Living bryophytes are represented by ~900 genera and nearly 24,000 species. They are not a conspicuous portion of the Earth's flora, although they may dominate the vegetation in certain special environments, for example Sphagnum in certain types of bogs. Most bryophytes are small plants, many <2 cm long. The largest forms rarely exceed 60 cm in length (e.g., species in the genus Dawsonia). In general, bryophytes are most abundant in relatively moist areas. They range throughout the world and can even be found in coastal areas of Antarctica. Bryophytes differ from true vascular plants by the absence of vascular tissue and by the presence of a nutritionally independent gametophyte generation in their life cycle (FIG. 5.1- . Although bryophytes do not contain true vascular tissue, some have specialized conducting elements (Hebant, 1977) (FIG. 5.8- . In certain mosses, the stem of the gametophyte and the seta (stalk) of the sporo-phyte contain elongate, non-lignified water-conducting cells called hydroids (analogous to xylem in vascular plants). Surrounding the hydroids are assimilate-conducting cells
termed leptoids, which are comparable to sieve elements in the phloem of higher plants.
The fossil record of the Bryophyta is based on both spores and macrofossil remains. Numerous spores have been described as bryophytic, but most of these are spo-rae dispersae, which limits their taxonomic usefulness and may obscure their biological affinities. One of these is Tetrapterites visensis, an unusual dispersed structure isolated from Mississippian rocks (Hibbert, 1967). It consists of a tet-rahedral, non-cellular membrane with winglike ridges; each ridge is attached to a single, trilete spore (FIG. 5.9). Sullivan and Hibbert (1964) compared Tetrapterites to the persistent tetrads of some liverworts, including Sphaerocarpus.
The Paleozoic macrofossil record of bryophytes is poor. The earliest accepted specimens come from the Carboniferous (e.g., Walton, 1925, 1928; Oostendorp, 1987), and a few examples have been found in rocks as old as the Devonian (discussed below). Just why there is no extensive record of bryophytes in the Carboniferous is debatable, since the coal-swamp forests of the Pennsylvanian should have provided a wealth of suitable habitats for bryophytes. In addition, the preservation of other fossils from these paleoenvironments is often excellent (e.g., in coal balls). Some believe that the preservational potential of bryophytes is so poor that they are simply not preserved in sufficient numbers to be accurately recorded. Experiments conducted
by Hemsley (2001), however, show that the preservational potential of bryophytic plant material is similar to that of vascular plants, which suggests that bryophytes were indeed rare elements of the Carboniferous coal-swamp ecosystems.
Until approximately 25 years ago, the fossil record of the Bryophyta consisted principally of vegetative remains of the gametophyte; with few exceptions, the sporophyte generation remained unknown, and almost no fossil bryophytes were known with identifiable sex organs. This situation has changed, however, because a number of well-preserved fossils of moss and liverwort sporophytes have been described from Mesozoic rocks (e.g., Konopka et al., 1997, 1998) and Cenozoic amber (e.g., Grolle, 1998; Frahm, 1999b, 2001b; Grolle and Schmidt, 2001) (Table 5.1; FIGS. 5.10-5.16). In several of the liverworts in amber, even the androecium, perianth, and gynoecium are preserved (Grolle, 1990, 1998). Although bryophyte remains in amber were noted as early as the nineteenth and early twentieth centuries (e.g., Goppert, 1853; Caspary, 1887; Dixon, 1922), only recently have these fossils received wider scholarly attention. Today, amber fossils represent the single most important source of evidence for the evolutionary history and biodiversity of bryophytes in the Cenozoic.
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