Atoposaurids were a clade of semiaquatic crocodyliforms known from the Late Jurassic to the latest Cretaceous. Tentative remains from Europe, Morocco, and Madagascar may extend their range into the Middle Jurassic. Here we report the first unambiguous Middle Jurassic (late Bajocian-Bathonian) atoposaurid: an anterior dentary from the Isle of Skye, Scotland, UK. A comprehensive review of atoposaurid specimens demonstrates that this dentary can be referred to Theriosuchus based on several derived characters, and differs from the five previously recognized species within this genus. Despite several diagnostic features, we conservatively refer it to Theriosuchus sp., pending the discovery of more complete material. As the oldest known definitively diagnostic atoposaurid, this discovery indicates that the oldest members of this group were small-bodied, had heterodont dentition, and were most likely widespread components of European faunas. Our review of mandibular and dental features in atoposaurids not only allows us to present a revised diagnosis of Theriosuchus, but also reveals a great amount of variability within this genus, and indicates that there are currently five valid species that can be differentiated by unique combinations of dental characteristics. This variability can be included in future broad-scale cladistics analyses of atoposaurids and closely related crocodyliforms, which promise to help untangle the complicated taxonomy and evolutionary history of Atoposauridae.

INTRODUCTIONIn late 2002, dinosaur footprints were discovered on loose blocks of sandstone, as well as in situ, on the foreshore at Lub Score, northwest Trotternish Peninsula, Isle of Skye, Scotland, UK (Fig. 1). The majority of these footprints were much smaller than any previously found in Scotland, and were closely associated with larger footprints of what seems likely to be the same species. These footprints are stratigraphically younger than any other dinosaur remains found in Scotland to date, and are different from those found elsewhere from the Middle Jurassic succession on the Isle of Skye (Fig. 2).

Synopsis The limb bone of a dinosaur was discovered from the Valtos Sandstone Formation (Bathonian, Middle Jurassic) near Valtos, Isle of Skye. The bone preserves the eroded proximal and distal ends and is missing the mid-shaft. It represents the first unequivocal Middle Jurassic dinosaur bone from Scotland. The bone is of a sauropod dinosaur and may be the femur of a cetiosaurid.