The Campanian-aged “Mesaverde” Formation occurs across much of Wyoming, representing nearshore and continental environments along the eastern coast of Laramidia. It is of significant interest biogeographically as an intermediate location between purported southern and northern vertebrate faunal assemblages. However, only a single radiometric date has been reported for the formation, a recalibrated age of 80.10±0.61 Ma from the middle of the unit in Elk Basin (northern Bighorn Basin). The age of the Teapot Sandstone, the uppermost member of the “Mesaverde” Formation, presents a puzzle. In 1966, Gill and Cobban inferred a major unconformity between the Teapot and the underlying portions of the “Mesaverde” Formation, based on their interpretation of missing ammonite zones. This suggests a late Campanian age for the Teapot (~74 Ma), versus a middle Campanian age for the underlying portions of the formation. These and subsequent authors noted, however, the lack of expected indicators of an unconformity, such as paleosol development or major channel incision into underlying rocks. A lack of radiometric and biostratigraphic dates has hampered an independent test of the unconformity hypothesis.

Since 2019, the Alf Museum has been documenting vertebrate fossil localities from the “Mesaverde” Formation of Oregon Basin, in the northwestern Bighorn Basin of Park County, Wyoming. Fossils occur throughout the upper two-thirds of the formation, including many in the Teapot Sandstone. Diagnostic helochelydrid turtle shell was collected at two sites in the Teapot Sandstone and occurs throughout the underlying portions of the formation at Oregon Basin. Elsewhere in North America, helochelydrids go extinct during the middle Campanian, no later than 78.6 Ma. This strongly suggests a middle Campanian (<78.6 Ma) age for the Teapot Sandstone in Oregon Basin and argues against a major unconformity within the formation at this location. Ideally, this will be tested by discovery of additional biostratigraphically significant fossils and/or radiometric dates. If the Teapot Sandstone at Oregon Basin is indeed middle Campanian in age, this creates an additional puzzle around the age of the overlying (but poorly exposed) Meeteetse Formation there. Although ammonites and direct radiometric dates are undoubtedly the gold standard for dating the Cretaceous sections of Wyoming, this example shows the significant potential for vertebrate fossils to resolve chronostratigraphic problems.