Pollen Grains Found in Pollen Cones of Schidolepium Heer (Pinopsida) from the Middle Jurassic of East Siberia, Russia

Premise of research. Jurassic localities of fossil flora in the Irkutsk Region (East Siberia) are rich in reproductive remains of gymnosperms with in situ pollen. Our aim was to understand the morphology of pollen grains from conifer cones of Schidolepium gracile in the context of the botanical affinity of their parent plant. Methodology. Pollen grains were studied in transmitted light as well as with SEM and TEM. Pivotal results. The pollen grains demonstrate an unusual combination of morphological and ultrastructural traits. In pollen masses, they appear circular, asaccate, and flattened in a polar position. Detached monads show a polar and equatorial position equally often; the outlines are rounded, oval, and irregular. An equatorial-distal saccus was revealed. A small trilete scar is occasionally present. The surface is fossulate. The ectexine is formed by structural elements that fused with each other by their lateral surfaces partially or completely. The element is a solid elongated cylinder with a rounded external end and narrowing internal end/ends. The elements are arranged along their length, perpendicular to the pollen surface. The endexine is more electron dense than the ectexine, and it is prominent and appears homogeneous. We revealed variations in pollen morphology that we consider preservational, although the existence of two species of Schidolepium is not excluded. One of the cones was contaminated by pollen grains of the Cycadopites type; electron microscopical data proved their ginkgoalean affinity. Conclusions. The exine ultrastructure excludes an araucariaceous affinity, in spite of a relatively close general morphology. Certain similarities to Cerebropollenites were found in the exine ultrastructure, but it possesses a distal aperture and proximal saccus-like extensions. Although the pollen cones have something in common with the Taxodiaceae and Voltziales, the palynological data do not support this relationship. The plant apparently represents an early member of an evolutionary line within conifers.

Automated summary

The research focused on the morphology and ultrastructure of pollen grains from Schidolepium gracile's cones, revealing some unusual traits and suggesting the possible existence of two species within Schidolepium. The exine ultrastructure of the pollen grains excludes an affinity with the araucariaceous, despite some morphological similarities. Additionally, the presence of pollen grains contamination from Cycadopites suggests a ginkgoalean affinity, highlighting the complexity of the botanical relationships within the conifer family.