The microfossil record of early land plants

Abstract

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.

Fragments of the earliest land plants

“…They have been reported from throughout the globe, exhibiting surprisingly little temporal and spatial variation 3,15 , and this uniformity suggests a worldwide cosmopolitan flora. The Oman dispersed spore assemblages provide further evidence for this hypothesis, as they are identical to coeval assemblages from elsewhere.…”

“…There are several lines of evidence indicating that Ordovician cryptospores derive from land plants [2][3] . First, the distribution of the spores is similar to that of extant spores/pollen (that is, they occur in non-marine sediments, and when found in marine sediments their abundance declines offshore).…”

“…The earliest fossil evidence for land plants comes from microscopic dispersed spores [1][2][3] . These microfossils are abundant and widely distributed in sediments, and the earliest generally accepted reports are from rocks of mid-Ordovician age (Llanvirn, 475 million years ago) 4 .…”

Resistant tissues of modern marchantioid liverworts resemble enigmatic Early Paleozoic microfossils

“…Some, such as tubular elements adhering to the sporangium of Tortilicaulis (22) or tubular features on sporangia of a tetrad-containing plant of putative affinity to liverworts (23), may represent fungal hyphae (5). Some might have been components of lichens or the resistant sheaths of cyanobacteria.…”

“…This material included tubes of at least two width types, apparent sheets of cells or adherent parts of cells (described as cuticle), and spores, the latter suggesting that the hypothetical plants reproduced by dispersed spores, as do modern basal embryophytes. Although similar tubes and cell sheets attributed to nematophytes have now been found in Cambrian-Devonian deposits, no later studies have confirmed that the tubes, cell sheets, and spores were attached (2)(3)(4)(5). Thus, the original concept of a nematophyte with a body that included all three components (1) remains hypothetical.…”