A second genus Regnellidium includes a single living species that grows only in southern Brazil[3] and neighboring parts of Argentina;[2] it has only two leaflets per leaf.

[4][5] However, both of these other fern families float freely on the surface of ponds or lakes instead of rooting in soil or mud.

The close relationship of these groups to the Marsileaceae is supported by both morphologic and molecular analysis,[4] as well as by the discovery of an intermediate fossil named Hydropteris.

[7] Until recently, Rodeites dakshinii was the oldest fossil member known; it is a preserved sporocarp containing spores, found in Tertiary chert of India.

[8] These fossils were assigned to the species Regnellidium upatoiensis, and pushed the known history of the Marsileaceae back into the Mesozoic.

The currently oldest known member of the family is Flabellariopteris, described in 2014 from isolated leaves dating to the Late Triassic in Liaoning, China.

[11] The leaves are the most easily observed characteristic for the Marsileaceae; they have a long slender leaf stalk ending in zero, two, or four (occasionally six) leaflets.

These plants also produce other leaves with shorter leaf stalks that are not long enough to reach the surface, and so the leaflets remain underwater.

This outer covering is tough and resistant to drying out, allowing the spores inside to survive unfavorable conditions such as winter frost or summer desiccation.

Despite this toughness, the sporocarps will open readily in water if conditions are favorable, and specimens have been successfully germinated after being stored for more than 130 years.

[13] Each growing season, only one sporocarp typically develops per node along the rhizome near the base of the other leaf-stalks, though in some species of Marsilea there may be two or occasionally as many as twenty.

Inside the sporocarp, the modified leaflets bear several sori, each of which consists of several sporangia covered by a thin hood of tissue (the indusium).

As this internal tissue swells with water, it pushes the halves of the hard outer covering apart, and emerges as a long gelatinous worm-like sorophore.

The sorophore is a sorus-bearing structure unique to the Marsileaceae; it may extend to more than ten times the length of the sporocarp inside which it was coiled.

However, the sporocarps contain toxic levels of thiaminase, so careful preparation methods must be used in order for the nardoo to be safe for consumption.