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The Rhynie Chert – our earliest terrestrial ecosystem revisited

Discussion meeting

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Starts:

March
062017

09:00

Ends:

March
072017

17:00

Location

The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG

Overview

Scientific discussion meeting organised by Professor Dianne Edwards CBE FRS, Professor Liam Dolan FRS and Dr Paul Kenrick

400 million year old fossil plant stem. Copyright Natural History Museum, UK

New discoveries in the fields of developmental and functional biology are shedding light on the origins of land plants. We explored how exceptional fossils from our earliest preserved terrestrial ecosystem – the 400 million year old Rhynie Chert – could be integrated into a neobiological understanding of the evolution of plants and their interactions with other organisms and their environment.

The schedule of talks, organiser/speaker biographies and abstracts is available below.

Attending the event

This meeting has taken place. Recorded audio of the presentations will be available on this page shortly. Meeting papers will be published in a future issue of Philosophical Transactions B.

Enquiries: Contact the Scientific Programmes team

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Schedule of talks

06 March

09:00-13:30

Session 1

7 talks Show detail Hide detail

Chairs

Professor Liam Dolan FRS, University of Oxford, UK

09:00-09:15 The Rhynie Chert - setting the scene

Professor Dianne Edwards CBE FRS, Cardiff University, UK

Abstract

The Rhynie Chert is a 407 million year old hydrothermal deposit that is world renowned as the earliest well-preserved terrestrial ecosystem.  Beginning in 1917, Kidston and Lang published a series of ground-breaking papers describing the permineralised remains of plants, algae, fungi and cyanobacteria, and they also proposed a broadly accurate interpretation of the depositional environment. Subsequently faunal elements were discovered and include arachnids, collembolans, nematodes and crustaceans. Following further excavations (1963-1971) blocks of fossiliferous chert became widely accessible leading to major discoveries that include new life cycle variants in land plants, mycorrhizal-like association, lichen-like associations, and the diversity and functions of the fungal, algal and cyanobacterial elements of the ecosystem. Beginning in the 1980s, an extensive programme of trenching, boreholes, geophysics, and geochemistry provided a much clearer understanding of the geology of the Rhynie Basin, the environments of deposition and the age of the sediments. The site is thus exceptional because of the quality of preservation of organisms and their associations and interactions. The plants have played a central role in our understanding of the early evolution of tissues and organ systems and they continue to do so in the modern era of developmental biology and genomics.

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09:15-09:55 Modern hot spring analogues

Dr Alan Channing, Cardiff University, UK

Abstract

Siliceous hot spring deposits, such as the Lower Devonian Rhynie Chert are the surface expression of terrestrial hydrothermal systems which create gold/silver bearing low-sulfidation epithermal mineral deposits deeper in the crust. Eruption of hot spring waters and precipitation of silica create a range of terrestrial and aquatic environments such as sinter apron complexes and areas of geothermally influenced wetland. These provide habitats for ecosystems that may be preserved in situ and, via permineralization of tissues, with exquisitely preserved anatomy. Sampling at active sinter-depositing hot springs allows characterisation of the physicochemical properties of the waters responsible for the Rhynie exceptional preservation. Broadly, thermal waters had elevated temperatures (100oC - ambient), relatively high sodium and chloride content (brackish - ca. 1.5 ppt salt), high pH (typically 8-9) and carried a suite of phytotoxic elements (e.g. arsenic, mercury, zinc, antimony). Fluctuating water availability, created by cyclic periods of hot spring eruption and quiescence, add further to these stresses. Observations of analogues for Rhynie such as Yellowstone show that physicochemical conditions plus the distribution of plants and animals profoundly affect not only ecosystem ecology and ecophysiology but also organism preservation potential. Such data raise questions of adaptation, specialization, endemism, and ecological vs. preservation biases.

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09:45-09:55 Discussion

09:55-10:35 How unique were the Rhynie plants?

