Introduction

The three lineages of bryophytes, mosses, liverworts, and hornworts, compose successful groups of early embryophytes. The mosses are estimated to include some 12 700 species (Crosby et al. 2000), the liverworts approximately 6000–8000 extant species (Crandall-Stotler & Stotler 2000, Chapter 1, this volume), and the hornworts about 100–150 species (Chapter 3, this volume). Mosses are comparable in species richness to the monilophytes, which are estimated to include about 11 500 species (Pryer et al. 2004). Among the extant land plants, therefore, only the angiosperms are currently more species-rich than are the bryophytes.

It is often stated that bryophytes are most diverse in the tropics and fit the general pattern found in many groups of organisms, with increasing species richness toward the equator (Rosenzweig 1995). However, a quantitative analysis of latitudinal diversity patterns in the mosses failed to detect any such latitudinal gradient, except perhaps a weak one in the Americas (Shaw et al. 2005a). It appears that liverwort diversity is highest at moderate to high latitudes of the Southern Hemisphere, although one family, the Lejeuneaceae, is hyperdiverse in wet tropical forests of both the New and Old Worlds (Gradstein 1979).

The fossil record for mosses, liverworts, and hornworts is too incomplete to assess whether these groups were more or less diverse in the geological past (Miller 1984, Oostendorp 1987).