Diversity of exine structure in Upper Carboniferous (Westphalian) selaginellalean megaspores

Studies of wall structure in Mesozoic and Recent selaginellalean megaspores have been well documented. However, Palaeozoic examples have received mini mal attention. The principal Palaeozoic megaspore genus of likely selaginel lalean affinity is Triangulatisporites, extending from the Upper Devonian t o the Upper Carboniferous. The particulate wall ultrastructure of a previou sly published Carboniferous (Duckmantian) megaspore assigned to this genus suggested that this form of wall construction may have been the ancestral w all structure of the group, an observation which posed difficulties in rela ting selaginellalean ultrastructure to that of other contemporaneous lycops id megaspores, Subsequent investigation showed that the genus also contains more laminate exines similar to those of other extinct lycopsids and extan t Selaginella species. Our new examples of Triangulatisporites ultrastructu re from the Langsettian, Duckmantian and Westphalian D yield more informati on regarding early variation of wall structure within Carboniferous selagin ellalean megaspores and suggest that a more laminate wall composition is at least as old as the particulate form. However, without further investigati on of Lower Carboniferous forms, we are unable to state which is indeed anc estral. The laminate structure reported here and elsewhere is, none the les s, more easily related to comparable ultrastructure in other groups of Carb oniferous lycopsid megaspores and could suggest a link with such genera as Zonalesporites and early Lagenicula. This would be in keeping with current concepts regarding the most primitive ultrastructural type within lycopsid megaspore walls. (C) 2000 Elsevier Science B.V. All rights reserved.