Professor Charles Wellman, University of Sheffield, UK
Professor Dianne Edwards CBE FRS, Cardiff University, UK

Abstract

The Rhynie Chert is our most complete example of an early terrestrial ecosystem, but the extent to which it is representative of the vegetation of the time remains conjectural. Physical, chemical and sedimentological evidence indicates that it was a geothermal wetland, and comparisons with similar modern environments (e.g., Yellowstone hot-springs system) highlight the presence of thermal gradients and brackish and potentially toxic surface water. The plants may therefore have been highly adapted and confined to the hot-springs system or perhaps they were more widespread elements of the regional biota but preadapted to survive in unstable stressed environments. It seems highly unlikely that the plants represented a relictual (primitive) population as is sometimes invoked for wetland communities. Comparison with contemporary megafossil remains found at other sites provides very limited support for a regional element to the flora. Integrated analysis of in situ and dispersed spores offers an opportunity to test these hypotheses and favours an interpretation of preadaptation for at least some of the plants.

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10:25-10:35 Discussion

10:35-11:00 Coffee

11:00-11:30 Aquatic microfauna of the Rhynie and Windyfield Chert: a biologist's view on an old ecosystem

Dr Carolin Haug, Ludwig Maximilians University of Munich, Germany

Abstract

Understanding ecosystems of the past is different from investigating modern day ones. Many properties of now-gone ecosystems can only be indirectly inferred, whereas in modern ecosystems simple observations or measurements would provide a direct access. Indirect access in fossil ecosystems is provided by the life habits of the extinct organisms that have been living in these ecosystems. The life habits of an extinct organism can finally be inferred by reconstructing the functional morphology, based on the morphological details observed in fossils in comparison to modern counterparts. Such investigations gain precision when exceptionally well-preserved fossils are studied, like those of the Lower Devonian Rhynie and Windyfield Chert. These deposits preserve a diverse range of arthropod groups, partly with terrestrial, partly with aquatic, non-marine lifestyle. Among aquatic forms especially crustacean fossils have become known, such as the famous Lepidocaris rhyniensis. The preservation of the crustacean fossils provides details of their feeding apparatus down to the sub-micron-range, including different types of setal structures. Based on these observations we can infer that crustaceans in the Rhynie and Windyfield Chert performed a variety of different feeding strategies. The biota thus contained a rich and differentiated non-marine aquatic fauna already 400 million years ago.

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11:30-11:45 Discussion

11:45-12:15 Terrestrial invertebrates in the Rhynie Chert ecosystem

Dr Jason Dunlop, Museum fur Naturkunde, Germany

Abstract

The Early Devonian Rhynie and Windyfield cherts remain a key locality for understanding early life on land. They host the oldest unequivocal nematode worm (Nematoda), which may also offer the oldest evidence for herbivory via plant parasitism. The trigonotarbids (Arachnida: Trigonotarbida) preserve the oldest book lungs and were probably predators with evidence for liquid feeding. The oldest mites (Arachnida: Acariformes) are represented by taxa known to include mycophages and predators on nematodes today. The oldest harvestman (Arachnida: Opiliones) includes the oldest tracheae and male and female genitalia. Myriapods are represented by a scutigeromorph centipede (Chilopoda: Scutigeromorpha), probably a cursorial predator on the substrate, as well as putative millipedes (Diplopoda). The oldest springtails (Hexapoda: Collembolla) probably acted as mycophages. The oldest true insects (Hexapoda: Insecta) are represented by a species known from chewing (non-carnivorous?) mandibles and another with a gut infill of phytodebris. Coprolites also offer insights into diet, and previous assumptions that several taxa were spore-feeders is challenged. Rhynie appears preserve a largely intact community of terrestrial animals, but some expected groups are absent. As has been argued for plants, Rhynie may be sampling an atypical ecosystem characterised by invertebrates adapted to life on the hot springs margins.

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12:15-12:30 Discussion

12:30-13:30 Lunch

13:30-17:00

Session 2

8 talks Show detail Hide detail

Chairs

Professor Jane Langdale FRS, University of Oxford, UK

13:30-14:00 The early evolution of land plants and their life cycles

Dr Paul Kenrick, The Natural History Museum, London, UK

Abstract

One of the most surprising discoveries in the Rhynie Chert was a completely new variant of the land plant life cycle in which both the gamete-bearing (haploid) and the spore-bearing (diploid) phases were independent, free-living plants of similar morphological and histological complexity. Sporophytes were distinguishable by their sporangia; gametophytes by their gametangia, which were often borne on distinctive bowl-shaped apices. In two species, gametophytes were significantly smaller than the sporophytes, whereas in others they were of comparable size. Otherwise, both phases were leafless, axial structures possessing rhizoids, vascular tissues and stomates. These discoveries have relevance to understanding the early evolution of the land plant life cycle, but their significance depends on precisely how the fossils relate to the plant tree of life. The leading hypothesis indicates that these fossils are stem group vascular plants, providing direct evidence that early life cycles were markedly different in form and ecology to those of modern pteridophytes. Divergence in size and growth architecture of gametophyte and sporophyte – a feature of modern pteridophytes – had begun by 407Ma, but the extent to which this life cycle variant persisted in early vascular plants floras remains an open question.

