Platyhypnidium pringlei [≡ Eurhynchium pringlei] is an uncommon, montane species of aquatic Brachytheciaceae occurring across central Mexico and in Guatemala. A morphologically distinct form of this moss is found disjunctively in some sheltered coves of the Blue Ridge Mountains in the southeastern United States. Both of these forms are sterile. A similar but more robust female plant is known from Arizona and California. In Himalayan India fertile forms of P. pringlei exist. These are apparently dioicous and have sporophytes with rostrate opercula and weakly roughened setae. Here we describe the sporophytes of P. pringlei for the first time and we discuss the variation, distribution and ecology of the species. Morphologically the several forms of P. pringlei intergrade, but in North America they appear to be geographically isolated.
We tested the monophyly of Platyhypnidium using nuclear ribosomal internal transcribed spacer (ITS) data from 47 collections of Brachytheciaceae. Although most of Platyhypnidium is an artificial segregate of the terrestrial genus Rhynchostegium, P. pringlei has its affinities with Oxyrrhynchium hians [≡ Eurhynchium hians]. Two other closely related aquatic species are E. selaginellifolium from Hawaii and Donrichardsia macroneuron from Texas, and at least one more aquatic relative is known from China. Based on morphological and molecular congruence between these aquatic mosses and terrestrial species of Oxyrrhynchium like O. hians, O. speciosum and O. savatieri, the new combinations O. pringlei, O. selaginellifolium and O. macroneuron are made. Morphologically these aquatic species are characterized by a dark green color, infrequent branching, a loose foliation, a homomallous leaf arrangement, short laminal cells and long costae.