Paleoecology and stratigraphy of the Morrison and Cloverly Formations, Bighorn Basin, Wyoming
The Morrison Formation in the Western Interior of North America represents a vast geologic region as well as a large period of time during the Late Jurassic. The dinosaurian portion of the fauna of the formation has been studied for over 100 years, and several attempts have been made to construct a dinosaurian biostratigraphy. The results of these studies, that the ecology and environment inhabited by the dinosaurs of the formation is homogeneous, have persisted despite serious questions as to the validity of the stratigraphic basis for these studies.
An examination of the vertebrate fossil quarries in the Morrison Formation in the Coyote Basin region of the Bighorn Basin, Wyoming, provided an excellent opportunity to test the hypothesis of faunal homogeneity. Careful lithostratigraphic work was completed and the five quarries in the area were placed in relative stratigraphic position. The quarries are all near the top of the Morrison Formation, and contain an almost identical suite of dinosaurian fossils. Examination of the palynoflora from the Two Sisters Quarry, just outside of the Coyote Basin, and comparison with previous published reports indicate that there are great differences in the taxonomic composition of the flora in different regions and stratigraphic positions within the entire formation.
The Coyote Basin area contains the type section for the Cloverly Formation. While studying the stratigraphy of the dinosaur quarries in the area, it became apparent that the base of the formation, and specifically its contact with the underlying Morrison Formation, needed to be redefined. The base of the Cloverly Formation is redefined herein as the base of a chalcedony-bearing, non-calcite cemented grey claystone.
The Cloverly Formation in this area also contains the Crooked Creek Locality, a vertebrate fossil locality originally identified by Ostrom. Subsequent to that work, another fossiliferous horizon was located that contained palynoflora, megaflora, and vertebrate microfossils. Comparison of the taxonomic components of this quarry to those of the Two Sisters Quarry (Morrison Formation) indicates there was a significant change in the climate of the area between the times of deposition of the two formations. This study has demonstrated the potential of palynofloras and microvertebrates to develop a biostratigraphic scheme independent of the macrovertebrate realm that can be applied to resolve the question of the homogeneity of dinosaurian faunas through the entire Morrison Formation depositional area. Re-analyses of existing palynofloras and new collections of microvertebrates in resolved local stratigraphic sections will be important to expanding this scheme.