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The Mesozoic era, spanning ~252 to 66 million years ago (Ma), was an important time period in the early evolution of the major plant lineages that dominate modern ecosystems. The origins of the seven living conifer families (e.g., cypresses, pines) can be traced back to at least the beginning of the Jurassic (>200 Ma). They were a diverse and prominent part of global vegetation throughout most of the Mesozoic, and the living members of these families still thrive in many areas today. The flowering plants appeared in the Early Cretaceous (~135 Ma) and rapidly diversified and rose to dominate most lowland warm ecosystems by the time the dinosaurs went extinct. The rise of flowering plants transformed plant communities into their more modern form, as forests once dominated by conifers and other groups became dominated by flowering plants. This body of work addresses three different aspects of the evolution of plants and ecosystems since the Mesozoic. Chapter one examines the evolution of conifer dispersal strategies from the perspective of the structural adaptations and functional morphology of conifer diaspores (the seed plus other dispersing structures), Seed dispersal is an important process that shapes the distribution of plants and the biotic interactions within ecosystems. This chapter examines macroevolutionary patterns in conifer dispersal ecology by mapping diaspore characteristics across the conifer phylogeny and evaluating the frequency and direction of transitions in dispersal strategies. These analyses reveal multiple examples of convergence and divergence in dispersal strategies, particularly for seed dispersal by wind and animals. The inferred ancestral dispersal characteristics are evaluated in the context of the Mesozoic fossil record, which demonstrates that dispersal strategies were varied and complex even during the early evolution of conifers. Chapter two investigates the early evolution of the Cupressaceae (cypress family), a diverse and ecologically important conifer family with pan-hemisphere distribution. The early diverging lineages of the family were once diverse and important components of Mesozoic ecosystems, but are now represented by relatively few species with disjunct distributions (e.g., redwoods). The fossil history of the Cupressaceae is investigated through the description of an Early Jurassic (~179 Ma) species from Patagonia, Argentina based on foliage, pollen cones, and seed cones. This species is one of the oldest records for the family based on multiple organs, and provides context for evaluating the early structural evolution and biogeographical history of the family.Chapter three focuses on the evolution of plant community structure during the ecological expansion of flowering plants. After their origination in the equatorial region, flowering plants increased in diversity as they spread towards higher latitudes. The timing of their ecological takeover of communities by abundance, or biomass, and the resulting changes in community structure have been less well understood than their diversity increases. This critical period in plant evolution is investigated by reconstructing a 74.6 Ma fossil forest from south-central New Mexico, which existed in a moist-wet megathermal (= "tropical") climate. The fossil flora is one of the most diverse known from leaf macrofossils, and is the oldest flora for which the ecologically dominance of flowering plants has been demonstrated across a floodplain. In contrast to modern forests in similar climates, conifers still formed a major component of the canopy in the fossil flora. This non-analog flora provides novel insights into geographic and temporal patterns in the ecological turnover of plant communities, and the early development of angiosperm-dominated forests in warm-wet climates.