 The closest relatives of the Marsileaceae are the <unk> , which are also aquatic and heterosporous . 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 , as well as by the discovery of an intermediate fossil named <unk> . In general , the <unk> and <unk> have a much better fossil record than the Marsileaceae . Until recently , <unk> <unk> was the oldest fossil member known ; it is a preserved sporocarp containing spores , found in Tertiary chert of India . In 2000 , the discovery of fossilized sporocarps from the Cretaceous of eastern North America was announced . These fossils were assigned to the species Regnellidium <unk> , and pushed the known history of the Marsileaceae back into the Mesozoic . The oldest fossil known for the Marsileaceae is <unk> <unk> from the Upper Jurassic or Lower Cretaceous of Japan . The fossils include leaves with visible veins , as well as sporocarps . 
