Alamosaurus and the sauropod hiatus in the Cretaceous of the North American Western Interior
Sauropod dinosaurs have a temporally disjunct distribution in the North American Western Interior during Cretaceous time, here referred to as the sauropod hiatus. Sauropod body and ichnofossils are present in inland basinal and coastal deposits of Aptian-Albian age in Wyoming, Texas, Oklahoma, and Arkansas. Body fossils of sauropods (the titanosaurid Alamosaurus) occur in inland basinal deposits of Maastrichtian age in Texas, New Mexico, Utah, and Wyoming. No sauropods of Cenomanian-Campanian age are known from a Western Interior sedimentary record dominated by coastal deposits and essentially devoid of inland basinal deposits. Dinosaur ichnofaunas from late Albian coastal deposits in New Mexico, Oklahoma, and Colorado lack sauropod footprints, and thus suggest sauropod disappearance from Western Interior coastal environments by the end of Early Cretaceous time.
These observations suggest two scenarios: (1) sauropods abandoned Western Interior coastal environments at the end of the Albian, but persisted in Western Interior inland basinal environments throughout Cretaceous time, or (2) sauropods became extinct in the Western Interior at the end of the Albian and reinvaded during the Maastrichtian. Choosing between these scenarios depends on an evaluation of negative evidence. However, the close phylogenetic relationship of Alamosaurus to South American titanosaurids, the absence of sauropod fossils in inland deposits of the Campanian Two Medicine and Judith River Formations, and the availability of a dispersal route between North and South America near the end of Cretaceous time support the second scenario. Thus, sauropods apparently became extinct in the Western Interior near the end of the Albian, then reinvaded from South America during the Maastrichtian but were only able to establish themselves in inland basinal environments. The extinction of sauropods in the Western Interior at the end of Early Cretaceous time may reflect an unrecognized terrestrial extinction coincident with the well-known severe marine extinction caused by a major late Albian regression.
Figures & Tables
- Judith River Formation
- Lower Cretaceous
- New Mexico
- North America
- sea-level changes
- Two Medicine Formation
- United States
- Upper Cretaceous
- Western Interior
- Alamosaurus sanjuanensis
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