On the exine ultrastructure of fossil ginkgoaleans: In situ pollen of Sorosaccus Harris

The morphology and exine ultrastructure have been studied of pollen grains from pollen cones of Sorosaccus sibiricus Prynada from the Ust'-Baley (Aalenian, Siberia), Sorosaccus sp. from the Vladimirovka (Aalenian-Bajocian, Siberia), and S. ex gr. sibiricus Prynada from the Tyrma (Tithonian-Berriasian, Russian Far East) locali-ties. The pollen grains are boat-shaped and monosulcate. A finely granulate pattern is discernible under high magnification of light microscope in pollen grains from Vladimirovka and Tyrma; SEM shows that this pattern is formed by flat verrucae. TEM shows that sculptural elements are present on the surface of pollen grains from Ust'-Baley, although it is not evident in LM and SEM. By previous data on modern and fossil members, we believed that ginkgoalean pollen grains can be differentiated from similar boat-shaped monosulcate pollen of other affinities by a ratio of ectexinal sublayers (a thick homogeneous tectum, a thin infratectum of one row of structural elements, and a thin foot layer) in combination with an ectexine that is greatly reduced in the aperture region. Freshly obtained data have revealed another set of characters, with a less prominent tectum by compar-ison to underlying sublayers, an infratectum with small granules within the alveoli, and a prominent verrucate surface pattern that is distinguishable even in transmitted light. Ginkgoaleans are characterized by more than one set of ultrastructural characters of the exine.(c) 2023 Elsevier B.V. All rights reserved.

Automated summary

This study investigated the morphology and exine ultrastructure of pollen grains from Sorosaccus sibiricus, Sorosaccus sp., and S. ex gr. sibiricus. The pollen grains were boat-shaped and monosulcate, with a finely granulate pattern on the surface. Scanning electron microscopy revealed flat verrucae, while transmission electron microscopy showed sculptural elements on the surface. Ginkgoalean pollen grains can be differentiated from similar boat-shaped monosulcate pollen by a combination of ectexinal sublayers and a greatly reduced ectexine in the aperture region.