On Porostrobus zeilleri Nathorst and its spores with remarks on the systematic position of P. bennholdi Bode and the phylogeny of Densosporites Berry

Abstract

This paper contains results of a reinvestigation of Porostrobus zeilleri Nathorst., a fructification from the Lower Carboniferous of Spitzbergen and, based thereupon, remarks on the systematic position of P. bennholdi Bode and the phylogeny of Densusporites Berry. The cone studied represents the lower portion including a part of the stalk. The cone shows rhomboid sporangia arranged spirally, sporangial sporophylls not visible but fine delicate hairs, apparently sporophyllends, seen on the matrix along one of the sides of the cone. Megasporate and microsporate sporangia occur intermixed. Megaspores are sack-like, bottom heavy and gulate or vestibuled with verrucose ornamentation of the exine. Microspores are cingulate, roundly triangular with spinose ornamentation. Inner body has 3 inter-ray papillae. Porostrobus shows lepidodendroid as well as sigillarioid characters. Porostorbus bennholdi Bode is shown not to be a Porostrobus at all. It is provisionally referred to Lepidocarpon. The microspores of Porostrobus agree morphographically as well as stratigraphically to the genotype of the dispersed spore genus Densosporites Berry. In view of the fact that Densosporites also occurs in strata where Porostrobus has never been found, it is postulated that presently circumscribed Densosporites contains spores of cone genera other the Porostrobus as well. PS. Porostrobus is considered distinct from Selaginellites and S. canonbiensis Chaloner is advised inclusion in Bothrostrobus (Nath.) Sew.