The Hypnales are one of the major moss groups: as currently recognised, about a third of mosses are Hypnales. They are a major subgroup of the clade of pleurocarpous mosses, i. e. those in which the reproductive sporophytes arise from the sides of gametophyte stems, as explained earlier in this post. In the past, the pleurocarpous mosses have been divided between three orders, the Hypnales, Hookeriales and Leucodontales, on the basis of features of branching habit and the peristome, the array of teeth surrounding the opening of the spore capsule. In the Hookeriales, the teeth of the endostome (the inner ring of the peristome) are connected by a high basal membrane, and molecular phylogenetic analyses have generally supported this order as monophyletic. The Leucodontales were defined by having reduced peristome teeth, and usually sympodial growth (as the primary shoot produces a side-branch, it ceases growing itself and the new branch becomes the new primary shoot). The Hypnales had well-developed peristome teeth, and their growth was generally monopodial (the primary shoot continues growing even after it produces side-branches). The distinction between these latter two orders also correlated with their choice of niches: Leucodontales were mostly epiphytes, whereas Hypnales mostly grew on the ground. However, molecular phylogenetic analyses have not supported the distinction between the Hypnales and Leucodontales, with features such as reduced peristome teeth apparently evolving multiple times with the united clade combining the two orders (Buck et al. 2000). As a result, recent authors have treated the Hypnales as including most members of both the prior orders Hypnales and Leucodontales. A smaller number of pleurocarpous mosses have been placed outside the clade including Hookeriales and Hypnales in the broad sense; there are now known as the Ptychomniales and Hypnodendrales. The broader Hypnales is less well defined morphologically, but its members tend to have differentiated alar cells (distinctly formed cells at the basal corners of the leaves) and smooth spore capsules (Huttunen et al. 2012).
This shuffling is not restricted to the higher levels, either. Relationships within the Hypnales remain poorly resolved; indications are that at some point this group went through a quite rapid diversification, resulting in a fairly high level of convergence between lineages and low support for molecular branches. Huttunen et al. (2012) found support for a large clade within the Hypnales including the majority of its Northern Hemisphere members, with a paraphyletic grade outside this containing mostly Southern Hemisphere taxa. Huttunen et al. suggested a Gondwanan origin for the Hypnales, with their diversification in the Northern Hemisphere (where the other pleurocarpous orders never made many inroads) related to the break-up of the Laurasian landmasses. Within the Northern Hemisphere clade, many previously recognised families appear to be polyphyletic; even the type genus of the order, Hypnum, contains species that seem to occupy widely separate places in the hypnalean family tree.
A good example of all this mess is the genus Echinodium, a small genus of six living species whose distinctive appearance lead to it being placed in a family all of its own. Echinodium species grow as fairly stiff plants with long leaves that taper to a narrow point and have thickened margins (the margins are two cell layers thick whereas the body of the leaf is only one cell thick). Echinodium mosses also have a very unusual distribution: two species are found in southeastern Australia and New Zealand, but the other four are restricted to the Macaronesian islands in the Atlantic (that is, the Canaries, the Azores and Madeira). When fossil Echinodium species were discovered in eastern Europe, it was suggested that the genus' current distribution could be a relict of a previously much wider one. However, a molecular analysis of the genus by Stech et al. (2008) identified another explanation: not only were the Australasian and Macaronesian Echinodium species widely separated geographically, they were widely separated phylogenetically. The Australasian species were placed in the family Neckeraceae, whereas the Macaronesian species were related to mosses of the family Lembophyllaceae. What is more, the Macaronesian species did not form a single clade within the Lembophyllaceae: at least one of the species was placed separately from the rest. The supposedly distinctive 'Echinodium' features, it seems, have evolved independently, possibly as an adaptation for wet habitats.
Buck, W. R., B. Goffinet & A. J. Shaw. 2000. Testing morphological concepts of orders of pleurocarpous mosses (Bryophyta) using phylogenetic reconstructions based on trnL-trnF and rps4 sequences. Molecular Phylogenetics and Evolution 16(2): 180–198.
Huttunen, S., N. Bell, V. K. Bobrova, V. Buchbender, W. R. Buck, C. J. Cox, B. Goffinet, L. Hedenäs, B.-C. Ho, M. S. Ignatov, M. Krug, O. Kuznetsova, I. A. Milyutina, A. Newton, S. Olsson, L. Pokorny, J. A. Shaw, M. Stech, A. Troitsky, A. Vanderpoorten & D. Quandt. 2012. Disentangling knots of rapid evolution: origin and diversification of the moss order Hypnales. Journal of Bryology 34 (3): 187–211.
Stech, M., M. Sim-Sim, M. G. Esquível, S. Fontinha, R. Tangney, C. Lobo, R. Gabriel & D. Quandt. 2008. Explaining the ‘anomalous’ distribution of Echinodium (Bryopsida: Echinodiaceae): independent evolution in Macaronesia and Australasia. Organisms Diversity & Evolution 8 (4): 282–292.