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14:00-14:15 Discussion

14:15-14:45 Effects of cellular changes to the evolution of land plant development and life cycles

Professor Mitsuyasu Hasebe, National Institute for Basic Biology, Japan

Abstract

Tissue and organs are composed of cells, and the development and resulted life cycle reflect the dynamics of cell behavior. Since the same or similar regulatory mechanisms at the cellular level can be repeatedly used in different developmental processes, changes of cellular characters and mechanisms can change multiple developmental processes with pleiotropic effects. Furthermore, new morphology originated from changes at the cellular level induces further morphological changes. However, genetic regulations of cellular characters and mechanisms especially deeply related to the evoluiton of land plant body plan have not been well studied. In this presentation, I will talk some examples to connect the evolution of development and life cycle to the changes at the cellular level. (1) The life cycle of Physcomitrella patens is a series of changes in stem cell characters. (2) Changes of stem cell characters switch body plants between gametophyte and sporophyte generations. (3) Sporophyte generation can be extended by the elongation of stem cell longevity. (4) Extended longevity of stem cells form new stem cells to be branches. (5) Periclinal cell divisions producing inner and outer tissue in antheridia, archegonia, and sporangia as well as water conducting tissue are regulated by the same molecular mechanisms, suggesting the partial homology of these organs.

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14:45-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 Organs and tissue systems of Rhynie Chert plants

Professor Hans Kerp, Institut fur Geologie und Palaeontologie, Germany

Abstract

The publication of Kidston and Lang’s monograph on the silicified plants from the Rhynie chert (1917–1921) is a real milestone in palaeobotany, because it was the first time that plants showing a great amount of anatomical detail were described from rocks as old as Early Devonian. The plants are often found in life position. Moreover, the often pristine preservation of very delicate short-lived developmental stages indicates a rapid preservation in silica. Therefore, the Rhynie chert provides a series of unique snapshots of an early terrestrial ecosystem. Between 1917 and 1920 Kidston and Lang described five land plants from the chert in great detail. Although all, except for Asteroxylon mackiei, look at first glance quite similar as they all consist of dichotomously branching naked axes, a detailed look shows striking differences between the individual taxa. However, several of these differences would never have been revealed by the much more common compression fossils. Even a century after the publication of the first part of Kidston and Lang’s monumental monograph the Rhynie chert continues to yield important new information. In the 1980s and 1990s Winfried Remy and his co-workers published several papers on gametophytes of Rhynie chert plants. In later years additional ones were described and life cycles could be reconstructed in more detail. Gametophytes are now known of four of the seven Rhynie chert plants, i.e. Aglaophyton major, Rhynia gwynne-vaughanii, Horneophyton lignieri and Nothia aphylla. The first two had rather simple gametophytes, whereas those of the latter two taxa are more complex. More recently published papers deal with vegetative structures in Rhynia gwynne-vaughanii and the sporangia-bearing axes of Asteroxylon mackiei, the largest and most complex plant from the Rhynie chert, which superficially looked quite similar to the modern lycopsid Huperzia selago. Currently, the development of the vascular strand in the underground parts and the aerial axes of Asteroxylon mackiei and the ontogenetic development of aerial axes of Aglaophyton major and Rhynia gwynne-vaughanii are subjects of research. Apart from the taxa described by Kidston and Lang, two other taxa have been described from the Rhynie and the nearby Windyfield chert localities. These are, however, not preserved in such great detail as the ones originally described by Kidston and Lang. This presentation will therefore focus on the well-studied taxa and give an overview of the various organs, rhizomatic and aerial axes, life stages and tissues.

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16:00-16:15 Discussion

16:15-16:45 The evolution of branching forms in plants

Dr Jill Harrison, University of Bristol, UK

Abstract

The evolution of vascular plants from their bryophyte-like precursors underpinned a ten-fold increase in species numbers and changes the course of plant and animal evolution. Whilst living bryophytes and vascular plants have widely disparate forms, fossil intermediaries reveal stepwise morphological innovations priming the radiation of vascular plant forms, including the innovation of branching. The genetic pathways that regulate branching are well characterised in flowering plants and are conserved within the land plants, predating the origin of branching. This talk will discuss the genes involved in the evolution of branching.

