Water Ferns

Ecology & Form

• Growing emergent in damp soil, sometimes submerged, or floating forms

• Azolla is known to form a symbiotic relationship with the cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen

  • This feature has been advantageous to rice farmers in China for thousands of years

Sporophyte (=spore-bearing phase)

Vegetative features

Stems

• Some forms possess rhizome roots in saturated or submerged soil

• Floating forms possess thin stem that connects leaflets

Leaves

• Few or no leaflets

• Leaf form is atypical for ferns 

Roots

• Marsilea, Pilularia, and Regnellidium have adventitious roots that emerge from the rhizome to anchor the plant in saturated soils

• Since Azolla and Salvinia are floating, they exhibit roots that hang in the water column

Geologic Range

• Jurassic - present

• Marsileaceae is first recorded in the Middle to Late Jurassic, and diversify in the Aptian–Albian of the Early Cretaceous (De Benedetti et al. 2021)

  • Diversity decreased during the Cenomanian–Santonian

• Salviniaceae is first recorded in the Early Cretaceous, and diversify during the Campanian–Maastrichtian of the Late Cretaceous (De Benedetti et al. 2021)

• Decrease in diversity for all Salviniales occurred during the Cretaceous–Paleogene mass extinction

• From the Paleocene onwards there is a steady decline in the fossil record of the group, with most remains belonging to the extant genera