426 FOSSIL PLANTS/Angiosperms product of relatively recent radiations that occurred during the second half of angiosperm evolution The evolutionary basis for the rapid diversification of specific eudicot clades remains unknown The rapid rise to prominence of angiosperms during the mid-Cretaceous marks the transition from Mesozoic ecosystems dominated by ferns and gymnosperms to those of the Late Cretaceous and Cenozoic, when flowering plants predominate The record of fossil pollen in the northern hemisphere shows that the initial increase in angiosperm diversity occurred at low palaeolatitudes (Figure 11) During the Early Cretaceous, low-latitude areas experienced semiarid or seasonally arid climates These conditions may have promoted a weedy life history with precocious reproduction favouring herbaceous annuals rather than long-lived woody perennials There is some fossil evidence to support this With the exception of one disputed record, angiosperm wood has not been recorded from rocks that predate the Albian Angiosperm leaves are also extremely rare in these older sediments Furthermore, it seems likely that some of the angiosperms from other Lower Cretaceous assemblages (e.g., Crato Formation, Brazil, and Portugal) were aquatic These data indicate that the earliest angiosperms were predominantly herbaceous and that there may have been considerable diversity in aquatic habitats By the end of the Cretaceous period, flowering plants represented about 50–80% of land plant species diversity, but pockets of gymnosperm- and fern-dominated vegetation persisted This notable Figure 11 Analyses of dispersed pollen grains indicate that the diversification of angiosperms began at low palaeolatitude Reproduced with permission from Crane PR and Lidgard S (1989) Angiosperm diversification and palaeolatitudinal gradients in Cretaceous floristic diversity Science 246: 675 678 change in floral composition predates the end of the Mesozoic Era by some 30 million years and was more radical than that which occurred at the Cretaceous– Tertiary boundary Some groups of flowering plants that are ecologically prominent today were completely absent from the Cretaceous Period Grasses, for example, did not become abundant and widespread until the Late Oligocene and Miocene Ferns may well have played a more prominent role as herbaceous ground cover in prairie vegetation prior to the mid-Cenozoic The diversification of angiosperms has often been linked to the evolution of various animal groups But proving cause and effect in the fossil record is often very difficult For example, the rapid diversification of angiosperms during the mid-Cretaceous coincides broadly with a transition from sauropod-dominated to ornithopod-dominated dinosaur faunas (see Fossil Vertebrates: Dinosaurs) It is difficult to show whether these changes are in some way linked or just coincidental A stronger case can be made for a link between insects (see Fossil Invertebrates: Insects) and flowering plants The diversification of pollenand nectar-collecting insects has been linked to flowering plants on the supposition that insect pollination provides new possibilities for reproductive isolation and therefore elevation of speciation rates Compared to wind pollination, which is widespread in gymnosperms, insect pollination may permit more effective out-crossing at lower population densities and in a greater range of environments, thereby reducing extinction rates Pollination biology may provide a plausible explanation for spectacular diversity in some angiosperms families (e.g., orchids), but the extent to which this provides a general explanation for the mid-Cretaceous rise of angiosperms is uncertain There is evidence that insect pollination was an aspect of the biology of some extinct gymnosperms (e.g., Bennettitales) and some living gymnosperms (e.g., Gnetales) Early angiosperms may therefore have had the opportunity to co-opt pollinators from previously established relationships with other groups of seed plants The fossil record of flowers from the Early Cretaceous clearly indicates that early angiosperms were insect pollinated Many of the features that have been documented in early fossil flowers are comparable to those found in modern insect-pollinated groups The anthers show that pollen output was low, and the pollen grains are often smaller than the most effective size for wind dispersal Release of pollen from anthers occurred via hinged flaps, and the individual grains are often covered by a pollenkitt-like substance The stigmatic surfaces of the carpels are generally unelaborated These features are consistent with