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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]

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]

New specimens of Ginkgoites skottsbergii Lundblad collected from the Albian Kachaike Formation at the type locality in Santa Cruz Province, Argentina are characterized with additional scanning and transmission electron microscopic observations and EDS elemental analysis. The micromorphology of the amphistomatic and 12-segmented leaf of G. skottsbergii shows a smooth lamina with straight and pitted anticlinal and slightly granulate periclinal walls, abundant epicuticular wax on the petiole surface, and monocyclic to partially dicyclic stomatal apparatuses with between six and eight papillate to non-papillate, finely striated subsidiary cells and guard cells with smooth walls. Palaeoenvironmental considerations suggest that G. skottsbergii was a mesophytic plant. The TEM studies of ordinary epidermal cell cuticles and subsidiary and guard cell cuticles reveal the general ultrastructural features of Ginkgoaceae. Using confidence intervals based on 30 statistical measurements and the Mann–Whitney test for five measurements of EDS ratios of the elements, 33 ultrastructural characters were identified that enabled discussion at different taxonomic levels. Comparisons with the previously studied Ginkgoites ticoensis Archangelsky reveal that 14 of these characteristics could be species features that reinforce the identity of G. skottsbergii , and 19 could be genus features. EDS analysis and comparisons with G. ticoensis yielded five ratios using Cl, N, K, S, and Ca, revealing seven ratios of potential species interest and eight ratios at the genus level among the three zones evaluated in the leaf cuticle of each species. A three-dimensional reconstruction of the cuticle and an ultrastructural identification key for each of the three types of cuticle are also provided, i.e., from the ordinary epidermal cell and the subsidiary and guard cells of the stomatal apparatus. Fruitful statistical comparisons with two previously studied ginkgos allow discussion of the relationships within the Ginkgoaceae with respect to 18 features, six of them being potentially significant at the genus level and four at the family level. [ABSTRACT FROM AUTHOR]