closely related, perhaps even deriving from a single species (Johnson 1985; Richardson
1988, 1992; Strother 1991; Hemsley 1994; Taylor 2001).
Studies of in situ spores provide the only direct link between the dispersed spore and
plant megafossil records, and are critical to our understanding of the affinities of dispersed
spore types. Unfortunately, however, plant megafossils are practically unknown until the
late Silurian and hence there are no in situ spore records for the first 50 million or so years
of the early land plant dispersed spore record. However, there are a handful of
exceptionally well-preserved plant megafossil assemblages from the latest Silurian-earliest
Devonian in which in situ spores are preserved in sufficient detail to enable detailed
analysis. Such exceptional preservation is known from two main localities: Ludford Lane
(Pridoli, late Silurian) and north Brown Clee Hill (Lochkovian, Early Devonian) from the
Welsh Borderland, UK, and the record of in situ early land plant spores is based primarily
on material from these localities (see reviews by Fanning et al. 1991; Edwards 1996, 2000;
Edwards and Richardson 1996; Edwards and Wellman 2001). At both localities extremely
small plant fragments (sometimes referred to as mesofossils) are preserved as relatively
uncompressed coalifications that preserve exquisite cellular detail (Edwards 1996).
At the Ludford Lane and north Brown Clee Hill localities, the vast majority of in situ
spores are trilete. Rare specimens, however, contain cryptospores. In fact most
cryptospore morphotypes have now been recovered in situ (naked and envelope-enclosed
permanent tetrads and dyads, and hilate monads). However, it is unclear if the parent
plants represent relict populations, and provide a true reflection of earlier cryptospore-
producing plants, or if the cryptospores are plesiomorphic in more advanced plants, or
perhaps even arose due to convergence (Gray 1991; Edwards 2000). It must be borne in
mind that the fossils occur some 65 million years after the earliest reported cryptospores
from the Llanvirn (Ordovician).
Interpretation of the parent plants is not always straightforward as the mesofossils are
fragmentary. Usually only terminal parts of the axes (+/−sporangia) are preserved, and
cellular detail is variable. Furthermore, many of the rhyniophytoid plants preserve
unusual character combinations, confusing considerations of affinities. However, trilete
spores have been recovered from the rhyniophyte Cooksonia pertoni (Fanning et al. 1988)
which is demonstrably a true tracheophyte (Edwards et al. 1992), and the vast majority of
trilete spore producers appear to have constituted plants with bifurcating axes, with
terminal sporangia, and often possessing stomata. Interestingly, some dyads and tetrads
derive from plants with bifurcating axes/sporangia (Edwards et al. 1995a, 1999; Wellman
et al. 1998a), a character not represented among extant bryophytes (see Edwards 2000).
Another interesting observation is the presence of stomata on plants containing in situ
hilate cryptospores (Edwards 2000; Habgood 2000). Stomata are absent from liverworts,
but present in most hornworts, mosses, and vascular plants (although losses are not
uncommon in these groups, and are generally considered to be related to ecological
factors and functional requirements) (see review in Kenrick and Crane 1997).
It is well established that analysis of spore wall ultrastructure characters can be
extremely useful when attempting to ascertain the phylogenetic relationships of extant
land plants, and similar research has been extrapolated back in time and is now routinely
undertaken on fossil spores (e.g. Kurmann and Doyle 1994). Such research is also of
paramount importance in studies of spore wall development. Recently there has been a
132 CHARLES H.WELLMAN