Wikipedia
The order Salviniales (formerly known as the Hydropteridales and including the former Marsileales) is an order of ferns in the class Polypodiopsida. Salviniales are all aquatic and differ from all other ferns in being heterosporous, meaning that they produce two different types of spores (megaspores and microspores) that develop into two different types of gametophytes (female and male gametophytes, respectively), and in that their gametophytes are endosporic, meaning that they never grow outside the spore wall and cannot become larger than the spores that produced them. In being heterosporus with endosporic gametophytes they are more similar to seed plants than to other ferns.
The ferns of this order vary radically in form from one another and do not look particularly fern-like. Species of the family Salviniaceae are natant (floating), while those of the family Marsileaceae are rooted. However, the natant species may temporarily grow on wet mud during times of low water, and the Marsileaceae may grow as emergent species, depending on species and location.
There is a well-known fossil member of the Marsileales, Hydropteris (incertae sedis).
The likely phylogenic relationships between the two families and five genera of the Salviniales are shown in the following diagram.