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.

The Middle Jurassic is a largely mysterious interval in dinosaur evolution, as few fossils of this age are known worldwide. In recent years, the Isle of Skye has yielded a substantial record of trackways, and a more limited inventory of body fossils, that indicate a diverse fauna of Middle Jurassic dinosaurs living in and around lagoons and deltas. Comparatively little is known about the predators in these faunas (particularly theropod dinosaurs), as their fossils are among the rarest discoveries. We here report two new isolated theropod teeth, from the Valtos Sandstone and Lealt Shale Formations of Skye, which we visualized and measured using high-resolution x-ray computed microtomographic scanning (µCT) and identified via statistical and phylogenetic analyses of a large comparative dental dataset. We argue that these teeth most likely represent at least two theropod species-one small-bodied and the other large-bodied-which likely belonged to one or several clades of basal avetheropods (ceratosaurs, megalosauroids, or allosauroids). These groups, which were diversifying during the Middle Jurassic and would become dominant in Late Jurassic, filled various niches in the food chain of Skye, probably both on land and in the lagoons.

Synopsis:The Middle Jurassic was a dynamic interval in dinosaur evolution, but the dinosaur fossil record from this time is extremely poor throughout the world. The Isle of Skye (Scotland) preserves marginal marine and terrestrial deposits of Middle Jurassic age, which have yielded sparse bones, teeth, footprints, and small segments of trackways belonging to dinosaurs. We report the discovery of the most extensive dinosaur fossil site yet known in Scotland: a coastal outcrop of the Duntulm Formation (Bathonian) at Cairidh Ghlumaig, Skye, that preserves numerous trackways of sauropod dinosaurs in multiple layers deposited in a lagoonal system. We present an initial description of these tracks and identify them as most likely belonging to a primitive, non-neosauropod species that retained a large claw on manual digit I and produced narrow-gauge trackways. They provide additional evidence that basal sauropods persisted deep into the Middle Jurassic, a time when the earliest members of larger and more derived sauropod lineages were radiating. The new Skye tracks document multiple generations of sauropods living with the lagoonal environments of Jurassic Scotland, and along with other tracks found over the past two decades, suggest that sauropods may have frequented such environments, contrary to their image as land-bound behemoths.