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Leaf architectural profiles of angiosperm floras across the Cretaceous/Tertiary boundary

Yale University, Department of Geology and Geophysics, P. O. Box 208109, New Haven, Connecticut 06520-8109; walton.green{at}yale.edu

The Cretaceous/Tertiary boundary extinction has long been considered one of the most important identifiable events in the course of Phanerozoic evolution. At times, the dramatic evidence for this has obscured the fact that any extinction event is selective and may not affect all groups of organisms in the same way. In this paper we examine a North American plant fossil database from the Mesozoic and Cenozoic eras in order to re-evaluate the evolutionary significance of the Cretaceous/Tertiary extinction on plants. When we compare the leaf architectural profiles of fossil floras in each stage of the Cretaceous and epoch of the Cenozoic, we find that the changes in leaf architecture at the Maastrichtian/Paleocene boundary cannot be statistically distinguished from the population of changes at other boundaries. To the extent that patterns in leaf architecture reflect ecosystem structure, we can therefore conclude that despite the local species or morphotype extinctions that are known to have taken place at the boundary, the effect of the extinction on the structure of plant ecosystems was either minor or short-lived. Certainly, the extinction seems insignificant compared with the dramatic changes in leaf architecture that accompanied the rise of angiosperms in the middle Cretaceous. This analysis also provides an example of the importance of time scales in the evaluation of macro-evolutionary pattern, and shows how the use of morphological categories instead of phylogenetic groups or simple diversity measures can produce rich and ecologically informative semi-quantitative proxy measurements of plant evolutionary patterns.