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Abstract: A scanning and transmission electron microscopy-based ultrastructural study of Arcellites humilisVillar de Seoane and Archangelsky, 2008, from the Kachaike Formation and Piedra Clavada Formation (Albian-Cenomanian) from several localities in Patagonia (Argentina), is presented in this paper. TEM-photographs of thin-sectioned specimens show a thick spore body wall composed of three layers: a thin and dense exine, an inner granular perine sublayer with irregularly disposed small channels and lacunae, and an outer alveolate perine sublayer with channels perpendicular to the external surface. Transverse sections of the A. humilis wall are compared with fossil and extant species of Marsileaceae. We show that the wall ultrastructure is similar in A. humilis, A. santacrucensis, A. disciformis, A. stellatus and Regnellidium upatoiensis. However, the body wall and acrolamella of A. humilis are more similar to those of R. diphyllum Lindman than to those of the Marsilea L. or Pilularia L. species. Water ferns such as members of the Marsileaceae played an important role in aquatic or semi-aquatic niches in Patagonian Cretaceous floras, suggesting that high humidity and temperature prevailed during the Albian-Cenomanian in this region of Argentina. [Copyright &y& Elsevier]

Abstract: The araucarian pollen grain Cyclusphaera radiata Archangelsky is emended based on newly discovered material from the Piedra Clavada Formation of Albian age in Santa Cruz province, Argentina. The general morphology and ultrastructure of the pollen were studied by means of transmitted light, scanning, and transmission electron microscopy. C. radiata is discoid with an equatorial thickening and two large opposite apertures, with radial thickenings in one of them and two central apertural occlusive opercula-like structures. Beneath there is a thick amorphous electron-dense nexine. The combination of characters seen in this pollen type is very unusual, and no parallels are known in fossil or extant gymnosperm grains. Functional interpretation of the apertural exine suggests that the radial thickenings and apertural membranes were involved in harmomegthy, the opercula-like structures in germination, and the apertural nexine perhaps as a strengthening layer. A hypothetical pollination mechanism is also presented for C. radiata. The paleobiogeography of this taxon shows that during the Mesozoic, the plants that produced these grains inhabited an extensive area of Gondwana that included South America, Africa, India and Antarctica. [Copyright &y& Elsevier]

Three new species of the cycadean pollen cone Androstrobus, viz. A. munku, A. patagonicus, and A. rayen, are described. All specimens are represented by microsprophylls or fragmentary cone pieces, and they have been found in the same fossil bed at the locality Bajo Grande, in Santa Cruz province, Argentina, which is referred to the Anfiteatro de Ticó Formation of Aptian age. This is the first record of the genus in Argentina and South America. The good preservation of the material allowed the study of cuticles and pollen with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is noted that Androstrobus is a poorly defined genus that needs a revision of the type material as well as new specimens from the type locality to certify the presence of pollen. Comparisons with other Androstrobus species are made, especially with those that have been described with cuticular characters and structure of pollen. Attention is paid to the similarity of the alveolate pollen ultrastructure of the Patagonian fossils with some extant cycads, especially with genera of the Zamiaceae and Cycadaceae. The variety of pollen cones found in the same bed agrees with the presence of different vegetative organs, leaves, and fronds found previously at the same place, and that were referred to the cycads on the base of their cuticular structure. The abundance of cycads during the mid-Cretaceous in southern Argentina is a contrasting evidence with the present-day absence of the group in southern South America. [Copyright &y& Elsevier]

The leaf cuticle of the Ginkgoites ticoensis Archang. type material from the Aptian Anfiteatro de Ticó Formation in Patagonia, Argentina, is fully characterized with additional scanning and transmission electron microscopic observations. Many new anatomical and ultrastructural cuticular features are identified in the four-lobed leaf of G. ticoensis: the leaf shows a hypostomatic and papillate laminae, straight and pitted anticlinal and granulate periclinal walls, actinocytic stomata with between five and seven papillate, striate subsidiary cells, and guard cells with anticlinal smooth walls. The TEM studies on ordinary epidermal cells, papillae, subsidiary cells, and guard cells reveal general ultrastructural features of Ginkgoaceae: an outer polylamellate layer A made with A1 and a granular inner layer A2; A1 with an upper part A1U with continuous and straight translucent lamellae; a lower part A1L with significantly disrupted and waving translucent lamellae; and the fibrillar cuticular layer B1 as the innermost part. Ten ultrastructural characters are detailed and ranked by the use of confidence intervals based on 30 statistical measurements. A three-dimensional reconstruction of the cuticle is also provided. Because of the anatomical and ultrastructural fine details shown in the G. ticoensis cuticle, new elements are given to suggest its probable family affinity and to enhance the specificities of Ginkgo and Ginkgoites. [ABSTRACT FROM AUTHOR]