The remarkably preserved Rhynie chert plants remain pivotal to our understanding of early land plants. The extraordinary anatomical detail they preserve is a consequence of exceptional preservation, ...by silicification, in the hot-springs environment they inhabited. However, this has prompted questions as to just how typical of early land plants the Rhynie chert plants really are. Some have suggested that they were highly adapted to the unusual hot-springs environment and are unrepresentative of ‘normal’ plants of the regional flora. New quantitative analysis of dispersed spore assemblages from the stratigraphical sequence of the Rhynie outlier, coupled with characterization of the in situ spores of the Rhynie chert plants, permits investigation of their palaeoecology and palaeophytogeography. It is shown that the Rhynie inland intermontane basin harboured a relatively diverse flora with only a small proportion of these plants actually inhabiting the hot-springs environment. However, the flora of the Rhynie basin differed from coeval lowland floodplain deposits on the same continent, as it was less diverse, lacked some important spore groups and contained some unique elements. At least some of the Rhynie plants (e.g. Horneophyton lignieri) existed outside the hot-springs environment, inhabiting the wider basin, and were indeed palaeogeographically widespread. They probably existed in the hot-springs environment because they were preadapted to this unstable and harsh setting.
This article is part of a discussion meeting issue ‘The Rhynie cherts: our earliest terrestrial ecosystem revisited’.
The timescale of early land plant evolution Morris, Jennifer L.; Puttick, Mark N.; Clark, James W. ...
Proceedings of the National Academy of Sciences - PNAS,
03/2018, Letnik:
115, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Establishing the timescale of early land plant evolution is essential for testing hypotheses on the coevolution of land plants and Earth’s System. The sparseness of early land plant megafossils and ...stratigraphic controls on their distribution make the fossil record an unreliable guide, leaving only the molecular clock. However, the application of molecular clock methodology is challenged by the current impasse in attempts to resolve the evolutionary relationships among the living bryophytes and tracheophytes. Here, we establish a timescale for early land plant evolution that integrates over topological uncertainty by exploring the impact of competing hypotheses on bryophyte−tracheophyte relationships, among other variables, on divergence time estimation. We codify 37 fossil calibrations for Viridiplantae following best practice. We apply these calibrations in a Bayesian relaxed molecular clock analysis of a phylogenomic dataset encompassing the diversity of Embryophyta and their relatives within Viridiplantae. Topology and dataset sizes have little impact on age estimates, with greater differences among alternative clock models and calibration strategies. For all analyses, a Cambrian origin of Embryophyta is recovered with highest probability. The estimated ages for crown tracheophytes range from Late Ordovician to late Silurian. This timescale implies an early establishment of terrestrial ecosystems by land plants that is in close accord with recent estimates for the origin of terrestrial animal lineages. Biogeochemical models that are constrained by the fossil record of early land plants, or attempt to explain their impact, must consider the implications of a much earlier, middle Cambrian–Early Ordovician, origin.
The evolutionary emergence of land plant body plans transformed the planet. However, our understanding of this formative episode is mired in the uncertainty associated with the phylogenetic ...relationships among bryophytes (hornworts, liverworts, and mosses) and tracheophytes (vascular plants). Here we attempt to clarify this problem by analyzing a large transcriptomic dataset with models that allow for compositional heterogeneity between sites. Zygnematophyceae is resolved as sister to land plants, but we obtain several distinct relationships between bryophytes and tracheophytes. Concatenated sequence analyses that can explicitly accommodate site-specific compositional heterogeneity give more support for a mosses-liverworts clade, “Setaphyta,” as the sister to all other land plants, and weak support for hornworts as the sister to all other land plants. Bryophyte monophyly is supported by gene concatenation analyses using models explicitly accommodating lineage-specific compositional heterogeneity and analyses of gene trees. Both maximum-likelihood analyses that compare the fit of each gene tree to proposed species trees and Bayesian supertree estimation based on gene trees support bryophyte monophyly. Of the 15 distinct rooted relationships for embryophytes, we reject all but three hypotheses, which differ only in the position of hornworts. Our results imply that the ancestral embryophyte was more complex than has been envisaged based on topologies recognizing liverworts as the sister lineage to all other embryophytes. This requires many phenotypic character losses and transformations in the liverwort lineage, diminishes inconsistency between phylogeny and the fossil record, and prompts re-evaluation of the phylogenetic affinity of early land plant fossils, the majority of which are considered stem tracheophytes.
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•Early land plant relationships are extremely uncertain•We resolve the “Setaphyta” clade of liverworts plus mosses•The simple body plan of liverworts results from loss of ancestral characters•The ancestral land plant was more complex
Puttick et al. resolve a “Setaphyta” clade uniting liverworts and mosses and support for bryophyte monophyly. Their results indicate that the ancestral land plant was more complex than has been envisaged based on phylogenies recognizing liverworts as the sister lineage to all other embryophytes.
The origin of trees and forests in the Mid Devonian (393–383 Ma) was a turning point in Earth history, marking permanent changes to terrestrial ecology, geochemical cycles, atmospheric CO2 levels, ...and climate. However, how all these factors interrelate remains largely unknown. From a fossil soil (palaeosol) in the Catskill region near Cairo NY, USA, we report evidence of the oldest forest (mid Givetian) yet identified worldwide. Similar to the famous site at Gilboa, NY, we find treefern-like Eospermatopteris (Cladoxylopsida). However, the environment at Cairo appears to have been periodically drier. Along with a single enigmatic root system potentially belonging to a very early rhizomorphic lycopsid, we see spectacularly extensive root systems here assigned to the lignophyte group containing the genus Archaeopteris. This group appears pivotal to the subsequent evolutionary history of forests due to possession of multiple advanced features and likely relationship to subsequently dominant seed plants. Here we show that Archaeopteris had a highly advanced root system essentially comparable to modern seed plants. This suggests a unique ecological role for the group involving greatly expanded energy and resource utilization, with consequent influence on global processes much greater than expected from tree size or rooting depth alone.
•The earliest fossil forest to date is recovered from the Devonian of New York•Three types of trees are identified from fossil soil evidence in plan view•Early lignophyte relatives of seed plants have surprisingly modern root systems•Advanced energetics in this group suggests a unique role in changing Earth history
Using data from a Middle Devonian fossil soil, Stein et al. report root systems from the earliest intact forest to date, including cladoxylopsids, possibly stigmarians and Archaeopteris. Striking seed plant-like features of the latter indicate a special role for this clade in the profound changes in Earth global systems that took place at that time.
The geochemical carbon cycle is strongly influenced by life on land, principally through the effects of carbon sequestration and the weathering of calcium and magnesium silicates in surface rocks and ...soils. Knowing the time of origin of land plants and animals and also of key organ systems (e.g. plant vasculature, roots, wood) is crucial to understand the development of the carbon cycle and its effects on other Earth systems. Here, we compare evidence from fossils with calibrated molecular phylogenetic trees (timetrees) of living plants and arthropods. We show that different perspectives conflict in terms of the relative timing of events, the organisms involved and the pattern of diversification of various groups. Focusing on the fossil record, we highlight a number of key biases that underpin some of these conflicts, the most pervasive and far-reaching being the extent and nature of major facies changes in the rock record. These effects probably mask an earlier origin of life on land than is evident from certain classes of fossil data. If correct, this would have major implications in understanding the carbon cycle during the Early Palaeozoic.
The spore Acinosporites macrospinosus Richardson 1965 is common in exceptionally well-preserved dispersed spore assemblages recovered from the Middle Devonian (Eifelian) Middle Old Red Sandstone ...deposits in the Orcadian Basin, Scotland. This article reports on a detailed LM, SEM, and TEM analysis of these spores. The spores are large and spinose with an apical prominence associated with the trilete mark. TEM analysis reveals that the wall consists of four layers that were all formed by white-line-centered lamellae that develop into laminae. The spore wall ultrastructure is most similar to that of extant and fossil lycopsids, and a lycopsid affinity is proposed for the parent plant. On the basis of this interpretation, a mechanism for spore wall development is presented. The nature and ecology of the parent lycopsid plant are unclear. However, on the basis of similarities to younger bona fide megaspores, it is suggested that A. macrospinosus was probably an incipient megaspore produced by one of the first groups of lycopsids to have experimented with a heterosporous reproductive strategy. This is the earliest report of an apical prominence on a lycopsid spore, a feature that went on to characterize many later lycopsid megaspores, leading to the development of extensive gulas and massas.
