The anatomy of the basal turtle Eileanchelys waldmani from the Middle Jurassic of the Isle of Skye, Scotland

Abstract

The stem turtle Eileanchelys waldmani is known from the Bathonian deposits of the Kilmaluag Formation, Isle of Skye, Scotland. A description of all the currently available material, including several skulls and shells, is provided herein. Eileanchelys waldmani is characterised by a mosaic of plesiomorphic and derived features. The former include a flat vomer, a reduced posterior extension of the pterygoid, the absence of flooring of the cavum acustico-jugulare, the absence of proper recessus scalae tympani, wide vertebral scales, a vertebral 3–4 sulcus on neural 6, a pair of mesoplastra meeting medially, and platycoelous cervical vertebrae. Derived features include a prefrontal–vomer contact, an elongate postorbital skull, a well-developed antrum postoticum, and a slender processus interfenestralis of the opisthotic. Eileanchelys waldmani also possesses unique features such as a possible ninth neural or supernumerary suprapygal of unusual shape, as well as a broad first suprapygal that contacts the peripherals laterally. Additionally, E. waldmani documents the evolution of the vomer and basicranium from basalmost turtles to the crown-group.