In recent years, isolated spores and spore tetrads of Ordovician and Silurian age have received considerable attention as potential proxy records for early land plants (Wellman and Gray, 1988b, 2000; Edwards and Wellman, 2001; Steemans and Wellman, 2004) (FIG. 6.23). Although spore-like microfossils have been reported from as early as the Middle Cambrian (Strother, 2000; Strother and Beck, 2000), to date the oldest spore assemblages believed to have been produced by some land-inhabiting plant occur in lower Middle Ordovician (Llanvirn) rocks (Vavrdovâ, 1984, 1990; Gray, 1985; Strother et al., 1996; Steemans, 1999; Le Hérissé et al., 2007). Nearly identical spore assemblages occur from the Llanvirn to the late Llandovery (Early Silurian) (Richardson, 1988; Steemans et al., 1996; Rubinstein and Vaccari, 2004; Richardson and Ausich, 2007), suggesting a period of relative evolutionary stasis 40 myr in duration (Wellman and Gray, 2000; Steemans, 2001).
Commonly referred to as cryptospores (Richardson, 1996), these microfossils occur as monads, dyads, and tetrads ( FIGS. 6.24, 6.25), some surrounded by a smooth, loosely attached membrane (FIG. 6.26 ) (Strother, 2000). Strother and Beck (2000) categorized cryptospores (FIG. 6.27) as a class of organic-walled microfossils of probable terrestrial origin, whereas Steemans (2000) restricted the term to spore-like microfossils that are unambiguously attributable to the Embryophyta (Le Hérissé et al., 2007). Individual spores show no clearly defined exit site and are termed inaperturate. Other spores that first appear in Silurian rocks are solitary and possess a distinct trilete laesura (FIG. 6.28) (Steemans, 2000). In younger rocks there is a
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