Mosses arguably possess the most structurally complex sporangia of any extant land plants, a consequence of being the monosporangiophyte lineage most strongly adapted to terrestrial environments. Morphological and functional variation in the mechanisms that regulate spore release in one of the major classes of mosses, the Polytrichopsida, is largely unexplored, while recent research indicates that the most distinctive structure, the peristome, has evolved independently in the Polytrichopsida and in other mosses. The genus Polytrichastrum was separated from Polytrichum on the basis of such sporangial characters, although the critical features had until recently only been examined using light microscopy, and strong evidence from molecular data indicated that Polytrichastrum as currently circumscribed is polyphyletic. Here we use Bayesian ancestral character state reconstruction in conjunction with extensive scanning electron micrographic studies to elucidate probable morphology at ancestral nodes and define natural taxa. As well as clarifying the structure, evolution, and aspects of development of the peristome-epiphragm complex in this highly prominent group of mosses, the results provide a basis for a revised phylogenetic taxonomy in which the species of Polytrichastrum sect. Aporotheca are recognized once more within Polytrichum.