adaptive node marking a distinct coevolution of early flowering plants and animal (insect) pollinators. The success of this involvement of insects in the reproductive biology of plants was not new. Dating back into the Paleozoic, insects most probably were involved in pollination of some of the seed ferns such as Medullosa (Dilcher, 1979; Retallack and Dilcher, 1988). During the Mesozoic, several non-angiospermous plants were certainly using animals for pollination as part of their reproductive biology. These include plants such as the Cycadoidea, (Delevoryas, 1968; Crepet, 1974) Williamsonia, Williamsoniella, and, perhaps, some seed ferns such as Caytonia. Insect diversity increased parallel to the increasing diversity of the angiosperms during the Mesozoic (Labandeira et al., 1994; Labandeira, 1998; Magallón et al., 1999). This node of evolution corresponds to the initial coevolution of animals and flowering plants in gamete transport. These early showy flowers came in many sizes, were displayed on the plant in many different ways, and were uniform in the types of organs they contained and the radial symmetry of these organs. They must have accommodated many different types of pollinators as evidenced by the variety of their anthers, stigmatic surfaces, nectaries, and the sizes and positions of the floral organs (Dilcher et al., 1976a; Dilcher, 1979; Crepet and Nixon, 1998a, b; Gandolfo et al., 1998a, b, c; Friis et al., 1999; Crepet et al., 2000). It was through the success of this coevolution that the angiosperms became the dominant vegetation during the early Late Cretaceous. Ordinal and family clades began to become identifiable during the later Early Cretaceous and the early Late Cretaceous (Crepet and Nixon, 1998a, b; Gandolfo et al., 1998a, b, c; Magallón et al., 1999; Crepet et al., 2000). However, at the same time, some of the angiosperms never developed showy flowers and used other means of gamete transport for cross-pollination such as wind (early platinoids) and water (early ceratophylloids).
The evolution of bilateral flowers happened about 60 million years after the origin of the angiosperms. This node in coevolution never affected the water- or wind-pollinated groups that were already established. The evolution of the bees late in the Late Cretaceous (Michener and Grimaldi, 1988a, b) was a coevolutionary event with the evolution of bilateral flowers. This occurred independently in many different clades of flowering plants that were already established by the mid-Late Cretaceous. The potential for flowers to further direct the behavior of insects to benefit their pollination had a profound influence on those clades that evolved during the late Upper Cretaceous and early Tertiary. Flowers not only presented their sex organs surrounded by sterile floral organs with attractive patterns and colors, exuding attractive fragrances and filled with nectar and pollen for food, but the bilateral flowers could show the animals which way to approach them and how to enter and exit them. This allowed flowers to maximize the potential for precise gamete ex-