Palaeoecology and palaeophytogeography of the Rhynie chert plants: further evidence from integrated analysis of in situ and dispersed spores

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'

Palaeontology: The Rhynie chert is the gift that keeps on giving

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

Palaeoecology of a billion-year-old non-marine cyanobacterium from the Torridon Group and Nonesuch Formation

A new chroococcalean cyanobacterium is described from approximately 1-billion-year-old non-marine deposits of the Torridonian Group of Scotland and the Nonesuch Formation of Michigan, USA. Individual cells of the new microfossil, Eohalothece lacustrina gen. et sp. nov., are associated with benthic microbial biofilms, but the majority of samples are recovered in palynological preparations in the form of large, apparently planktonic colonies, similar to extant species of Microcystis. In the Torridonian, Eohalothece is associated with phosphatic nodules, and we have developed a novel hypothesis linking Eohalothece to phosphate deposition in ancient freshwater settings. Extant cyanobacteria can be prolific producers of extracellular microcystins, which are non-ribosomal polypeptide phosphatase inhibitors. Microcystins may have promoted the retention and concentration of sedimentary organic phosphate prior to mineralization of francolite and nodule formation. This has a further implication that the Torridonian lakes were nitrogen limited as the release of microcystins is enhanced under such conditions today. The abundance and wide distribution of Eohalothece lacustrina attests to the importance of cyanobacteria as oxygen-producing photoautotrophs in lacustrine ecosystems at the time of the Mesoproterozoic–Neoproterozoic transition

Middle Jurassic vegetation dynamics based on quantitative analysis of spore/pollen assemblages from the Ravenscar Group, North Yorkshire, UK

Quantitative analysis of the distribution of dispersed spores and pollen (sporomorphs) has been used to assess temporal floral variation through the Middle Jurassic Ravenscar Group (Aalenian–Bathonian), North Yorkshire, UK. Aalenian, Bajocian and Bathonian strata possess relatively distinct sporomorph and palynofacies assemblages, which potentially reflect a dynamic history regarding the nature of parent vegetation. Specifically, Aalenian palynofloras are composed of a heterogeneous mixture of conifers, ferns, simple monosulcate pollen producers, sphenophytes and Caytoniales; Bajocian palynofloras are codominated by conifers and ferns; and Bathonian palynofloras are highly rich and contain assemblages of abundant ferns, conifers, lycophytes, pteridosperms/conifers and Caytoniales. Individual- and sample-based rarefaction demonstrates that Bathonian samples are richer than Aalenian and Bajocian samples. Temporal variations in assemblages are a result of long-term depositional and possible climatic fluctuations through the Middle Jurassic. Ordinations of sporomorph data using non-metric multidimensional scaling (NMDS) demonstrate that short-term variations between samples are largely governed by taphonomic biases as a result of slight changes in depositional processes, which give rise to highly variable catchment areas that supply deposits with sporomorphs. Long-term compositional changes are apparent in sporomorph assemblages regardless of lithology/local depositional environments, suggesting that long-term variations are more substantial than short-term variations and potentially include genuine regional temporal changes in parent vegetation. Relating sporomorph assemblages with their respective depositional environments and relative catchment area sizes using lithological and palynofacies information suggests that the basin interior was occupied by mostly low-standing species and extrabasinal vegetation was dominated by coniferous taxa. Comparisons of the dispersed sporomorph and plant megafossil records indicate that both fossil assemblages reflect different aspects of the palaeoflora due to a multitude of taphonomic and ecological biases. Such biases include variation in sporomorph production levels, depositional environment and differential sporomorph and parent plant durability

Palynological analysis of Upper Ordovician to Lower Silurian sediments from the Diyarbakir Basin, southeastern Turkey

This paper reports on a palynological analysis of 41 core and 21 cutting samples from a well drilled through an Upper Ordovician–Lower Silurian sequence, belonging to the Bedinan and Dadas formations, of the Diyarbakır Basin of southeastern Turkey. The samples yield abundant and well-preserved marine palynomorphs (acritarchs, chitinozoans and scolecodonts) although non-marine palynomorphs (spores/cryptospores) are extremely rare or absent. The Upper Ordovician sediments of the Bedinan Formation have low organic content but contain abundant palynomorphs whereas the Lower Silurian sediments of the Dadas Formation have high organic content, dominated by amorphous organic matter, with relatively rare palynomorphs. Three chitinozoan assemblages are identified and attributed a late Katian (merga Biozone), Hirnantian (moussegoudaensis Biozone) and Llandovery (?alargada Biozone) age. Two acritarch assemblages are identified and attributed a Katian–Hirnantian and Llandovery (Aeronian–Telychian) age. The chitinozoan and acritarch age determinations are compatible and suggest that the Bedinan Formation is of Katian–Hirnantian age and is separated by an unconformity from the Dadas Formation that is of Llandovery (Aeronian–Telychian) age. These findings confirm the presence of an unconformity at the Ordovician–Silurian transition in the southeastern Turkey. Palynofacies analysis suggests that the Bedinan Formation accumulated on an offshore shelf that was initially well oxygenated but became increasingly anoxic, whereas the Dadas Formation accumulated in an offshore basin that was anoxic. Palynomorph assemblages recorded in the Bedinan and Dadas formations indicate northern Gondwana affinity

Reply to discussion on 'A high-precision U-Pb age constraint on the Rhynie Chert Konservat-Lagerstatte: time scale and other implications': Journal, 168, 863-872

