The fine morphology of pollen grains from the pollen chamber of a supposed ginkgoalean seed from the Middle Jurassic of Uzbekistan (Angren locality)

Abstract

Pollen grains of Cycadopites-type were found in the pollen chamber of a supposed ginkgoalean seed Allicospermum sp. from the Middle Jurassic deposits of Uzbekistan (Angren locality). The pollen grains were studied with help of LM, CLSM, SEM, and TEM. All pollen grains show the identical morphology and exine ultrastructure allowing us to suppose the same botanical affinity. The pollen morphological data do not contradict the ginkgoalean interpretation of the seed; therefore, the pollen grains and the seed most probably did belong to the same parent plant. The pollen grains are monosulcate, the non-apertural surface is nearly psilate, with low short elements, which are occasionally scattered over the surface or more densely distributed. The aperture and adjacent areas appear to bear more distinct sculpturing. The ectexine is composed of a prominent solid tectum, a thin infratectum, and a thin foot layer. The infratectum is formed of one row of alveolae, which are more voluminous laterally, where the ultrastructure is more easily understandable. The endexine is multilamellate, although it is evident only in some regions of stained sections. Towards the aperture the ectexine becomes gradually thinner; over the aperture no sublayers can be discerned within the ectexine. The ectexine of the apertural region repeatedly varies in thickness, reflecting a sculpturing surface of this region. The obtained data contribute to the knowledge about the exine ultrastructure of ginkgoaleans; nonetheless, a TEM study of ginkgoalean pollen grains extracted from pollen organs is still highly desirable. We also considered pluses and minuses of CLSM: it failed to substitute SEM, since the surface pattern under study was too fine, but demonstrated the general morphology of the pollen grains under study better than conventional LM. The possibility of viewing virtual sections of any area of the pollen grain was profitable for later interpretation of TEM sections. CLSM would give better results in interpreting relatively large palynological objects with distinct sculptural elements, a complicated architecture, variously arranged appendages, or possessing cameras.