EXINE ULTRASTRUCTURE OF POLLEN GRAINS FROM DELEMAYA SPINULOSA, A STRUCTURALLY PRESERVED CYCAD POLLEN CONE FROM THE TRIASSIC OF ANTARCTICA

The Cycadales are a small gymnospermous group that represent one of the most ancient lineages of seed plants. They are thought to have originated from Paleozoic pteridosperms and reached the height of their diversity and distribution in the Mesozoic. Historically, vegetative fossils have been difficult to identify as Cycadalean due to morphological similarities with other distantly related groups (e.g., pteridosperms and Bennettitales). Because of this, little is known about the evolutionary history of these plants. A few fossil cycad reproductive structures have been described from China, but until recently, no fossil cones could provide details of internal organization. Delemaya spinulosa, from the Triassica of Antarctica, represents the first structurally preserved cycad pollen cone. The internal anatomy of the cone as well as the overall morphology of the pollen have been described. The objective of this research was to comprehensively study pollen ultrastructure in D. spinulosa using combined light, scanning electron, and transmission electron microscopy. Pollen grains are ellipsoidal and longitudinally monosulcate. The surface of the grains is psilate, although some pits have been observed. The exine is tectate and exhibits the alveolar structure typical of modern cycads, although no evidence of a lamellate endexine has been detected. These ultrastructural characters will be discussed regarding phylogenetic interpreations of both fossil and modern Cycadales.