The rise of the first land plants
TL;DR: In this paper, the authors propose a method to solve the problem of the problem: this paper...,.. ].. ).. ]... )...
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TL;DR: Comparisons of dated pollen floras of other regions indicate that one major subgroup of angiosperms, tricolpate-producing dicots (i.e., excluding Magnoliidae of Takhtajan) originated in the Aptian of Africa-South America at a time of increasing aridity and migrated poleward into Laurasia and Australasia.
Abstract: Morphological, stratigraphic, and sedimentological analyses of Early Cretaceous pollen and leaf sequences, especially from the Potomac Group of the eastern United States, support the concept of a Cretaceous adaptive radiation of the angiosperms and suggest pathways of their initial ecological and systematic diversification. The oldest acceptable records of angiosperms are rare monosulcate pollen grains with columellar exine structure from probable Barremian strata of England, equatorial Africa, and the Potomac Group, and small, simple, pinnately veined leaves with several orders of reticulate venation from the Neocomian of Siberia and the basal Potomac Group. The relatively low diversity and generalized character of these fossils and the subsequent coherent pattern of morphological diversification are consistent with a monophyletic origin of the angiosperms not long before the Barremian. PatuxentArundel floras (Barremian-early Albian?) of the Potomac Group include some pollen and leaves with monocotyledonous features as well as dicotyledonous forms. Patuxent angiosperm pollen is strictly monosulcate and has exine sculpture indicative of insect pollination. Rare Patuxent-Arundel angiosperm leaves are generally small, have disorganized venation, and are largely restricted to sandy stream margin lithofacies; the largest are comparable to and may include ancestors of woody Magnoliidae adapted to understory conditions. Patapsco floras (middle to late Albian?) contain rapidly diversifying tricolpate pollen and several new complexes of locally abundant angiosperm leaves. Ovate-cordate and peltate leaves in clayey pond lithofacies may includeancestors of aquatic Nymphaeales and Nelumbonales. Pinnatifid and later pinnately compound leaves with increasingly regular venation which are abundant just above rapid changes in sedimentation are interpreted as early successional “weed trees” transitional to but more primitive than the modern subclass Rosidae. Apparently related palmately lobed, palinactinodromous leaves which develop rigidly percurrent tertiary venation and become abundant in uppermost Potomac stream margin deposits (latest Albian-early Cenomanian?) are interpreted as riparian trees ancestral to the order Hamamelidales. Comparisons of dated pollen floras of other regions indicate that one major subgroup of angiosperms, tricolpate-producing dicots (i.e., excluding Magnoliidae of Takhtajan) originated in the Aptian of Africa-South America at a time of increasing aridity and migrated poleward into Laurasia and Australasia. However, the earlier (Barremian) monosulcate phase of the angiosperm record is represented equally in Africa-South America and Laurasia before marked climatic differentiation between the two areas. These trends are considered consistent with the hypothesis that the angiosperms originated as small-leafed shrubs of seasonally arid environments, and underwent secondary expansion of leaf area and radiated into consecutively later successional stages and aquatic habitats after entering mesic regions as riparian “weeds,” as opposed to the concept that they arose as trees of mesic forest environments.
419 citations
TL;DR: Finer structures involved in the uptake of nutrients of low diffusivity in soil evolved at least 400 million years ago as arbuscular mycorrhizas or as evaginations of "roots" ("root hairs").
Abstract: Roots, as organs distinguishable developmentally and anatomically from shoots (other than by occurrence of stomata and sporangia on above‐ground organs), evolved in the sporophytes of at least two distinct lineages of early vascular plants during their initial major radiation on land in Early Devonian times (c. 410–395 million years ago). This was some 15 million years after the appearance of tracheophytes and c. 50 million years after the earliest embryophytes of presumed bryophyte affinity. Both groups are known initially only from spores, but from comparative anatomy of extant bryophytes and later Lower Devonian fossils it is assumed that, during these times, below‐ground structures (if any) other than true roots fulfilled the functions of anchorage and of water and nutrient acquisition, despite lacking an endodermis (as do the roots of extant Lycopodium spp.). By 375 million years ago root‐like structures penetrated almost a metre into the substratum, greatly increasing the volume of mineral matter subject to weathering by the higher than atmospheric CO2 levels generated by plant and microbial respiration in material with restricted diffusive contact with the atmosphere. Chemical weathering consumes CO2 in converting silicates into bicarbonate and Si(OH)4. The CO2 consumed in weathering ultimately came from atmospheric CO2 via photosynthesis and respiration; this use of CO2 probably accounts for most of the postulated 10‐fold decrease in atmospheric CO2 from 400–350 million years ago, with significant effects on shoot evolution. Subsequent evolution of roots has yielded much‐branched axes down to 40 μm diameter, a lower limit set by long‐distance transport constraints. Finer structures involved in the uptake of nutrients of low diffusivity in soil evolved at least 400 million years ago as arbuscular mycorrhizas or as evaginations of ‘roots’ (‘root hairs’).
395 citations
TL;DR: Functional considerations suggest that stomata evolved from pores in the epidermis of plant organs which were at least three cell layers thick and had intercellular gas spaces and a cuticle; an endohydric conducting system would not have been necessary for low-growing rhizophytes, especially in early Palaeozoic CO2 -rich atmospheres.
