. "14 Toward a New Synthesis: Major Evolutionary Trends in the Angiosperm Fossil Record." Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years after Stebbins. Washington, DC: The National Academies Press, 2000.
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Variation and Evolution in Plants and Microorganisms: TOWARD A NEW SYNTHESIS 50 YEARS AFTER STEBBINS
helically on an elongated axis with pollen organs if present, subtending and helically arranged on the same axis (Sun et al., 1998). Similarly, the early small flowers (Friis et al., 1999), unisexual or bisexual, have axes with radially arranged organs. In small flowers the elongation of many early flowering axes is compressed so that the organs appear radially arranged. This organization is clearly seen in larger flowers such as Archaeanthus (Dilcher and Crane, 1984) and the Rose Creek flower (Basinger and Dilcher, 1984). This radial arrangement of organs dominated floral form until late into the Late Cretaceous or the Paleocene and still persists in many flowers today.
Bilateral symmetry. By Paleocene and Eocene time, there are several evidences in the fossil record of bilateral flowers. This evolution probably began during the Upper Cretaceous. The evolution of bilateral flowers is associated with the presence of social insects in the Upper Cretaceous (Michener and Grimaldi, 1988a, b) and the coevolution of bilateral flowers occurred at different stages in the evolution of several living families. In some angiosperm families, bilateral symmetry may be present in only a part of the family, while in other families the entire family, is characterized by bilateral symmetry. As discussed below, this must relate to the time at which different groups evolved in relation to these coevolutionary events.
Evolution of Small and Large Flowers
Flower size in living angiosperms is quite variable. Only during the past 25 years have numerous new fossil flowers been discovered from the Cretaceous. The record that has been developed demonstrates that both medium- and small-sized flowers are present very early. Certainly, flower size must relate to pollinator size. The variability in size of the early flowers suggests that a variety of pollinators were involved in their pollination biology (Grimaldi, 1999). In addition to insect pollinators, both wind and water were important in the pollination of early angiosperms. Because the wind and the water have changed very little since the Cretaceous, there has been little change in the floral anatomy and morphology of these plants. Therefore, they are examples of some of the most ancient lines of living flowering plants. Those angiosperms that have modified their pollination biology to accommodate new or different animal pollinators are plants that probably have undergone the most extensive changes and whose fossil ancestors should be the most different from their modern descendants. These would include bird and bat pollinated flowers.