Pseudocrossidium obtusulum differs from P. revolutum (Bridel) R. H. Zander of Europe and Africa (absent in North America), by not or weakly differentiated perichaetial leaves, and cauline leaves ovate to deltoid, broadly acute, once-revolute, and revolute only in distal 2/3 of leaf. Discussions of the morphology and distribution of P. obtusulum in the New World have been published by G. R. Brassard (1971), P. M. Eckel et al. (1996), B. C. Tan et al. (1981), and R. H. Zander (1979). It has been collected as subfossil in the Yukon (J. A. Janssens and R. H. Zander 1980) and Vermont (N. G. Miller 1987). Fruiting material has been collected in Greenland—the peristome generally comes off with the operculum. A collection from Oregon (McIntosh 4939, UBC) has gemmae on the adaxial surface of the costa (as does P. revolutum). Didymodon brachyphyllus and D. nevadensis have somewhat the same appearance, but have merely strongly recurved distal laminal margins and an indistinct apex. This species has been collected widely in Alaska (B. M. Murray 1992), California (D. H. Norris and J. R. Shevock 2004), and Washington (T. T. McIntosh, unpubl.).
