12. Syntrichia obtusissima (Müller Hal.) R. H. Zander, Bull. Buffalo Soc. Nat. Sci. 32: 269. 1993.
Barbula obtusissima Müller Hal., Syn. Musc. Frond. 1: 640. 1849; Tortula obtusissima (Müller Hal.) Mitten
Stems (5-)10-20 mm. Leaves clasping at base, infolded, and twisted around the stem when dry, wide-spreading to squarrose when moist, lingulate to spatulate, 3-4 × 1-1.5 mm, keeled; margins revolute in the proximal 3/4-7/8, entire, often laxly undulate; apices emarginate to retuse, occasionally truncate or acute; costa excurrent into a toothed, hyaline awn (often brown at base) usually longer than the leaf, yellow or brown, often minutely papillose abaxially (but not serrate because of projecting cell ends); basal cells abruptly differentiated, rectangular, 70-90(-100) × 22-40 µm, often with yellow walls, narrowly rectangular at the margins; distal cells quadrate to polygonal, 15-23 µm, thin-walled, pellucid, bulging and bearing 4-7 papillae per cell. Specialized asexual reproduction absent. Sexual condition autoicous, although perigonia often lacking, apparently sometimes dioicous. Seta red-brown, 10-15 mm. Capsule yellowish or reddish brown, 3-4.5 mm, straight or slightly curved, with an abrupt neck; operculum 1.5-2 mm, reddish brown; peristome ca. 1.5 mm, the distal divisions twisted ca. 2 turns, yellow or red, the basal membrane white, 1/3 the total length. Spores 9-16 µm, papillose.
Soil, rock, rarely the bases of trees; moderate to high elevations; Ariz., N.Mex., Tex.; Mexico; South America (Bolivia, Peru).
Syntrichia obtusissima, with a distinctive Southwest and Mexican-Andean disjunction, has been much confused with S. ruralis, leading to many misidentified specimens in herbaria, but the large, pellucid leaf cells (about 15-23 µm) contrast with those of S. ruralis (about 8-12 µm), and the back of the costa lacks serrations from projecting cell ends. Furthermore, it has a clear central strand in the stem, and hydroids in the costa, neither of which is found in S. ruralis.