DISCORDANT MORPHOLOGICAL DISPARITY AND TAXONOMIC DIVERSITY DURING THE CRETACEOUS ANGIOSPERM RADIATION : NORTH AMERICAN POLLEN RECORD

Abstract

The Cretaceous angiosperm radiation offers an opportunity to examine patterns of morphological evolution in terrestrial plants and to compare them with patterns previously observed during radiations of marine animals. Focusing on evolution among angiosperm pollen types, I used average pairwise dissimilarity and total variance to describe changing morphological varieties (disparity) through the Cretaceous and Paleocene in North America. Angiosperm species diversity shows an approximately tenfold increase through this interval, but this taxonomic diversification is not matched by a comparable change in morphological disparity. Partitioning of morphological disparity among major pollen groups shows that most of the variance is contributed by eudicots from the Albian onwards. Constant disparity across the Cretaceous/Tertiary boundary despite decreased taxonomic diversity suggests that the Cretaceous/Tertiary extinction was not selective with respect to the pollen morphological characters analyzed here. The two measures of disparity show similar patterns. The overall pattern is robust to changes in character weighting, indicating that no one set of characters or weighting scheme is driving the pattern. Analyses of older data indicate that the initial burst of disparity in the Aptian could be due in part to analytical time-averaging. The observed incongruence between taxonomic diversity and morphological disparity suggests that morphological evolution in pollen was characterized by larger jumps early and smaller jumps later on, and is similar to that found in several groups of marine invertebrates.