The ultrastructure of fossil ephedroid pollen with gnetalean affinities from the Lower Cretaceous of Brazil

Abstract

The micromorphology and fine structure of several dispersed polyplicate, “ephedroid” palynomorphs, originally assigned to Equisetosporites spp., are described from the Lower Cretaceous (Aptian-Albian) Santana Formation of northeastern Brazil. Pollen grains are elliptic in shape and have a variable number of longitudinally oriented plicae, but lack a distinct aperture. Plicae do not reach the grain tips and are psilate to faintly scabrate in surface ornamentation. The exine is distinctly two-parted with the sexine typically staining more lightly than the nexine. The sexine is on average 1.5 times thicker than the nexine, and has a tectate granular fine structure with the granular infrastructural layer having a graded organization. Small granules characterize the lower region of the infratectum and are directly contiguous with nexine; these gradually grade into larger granules that appear to fuse with a thick, homogeneous tectum. The nexine is thick and lamellate throughout. Both sexine and nexine sporoderm components are markedly thinner between the plicae (i.e. within the wall furrows), with the granular infratectum laterally thinning until completely absent. An additional wall layer, external to the tectum, is also present within sporoderm furrows; both the tectum and this layer typically occur in a folded, hinge-like fashion in the furrow. This unique feature as well as several other structural characters of the sporoderm are discussed with regard to their possible functional significance. Exine architecture is also compared with that of the only other three fossil polyplicate palynomorphs known at the ultrastructural level ( Ephedripites sp., Equisetosporites chinleana, and Cornetipollis reticulata) as well as with that of the pollen of the extant Gnetales ( Ephedra, Welwitschia, and Gnetum). By comparison, the Brazilian grains are most similar to Ephedripites using fine structural features.