Plants to 10 mm tall. Leaves often caducous; lateral leaves elliptic to oblanceolate, often curling inwards dorsally when dry, 0.75–1.25 mm long; dorsal leaves broadly ovate to orbicular, c. half the length of the lateral leaves; in both types margin unistratose, plane, minutely and regularly crenulate; costa ending a little below the apex; laminal cells 7–10 µm diameter.

Perichaetial leaves crowded and overlapping, linear-spathulate, 2.5–3.0 mm long. Setae 2–3 mm long. Capsules ovate-cylindrical, exserted, c. 1 mm long. Spores c. 14 µm diameter, brown.

Native.

Very rare on the trunks of trees ferns in humid gullies in New South Wales, Victoria and Tasmania. Also in New Zealand.

Gymnostomum complanatum Hook.f. & Wilson, London J. Bot. 3: 538 (1844). Calomnion complanatum (Hook.f. & Wilson) Lindb., Acta Soc. Sci. Fenn. 10: 240 (1872). Type: Bay of Islands, North Island, New Zealand, [1841], J.D.Hooker; lecto: BM, fide D.H.Vitt, Bryologist 98: 340 (1995); isolecto: BM, FH.

Taxonomic synonyms

Calomnion laetum Hook.f. & Wilson, in J.D.Hooker, Fl. Nov.-Zel. 2: 97 (‘1855’) [1854], nom. illeg., incl. spec. prior. Type: “Northern Island”, Bay of Islands, New Zealand, [1841], J.D.Hooker; syn: BM-Wilson n.v.; Waikehi, Sinclair; syn: BM-Wilson n.v.

Listed as endangered in Australia (Scott et al., 1997) and, in Victoria, as a Threatened taxon under the Flora and Fauna Guarantee Act 1988 (Vic.).

N.S.W.: Cambewarra Mtn, W.W.Watts 6568 (NSW); Waterfall Track, Mt Wilson, 25 Sept. 2001, E.A.Brown (AD, NSW).

Vic.: Ferny Glade, on track to Sealers Cove, 6 Apr. 1994, D.A.Meagher (MEL).

Tas.: Dip Falls, S of Stanley, I.G.Stone 25273 (HO, MEL).

Sporophytes are apparently rare in Australian material, only one fertile specimen having being observed. This species is readily confused with other small mosses (e.g. Rhizogonium spp. and Hymenodon pilifer) that project horizontally from tree fern trunks, so it has possibly been overlooked.

Scott, G.A.M., Entwisle, T.J., May, T.W. & Stevens, G.N. (1997). A Conservation Overview of Australian Non-marine Lichens, Bryophytes, Algae and Fungi. Environment Australia: Canberra. p. 100.

 D.G.Catcheside & G.H.Bell (2012), Australian Mosses Online 44. Calomniaceae. Australian Biological Resources Study, Canberra. Version 13 June 2012. 

First published as: D.G.Catcheside & G.H.Bell (2006), Calomniaceae, Fl. Australia 51: 367–368. Australian Biological Resources Study, Canberra & CSIRO Publishing, Melbourne.