most cases the animals benefited as well from this coevolutionary association. Wind and water pollination syndromes also allowed for outcrossing and have continued to exist since the Early Cretaceous. However, they have never developed the diversity of those angiosperms pollinated by animals. Also several abiotically pollinated angiosperms, for example the Fagaceae (Quercus or oaks) and the Juglandaceae (Carya or pecans), later accommodated themselves for animal dispersal of their fruits or seeds. The importance of outcrossing cannot be underestimated as a driving force in the evolution of the angiosperms (Dilcher, 1995, 1996).
The ability of the angiosperms to accommodate and maximize benefits from animal behavior has been responsible for the evolutionary success of the group. As individual clades made use of particular coevolutionary strategies the diversity of both the angiosperms and animal groups increased. The benefits to the angiosperms were the benefits of the genetics of outcrossing. Because this is a sexual process, it was accomplished by means of evolutionary changes to flowers and fruits and seeds. This is why these particular organs have been centers of angiosperm evolution and why they are so useful in angiosperm systematics today.
I acknowledge with thanks the help of Terry Lott and Katherine Dilcher in the preparation of this manuscript. Thanks to Peter Raven who read and commented on this paper and also to the many students and J. William Schopf and colleagues who shared perspectives of angiosperm evolution with me. I thank the organizers of the symposium at which this paper was presented: Francisco Ayala, Walter Fitch, and Michael Clegg.
Axelrod, D. I. ( 1966) The Eocene Copper Basin Flora of Northeastern Nevada. Univ. Calif. Pub. Geol. Sci. 59, 1–83.
Bandulska, H. ( 1924) On the cuticles of some recent and fossil Fagaceae. Linn. Soc. Bot. (London) 46, 427–441.
Basinger, J. F. & Dilcher, D. L. ( 1984) Ancient bisexual flowers. Science 224, 511–513.
Bell, W. A. ( 1957) Flora of the Upper Cretaceous Nanaimo Group of Vancouver Island, British Columbia. Geol. Surv. Canada Memoir 293, 1–84.
Berry, E. W. ( 1924) The Middle and Upper Eocene Floras of Southeastern North America. U.S. Geol. Sur. Prof. Paper 92, 1–206.
Brown, R. W. ( 1962) Paleocene Flora of the Rocky Mountains and Great Plains. U.S. Geol. Surv. Prof. Paper 375, 1–119.
Call, V. & Dilcher, D. L. ( 1992) Early angiosperm reproduction: survey of wind-dispersed disseminules from the Cretaceous-Paleogene of North America. In Organisation Internationale de Paleobotanique 4eme Conference (Organisation Francaise de Paleobotanique Information N: Special 16B, Paris), p. 36.
Crane, P. R. ( 1985) Phylogenetic analysis of seed plants and the origin of angiosperms Ann. Missouri Bot. Gard. 72, 716–793.
Crepet, W. L. ( 1974) Investigations of North American cycadeoids: the reproductive biology of Cycadeoidea. Paleontographica B 148, 144–169.