An order of flowering plants (angiosperms) previously thought to be related closely to the waterlilies because one genus of the latter, Cabomba (Nymphaeaceae), has similarly highly dissected leaves. Studies of deoxyribonucleic acid (DNA) sequences have revealed that the single genus of the order, Ceratophyllum (Ceratophyllaceae), has no close relationship to any other extant group of flowering plants. It is an old, highly specialized plant, modified for a fresh-water aquatic habitat. Fossil fruits attributed to it are more than 120 million years old, which makes it the oldest extant angiosperm genus. The plants are submersed, rootless aquatics with highly dissected, branching leaves; they have reduced, separately sexed, petalless flowers. There are probably only six species distributed throughout fresh-water systems worldwide.