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Evolution of Floral Presentation

In flowers that are insect pollinated, the display of the flower is critical. There seem to be clear distinctions between the presentation of the large Archaeanthus flower and the small fossil dichasial (Dilcher and Muller, 2000) flowers. The large Archaeanthus flower appears to have been terminal on a moderately large axis similar to the flowers of Liriodendron or Magnolia today. This allows for sturdy support and a colorful display to attract a pollinator. The dichasial flower, in contrast, is small and clustered into an umbel-like arrangement. This allows for a showy display of flowers in different stages of maturity and a broad area of clustered flowers upon which a pollinator can land and move about. However, small unisexual florets such as those of wind pollinated platanoid-like inflorescences and water pollinated ceratophylloid-like plants have been little affected by animal pollinators. For this reason, they persist today only slightly changed from their forms in the Early Cretaceous.

Unisexual vs. Bisexual Flowers

The earliest flowers now known appear to be gynodioecious. One axis has only carpels with a clear indication that no other organs subtended them, while an attached axis has both carpels and stamens (Sun et al., 2000). So, was the first flower unisexual or bisexual? It appears to have had the potential to be both. Some early flowers, such as the platanoids and ceratophylloids, appear to be unisexual and never to have had a bisexual ancestry. Others such as Archaefructus, many of the small flowers from Portugal and the larger flowers from the Dakota Formation, are certainly bisexual. I suggest that the ancestral lineage of the angiosperms was most likely unisexual, and that with the availability of insect pollinators the efficiency of bisexual flowers won the day.

WHAT ARE THE SIGNIFICANT NODES OF ANGIOSPERM EVOLUTION?

There are three major nodes or events through time that resulted in major radiations of the angiosperms. These nodes include the evolution of showy flowers with a closed carpel, the evolution of bilateral flowers, and the evolution of nuts and fleshy fruits. At each of these events, there is a burst of adaptive radiation within the angiosperms that can be interpreted as an attempt to maximize the event for all of the diversity possible and to use the event for increased reproductive potential.

The evolution of the closed carpel and the evolution of the showy radial flower must have occurred at nearly the same time. This was the first



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