Morphology, histology and identification of the ‘granicones’ from the Purbeck Limestone Formation (Lower Cretaceous: Berriasian) of Dorset, southern England

The identification of the ‘granicones’, small, conical and shield-like phosphatic structures with a pustulate surface ornamentation, from the Purbeck Limestone Formation of Dorset, has been problematic for over 130 years. They have been interpreted as the osteoderms of theropod or ornithischian dinosaurs, lizards or crocodiles, though only circumstantial evidence was presented in support of these hypotheses. Here, we provide anatomical, histological and geological evidence that demonstrates the true affinities of the granicones: they are the limb osteoderms of turtles and may be referable to either Helochelydra (=Tretosternon) anglica or ‘Tretosternon’bakewelli .