Marsileales and Salviniales – “Living fossils”?

According to current opinion (see, e.g., Christensen 1938, Reed 1954, Zimmermann 1959, Pichi-Sermollx 1959) there is no close relationship between the Marsileales and the Salvineales, so that they should not be united into one class as “Hydropteridales”. Their taxonomic position with regard to the Filices Leptosporangiatae, to which class they have up to now usually been referred, has always been unsatisfactory. Not only has a close relationship with any group of real ferns never been demonstrated, but their heterospory—to my mind an extremely fundamental character—and the absence of a true annulus distinguish them sharply from the Leptosporangiate ferns. The resemblance between certain fossil “seeds”, later identified as fructifications of the Caytoniales (Thomas 1925, 1927), and the sporocarps of Marsilea has in the past lead to the interpretation of these Caytonialian fructifications and the associated Sagenopteris leaves as remains of Marsileaceae. When Thomas recognised these remains as those of advanced Pteridosperms, this interpretation was abandoned. However, Zimmermann (1930) again suggested that Marsileales might be descendants of Caytonialian stock in the same way as Isoëtes IS a dwarfed “survivor” of the Lepidodendrales. Thomas and Harris (1951) critisized this idea and mentioned several differences between the two groups under discussion. Indeed it is not very likely that Marsilea, which is undoubtedly more primitive than the Caytoniales in several respects, could have descended from the latter. Among other things, the Caytoniales produced unisexual strobili and seed-like fructifications, whereas Marsilea has bisexual sporocarps which are shed before fertilisation takes place and do not produce “seeds”, the embryo developing at once into a young plant.