Ecology. Open Yungas montane to humid puna; boggy or seepy sites, in and along margin of ponds or lakes, on humus and rocks. Elevation. 2400-4950 m. Distribution. Bolivia: Cochabamba, La Paz, Santa Cruz. World range: Widespread in the Northern Hemisphere; Mexico, Venezuela to Argentina, and southeast Brazil.
Warnstorfia exannulata is characterized by the ovate-lanceolate leaves, gradually curved and long acuminate, auriculate leaf base with a group of inflated hyaline alar cells extending costa, costa 3/4-4/5 the leaf length, denticulate or finely serrulate margins (above and/or just above the alar region), rarely entire, and dioicous sexual condition. Further distinctions include the reddish coloration sometimes exhibited by the plants and presence of rhizoid initials on at least some of the leaves especially toward the apex. These latter features aid to differentiate Drepanocladus aduncus which lacks rhizoid initials or red coloration; furthermore the leaf margins are not denticulate, but entire.