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The early diversification of land plants; clues from Lower Devonian mesofossils.
Dr. Jennifer Morris
School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol,
BS8 1TQ
The origination of land plants (embryophytes) during the Mid-Ordovician period (~480 Myrs ago)
was a major landmark in the evolution of life. They transformed landscapes, providing new habitats
and energy for emerging terrestrial ecosystems. It is thought that the embryophyte lineage, which
including liverworts, hornworts, mosses (collectively referred to as bryophytes) and the vascular
plants (tracheophytes), emerged from a green algal ancestor. Through a series of innovations (e.g.
fundamental changes to their life cycle, cuticles, stomata, vascular tissues) plants were able to adapt
to life on land.
The key to understanding the transition from aquatic algae to terrestrial vascular plants is to
understand the evolutionary relationships between the main lineages within the embryophytes, in
particular between the bryophytes and tracheophytes. One approach is via the creation of
phylogenetic trees from two main data sources: molecular and morphological data on living taxa
and/or the morphological data of plant fossils i.e. extinct taxa. However, these analyses have yet to
be successful in achieving a unifying consensus. This is in part due to the relative lack of data from
the fossil record.
The plant fossil record comprises of body fossils and a separate record of their spores that have been
dispersed and subsequently preserved within the matrices of sedimentary rocks. The earliest
unequivocal body fossil is that of Cooksonia, a simple vascular plant of leafless stems that are
branched with equal division and terminate with spore-bearing organs (sporangia), found in rocks of
Silurian age (~ 425 Myrs ago). However, this record is highly biased towards plants that contain
recalcitrant tissues (e.g. lignin). In contrast, the spore record is much older, in part because their
walls are coated with decay-resistant sporopollenin. Trilete spores appear around ~450 Myrs ago
(Upper Ordovician) while the more elusive cryptospores (dyads and tetrads) go back to ~470 Myrs
ago. There is some debate over the affinity of the cryptospores, but it is noted that the wall
ultrastructure of some cryptospore taxa bear resemble to those of extant liverworts. However the
earliest unequivocal record of liverworts is not until ~385 Myrs ago.
Much of our understanding of the anatomy of the earliest land plants comes from the study of
exceptionally well preserved, minute charcoalified fossils from the Lower Devonian, termed
mesofossils. Despite representing only a facet of early land vegetation, recent studies with Prof.
Dianne Edwards at Cardiff University indicate that they continue to make significant contributions to
the understanding of morphological and anatomical diversity, giving clues to the evolution of key
characters. Some fossils possess a combination of characters that are not known in plants today and
may represent stem group embryophytes. Thus it is hoped that with this data the relationships
between bryophytes and the early tracheophytes may be elucidated.