Dispersed microfossils (spores and phytodebris) provide the earliest evidence for land plants. They are first reported from the Llanvirn (Mid-Ordovician). More or less identical assemblages occur from the Llanvirn (Mid-Ordovician) to the late Llandovery (Early Silurian), suggesting a period of relative stasis some 40 Myr in duration. Various lines of evidence suggest that these early dispersed microfossils derive from parent plants that were bryophyte-like if not in fact bryophytes. In the late Llandovery (late Early Silurian) there was a major change in the nature of dispersed spore assemblages as the separated products of dyads (hilate monads) and tetrads (trilete spores) became relatively abundant. The inception of trilete spores probably represents the appearance of vascular plants or their immediate progenitors. A little later in time, in the Wenlock (early Late Silurian), the earliest unequivocal land plant megafossils occur. They are represented by rhyniophytoids. It is only from the Late Silurian onwards that the microfossil/ megafossil record can be integrated and utilized in interpretation of the flora. Dispersed microfossils are preserved in vast numbers, in a variety of environments, and have a reasonable spatial and temporal fossil record. The fossil record of plant megafossils by comparison is poor and biased, with only a dozen or so known pre-Devonian assemblages. In this paper, the early land plant microfossil record, and its interpretation, are reviewed. New discoveries, novel techniques and fresh lines of inquiry are outlined and discussed.

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The plant spore record indicates two major phases of adaptive radiation of land plants before the Devonian; these can be interpreted to correspond to different reproductive, vegetative, and ecophysiological strategies of these early terrestrial plants. The first major adaptive radiation by plants onto land occurred in the mid Ordovician. These early plants are represented by abundant obligate spore tetrads; this assemblage persists from the mid Ordovician to about the mid-late Early Silurian. The fossil spore records indicate that these primary producers were widespread by the end of the Ordovician and the beginning of the Silurian. The close similarity of the fossil tetrads with obligate spore tetrads produced by some hepatics and mosses suggests a non-vascular vegetative grade of organization for plants of this interval. The second major adaptive radiation begins with the replacement of the monotonous spore tetrad assemblage by single trilete spores in the mid-late Early Silurian. These trilete spores find morphological counterpart with spores produced by vascular cryptogams; they suggest a vegetative grade of organization at the vascular level for plants of this interval. The initially smooth-walled trilete spores of this radiation are followed by diverse assemblages of trilete spores with variously ornamented spore coat patterns and varied laesurae morphologies by the mid-Late Silurian. The interval from the mid Ordovician to the mid-late Early Silurian is hypothesized on the basis of the abundant and widespread spore records to be one of rapid colonization by founder populations with limited genetic diversity and with life-history strategies that included an ecophysiological tolerance to desiccation and a short vegetative life cycle. The interval from the mid-late Early Silurian to the Pridoli largely coincides with the appearance of vascular plant megafossils. It is hypothesized on the basis of the spore assemblages to be one of major establishment of large populations of genetically diverse plants exploiting a broad spectrum of ecological sites that have escaped representation in the meagre megafossil record.

Megafossil evidence does not fill the 'evolutionary gap' between land plants and their hypothetical green algal ancestors. Rare Late Silurian vascular plant megafossils provide little information about the morphological, physiological, biochemical, and ecological steps that preceded their evolution. Dissociated trilete spores, spore tetrads, cuticle-and tracheid-like structures far exceed the abundance and diversity of Silurian vascular plant megafossils, and appear millions of years before them. In reference to whole-bodied organisms, these or analogous structures belong to land plants or emergent aquatics; they may represent plants evolutionarily intermediate between green algae and descendent vascular plants at the bryophyte or pre-bryophyte stages. Changes in the cellular biochemistry of pre-Devonian land plants in response to the selective pressures of terrestrial life may have led to the origin of lignin and cutin, neither of which has any counterpart among the algae, and to the evolutionary surge of the vascular plants in the Early Devonian represented by the plant megafossil record. Positive correlation between abundance and diversity of trilete spores and shallow-water, nearshore sites reinforces conclusions based on morphology that a terrestrial flora existed well prior to the appearance of vascular plant megafossils.

Among the abundant plant microfossils obtained from the late Silurian Burgsvik Sandstone are spores and filaments whose morphology suggests a fungal origin. These include large multiseptate spores resembling conidia of present-day Fungi Imperfecti; ovate, reniform, rugulately ornamented unicellular spores; branched filaments with perforate septa; and filaments with flask-shaped appendages resembling phialides. We suggest that these microfossils represent the remains of the imperfect stages of terrestrial Ascomycetes, and provide evidence for an origin of this group at least contemporaneous with the earliest land plants. Larger, ovoid and cylindrical heterogeneous bodies composed of hyphal fragments resemble the fecal pellets of mycophagous microarthropods. The implications of these in terms of early terrestrial ecosystems are discussed. 0 Fossil fungi, Ascomycetes, Fungi Imperfecti, fungal phylogeny, terrestrial ecosystems, fecal pellets.