Plants gregarious, small, dull, often encrusted with soil. Stems to 1 cm tall but mostly much shorter, distal cell of axillary hairs swollen, somewhat asymmetric, colorless, often encrusted. Leaves soft, commonly shriveled when dry, 0.6--0.8 mm long, oblong to obovate or spatulate, sometimes decurrent from costa and margins, margins plane, or somewhat recurved proximally; costa weak to strong, 1/2 leaf length to nearly percurrent, sometimes forked distally; cells in distal part of leaf in ascending rows diverging from costa. Asexual propagation by gemmae on axillary rhizoids rare; rhizoid tubers inconspicuous, of several bulging cells linearly arranged.

Sporophytes not produced in the flora area. Exposed sites on damp or periodically wet limestone, marl, calcareous soil, mortar-work; 0--1000 m; Ariz., Fla., La., Okla., Tex.; Mexico; West Indies; Central America; South America; Africa; Pacific Islands in Hawaii.

This species is almost entirely restricted to base-rich substrates in the flora area and probably elsewhere in its broad range. It is an obscure moss, difficult to find in the field because of its small size, drab aspect, and lack of field characters. Most specimens from the flora area are comprised of tiny, poorly developed plants with short stems and small leaves. The typical habitat is open areas on limestone or other limey substrates along rivers and streams; in the latter habitat S. obtusum may form minor tufa deposits, sometimes mixed with other calciphilic mosses. In the flora area, underdeveloped specimens of various other mosses, mostly Bryaceae and Pottiaceae, are sometimes misidentified as Splachnobryum. Gymnostomiella orcuttii is superficially similar to S. obtusum but has papillose leaf cells; the type material of Splachnobryum kieneri R.S. Williams is a Bryum. Under the microscope the leaf shape, crenulate distal leaf margin, and distal leaf cells in ascending rows diverging from the costa, are helpful for identification. The oddly shaped axillary hairs are helpful for identification, but often difficult to demonstrate. The leaves of many specimens of this moss are difficult to rehydrate after drying. Plants with archegonia are common in at least some of the material distributed as Grout's North American Musci Perfecti 448, collected by Faith Pennebaker Mackaness on mortar work in old cemeteries in New Orleans. However, neither archegonia nor antheridia have been found in field-collected specimens of S. obtusum from the flora area; limey substrates do not occur naturally in the area of New Orleans, suggesting that the cemetery populations might have been introduced. Rhizoid tubers, as reported by Arts (1996) for African specimens of S. obtusum, are present in at least some of the specimens from the flora area, including the New Orleans material. Splachnobryum obtusum has been reported from Georgia, from the top of Stone Mountain. The specimens are correctly determined but it is doubtful that they actually came from Stone Mountain.