EARLY CRETACEOUS PLANT DIVERSITY BEFORE TO THE APPEARANCE OF ANGIOSPERMS IN NORTH AMERICA

A comprehensive understanding of the evolution of flowering plants requires an understanding of the plant communities into which the angiosperms radiated. Unfortunately, there are very few studies of Early Cretaceous "pre-angiosperm" (Berriasian-Aptian) floras from North America. Here, I use evidence from >40 plant megafossil collections compiled from literature sources and museum collections to understand the diversity and systematic composition of "pre-angiosperm" Early Cretaceous floras from the United States. Collections range from Texas to Montana and Oregon to Arkansas. Geologic evidence indicates a strong latitudinal climate gradient in the western United States as indicated by plant-bearing limestone beds in Texas, vertic paleosols in Nevada and Wyoming, and coals up to 2 meters thick in Montana (and Canada). This gradient may have been paralleled by a shallow and inverted latitudinal diversity gradient. In general, the collections have low species richness (<15 species) regardless of original collection size and facies. However, collections from along the western margin of North America (southern Oregon & northern California) have the highest species richness (~15-25 species), but these collections represent allochthonous terranes. A variety of plant groups are represented among the species that dominate these collections including Equisetum, ferns, various cycadophytes, Sagenopteris spp., Ginkgo, and several conifer groups.