Pollen grains associated with Karkenia irkutensis Nosova (Ginkgoales) from the Jurassic of Siberia

Abstract

Pollen grains found on the nucellar cuticle and the outer cuticle of the integument of Karkenia irkutensis ovules from the Middle Jurassic of the Irkutsk Coal Basin in Siberia were studied with light and transmission electron microscopies. The overwhelming majority of the pollen grains are non-saccate, boat-shaped and monosulcate and can be ascribed to Cycadopites, which fits to what we expected to find in association with ginkgoalean macrofossils. In addition, several non-saccate and bisaccate pollen grains were also detected and interpreted as alien pollen of wind-pollinated conifers. The Cycadopites pollen grains showed from moderate to poor preservation of the exine ultrastructure, that restricted us from decisive conclusions. Differences in the exine ultrastructure among the sectioned Cycadopites pollen grains are probably due to the insufficient preservation and different angles under which the pollen grains were sectioned, and the Cycadopites pollen most probably derived from the same plant. The ectexine is more electron-dense and much thicker than the endexine, which is not always preserved. The ectexine is predominantly homogeneous, but rare cavities of irregular size and shape (=alveoli) are present in a row in the deep layers of the ectexine. Towards the aperture, the ectexine becomes thinner and can acquire a crenulate outer contour (that means a more distinct surface sculpturing). The exine is usually broken over the aperture. This set of characters is more similar to Ginkgo-type of the exine ultrastructure than to Aegianthus- and Sorosaccus sp.-types, which we defined for ginkgoaleans.