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16:45-17:00 Discussion

07 March

09:00-13:30

Session 3

8 talks Show detail Hide detail

Chairs

Professor Alistair Hetherington, University of Bristol, UK

09:00-09:30 Evolution of rooting systems

Professor Liam Dolan FRS, University of Oxford, UK

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09:30-09:45 Discussion

09:45-10:15 Evolution and palaeophysiology of the vascular system

Professor John Raven FRS, University of Dundee, UK

Abstract

Photolithotrophic growth on land using atmospheric CO2 inevitably involves H2O vapour loss. Embryophytes ≥ 100 mm tall are homiohydric and endohydric with mass flow of aqueous solution through the xylem in tracheophytes. The Rhynie sporophytes provide structural detail enabling analysis of the hydraulics of H2O supply to the transpiring surface, and the potential for gas exchange with the Devonian atmosphere, allowing modelling of the fluxes of H2O and CO2 in the plant-atmosphere system. Xylem carrying H2O under tension involves programmed cell death, rigid cell walls and embolism repair; evidence from fossils provides little evidence on these functions. The phenylalanine ammonia lyase essential for lignin synthesis was derived from horizontal gene transfer. The absence of an endodermis in the Rhynie plants poses limits on the regulation of supply of soil nutrients to the shoots. The transfer of organic solutes from the sites of photosynthesis to growing and storage tissues involves mass flow through phloem in extant tracheophytes, although the Rhynie plants show little evidence of phloem. Extant examples of the arbuscular mycorrhizas found in Rhynie plants exchange soil-derived nutrients (especially P) for plant-derived organic matter involving bidirectional mass flow along the hyphae.

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10:15-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 Evolution and palaeophysiology of stomata

Professor Jeff Duckett, The Natural History Museum, London, UK

Abstract

Stomata are one of the key innovations of land plants. The long held notions of unitary origin and conservation of function, viz. active regulation of gaseous exchange via aperture changes mediated by a potassium pump, from mosses and hornworts to angiosperms, and including Palaeozoic fossils  over 400 million years old, receives increasing support today  from the identification of a growing army of genes common to all extant land plant clades. However, recent functional studies present highly conflicting scenarios: on the one hand aperture changes in response to a range of experimental treatments in the moss Physcomitrella point to early acquisition of aperture regulation, on the other very different responses in most pteridophytes are indicative of a gradual evolution. New structural, developmental and physiological data on mosses and hornworts increasingly point to the primeval function of stomata as facilitators of sporangium desiccation and spore dispersal. In hornworts, absence of potassium pumps together with apertures that are non-responsive to environmental stimuli support this notion. Similarly in mosses developmental changes in guard cell wall biochemistry render the apertures immovable.  In addition, the sporophytic intercellular spaces in both these groups are initially liquid- filled whereas in vascular plants they are gas-filled from the outset.

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11:30-11:45 Discussion

11:45-12:15 Exploring the geochemical distribution of organic carbon in early land plants

Dr Geoff Abbott, Newcastle University, UK

Abstract

Phytoterrestrialisation resulted in anatomical innovations which were accompanied by the evolution of biosynthetic pathways for secondary metabolites such as lignin. The question is whether or not the high resolution spatial distribution of the diagenetic products of such metabolites and cell wall materials can be correlated with microscopic structures. Surface analytical techniques including helium ion microscopy (HIM), X-ray photoelectron spectroscopy (XPS) and time-of-flight mass spectrometry (ToF-SIMS) have been used to meet this challenge. An argon gas cluster ion beam source was rastered over analysis areas to remove any post-depositional ingress from the top 3 nm of the specimen surface. The resolution of the HIM is 0.3 nm. The bonding environments of the organic carbon were identified using XPS on Rhynia gwynne-vaughanii and then ToF-SIMS was used to map the distribution of positively charged fragment ions for both aromatic and aliphatic hydrocarbons as well as silicon across the diameter of a Rhynia stem from the Rhynie Chert. This presentation will give an overview of high resolution surface analytical techniques that may help us gain a deeper understanding of the distribution of metabolites within the organs and tissues of early land plants and associated microbial systems.