We welcome the opportunity to address the points raised by Mark et al. in their discussion of the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA–ID–TIMS) U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte presented by Parry et al. (2011) and also to make some further observations of our own. We begin by briefly providing some context for the benefit of the wider readership. Two radio-isotopic age constraints on the Rhynie Chert Konservat-Lagerstätte and, by corollary, its parental hydrothermal (hot-spring) system have recently been published. The first of these is a weighted mean 40Ar/39Ar plateau age of 403.9 ± 2.1 Ma (2σ) derived from the analysis of two samples of vein-hosted hydrothermal K-feldspar and a single sample of hydrothermally altered andesite (Mark et al. 2011). In order to account for systematic uncertainties associated with the 40Ar/39Ar geochronometer, Mark et al. (2011) recalculated their individual sample ages with reference to the Fish Canyon Tuff sanidine (FCs) age of 28.201 Ma (Kuiper et al. 2008), thereby producing a ‘U–Pb comparable’ mean age of 407.1 ± 2.2 Ma (2σ). An alternative ‘preferred age’ for the Rhynie hot-spring activity (407.6 ± 2.2 Ma (2σ)) has now been produced from the ‘raw’ data using the optimization model of Renne et al. (2010, 2011) (this discussion). The 40Ar/39Ar system calibrations on which these various ages are based are summarized in Table 1. The second radio-isotopic age constraint in question is a weighted mean 206Pb/238U zircon age of 411.5 ± 1.3 Ma (2σ, including decay constant- and tracer calibration-related uncertainties; MSWD = 0.12, n = 4) yielded by the Milton of Noth Andesite, a moderately altered basaltic andesite lava flow (cum near-surface sill?) that lies along the northwestern margin of the Rhynie Outlier (Parry et al. 2011). U–Pb titanite data corroborate the zircon data, and c. 411.5 Ma is interpreted as the crystallization-eruption age of the Milton of Noth Andesite. Lavas and tuffs of andesitic composition occur elsewhere within the northern half of the Rhynie Outlier (Rice & Ashcroft 2004) and a holistic view of the available evidence would suggest that these volcanic rocks represent the surficial expression of the thermal drive for the Rhynie hot-spring system. Parry et al. (2011) therefore concluded that that the U–Pb zircon age yielded by the Milton of Noth Andesite dates the Rhynie hydrothermal activity within error [our italics]

Early land plant phytodebris

Historically, phytodebris (often considered a type of non-pollen palynomorph – NPP) has played a prominent role in research into the fossil record of early land plants. This phytodebris consists of cuticles and cuticle-like sheets, various tubular structures (including tracheids and tracheid-like tubes) and sundry other enigmatic fragments. Initial research focused on elucidating their morphology, attempts to identify them in situ in plant megafossils and comparisons with potentially homologous structures in extant plants. The fragmentary nature of these remains, and associated difficulties in positively identifying their presence in fossil/extant plants, resulted in vigorous debate regarding what many of these microfossils actually represented and their relevance to early land plant studies. More recently a wider array of analytical techniques has been applied (e.g. ultrastructural analysis, geochemistry and taphonomic experiments). However, positive identification of the affinities of at least some of these enigmatic fossils remained elusive. Ongoing investigations based on exceptionally preserved material from Lagerstätten (charcoalified and silicified) seem to have finally demonstrated that the more enigmatic of these remains derive from nematophytes that probably represent fungi and possibly also lichenized fungi

The Nonesuch Formation Lagerstätte: a rare window into freshwater life one billion years ago

The Nonesuch Formation in the clastic sedimentary Oronto Group on the Keweenaw Peninsula of the Upper Peninsula, Michigan, USA most likely represents an ancient lake that formed between 1083 and 1070 Ma. Exceptional preservation, seen in palynological preparations, provides a snapshot of cell morphology, biological complexity and ecology at an early stage in the evolution of the eukaryotes. A wide range of unicellular organization is documented in both vegetative and encysted cell morphologies, but the extent to which multicellularity is developed seems very limited at this time. Overall, the Nonesuch microbiota, when viewed as a Lagerstätte, opens up a window onto the early evolution of unicellular eukaryotes, presenting an essential baseline of both eukaryotic diversity and cell structure well in advance of eukaryotic diversification documented in marine deposits from the later Neoproterozoic

Reconstructing the terrestrial flora and marine plankton of the Middle Devonian of Spain : implications for biotic interchange and palaeogeography

A rich and well-preserved palynomorph assemblage from the Middle Devonian of northern Spain is analysed with regard to palaeobiogeography and palaeocontinental reconstruction. The communities of terrestrial plants (dispersed spores), marine zooplankton (chitinozoans) and marine phytoplankton (acritarchs and prasinophytes) all show significant endemism. They are depauperate in some respects, missing common species found elsewhere, but also containing many endemic taxa. Measures of similarity and cluster analysis reveal little relatedness to other contemporary assemblages, with both the spore and phytoplankton communities being sister groups to all other communities except one. Attempted correlation with local, regional and global sea-level and transgression–regression schemes is complicated by the absence of key index palynomorphs in this unusual assemblage. The distinctive sedimentary sequence of northern Spain may reflect an increased clastic input to the marine shelf resulting from an increasingly monsoonal climate, possibly connected to the Kačák extinction event. The assemblage's unusual, endemic character requires restrictions on dispersal. The terrestrial spore assemblage suggests that large tracts of ocean existed, without appreciable land bridges, between the Armorican Terrane Assemblage and Laurussia to the north and Gondwana to the south. The phytoplankton communities support existing evidence for significant east to west ocean currents through the Rheic Ocean