MORPHOLOGICAL DISPARITY OF DICOTYLEDONOUS ANGIOSPERM LEAVES IN THE CRETACEOUS AND PALEOGENE

Patterns of change in morphological diversity (“disparity”) offer insight into the evolutionary history of a clade that studies of taxonomic diversity cannot alone provide. While marine invertebrate paleontologists have discovered fascinating decouplings between taxonomic diversity and morphological disparity (for example, disparity frequently increases rapidly and peaks much sooner than taxonomic diversity, suggesting an early filling of the available morphospace), this approach has not yet been widely applied to fossil angiosperm leaf floras.

In order to understand the evolution of morphological diversity in dicot angiosperm leaves, fossil megafloras spanning the early Late Cretaceous through the early Miocene were scored for a set of characters previously demonstrated to capture diverse aspects of leaf morphology. Scored data were subjected to a principle components analysis (PCO), and the resulting axes were then used to calculate the mean pairwise distance (disparity) and sum of univariate ranges (approximate volume) of morphospace occupied through time. The floras examined demonstrated a lack of correspondence between changes in diversity and disparity: while diversity increased steadily through the Oligocene, disparity showed no steady pattern of increase and in fact decreased sharply between the Eocene and the Oligocene. For example, the early Oligocene Goshen flora, despite having a high taxonomic diversity compared to other floras in the study, had disparity values more similar to relatively depauperate Cretaceous floras than to either the Eocene or Miocene floras. This may be due to the tropical nature of the Goshen flora: a preliminary study of modern floras indicated a similar pattern of relatively low morphological disparity in high-diversity tropical floras as contrasted with lower diversity but more morphologically disparate temperate floras.

Expanding this study to include more floras from a wider range of environments will provide further insight into the morphological evolution of angiosperm leaves.