The earliest record of a terrestrial testate amoeba is reported. This provides further evidence that early terrestrial ecosystems were more complex and modern in aspect than previously considered, in ...terms of biota, ecological interactions and biogeochemical cycling.
The earliest record of a terrestrial testate amoeba is reported. This provides further evidence that early terrestrial ecosystems were more complex and modern in aspect than previously considered, in terms of biota, ecological interactions and biogeochemical cycling.
It is often assumed that life originated and diversified in the oceans prior to colonizing the land. However, environmental constraints in chemical evolution models point towards critical steps ...leading to the origin of life as having occurred in subaerial settings. The earliest fossil record does not include finds from terrestrial deposits, so much of our understanding about the presence of a terrestrial microbial cover prior to the Proterozoic is based on inference and geochemical proxies that indicate biospheric carbon cycling during the Archaean. Our assessment is that by 2.7 Ga, microbial ecosystems in terrestrial settings were driven by oxygen‐generating, photosynthetic cyanobacteria. Studies of modern organisms indicate that both the origin and primary diversification of the eukaryotes could have occurred in terrestrial settings, shortly after 2.0 Ga, but there is no direct fossil evidence of terrestrial eukaryotes until about 1.1 Ga. At this time, it appears that the diversity of life in non‐marine habitats exceeded that found in marine settings where sulphidic seas may have impaired eukaryotic physiology and retarded evolution. Geochemical proxies indicate the establishment of an extensive soil‐forming microbial cover by 850 Ma, and it is possible that a rise in atmospheric oxygen at this time was due to the evolutionary expansion of green algae into terrestrial habitats. Direct fossil evidence of the earliest terrestrial biotas in the Phanerozoic consists of problematical palynomorphs from the Cambro‐Ordovician of Laurentia. These indicate that the evolution of the first land plants (embryophytes) during the Middle Ordovician took place within a landscape that included aeroterrestrial algae which were actively adapting to selection in subaerial settings.
Sediments of the Torridonian sequence of the Northwest Scottish Highlands contain a wide array of microfossils, documenting life in a non-marine setting a billion years ago (1 Ga).1–4 Phosphate ...nodules from the Diabaig Formation at Loch Torridon preserve microorganisms with cellular-level fidelity,5,6 allowing for partial reconstruction of the developmental stages of a new organism, Bicellum brasieri gen. et sp. nov. The mature form of Bicellum consists of a solid, spherical ball of tightly packed cells (a stereoblast) of isodiametric cells enclosed in a monolayer of elongated, sausage-shaped cells. However, two populations of naked stereoblasts show mixed cell shapes, which we infer to indicate incipient development of elongated cells that were migrating to the periphery of the cell mass. These simple morphogenetic movements could be explained by differential cell-cell adhesion.7,8 In fact, the basic morphology of Bicellum is topologically similar to that of experimentally produced cell masses that were shown to spontaneously segregate into two distinct domains based on differential cadherin-based cell adhesion.9 The lack of rigid cell walls in the stereoblast renders an algal affinity for Bicellum unlikely: its overall morphology is more consistent with a holozoan origin. Unicellular holozoans are known today to form multicellular stages within complex life cycles,10–13 so the occurrence of such simple levels of transient multicellularity seen here is consistent with a holozoan affinity. Regardless of precise phylogenetic placement, these fossils demonstrate simple cell differentiation and morphogenic processes that are similar to those seen in some metazoans today.
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•The multicellular microfossil Bicellum brasieri possesses two distinct cell types•3D preservation in phosphate preserved different life cycle stages•Differential adhesion may have contributed to cell segregation during morphogenesis•This billion-year-old freshwater protist shows evidence of holozoan affinity
Strother et al. describe life cycle morphogenesis in a new billion-year-old microfossil, which may provide clues to the evolutionary roots of embryonic development in animals.
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