Origin of the Sauropod Hiatus

For over 60 million years North America harbored sauropods. At times, the iconic beats attained gigantic size, high diversity and ecological dominance. Even after the demise of Morrison forms, Mierasaurus, Cedarosaurus and others document the survival of North American sauropods to the Cenomanian. The group then vanished, however, not to reappear until middle Maastrichtian time. Sauropods remained absent for almost the entire late Cretaceous (roughly 98 to 69 Ma).

What caused this loss of sauropods, known as the North American sauropod hiatus? Locally, two factors doomed the group. The first, or primary cause, was the late Albian highstand. Just prior to the end of the Early Cretaceous, sea levels rose precipitously, inundating a great deal of land and habitat. Naturally the most severely affected taxa were the largest ones, requiring the most habitat and food resources. No doubt, a key victim was the huge Abydosaurus. Existing just prior to Mussentuchit or Cenomanian time, Abydosaurus was bereft of resources needed for survival and died out. This extinction, however, did not mark the onset of the hiatus.

The lower horizon of the Mussentuchit has yielded tiny sauropod teeth. These teeth indicate the presence of a dwarf sauropod probably related to Astrodon. At a time of greatly diminished resources, only a diminutive sauropod, requiring minimal habitat and food, could persist. The lower Mussentuchit taxon certainly outlasted the giant Abydosaurus. It did not not last much longer, however. The highstand was to ultimately prove fatal, for the survival prospects of a small sauropod were not good.

The Mussentuchit documents an influx of Asiatic taxa into North America. Among these were Eolambia a hadrosauroid. Naturally the Mussentuchit sauropod was a low feeder. It could not reach the higher tree foliage. Dependent on low vegetation, it now faced competition from ornithopods. Even worse, it may have faced a large predator, Siats. Generally sauropod behemoths had little trouble surviving theropods, even big ones. In contrast, the little taxon stood no chance. Faced with invasive species, it appears to have succumbed quickly. The little sauropod existed just prior to the hiatus.

The fact a dwarf sauropod survived the Albian highstand, however briefly, indicates sea level changes were not the actual cause of extinction. Most likely, competitors and predators were. Nevertheless, by compelling sauropods to become small, the highstand effectively doomed them. Unless they were island dwellers, inaccessible to competitors or dangerous hunters, they lived on borrowed time.

Astrodon, a close relative of the later, Mussentuchit astrodont.