It is often difficult to interpret plant life form and position within a forest based on fossils of isolated plant organs. Here we propose leaf vein density as a new tool to interpret fossil angiosperm life form, and in particular to trace the emergence of angiosperms as members of the canopy. Angiosperm leaf vein density was analyzed in two tropical forests and one temperate forest. Comparisons of vein density between canopy and understory plants showed that vein density variation mainly reflected the position of the leaf in the canopy, independent of ecological strategy (shade tolerant versus sun demanding), phylogenetic position, and site (tropical versus temperate). Vein density values of a standing forest were reflected in its leaf litter, suggesting that fossil leaf assemblages are representative of past forest ecosystems. Comparison of vein density distributions of Cretaceous–Paleocene paleofloras (132.35–58.0 Ma) to those of a modern tropical leaf litter assemblage suggests that angiosperms emerged in forest canopies by at least 58 Ma.