Coevolution of early angiosperms and their pollinators: Evidence from pollen

Pollen morphology has traditionally been used to distinguish wind from animal pollination in angiosperms. In addition, the body of literature on Cretaceous angiosperm pollen clumps that may have had either a botanical or zoological origin is growing. Modern pollination biology studies suggest that the evolution of pollen clumping in flowers, resulting in floral pollen clumps, is attributable to insect pollination and that sticky pollen may be ancestral in angiosperms. The study of feces from pollen-eating insects suggests that pollen-containing coprolites (fossil insect fecal pellets) may provide information on both the organisms consuming the pollen and the identity of potential pollinators. We present here a preliminary study on dispersed pollen samples from Aptian to Campanian strata in the U.S.A. to examine the frequency of pollination modes based on dispersed pollen and to track the presence of pollen clumps. Criteria were developed to distinguish between floral pollen clumps and pollen-containing coprolites that show that fossil floral pollen clumps are irregularly shaped with rough surfaces and are composed of a single species of well-preserved grains, while the pollen-containing coprolites are regularly shaped with smooth surfaces and contain damaged grains. The preliminary data on the frequency of dispersed pollen support previous hypotheses that the Early Cretaceous angiosperms were insect-pollinated and that wind pollination became important later during the mid-Cretaceous. These pollen data also support the idea that ambophilous pollination, by both animals and wind, may have also existed early on. Floral pollen clumps appear only rarely until the mid-Cretaceous. The temporal distribution of clumps suggests that the pollen from early flowers were not sticky and that stickiness may have evolved with an adaptation to more advanced pollinators. Pollen-containing coprolites are found throughout the Cretaceous and vary in overall form, as well as in the preservation of individual grains. Based on previous studies of pollen-containing coprolites of insect origin, it seems likely that a number of insect digestive strategies, similar to those found in living members of Coleoptera, Diptera, and Hymenoptera were already in place by the late Mesozoic.