THE ULTRASTRUCTURE AND BOTANICAL AFFINITY OF THE PROBLEMATIC MID-MESOZOIC PALYNOMORPH RICCIISPORITES TUBERCULATUS LUNDBLAD

Ricciisporites tuberculatus Lundblad is a prominently sculptured palynomorph that is permanently united in tetrads. R. tuberculatus first appears in the Norian and ranges into rocks of Hettangian age. This palynomorph has a wide geographic distribution and reaches a stratigraphically important acme in the Late Triassic of Europe, Greenland and Arctic Canada. This palynomorph has not yet been found in situ in a fossilized reproductive structure and consequently the botanical affinity of R. tuberculatus is unclear. Some authors have suggested that R. tuberculatus was produced by a seed plant, but others have suggested that this palynomorph was produced by a liverwort comparable to Riccia. This uncertainty hampers attempts to reconstruct vegetation history during the Late Triassic, which was a time of major environmental and biotic change. In order to clarify the botanical affinity of R. tuberculatus we have analysed the morphology of this palynomorph using scanning electron microscopy and transmission electron microscopy. Individual grains within the R. tuberculatus tetrad are equipped with single distal colpus, and the ultrastructure of the palynomorph wall is characterised by a granular inner sexine. These morphological features ally R. tuberculatus to the gymnosperms and there are some ultrastructural similarities between R. tuberculatus and pollen produced by the bennettite Cycadeoidea dacotensis. The combination of strongly ornamented monocolpate pollen grains permanently united in tetrads is unusual and distinctive, and to the best of our knowledge has not been reported previously in a gymnosperm. This may represent an extinct strategy to promote the fertilization of several archegonia in an ovule leading to simple polyembryony.