Miospores : A key to understanding the early terrestrialisation of plants
Steemans, P. 1* & Gerrienne, P. 1
1 NFSR Research Associate, Paléobotanique–Paléopalynologie–Micropaléontologie,
University of Liège, Bât. B-18, Parking 40, 4000 Liège 1, Belgium
* Corresponding author
It is now widely accepted that the cryptospores are the oldest witnesses of the earliest
continental vegetation. Indeed, cryptospores are known since the Ordovician (Strother et al.,
1996) or even, more controversially, since the Cambrian (Baldwin et al., 2004; Strother et al.,
2004) whereas the oldest axial remains of land plants are Silurian in age. To understand when
land plants first evolved and how their terrestrialisation occurred, the study of miospores
(sensu Steemans 2000) is the only plausible palaeontological option. Miospores provide a
time framework and an outline of the evolution of the flora through their morphological
innovations.
Richardson et al. (1984) defined cryptospores as: "Non-marine sporomorphs which consist of
"permanent" tetrads, dyads and alete monads (the latter have no contact features)." This
original definition was extended by Richardson (1988) to include "... spores separated from
dyads. Such spores are alete with a distinct, more or less circular contact area which is
typically smooth and diaphanous and may be wrinkled, simulating trilete folds, or more
commonly collapsed but which may be sculptured". Strother (1991) proposed a new
definition: "Non-marine sporomorphs (non-pollen grains) without those typical visible
haptotypic features such as trilete marks or furrows which characterize trachaeophyte spores
and pollen grains. Single grains or monads, "permanent" dyads and tetrads are included as are
sporomorphs from polyads which may or may not preserve contact areas". According to
Steemans (2000), the terms "non-marine sporomorph" are not precise enough; Strother’s
cryptospore definition cannot help to distinguish cryptospores from acritarchs which are
"Small microfossils of unknown and probably varied biological affinities" (Evitt, 1963).
Thus,. Consequently, Steemans (2000) proposed the following definition (here slightly
emended) for cryptospores: “Alete miospores (non-pollen grains) produced by basal
embryophytes. Single grains or monads, "permanent" dyads and tetrads, and sporomorphs
from polyads which may or may not preserve contact areas, are included”. In addition, the
term cryptospore should be included in the following emended miospore definition:
“Miospore is a general term used for all fossil plant spores smaller than 200 µm, regardless of
whether they are cryptospores, isospores, microspores, small megaspores, pre-pollen or pollen
grains”.
So far, estuarial palynomorphs, very similar to younger cryptospores, have been found by
Strother (1998) from the Middle Cambrian Bright Angel Shale, Grand Canyon, USA. An
algal affinity for those palynomorphs cannot be excluded (Taylor, 1999).
The first unequivocal cryptospore assemblage has been recorded from the Llanvirn Hanadir
Shales of Saudi Arabia (Strother et al., 1996). Higher in the stratigraphy, from the Caradoc up
to the Lower Devonian, observations of cryptospores become numerous (Steemans, 2000).
The oldest trilete spore has been observed in Ashgillian layers from Turkey (Steemans et al.,
1996). Trilete spores progressively become more abundant since the Middle Silurian.
It is noteworthy that the cryptospore biodiversity does not seem to be affected by the
Hirnantian glaciations, while the biodiversity of other palynomorphs (chitinozoans and
acritarchs) was strongly reduced during this important climatic event (Fig. 1). This has been
explained by the cosmopolitanism of the cryptospore assemblages around the
Ordovician/Llandovery boundary (Steemans, 1999; Steemans, 2000), from high to low
latitudes, meaning that they were able to live under a wide range of climates. On the opposite,
a marked decrease of cryptospore species occurred during the Llandovery. Miospores
assemblages before and after this decrease are very different. The pre-decrease assemblages
are dominated by cryptospores enclosed in a membrane (e.g. Segestrespora,
Laevigatasporites …), tetrads and dyads have spores intimately connected together, and trilete
spores are extremely rare. The post-decrease assemblages contain much more trilete spores,
cryptospore enclosed in a membrane become very rare, spores of dyads and tetrads are loosely
attached together, and monads become more abundant. This modification could be the
consequence of the global transgression during the Llandovery that may have destroyed or
displaced the habitats of the cryptospore producing plants.
The cryptospore curve of the fig.1 is based on observations from nearshore sediments.
However, recent discoveries from continental sediments around the Siluro-Devonian
boundary show that during that period the amount of species is underestimated. For example,
Wellman and Richardson (1996) identified thirty species from UK; (Steemans et al., 2007)
also describe thirty species from Saudi Arabia. Results obtained around the S/D boundary
(reference?) (Steemans et al., 2007)show that the cryptospore diversity is much lower in
nearshore than in continental sediments, suggesting that plant producing ubiquist species of
cryptospores (e.g. Tetrahedraletes medinensis) lived in a wide range of biotopes whereas the
plants producing other cryptospore species grew in more confined or remote habitats, with a
weak dispersion potential. Comparing results on Lochkovian miospore assemblages from UK
and Saudi Arabia shows that the cryptospores are very similar in both localities whereas the
assemblages of trilete spores are different, having rather strong affinities with their respective
palaeoplates. This suggests that cryptospore producing plants had a wide range of climatic
tolerance, and that the trilete spore producing plants were presumably more sensitive to
climatic variations. In addition, the high degree of similarity between UK and Saudi Arabian
cryptospore assemblages favours those of the palaeogeographic reconstructions that show
close vicinity between Western Gondwana and Euramerica and/or a land connection between
these palaeoplates.
It has been shown that, at the same time or somewhat later in the Lochkovian. (Breuer et al.,
2005; Steemans and Lakova, 2004) a palaeophytogeographic province characterised by
miospores belonging to the Emphanisporites micrornatus – zavallatus phylogenic lineage
existed in the eastern area of the Old Red Sandstone continent. Surprisingly, Lochkovian
sediments from the Moesian Platform contain miospore assemblages typical of this
palaeophytogeographic province whereas marine palynomorphs show strong affinities with
the Gondwana palaeoplate. Those different palaeogeographic affinities are not necessarily
contradictory if we accept the presence of a narrow Rheic Ocean which did not act as a
hermetic barrier for transoceanic chitinozoan and acritarch exchanges.
Higher in the stratigraphy, during the Middle Devonian, interesting results have been recently
obtained on miospores and megaspores from Libya and Saudi Arabia (Steemans et al, in
preparation ; (Ville de Goyet et al., 2007). The richest megaspore assemblages of latest
Eifelian and Givetian ages are currently under investigation. About twenty megaspores
species have been identified. Among them, six species are also present in Poland and in
Arctic, suggesting that heterosporous plants were widespread during the Middle Devonian and
that Euramerican and Gondwana plates were close to each other.
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Fig 1: Biodiversity curve of acritarchs, chitinozoans, cryptospores and trilete spores from the
Ordovician up to the Devonian. Data based on the former international stratigragraphic
chart.