naked laevigate monads naked monads with pseudo-trilete mark naked fused dyads (i.e., Pseudodyadospora)
naked unfused dyads (i.e., Dyadospora)
naked fused tetrads (i.e., Tetrahedraletes)
naked unfused tetrads (i.e., Stegambiquadrella)
smooth envelope enclosed dyads (i.e., Segestrespora)
envelope enclosed ornamented tetrads ornamented envelope enclosed dyads smooth envelope enclosed tetrads ornamented envelope enclosed tetrads envelope enclosed monads
figure 6.23 Stratigraphic ranges of numerous miospore morphologies showing evolution of biodiversity from the Ordovician into the Silurian. (From Steemans and Wellman, 2004.)
figure 6.25 Quasiplanar cryptospore tetrad (Cambrian). Bar = 5 pm. (Courtesy P. Strother.)
figure 6.27 Dicryptosporites radiatus monad (Silurian). Bar = 20 pm. (Courtesy P. Strother.)
pronounced increase in the diversity of spore types, as well as in the complexity of ornamentation (Gray et al., 1974; Wood, 1978; Beck and Strother, 2001), suggesting a major radiation of land plants during this period. Based on comparisons of Late Silurian plant microfossils from China with assemblages reported from other parts of the world, Wang et al. (2005) hypothesized that Late Silurian floras were cosmopolitan and exhibited little paleogeographic differentiation.
Although a few fragments of sporangia (FIG. 6.29) have been found containing these early spores (Wellman et al., 1998, 2003), their biological affinity and significance has continued to be challenged. Some solitary spores are believed to have been produced by algae, whereas the dyads and tetrads are thought to have represented a liverwort grade of land plant, or perhaps even vascular plants (Steemans and Wellman, 2004). The presence of land plants with a bryophytic level of organization as early as the Ordovician is supported by both paleobotanical and molecular evidence (Bateman et al., 1998; Qiu et al., 2006; Renzaglia et al., 2007). The ultrastructure (FIGS. 6.30, 6.31) of some of these spores is also expanding the database of wall organization patterns (Taylor, 2002, 2003) and, as a result, is opening new lines of evidence directed at determining the affinities of
major plant groups. These studies will also serve to distinguish phylogenetically important features from those that are developmental in scope (Wellman, 2004).
Despite the fact that most early spore types cannot be traced with certainty to major groups of plants, they are still a significant source of information with regard to biostratigra-phy, paleophytogeography, and paleoecology (Steemans et al., 2007). An excellent synthesis is the study by Gray (1985) who identified three microfossil zones based on dispersed spores. The first zone extends from the Middle Ordovician to the Lower Silurian, and is dominated by spores in tetrads
(FIG. 6.24), some of which possess an outer membrane. Smooth-walled, solitary, trilete spores mark the second zone, which extends from the Lower to Middle Silurian. The third zone (Middle and Upper Silurian) contains spores with various types of external ornament, suggesting increasing levels of diversity. Tetrads of spores are also found throughout this last zone, but without enclosing membranes.
Despite the uncertainties regarding the affinities of various cryptospores, their presence, together with other types of fragmentary debris as early as the Middle Cambrian, indicates the presence of some type of subaerial photosynthetic cover during this time period (Strother et al., 2004). These authors also noted that at least some of the cryptospores are no doubt homologous with younger Ordoviocian and Silurian forms, but that the record to date suggests that during the Cambrian there was a diverse mesoflora of eukaryo-tic photoautotrophs that were derived from chlorophytes and/or charophytes, together with various thalloid organisms constructed of filaments.
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