The Early Cretaceous diversification of flowering plants was not preceded by a mass extinction event. This suggests that biotic factors intrinsic to flowering plants played an important role promoting diversification, but the rarity of fossils of early flowering plants makes identifying the important features difficult. Here, I present the results of my specimen-based analysis of plant megafossil collections from Lower Cretaceous deposits of the United States. First, I describe previously unrecognized eudicot leaf fossils from a historically important Aptian (Lower Cretaceous) plant fossil site in the Potomac Group, and I provide a set of characters for recognizing the fossil leaves of these plants. Then, I present a morphotype catalog for the fossil plants from and Aptian-early Albian (Lower Cretaceous) site in the Potomac Group. This collection includes one angiosperm morphotype. Next I describe the angiosperm morphotype identified in the previous chapter. I show that it is widely distributed among coeval collections of the Potomac Group and some specimens were previously described as ferns. The preservation of attached stems leaves and root provides direct evidence of weedy, fast-growing, herbaceous angiosperms in the Aptian-early Albian. In the following chapter, I use megafossil data from the literature and museum collections to test the hypothesis that the diversification of flowering plants is associated with an increase in alpha diversity during the Early Cretaceous. Despite the evidence for a high diversification rate among early flowering plants, I found no relationship between collection age and collection richness, but I found strong evidence that angiosperms were consistently rare during the Aptan-middle Albian, and that locally abundant angiosperms became common during the late Albian, long after the initial diversification. Finally, I use new plant megafossil collections that I made from the Cloverly and Sykes Mountain Formations in Wyoming, USA, for a more high-resolution study of early angiosperm diversity, distribution, and abundance. I show that the Cloverly Formation records the appeareance of flowering plants in North America, and that by the Albian angiosperms were widely distributed among available habitats. I test the hypothesis that variation in community composition (beta diversity) increased with the appearance of angiosperms. I did not find strong support for the hypothesis that angiosperms increased beta diversity; however, rarefaction analysis shows that the rate of morphotype discovery in both the pre-angiosperm and the angiosperm interval is high, which means that additional sampling may reveal a difference in beta diversity between the two intervals. Together my findings indicate that flowering plants diversified during the Early Cretaceous not because they had features that allowed them to displace other plant groups, but because they were uniquely able to maintain high diversification rates in the face of rarity and dispersed populations.