Geoscience Reference
In-Depth Information
and culminated in the origin of tracheophytes that
dominate subaerial habitats today.
The tracheophytes (vascular land plants) have a
fossil record beginning in the Wenlock (Edwards
et al. 1983). Our understanding of the kinds of
plants that lived on the land surface prior to that
time is based almost entirely on the fossil record
of cryptospores, dispersed spores of early land
plants and probably ancestral algal forms that are
widely distributed in marginal marine settings by
Middle Cambrian (Strother & Beck 2000; Strother
et al. 2004) and later (Steemans 2000; Wellman &
Gray 2000; Steemans & Wellman 2003).
The simplest model for the estimation of the
evolution of terrestrialization is the notion of four
successive terrestrial autotrophic floras or phases:
a cyanobacterial-dominated microbial landscape
(microbial mats), a bryophyte-dominated subae-
rial phase similar to posterlands sensu Retallack
(1992) (thalloid bryophytes), a polysporangiophytic
phase that includes both rhyniophytoids and tra-
cheophytes which do not possess secondary xylem
(tracheophtyes) and a forested phase composed
of plants that possessed secondary xylem (ligno-
phytes). These four phases represent the known
stages in vegetative evolutionary succession that
are distinct enough from each other that they can
be modelled for the two factors that are of interest
in quantifying the impact of terrestrialization on
marine ecosystems: the sequestration of carbon in
the stored biomass of vegetative tissues (biomass)
and the effect of vegetative cover on weathering
of parent rock (depth of the rhizosphere).
Homerian (Wenlock, Silurian) which spans 423-
426 Ma. This timing is based on the occurrence of
small plant axes that occur by Wenlock time
(Edwards et al. 1983). The Homerian origin of tra-
cheophytes is supported by the rise of ornamented
trilete spores, which appear synchronously in both
Avalonian and Laurentian terrains (Beck & Strother
2008). These authors have argued that the synchro-
nous occurrence of ornamented trilete spores, cryp-
tospores and megafossil plant axes indicates that the
origin of ornamentation in spores is tracking the
origin of tracheophytes. However, Steemans et al.
(2009) have recently recovered ornamented trilete
spores from pre-Hirnantian sediments from the
Arabian plate. This would then drop the timing of
the tracheophyte phase to the late Katian, following
the arguments in Steemans et al. (2009) that such
spores are tracking the occurrence of tracheophytes.
Although we have retained the Homerian interval as
the estimated time of tracheophytes origins, it is
possible that this phase in plant evolution began
much earlier, perhaps by as much as 20 Ma.
The last phase in the developing terrestrializa-
tion process is the origin of secondary wood and
the rise of a forested landscape. This is likely to
have taken place by the end of the Givetian (see dis-
cussions in Meyer-Berthaud et al. 2010). We have
therefore estimated this phase to begin between
375-385 Ma, corresponding to the Frasnian stage.
The rise of lignophytes is the last stage in the evol-
utionary primary succession of terrestrialization, but
it is the only phase to be incorporated into the
GEOCARB model for the Palaeozoic Eon. It is
important to point out that the drop in pCO
2
that is
seen in the model is set by the parameters used in
GEOCARB.
Timing of land plant origins
We know enough from the fossil record to estimate
the origination time duration of each of these
successive phases. The microbial mat phase is of
Precambrian origin. Perhaps terrestrial microbial
mats were established by the time of the rise in O
2
at 2.2 Ga, but there is very little direct evidence of
organisms occupying terrestrial landscapes before
about 1 Ga. The thalloid bryophyte phase is
the least well known of the terrestrial phases, but
there is evidence of bryophyte-grade structure in
laminated cryptospore walls (Strother et al. 2004;
Taylor & Strother 2008) in addition to other
organic structures; this indicates the likelihood of
thalloid bryophytes by the end of the Early Cam-
brian. We have placed the estimate of the beginning
of plants of a bryophytic grade at the Early to
Middle Cambrian boundary (513 Ma), but have
plotted the origin as occurring in the interval
523-513 Ma to account for the likely late Early
Cambrian origin of related cryptospores.
The origin of tracheophytes in the fossil record
was, until recently, very tightly constrained to the
Assessing the effects of the primary radiation
of the land plants on the carbon cycle
Each successive phase represents a potential relative
increase in standing carbon biomass. Even without
knowing the actual biomass represented by the
global values, we can predict a stepwise discrete
increase in each phase. In other words, each tran-
sition represents a cumulative increase in biomass
retention in the terrestrial landscape. This is not
the case for the rhizosphere, because microbial
and bryophytic cover would have had significant
differences in producing roots or analogous struc-
tures that specifically penetrate parent rock. True
roots probably evolved during the tracheophyte
phase, but the significance of the earliest roots is
extremely difficult to assess. The earliest fossil
evidence of rooting organs in plants is found in
the Baffin Island floras of Pragian-Emsian age
described in Gensel et al. (2001). However, there
is indirect evidence of rooting from palaeosols of
