Origin of the Sauropod Hiatus.

Throughout the late Jurassic and early Cretaceous, North America harbored many sauropods. They attained great size, high diversity and ecological dominance. Even after the disappearance of iconic Morrison taxa, Mierasaurus, Cedarosaurus, Venenosaurus, Moabasaurus, Sauroposeidon and Abydosaurus continued the sauropod record to mid Cretaceous. The group then vanished, however, not to reappear until middle Maastrichtian time. Sauropods remained absent for almost the entire late Cretaceous.

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 very small sauropod teeth. The 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 sauropds 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