The origin and early evolution of the bryophytes is a complex and perplexing problem (Goffinet, 2000). Phylogenetic evidence suggests that bryophytes in general, and liverwort-like plants in particular, should have been important components of early terrestrial floras (Bateman et al., 1998; Renzaglia et al., 2007). The fossil record of bryophytes, however, especially of early bryophytes, is meager and those known from fossils appear comparable in many ways to extant taxa. Even the earliest bryophyte-like fossils have the basic thallus organization also seen in many living forms. Based on this evidence, it is possible that the bryophytes evolved far earlier than the fossil record suggests, and fossils of the earliest bryophytes have not been found to date. However, it is possible that paleobotanists simply may not recognize the earliest bryophytes, because morphologically they do not resemble modern forms.
One interesting hypothesis suggests that several of the enigmatic Cambrian to Devonian fossils traditionally included in the nematophytes (see Chapter 6) may represent remains of ancient liverworts which shared certain features with modern marchantioids (Graham et al., 2004). Some of these fossils consist of sheets of (pseudo-)cells (so-called cuticle) and tubes of some resistant material. Cosmochlaina (Silurian-Devonian) was originally considered to be of
uncertain affinity, but has been interpreted as the lower epidermal surface of a marchantioid liverwort (FIG. 5.4 ) . At least some of the tubular aggregations assigned to nemato-phytes (and possibly to Nematothallus) have been reinterpreted as masses of resistant liverwort rhizoids, and are sometimes attached to fragments of lower epidermal tissue. It has also been suggested that some of the dispersed Early Silurian cell sheets and tubes may represent fragmentary remains of bryophyte sporangia (Graham and Gray, 2001).
One of the oldest reports of bryophyte-like megafossils is Parafunaria sinensis, a compression from the Early-Middle Cambrian Kaili Formation of China (Yang et al., 2004). The specimen is up to 2 cm long and comprises 4-5 leaves (each 5-15 mm long by 5 mm wide) that are densely borne around a short stem. The basal portion of the stem consists of a footlike structure, whereas distally it bears what is interpreted as a short seta with capsule. The authors suggest that the arrangement of leaves is similar to that in the extant moss Funaria hygrometrica (Bryopsida). Due to the age of these fossils, additional supporting evidence will be important in confirming their assignment to the Bryopsida.
The study of dispersed spores (sporae dispersae) is an important source of data on the composition of the earliest land floras (e.g., Wellman and Gray, 2000; Steemans and Wellman, 2004). Although in many instances it is difficult to distinguish dispersed bryophyte spores from those of vascular plants, there is increasing support for the suggestion that some of the spores in Ordovician and Silurian rocks resemble those of modern liverworts. The spores most often suggested as bry-ophytic occur in permanent (sometimes envelope-enclosed) tetrads in these assemblages; it is hypothesized that such tetrads came from plants at a bryophytic, most likely a liverwort, grade of organization (Gray, 1985). Additional support for this hypothesis comes in the form of tiny spore-containing plant fragments from Ordovician rocks of Oman (Wellman et al.,
2003). These fossils indicate that the spore producers, although diminutive in size, were true land plants, which produced sporangia containing large numbers of spores—a minimum of 7450 tetrads in some specimens. Ultrastructural features of the spore wall also suggest affinities with the liverworts.
In addition to microfossil remains, there are several types of meso- and macrofossils that have been considered important in understanding the early history of bryophytes (Edwards, 2000), including isolated sporangia with i n situ spores and axes with conducting elements similar to those in extant bryophytes. The Early Devonian compression fossil Sporogonites (Halle, 1916a, 1936) morphologically resembles a bryophyte. This plant was originally found in Norway and consists of stalks ~5cm long which terminate in elongate capsules (FIG. 5.5). Several longitudinal furrows ornament the base of the sporangium and extend onto the stalk. The sporangium is multilayered, and there is some suggestion that it may contain a central columella-like projection. Inside the sporangia are trilete spores that range up to 30 pm in diameter. Many specimens of S. exuberans are preserved with the sporangial stalks in a more or less parallel orientation (Andrews, 1960), suggesting that they were produced from a common thallus; some stalks appear to be attached at the base to an irregularly shaped, carbonaceous film ~15cm long (FIG. 5.5). The fact that not all stalks bear
figure 5.6 Tortilicaulis transwalliensis sporangium with twisted stalk (Devonian). (From Taylor and Taylor, 1993.)
sporangia suggests that they were perhaps easily detached or abscised once the spores were mature. Although the compressed nature of the specimens of Sporogonites reveals little about the internal organization of the tissues, it appears that vascular elements are lacking, thus supporting its classification with the bryophytes. It is suggested that Sporogonites may represent a compressed gametophytic thallus bearing upright sporophytes of a primitive moss or perhaps an early hornwort (Poli et al., 2003). Both interpretations are supported by the suggestion of a columella-like projection within the sporangium, because this structure is exclusively known in hornworts and the putative primitive lineages of extant mosses (Goffinet, 2000). Crandall-Stotler (1984) suggested that the basal thallus may represent a persistent pro-tonema, or even a small leafy gametophore, a hypothesis consistent with affinities to the mosses.
Tortilicaulis is a Late Silurian-Early Devonian fossil that shares morphological features with Sporogonites and certain extant bryophytes (Edwards, 1979, 1996; Edwards et al., 1994). Specimens consist of unbranched or isotomously branched axis fragments that terminate in solitary or branched, elongate sporangia (FIG. 5.6 ) containing trilete spores. The sporangia as well as the subtending axis are characteristically twisted (FIG. 5.6 ) . Sporangial dehiscence is unknown, but suggested to have occurred along a preformed line that split the sporangium into two vertical valves. The morphology and spiraled architecture of the axis and sporangium closely resemble those in the living moss, Takakia cerato-phylla (Takakiopsida) (Renzaglia et al., 1997). Morphological correspondence with members of the modern liverwort genus Pellia (Jungermanniopsida), however, have also been noted. Gerrienne (1997) pointed out that Tortilicaulis shares a number of important features with trimerophytes, and suggested that this taxon might therefore be ancestral to the Trimerophytina (Chapter 8).
Although the affinities of many of these early fossils will continue to be controversial, fossil organisms that possess bryophytic characters (dominant gametophyte and parasitic, inconspicuous sporophyte) will play an increasingly i mportant role, as paleobotanists continue to decipher steps leading to the colonization of the terrestrial realm.
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