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12:15-12:30 Discussion

12:30-13:30 Lunch

13:30-17:00

Session 4

7 talks Show detail Hide detail

Chairs

Professsor Rosmarie Honegger, University of Zurich, Switzerland

13:30-14:00 Microfungi and microfungal interactions: diversity and ecological roles

Professor Michael Krings, Bayerische Staatsammlung fur Palaeontologie and Geologie, Germany

Abstract

The Lower Devonian Rhynie chert is paramount to our conception of the roles of fungi in early continental ecosystems. However, this idea is based on a small number of well-documented specific interactions, whereas the majority of Rhynie chert fungi have received little or no attention. Hyphae, mycelial cords, vesicles, and reproductive units are almost ubiquitous in the Rhynie chert matrix, in litter layers, and within plant parts. The abundance and morphological variability of these remains suggest that fungi were important components of many vital processes that sustained the Rhynie ecosystem. Attempts to estimate fungal paleobiodiversity and specify what ecological roles these organisms played are generally hampered by the fact that the vast majority of fungal remains occur detached from the systems on or in which they were produced, and thus do not provide a complete range of features necessary to determine their systematic affinities. Nevertheless, certain fungal remains display structural features that, albeit not diagnostic, are consistent among specimens, and thus make it possible to recognize distinctiveness. At high magnification, others reveal consistent patterns of association suggestive of specific organismal interactions. These fossils contribute greatly to a more accurate understanding of the complex microbial interrelatedness in early continental ecosystems.

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14:00-14:15 Discussion

14:15-14:45 The origin and evolution of plant/fungal interactions

Dr Silvia Pressel, The Natural History Museum, London, UK
Dr Christine Strullu-Derrien, The Natural History Museum, London, UK

Abstract

Associations between plants and fungi are ancient, dating back at least 460 million years, and are widely considered to have had a fundamental role in the evolution of terrestrial ecosystems.  A key idea, first proposed by Pirozynski and Malloch in 1975, is that the evolution of symbioses between semi-aquatic green algae and aquatic fungal-like organisms propelled plant terrestrialisation, enabling both partners to overcome the novel challenges of life on land. Multidisciplinary approaches to studying fungal partnerships in extant ‘lower’ land plants, the closest living relatives to the first land colonisers, are providing novel insights into the origin and evolution of plant-fungus associations, showing these to involve diverse fungal partners, including members of the Mucoromycotina - an ancient but poorly understood lineage.  Distinct cytological features of colonization, separating for example Mucoromycotina and Glomeromycotina symbionts in living basal land plants, allow for a re-assessment of fungal associations in fossil plants.  Conversely, well-defined fossils are required for calibrating molecular clocks, providing more accurate date estimates for the origin and diversification of clades as well as revealing extinct and sometimes novel combinations of features and associations.

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14:45-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 Plant-fungal symbioses in relation to early land environment

Dr Katie Field, University of Leeds, UK

Abstract

The colonisation of Earth’s land masses by plants >475 Ma marked a turning point in the development of the biosphere and atmosphere, with widespread consequences for the future of terrestrial life. Rhynie Chert fossils indicate early plants hosted diverse fungal endophytes structurally analogous to mycorrhizal associations in modern plants. Recent research has suggested greater diversity in fungal symbiont identity, structure, and functioning in the earliest diverging clade of extant land plants – liverworts – giving rise to the hypothesis that symbiotic options were available to the earliest land plants. The rise of plants likely drove the long-term shift towards the modern climate, but little is known about how early plant-fungal symbiosis may have influenced this. Experiments show that liverwort-fungal symbiotic function varies according to fungal identity, with carbon-for-phosphorus exchange either enhanced or suppressed under simulated-Paleozoic [CO2]. By incorporating these findings into biogeochemical models, we show that differences in symbiotic nutrient acquisition strategies can greatly alter the Paleo-environmental conditions sufficient to drawdown [CO2] to glacial levels and to either promote or delay the rise of oxygen. The group conclude that an accurate depiction of plant-fungal systems, informed by experiments, is key to resolving the question of how the first terrestrial ecosystems altered our planet.

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16:00-16:15 Discussion

16:15-17:00 Panel discussion

Professor Dianne Edwards CBE FRS, Cardiff University, UK

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The Rhynie Chert – our earliest terrestrial ecosystem revisited The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK