Diversity of the dispersed Gnetalean pollen record from the Lower Cretaceous Crato Formation, Brazil: Entomophily, harmomegathy and habitat heterogeneity

Abstract

Despite that there are only three morphologically isolated genera of Gnetales (gymnosperm) plants surviving today, they are thought to have had a major peak in diversity during the Early Cretaceous, with the Crato Formation being one of the richest sources for these plants. The reinvestigation of a Crato Formation palynoflora sample with LM and SEM here gives further insights in to the wider diversity of fossil gnetaleans present, and their pollination types, which is not preserved in the corresponding macroflora. Here we present 21 different pollen types of gnetaleans from a single sample of Prof. David J. Batten's legacy: 11 Ephedripites and seven Gnetaceaepollenites taxa, one Welwitchiapites taxon and Regalipollenites amphoriformis. We suggest that this sample, originating from the lowermost Crato Formation, corresponds to Lima's palynozone 1a. The high resolution scanning electron microscopy reveals several types of conspicuous ornamentation of the pollen wall, i.e., implying a more insect-pollinated rather than wind-pollinated (i.e., mixed-ambophilous) pollination syndrome for many of these gnetalean plants. The ornamentation also provides clear indications for harmomegathic (shrinking and deshrinking) behaviour of many of the pollen grains as a likely adaptation to drier climate conditions. The pollen described here shows a far greater diversity of species and morphologies than seen today and in the gnetalean macroremains present in the Crato Formation flora, re-emphasising the importance of the Crato Formation as a source for both the macro-, and importantly, the micro-fossil diversity of gnetalean plants, not matched elsewhere.