Occurrence of angiosperm-like ultrastructural features in gymnosperm pollen from the Permian of Russia

The morphology and ultrastructure of some dispersed pollen grains from the Permian of the Russian Platform were studied using light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Although being morphologically similar in LM (more or less circular in polar view with reticulate structure), these pollen grains demonstrate strong differences when studied under SEM and TEM, showing that they correspond to two different pollen taxa. The first one exhibits under SEM a coarser reticulum over the poles of the pollen grain and a finer peripheral reticulum. The ectexine includes a perforated tectum, spongy infratectum with rather regular short partitions, and a supposed foot layer. The thick inner layer (supposed endexine) appears nearly homogeneous, but in places lamellate structures are distinguishable suggesting that this layer was originally lamellate. Such pollen grains may be identified as Reticulatina microreticulata. The second pool of specimens was assigned to Samoilovitchisaccites turboreticulatus. The pollen grains of S. turboreticulatus demonstrate a continuous tectum completely covering the underlying exinal layers; under SEM these pollen grains appear nearly smooth. Differing in ectexine ultrastructure, they are similar to Reticulatina in the ultrastructure of the innermost layer. Although both taxa are of gymnospermous (pinopsid) affinity, the similarity between the surface of Reticulatina exine and that of Cretaceous angiosperm pollen is fascinating.