A river runs through it—the Quarry Sandstone and adjacent strata, Dinosaur National Monument, Utah

Currently known from two valid species, Haplocanthosaurus priscus and H. delfsi, the Late Jurassic sauropod Haplocanthosaurus (Morrison Formation, Western United States) has often been described as an enigmatic sauropod taxon due to its unstable phylogenetic position and paucity of specimens. Here, we quantify the number of Haplocanthosaurus specimens known from the literature and in collections. Although most regions of the postcranial skeleton are known, the most commonly found elements of Haplocanthosaurus are vertebrae (dorsals and caudals) and tibiae. Our investigation identified twelve individuals of Haplocanthosaurus from ten localities across four states, Colorado, Utah, Montana (private specimen), and Wyoming, making Haplocanthosaurus spatially widespread in the central part of the Morrison Formation. The existence of twelve individuals across four states indicates this genus was widely distributed and more abundant than historically thought. Haplocanthosaurus has been characterized as a ‘primitive’ sauropod restricted to the lower half of the Morrison Formation, but the identification of Haplocanthosaurus in the Dry Mesa Dinosaur Quarry confirms that the genus was also present within the upper part of the Morrison Formation.