Mesofossils of an unrevealed affinity from the Jurassic of siberia

Enigmatic mesofossils were obtained via maceration of a microsporangium detached from a ginkgoalean pollen cone of Sorosaccus sibiricus Prynada from the Aalenian Ust’-Baley locality in Irkutsk Coal Basin, Siberia. The organic-walled remains were constituted by rounded oval bodies of sporopollenin-like colour, overlapped with their margins and arranged in several layers. The bodies had jointed continuous walls of variable thickness, which bifurcated, fused and formed inseparable structures. LM, SEM and TEM observations showed that the find cannot be in situ ginkgoalean pollen or other remains of a ginkgoalean plant. The further comparison also excluded the possibility that these mesofossils were alien pollen or spores, trapped in an open sporangium or fossilized in its close vicinity. Some slight ultrastructural similarities were only revealed to cryptospores, which led the search to algae and bryophytes. However, although fresh-water colonial algae and bryophytes theoretically could have been present in these lacustrine deposits, they differ from the mesofossils by smaller cells, which are grouped differently and show dissimilar outlines and wall ultrastructure. There is a possibility that the find represents wall fragments of some resting stage of an unknown organism. So far, no close analogues to these mesofossils have been found.