The Cretaceous (145-66 Ma) origin and diversification of flowering plants (angiosperms) heralded the assembly of modern terrestrial ecosystems. During the Late Cretaceous (100-66 Ma), over half of modern angiosperm families rapidly evolved. This led to the shift from gymnosperm (e.g., conifers, cycads) dominated forests to angiosperm dominated forests in which landscapes began to appear more similar to the modern. In this talk, I will discuss how my research group investigates this critical transition by analyzing three-dimensionally preserved plant fossil from the Late Cretaceous on western North America and Antarctica. The exceptional preservation of these fossils allows us to reconstruct phylogenetic relationships and the paleobiology of these fossils in outstanding detail, which enables new perspectives on the Late Cretaceous diversification of angiosperms and the concomitant development of modern terrestrial ecosystems.