Discordant morphological disparity and taxonomic diversity during the Cretaceous angiosperm radiation: North American pollen record

The Cretaceous angiosperm radiation offers an opportunity to examine patter ns of morphological evolution in terrestrial plants and to compare them wit h patterns previously observed during radiations of marine animals. Focusin g on evolution among angiosperm pollen types, I used average pairwise dissi milarity and total variance to describe changing morphological varieties (d isparity) through the Cretaceous and Paleocene in North America. Angiosperm species diversity shows an approximately tenfold increase through this int erval, but this taxonomic diversification is not matched by a comparable ch ange in morphological disparity. Partitioning of morphological disparity am ong major pollen groups shows that most of the variance is contributed by e udicots 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 poll en morphological characters analyzed here. The two measures of disparity sh ow 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-averagi ng. The observed incongruence between taxonomic diversity and morphological disparity suggests that morphological evolution in pollen was characterize d by larger jumps early and smaller jumps later on, and is similar to that found in several groups of marine invertebrates.