THE PALEONTOLOGICAL CONTEXT OF ANGIOSPERM VEGETATIVE EVOLUTION

Angiosperms represent more than three-quarters of all land plant species despite existing for only a quarter of land plant history, inspiring much interest in understanding what aspects of angiosperm biology may have enabled their unparalleled success. However, comparative study of extant plants cannot fully address the basis of angiosperm dominance because most living nonflowering plants are members of recently radiating lineages as young or younger than the flowering plants and, thus, are a highly biased sampling of the full 400 Myr history of vascular plant diversity. Here, a survey of the fossil record demonstrates that most anatomical traits that are now unique to the angiosperms were more broadly distributed among extinct lineages. However, the growth capacities of angiosperms are likely to have been uniquely high for all of vascular plant history and may have enabled a greater range of architectural possibilities, including the fast-growing annual herbs that represent a large proportion of angiosperm diversity. Many of the gymnosperm lineages lost to extinction during the angiosperm radiation may have been understory and earlier successional plants with advantages relative to conifers for early establishment but without the capacity to match either the high leaf area of mature conifers or the high productivity per unit leaf area of angiosperms. Vascular plants had already occupied all terrestrial climate zones before the angiosperm radiation, but the spread of flowering plants may have substantially changed the vegetation structure of many ecosystems and may have actually changed the temperature and rainfall patterns that determine ecosystem distributions.