THE END OF THE CRETACEOUS 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 Upper Cretaceous. This ca. 25–30 million-year long sauropod hiatus has been attributed to either a true extinction or a false extinction related to non-preservation of formations representing sauropod habitats. The duration of the sauropod hiatus remains in question due to uncertainty in the ages and affinities of the specimens bounding the observed gap. Here, 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 sauropods, but to hadrosaurid dinosaurs. Based on this reassessment, the only definitive Late Cretaceous records of sauropods in North America are Maastrichtian, and the sauropod hiatus extended until the last five million years of the Cretaceous. Reintroduction of sauropods into North America is first registered ca. 15–45 million years after some other groups of dinosaurs that likely dispersed to the continent from Asia (e.g., ceratopsians, pachycephalosaurids), but nearly coincident with the arrival of derived hadrosaurids that likely dispersed from South America (e.g., Secernosaurus). Resolving whether derived sauropods dispersed between the Americas using the same landbridge as derived hadrosaurids or via another landbridge awaits a comprehensive phylogeny of the sauropod taxa involved.