An ultrastructural study of the araucarian pollen grain Cyclusphaera radiata Archangelsky from the Albian of Patagonia

Abstract

The araucarian pollen grain Cyclusphaera radiata Archangelsky is emended based on newly discovered materialfrom the Piedra Clavada Formation of Albian age in Santa Cruz province, Argentina. The general morphologyand ultrastructure of the pollen were studied by means of transmitted light, scanning, and transmissionelectron microscopy. C. radiata is discoid with an equatorial thickening and two large opposite apertures,with radial thickenings in one of them and two central apertural occlusive opercula-like structures. Beneaththere is a thick amorphous electron-dense nexine. The combination of characters seen in this pollen type isvery unusual, and no parallels are known in fossil or extant gymnosperm grains. Functional interpretationof the apertural exine suggests that the radial thickenings and apertural membranes were involved in harmomegthy,the opercula-like structures in germination, and the apertural nexine perhaps as a strengtheninglayer. A hypothetical pollination mechanism is also presented for C. radiata. The paleobiogeography of thistaxon shows that during the Mesozoic, the plants that produced these grains inhabited an extensive areaof Gondwana that included South America, Africa, India and Antarctica.