The end of the sauropod dinosaur hiatus in North America

Sauropod dinosaurs reached their acme in abundance and diversity in North America during the Late Jurassic. Persisting in lesser numbers into the Early Cretaceous, sauropods disappeared from the North American fossil record from the Cenomanian until the Campanian or Maastrichtian stage of the Late Cretaceous. This ca. 25–30 million-year long sauropod hiatus has been attributed to either a true extinction, perhaps due to competition with ornithischian dinosaurs, or a false extinction, due to non-preservation of sediments bearing sauropods. The duration of the sauropod hiatus remains in question due to uncertainty in the ages and affinities of the specimens bounding the observed gap. In this paper, we re-examine the phylogenetic affinity of materials from Campanian-aged sediments of Adobe Canyon, Arizona that currently mark the end of the sauropod hiatus. Based on the original description of those remains and new specimens from the same formation, we conclude that the Adobe Canyon vertebrae do not pertain to titanosaurs, but to hadrosaurid dinosaurs. Other reports of Campanian North American titanosaurs are imprecisely or erroneously dated and/or identified. Based on this reassessment, the sauropod hiatus extended until the last 5 million years of the Cretaceous. Reintroduction of sauropods into North America in the Maastrichtian is first registered ca. 30 million years after the first recorded dispersals of dinosaur taxa between the continent and Asia (e.g., ceratopsians, pachycephalosaurids, theropods), but is nearly coincident with the dispersal of derived hadrosaurids to South America.