The Rhynie Chert - setting the scene
Professor Dianne Edwards CBE FRS, Cardiff University, UK
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.
Modern hot spring analogues
Dr Alan Channing, Cardiff University, UK
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.
How unique were the Rhynie plants?
Professor Charles Wellman, University of Sheffield, UK
Professor Dianne Edwards CBE FRS, Cardiff University, UK
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.
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
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.
Terrestrial invertebrates in the Rhynie Chert ecosystem
Dr Jason Dunlop, Museum fur Naturkunde, Germany
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.