The lines of evolution of the Bacillariophyta. I. Origin

Abstract

The evolution of the diatoms has never been considered in any detail and this paper sets out some of the problems and discusses briefly the fossil evidence, the likelihood of monophyletic versus polyphyletic origin and the possible habitats of the ancestral forms. We assume that diatoms arose at a fairly early period and certainly long before the Cretaceous, when the first fossil diatoms were deposited. A ‘pre-diatom’ stage is envisaged during which a naked photosynthetic cell acquired a coating of siliceous scales. This was followed by an ‘Ur-diatom’ stage when the siliceous scales differentiated to form valves and girdle bands, to give a recognizable but extremely simple diatom. Evidence for such development is indirect and we discuss this from the standpoint of the structure of scales of modern diatom auxospores, symmetry of initial valves, homology of valves and girdle bands, comparative morphology of auxospore scales/valves/girdle bands and the developmental sequences of modern diatom valve formation. From a discussion of the available data and leaning heavily on electron microscope studies we suggest that there is very considerable evidence to substantiate our hypothesis.