Pollen morphology and ultrastructure of the corystospermales: permineralized in situ grains from the triassic of Antarctica

Abstract

Corystosperms, represented by Dicroidium leaves and Pteruchus-like pollen organs, are major components of the Early-Middle Triassic silicified flora from the Fremouw Formation of Antarctica. The micromorphology and ultrastructure of the in situ pollen contained within these organs are described. Pollen sacs of varying ontogenetic ages have been isolated. Mature grains are monosulcate and bisaccate, with large, crescent-shaped eusacci. The exine is relatively thick in the cappa region and thins toward the distal sulcus; surface ornamentation is psilate. In medial positions of the proximal wall, the exine is homogeneous but becomes tectate-alveolate in more lateral regions of the cappa. The alveolar units extend into the sacci forming an endoreticulum; however, the endoreticulations are discontinous and only attach to the outer walls of the sacci. A wedge-shaped unit, where the sacci attach to the corpus, characterizes both the proximal and distal poles. The sulcus is broad, extends the entire width of each grain, and is longitudinally flanked by elevated lips. The structural features of these grains are discussed with respect to other fossil and extant saccate pollen. The grains are systematically compared with those of other bisaccate pollen-producing plants with which the Corystospermales have been suggested to be closely related, including Glossopteridales, Caytoniales, and angiosperms (Lactoridaceae). The permineralized in situ grains are also compared with other compressed Pteruchus species known at the ultrastructural level and with morphologically similar dispersed palynomorphs known from Antarctic sediments.