessment of the available material with a discussion of the exceptional preservation of isolated scales. We conclude that: 1) no satisfactory diagnosis of this taxon can be proposed and Protochelys blakii (Mackie, 1863) has to be considered nomen dubium; 2) the carapace of the Stonesfield turtle has a plesiomorphic morphology (vertebral scales twice as wide as long; fifth vertebral scale as wide anteriorly as it is posteriorly; pleural scales longer than wide) shared with numerous basal turtles. The fossilisation of turtle epidermal scales is extremely rare (only two other examples are known). The Stonesfield material is unique in that the scales are isolated, without underlying bone. A review of the literature shows that isolation of shell scales occurs as a result of two processes: shedding of old scale layers during growth or post mortem disarticulation. We favour the disarticulation hypothesis because complete scales are thicker and more likely to preserve the well-developed ornamentation shown by the Stonesfield scales.