Early land plant megafossil recordThe fossil record for early land plant megafossils is relatively poor and highly biased. This
is primarily a reflection of the low preservation potential of the primitive land plants,
particularly the earliest bryophytes that almost certainly lacked recalcitrant lignified
tissues. To date there are only approximately 25 reported plant megafossil assemblages
from the Silurian. These are all from the late Silurian, and probably post-date the
acquisition of lignin (i.e. the origin of the polysporangiophytes) (see Figure 7.1). Some
workers (e.g. Kenrick 2000) have suggested that representational bias is important. That
is, plant megafossils are poorly known pre-Lower Devonian due to the paucity of non-
marine strata available for exploration. It should be noted, however, that plants are easily
transported into marine environments where conditions are often favourable for
preservation. While plants are most commonly preserved in terrestrial environments, it is
not uncommon for them to occur in marine deposits. All of the known late Silurian plant
megafossil assemblages are from marine deposits (i.e. the plants are allochthonous and
have been transported into the marine environment). Convincing plant megafossils simply
have not been recovered below this level, despite extensive exploration that has included
rare examples of non-marine Ordovician-Silurian strata that appear suitable for plant
megafossil preservation. Thus I consider that the plant megafossil record begins once
plants had evolved recalcitrant tissues that enhanced preservation potential. Earlier plants
were of low preservation potential and most likely will only occur as megafossils in cases
of exceptional preservation (if any exist and can be located). The early land plant
megafossil record has been reviewed most recently by Edwards and Wellman (2001).
Convincing early land plant megafossils are known from the late Wenlock (late Silurian)
(Edwards et al. 1983), and consist of rhyniophytoid plants. These diminutive plants
comprise a naked, bifurcating axis with terminal sporangia. Bifurcating axes, that probably
represent plants of a similar grade of organization, but lack preserved sporangia, are known
from the Llandovery (Schopf et al. 1966). The enigmatic vascular plant, Pinnatiramosus
qianensis, of Wenlock (late Silurian) age from southwest China is remarkable in its peculiar
morphology and advanced anatomy (Geng 1986; Cai et al. 1996). Recently, however,
Edwards et al. (2001) have suggested that these fossils may represent roots of Permian
plants growing on (and infiltrating) a Silurian substrate.
Most of the reported Silurian land plant assemblages are dominated by simple
rhyniophytoids. These are plants comprising a bifurcating axis (sometimes bearing
stomata) with terminal sporangia (sometimes containing in situ spores). However, none
possess a well preserved stele, and thus the presence of tracheids and vascular status
cannot be confirmed. Consequently they are termed rhyniophytoid, rather than included
in the vascular rhyniophyte plants, that are proven from earliest Devonian deposits
(Edwards et al. 1992). Although simple, Silurian rhyniophytoid plants exhibit extensive
diversity in terms of variations within in situ spores and sporangial characters (e.g. Fanning
et al. 1988). Some assemblages contain more complex plants, including lycophytes
(summarized in Edwards and Wellman 2001).
128 CHARLES H.WELLMAN