For the past century, paleontologists have been trying to figure out one of the most puzzling disappearing acts in the fossil record.

In both Europe and North America, the Jurassic was the heyday of the sauropod dinosaurs. After the beginning of the Cretaceous period 145 million years ago, however, the number of these dinosaurs dwindled and they ultimately disappeared. It was only much later that other sauropods from elsewhere reestablished the presence of these dinosaurs in Europe and North America.

But, as explained in an in-press Palaeogeography, Palaeoclimatology, Palaeoecology paper by paleontologists Philip Mannion and Paul Upchurch, this great "sauropod hiatus" is probably an illusion. Over the past few years new discoveries have begun to fill in the sauropod gap on both continents. Sauropods may be missing in North America only between about 90 to 75 million years ago, and they are absent in Europe during two short intervals between 95 and 83 million years ago. That is quite a different picture than the near-total elimination of sauropods at the end of the Jurassic I learned about as a kid.

Of course, there are a few uncertainties about with these dates. Some of the 80- to 70-million-year-old sauropod remains from North America have been found to actually belong to hadrosaurs, and these fossils have been difficult to get dates for. Even so, recent discoveries of tracks and bones have extended the range of sauropods in Europe and North America through at least the first 50 million years of the Cretaceous, and scrappy remains between 90 and 75 million years ago may further fill in the gaps.

Frustratingly, though, the recovery of better fossil remains from the latter half of the Cretaceous may be a difficult task. The reason why explains the apparent sauropod gap.

Sauropod fossils are found primarily among inland deposits, perhaps indicating that these dinosaurs preferred inland habitats. During the middle of the Cretaceous, there was an increase in coastal deposits in both Europe and North America. (In North America, for example, the Western Interior Seaway split the continent in half and created long stretches of coast in what is now the middle of the continent.) The new research by Mannion and Upchurch shows that the number of sauropod species rises and falls with the amount of known inland habitat, meaning that the rarity of Cretaceous sauropods may indicate a lack of well-sampled, inland fossil sites from the right age. In other words, sauropods may have moved away from the better-sampled coastal areas during the mid-Cretaceous and only spread out again when the coastal areas receded.

The closure of the great sauropod gap also has important implications for where the last of these dinosaurs in both Europe and North America came from. In North America, for instance, Alamosaurus is recognized as one of the very last sauropod dinosaurs and has been thought to be a descendant of a South American ancestor. A paper about the same topic published in the same journal just last year supported this hypothesis, but Mannion and Upchurch dispute the idea that Alamosaurus was an immigrant from the south.

Alamosaurus was a titanosaur, a group of sauropods which were recognized mainly thanks to discoveries made from the southern hemisphere. As Mannion and Upchurch note, this has created the impression that titanosaurs are a group of southern dinosaurs and that the titanosaurs of the Late Cretaceous of Europe and North America must have migrated from the south. Based upon what we now know, however, titanosaurs had spread around the world by the Early Cretaceous. In the case of Alamosaurus, its ancestors may have come from Asia or may have been present already in North America during an earlier time, although the origin of the European titanosaurs such as Ampelosaurus and Lirainosaurus is less clear.

There is still much that is unknown about the history of North American and European sauropods during the Cretaceous. Nevertheless, the new study does point researchers in directions of future research. If inland fossil sites during the mid-Cretaceous can be identified and systematically sampled, paleontologists may find the missing dinosaurs.

References:

Mannion, P., & Upchurch, P. (2010). A re-evaluation of the ‘mid-Cretaceous sauropod hiatus’ and the impact of uneven sampling of the fossil record on patterns of regional dinosaur extinction Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2010.12.003