The Late Jurassic was the heyday of sauropod dinosaurs in prehistoric North America. Apatosaurus, Diplodocus, Barosaurus and Brachiosaurus were among the titans found in the 156- to 146-million-year-old Morrison Formation. But after this slice of geologic time, North American sauropods all but disappear.
There have been just a few discoveries of Cretaceous sauropods in North America. The recently described Abydosaurus was found in the 127- to 98-million-year-old Cedar Mountain Formation of Dinosaur National Monument, and the existence of the Late Cretaceous sauropod Alamosaurus has been known for nearly a century, but the post-Jurassic record of North American sauropods is sparse and discontinuous. Sauropods thrived in South America and elsewhere, but in North America their diversity declined, they disappeared about 100 million years ago, and Alamosaurus showed up on the continent about 30 million years later, just in time for the curtain call of the non-avian dinosaurs. In a study just published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, scientists Michael D'Emic, Jeffrey Wilson and Richard Thompson went back to the record of these animals in an attempt to better outline their history on this continent.
The proper identification of sauropod remains is key to figuring out the ups and downs of sauropod evolution in North America. As D'Emic and colleagues propose, some specimens previously thought to belong to sauropods should actually be attributed to other kinds of dinosaurs and therefore widen the gap between the species that hung on during the Early Cretaceous and those that reappeared on the continent toward the close of the period. More specifically, the authors of the new study looked at putative sauropod fossils of Campanian age—the time period just before that of Alamosaurus—to see whether there was an yet-unknown species of sauropod in North America during that time.
The specimens the scientists investigated were two neck vertebrae and one back vertebra from a single animal discovered in the circa 75-million-year-old rock of Arizona's Santa Rita Mountains. They do not look very much like sauropod vertebrae. Instead they most closely resemble the vertebrae of hadrosaurs such as Gryposaurus, a species already well known from the southern United States during this time period. The same can be said of similar partial vertebrae found from the same span of time throughout the southwest. If this new study is correct, then, there is no sign that sauropods made it back to North America until the arrival of Alamosaurus a few million years later.
As outlined by the authors of the study, there was a 30-million-year gap in which sauropods did not exist in North America. The question that remains is where Alamosaurus came from. The authors propose that its lineage could have traveled to North America from Asia thanks to an east-west connection between the continents during the last 35 million years of the Cretaceous that allowed the dispersal of tyrannosaurs, horned dinosaurs, hadrosaurs and other lineages present on both continents during the Cretaceous. Given the last-minute appearance of Alamosaurus, however, this seems unlikely. A more plausible explanation may be that it migrated north from South America when a north-south landbridge opened up around five million years before the end of the Cretaceous. Either way, the arrival of Alamosaurus was not so much a triumphant return as a fleeting hint of a landscape once dominated by long-necked giants.
D'Emic, M., Wilson, J., & Thompson, R. (2010). The end of the sauropod dinosaur hiatus in North America Palaeogeography, Palaeoclimatology, Palaeoecology, 297 (2), 486-490 DOI: 10.1016/j.palaeo.2010.08.032