care, and the infamous vagaries of the fossil/stratigraphical record taken into account.
Clearly, only minimum age estimates for groups are possible (i.e. preservation of the first
fossilized remains following the origin of embryophytes), and this is dependent upon the
evolution of structures likely to leave a fossil record (disregarding any hitherto
undiscovered exceptionally preserved floras), a suitable stratigraphical record preserving
relevant fossils, and the actual discovery and correct interpretation of such fossils.
The earliest generally accepted evidence for embryophytes is dispersed cryptospore
assemblages from the Llanvirn (Ordovician). We must judge to what extent this reliably
represents the timing of the origin of land plants. There is convincing evidence that they
derive from embryophytes of bryophytic grade (see discussion above). However, we must
consider some potential problems: (i) Were there earlier embryophytes that did not
produce fossilizable parts? (ii) Is there an earlier, but unrecognized, fossil record for
embryophytes? (iii) Is there an earlier, but undiscovered, fossil record for embryophytes?
The first problem addresses the possibility that the earliest embryophytes did not leave
a fossil record because they lacked any recalcitrant parts with high fossilization potential.
This is unlikely because some of the features considered to be essential for successful
colonization of the subaerial environment would almost certainly have included parts
composed of recalcitrant materials with high fossilization potential. Spores are a case in
point. It has been argued (Wellman, in press) that successful exploitation of the subaerial
environment by plants was impossible without spores enclosed within a resistant
sporopollenin wall. It follows that such spores must have evolved coincidently with (or
possibly before) the invasion of the land, and populations of spore-bearing plants would
have spread rapidly due to the wide dispersal potential of such spores.
The second problem addresses the possibility that there is a pre-Llanvirn fossil record of
embryophytes but we simply do not recognize these fossils as deriving from land plants.
For example, could the Middle Cambrian palynomorphs described by Strother and Beck
(2000) derive from embryophytes? At the heart of this problem is the ability to recognize
embryophytes unequivocally. We need to recognize structures that clearly belong to
embryophytes (i.e. autapomorphies). Sporopollenin-walled spores are considered to be an
embryophyte autapomorphy and consequently are a good example. However, the onus is
on the researcher to identify unequivocal evidence for embryophyte affinities. Hence,
when identifications are uncertain (as is the case with the Middle Cambrian material) it is
best to err on the side of caution.
The third problem addresses the possibility that pre-Llanvirn embryophytes, capable
of, or indeed, producing fossils existed, but we simply have not discovered them. This is
unlikely. In the past half century a vast literature has accumulated based on a massive quantity
of palynological research undertaken on Precambrian and Early Palaeozoic deposits, that
is both stratigraphically and palaeogeographically extensive. Considering the amount of
pre-Llanvirn sediments palynologically analysed, it can be argued that if the embryophytes
were widespread we would have discovered them (unless we simply do not recognize them
—see above). Thus we are only likely not to have discovered pre-Llanvirn fossil-
producing embryophytes if either the rock record for the relevant time is missing or
inadequate, or the embryophytes comprised small populations that were
palaeogeographically restricted. The former is unlikely as nearshore marine deposits (that
should contain dispersed microfossils) are common in pre-Llanvirn successions and have
136 CHARLES H.WELLMAN