Pollen of the Triassic cycad Delemaya spinulosa and implications on cycad evolution

The Cycadales are an order of gymnosperms that represent one of the oldest lineages of seed plants. Cycads are thought to have originated in the Carboniferous, and subsequently diversified and geographically expanded throughout the Mesozoic. Despite the geologic diversity of the group, the evolutionary history of cycads remains unresolved. To a large degree this is because the leaves of cycads and various other fossil groups (e.g., pteridosperms and Bennettitales) are morphologically similar. To date there are relatively few fossil cycad reproductive structures. Several are known from the Permian of China, but these compression specimens provide little detailed information useful in tracing the evolution of cycad cone morphology and anatomy. A permineralized cycad pollen cone from the Triassic of Antarctica, Delemaya spinulosa, contains in situ pollen. The objective of the current study is to further elaborate the structure of the pollen grains in this species, with particular emphasis directed at the structure and organization of the pollen wall. The elliptical shape, monosulcate aperture, and small size of the pollen grains are like those of extant Cycadales. The exine, although originally described as homogenous, appears to possess an alveolar organization. Pollen and cone features are compared to those in extant Cycadales.