The Ultrastructure and Botanical Affinity of the Problematic Mid-Mesozoic PalynomorphRicciisporites tuberculatusLundblad

Ricciisporites tuberculatusLundblad is a prominently sculptured palynomorph that is permanently united in tetrads. 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 its botanical affinity is unclear. Some authors have suggested thatR. tuberculatuswas produced by a seed plant, but others have suggested that this palynomorph was produced by a liverwort comparable toRiccia. In order to clarify the botanical affinity ofR. tuberculatus, we have analyzed its morphology using SEM and TEM. Individual grains within the tetrad are equipped with a single distal colpus, and the ultrastructure of the palynomorph wall is characterized by a granular inner sexine and an electron-dense laminated nexine. These morphological features allyR. tuberculatusto the gymnosperms. The combination of strongly ornamented monocolpate pollen grains permanently united in tetrahedral tetrads is unusual 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.