PALEOBOTANY AND PALEOECOLOGY OF LOCALITIES FROM THE CRETACEOUS CLOVERLY FORMATION, BIGHORN BASIN, WYOMING

Despite the importance of the Cloverly Formation for our understanding of dinosaur and mammal paleoecology during the “middle” Cretaceous, relatively little has been documented regarding its vegetation. A new locality along Crooked Creek in the Bighorn Basin has produced a macroflora and microflora that shed light on the vegetation of the Cloverly Formation in north-central Wyoming. The Crooked Creek locality is in the Little Sheep Mudstone Member of the Cloverly Formation, Ostrom’s Unit V. The age of this unit has been considered to be Aptian based on fission track dating in northeastern Wyoming by Chen and Lubin. However, Sames et al. reported an age range of (Late?) Berriasian to Late Barremian–Aptian for the base of depositional correlates in the Western Interior.

In this study we describe these floras and compare and contrast macro- and microfloras. The macroflora is dominated by ‘taxodiaceous’ cones and foliage, sometimes completely covering a bedding plane. Their abundance corresponds well to the results from the palynological study in which inaperturate pollen, consistent with ‘taxodiaceous’ plants, are by far the dominant palynomorph. Ferns are rare components of the macroflora, but more common, and far more diverse in the palynoflora. Furthermore, there are no angiosperm remains in the macroflora but rare pollen grains attributable to four angiosperm form-genera, including Tricolpites, ‘Retimonocolpites’, Lilliacidites, and Stellatopollis, are present.

The palynoflora recovered in this study is not age-diagnostic on its own due to poor biostratigraphic control over palynomorphs that span the Late Jurassic–Early Cretaceous. Nevertheless, in possessing angiosperms (including a eudicot), the palynoflora is broadly consistent with an Aptian-Albian age based on published studies along the Atlantic Coast and in the Western Interior and Alberta.

The low diversity and abundance of flowering plants, especially eudicots, in Crooked Creek contrasts with Singh’s work in the middle Albian of Alberta, but may reflect either an older age (i.e., Aptian), local environmental controls or regional heterogeneity in vegetation. Of note, initial studies of a macroflora within sandstone beds at the base of the Himes Member, Ostrom’s Unit VI, reveal both diversity and abundance of angiosperm remains.