Abstract: Summary
Fossil evidence shows that stomata have occurred in sporophytes and (briefly) gametophytes of embryophytes during the last 400 m yr. Cladistic analyses with hornworts basal are consistent with a unique origin of stomata, although cladograms with hornworts as the deepest branching embryophytes require loss of stomata early in the evolution of liverworts. Functional considerations suggest that stomata evolved from pores in the epidermis of plant organs which were at least three cell layers thick and had intercellular gas spaces and a cuticle; an endohydric conducting system would not have been necessary for low-growing rhizophytes, especially in early Palaeozoic CO2-rich atmospheres. The ‘prestomatal state’ (pores) would have permitted higher photosynthetic rates per unit ground area. Functional stomata, and endohydry, permit the evolution of homoiohydry and the loss of vegetative desiccation tolerance and plants > 1 m tall. Stomatal functioning would then have involved maintenance of hydration, and restricting the occurrence of xylem embolism, under relatively desiccating conditions at the expense of limiting carbon acquisition. The time scale of environmental fluctuations over which stomatal responses can maximize carbon gain per unit water loss varies among taxa and life forms.
254 citations
Additional excerpts
...Among these are: Church (1919), Whitehouse (1952), Corner (1964), Meidner & Mansfield (1968), Chaloner (1970), Raven (1977, 1984a, 1993, 1995, 1996, 1999a,b), Ziegler (1987), Edwards (1993, 2000), Edwards & Axe (1992), Graham (1993), Edwards et al. (1996), Willmer & Fricker (1996), Edwards et…...
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TL;DR: 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.
Abstract: 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.
226 citations
TL;DR: This chapter describes several aspects of the significance of transport processes in the evolution of vascular land plants from their putative ancestors—the green algae.
Abstract: Publisher Summary This chapter describes several aspects of the significance of transport processes in the evolution of vascular land plants from their putative ancestors—the green algae. The vascular land plants have, in addition to the transport processes at the cell level, which are common to all organisms, important transport processes at the supracellular level, which involve complex anatomical features. The homoiohydric vascular land plant has three major transport systems at the supracellular level—the apoplast, the symplast, and the intercellular gas space system. The aquatic green algal ancestors of these plants use only the symplast for transport at the supracellular level. The other two transport systems, together with the elaboration of the symplast into the much more efficient phloem, are essential components of the homoiohydric land plants.
211 citations
References
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TL;DR: In this paper, the relative influence of location of source area and transport by air and water currents on pollen dispersal in the Orinoco [Venezuela] sediments has been studied.
Abstract: Palynological study of Recent Orinoco [Venezuela] sediments shows the relative influence of location of source area and transport by air and water currents on pollen dispersal. Distribution of fungi, cuticles, reworked material, Hystrichosphaeridae, and foraminifera have also been studied. Variations in microfossil content with depth in core samples are found to be related to depositional history. The significance of the results in palynological and paleogeographical studies of older sediments is discussed.
473 citations
TL;DR: The distribution of pine pollen has proved especially significant. as mentioned in this paper showed that pine pollen in the water northwest of Andros Island was most abundant at the very edge of the Bank, showing an apparently inverse relationship to distribution of Pine pollen in sediment.
217 citations
TL;DR: The strata from which the plant-remains to be described in this paper are those which are grouped as Downtonian in the extended sense of this term employed by Mr. W. Wickham King as discussed by the authors.
Abstract: The strata from which the plant-remains to be described in this paper are derived are those which are grouped as Downtonian in the extended sense of this term employed by Mr. W. Wickham King. They therefore include not only the Ludlow Bone-bed, the Downton Sandstone, and the Temeside Shales, i.e ., the Grey Downtonian, but a much greater thickness of red marls and sandstones that were formerly classed with the Lower Old Red Sandstone, but are now spoken of as the Red Downtonian. This way of regarding the rocks of this horizon has been fully discussed in the light of the history of opinions by O. T. Jones (1929, pp. 110- 121) and need not be considered further here. These strata have been included at different times in the Silurian or in the Old Red Sandstone. It is not necessary here to enter into the question as to the best limit between the Silurian and the Devonian. Reference may be made to Stamp (1923), O. T. Jones (1929), and to Wickham King’s recent paper (1934). It is sufficient for the purposes of this study of the plants to recognize that the Downtonian strata come above the more definitely marine Ludlovian rocks and below the more definitely continental beds of the Lower Old Red Sandstone. The transitional nature of the Grey Downtonian has long been accepted and can readily be extended to the succeeding beds included in the Downtonian by Wickham King.
202 citations
TL;DR: The Muir of Rhynie chert was discovered by Dr W. Mackie of Elgin while investigating the sedimentary and volcanic rocks of Craigbeg and Ord Hill which occur in that area as discussed by the authors.
Abstract: The chert of the Muir of Rhynie, containing plant-remains, was discovered by Dr W. Mackie of Elgin while investigating the sedimentary and volcanic rocks of Craigbeg and Ord Hill which occur in that area. The original discovery was made on loose specimens, built into the dykes or scattered over the fields, especially those lying to the north of the road which runs from Rhynie to Cabrach, and east and west of the right-of-way that here connects Windyfield Farm with the public road.
179 citations
TL;DR: Asteroxylon Mackiei as discussed by the authors was a plant of more complicated organisation and larger size than either Rhynia or Hornea, which have been described from the silicified peat-bed at Rhynie in the two preceding papers of this series.
Abstract: Asteroxylon Mackiei was a plant of more complicated organisation and larger size than either Rhynia or Hornea, which have been described from the silicified peat-bed at Rhynie in the two preceding papers of this series. The generic name refers to the stellate outline of the xylem of the stem as seen in cross section, while the specific name commemorates the original discovery of the plant remains by Dr Mackie.
138 citations