| In recent years, several phylogenetic (cladistic) studies of both extant and extinct seed plants have suggested close evolutionary relationships between angiosperms and several major groups of gymnosperms. In these studies, angiosperms are linked together with Bennettitales, Pentoxylales, and Gnetales into a single, informally designated clade referred to as anthophytes. Three other gymnospermous orders are variously suggested as most closely related to the anthophyte clade, including Caytoniales, Corystospermales, and Glossopteridales. In the present investigation, pollen wall micromorphology and fine structure are examined from these six gymnosperm groups. The taxa studied include: in situ pollen of Cycadeoidea dacotensis (Bennettitales), in situ pollen of Sahnia laxiphora (Pentoxylales), dispersed, polyplicate ephedroid pollen grains (Gnetales), in situ pollen of Caytonanthus arberi (Caytoniales), permineralized, in situ Pteruchus-like pollen (Corystospermales), and dispersed, Protohaploxypinus-type grains (Glossopteridales). Pollen of C. dacotensis, S. laxiphora, and the dispersed gnetalean grains is nonsaccate, elliptic, and has a two-parted exine with a tectate-granular ultrasturcture. Pollen of C. arberi, the Pteruchus-like specimens, and the dispersed Protohaploxypinus-type grains is saccate with varying exine organization. As a result of comparative analyses, many palynological characters are elucidated, including several developmental features because in situ pollen grains are studied. Exine architecture from the six taxa examined is compared with other ultrastructurally known gymnosperm pollen, particularly with other anthophyte grains, and interpreted with regard to its systematic and phylogenetic relevance. |
