The record of Ordovician Eurypterida from New York State, USA, is shown to be largely false. Twenty-nine species in 17 genera are here recognised as pseudofossils, reducing by more than 75% the total number of named Ordovician eurypterid taxa. Consequently, 10 families now have their first occurrence either later in the Ordovician or in the Early Silurian. The implications for eurypterid palaeoecology, diversity and evolution are not as straightforward as would be expected from such a drastic taxonomic revision. All Ordovician eurypterids are now known to occur in shallow-water, near-shore shales or fine-grained carbonates. Diversity measures indicate that the end-Ordovician extinction event appears to have had less effect on eurypterids than previously known, and their turnover is level in the Ordovician.

A new aquatic crustacean is described from the Early Devonian Rhynie chert. The small, broadly oviform carapace consists of a univalve symmetrical about its longitudinal axis, formed by a continuous cuticle with no fold or hinge. In transverse cross-section, the ventral surface exhibits a broadly ‘W’-shaped outline. Areas of fine, setose objects are preserved, positioned antero-ventrally on either side of the mid-line axis. In transverse sections, a ring-shaped internal structure is also visible. In longitudinal cross-section, this structure is resolved into an axially positioned, internally subdivided tube occupying the mid- and posterior part of the carapace. A small, anteriorly positioned rostrum has a deep pit on each side, perhaps indicative of the socket of an antennal appendage. The arthropods are commonly found clustered together around plant axes, comprising groups of up to 25 or more similarly sized individuals. The chert texture enclosing many individual specimens indicates a sub-aqueous preservational environment. The same organism has recently been discovered in preparations from the nearby and equivalently aged Windyfield hot spring deposit. The morphology of this new arthropod suggests affinity with Branchiopoda and it is tentatively placed within the Diplostraca (an order which contains both ‘conchostracans’ and ‘cladocerans’, or water fleas). The presence of a univalve test suggests a cladoceran affinity, but the lack of diagnostic appendages in the present material does not allow us to take the classification any further. This is potentially the earliest known occurrence of the group and would extend their fossil record back from the Early Cretaceous. The remarkable similarity between these arthropods and their present-day representatives strongly indicates morphological stasis within the group from early on in their evolutionary history

Study of abundant phalangiotarbid (Arachnida: Phalangiotarbida) material – provisionally assigned here to Bornatarbus mayasii (Haupt in Nindel 1955) – from the Upper Carboniferous of Writhlington, UK, has revealed new information about some previously equivocal characters. The present authors report a trifurcate apotele, possible spiracles on sternite 5, and confirm the presence of 10 opisthosomal tergites plus a dorsal anal operculum. The affinities of phalangiotarbids are obscure, with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks). Phalangiotarbida is scored for characters used in previous studies of arachnid relationships. A cladistic analysis based on 63 characters using 13 terminal arachnid taxa (plus a hypothetical outgroup), resolves Phalangiotarbida as sister group to (Palpigradi + Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990). Even under cladistic analysis, the position of the Phalangiotarbida remains hard to resolve, but a prosomal sternite with distinct sclerites potentially groups them with the Megoperculata.

Fossils in the Pragian Rhynie cherts, Aberdeenshire, Scotland, are preserved with exquisite cellular detail, and provide much information on Early Devonian terrestrial and freshwater ecosystems. The fossils include abundant and diverse coprolites which demonstrate the existence of consumers differing in life-habit and diet. The coprolites are small (0.5–3 mm) and diverse in morphology and content, including groups of amorphous coprolites as well as coprolites with identifiable, particulate content. The present authors define three new ichnogenera to accommodate these coprolites:

• • Lancifaex encompasses elongate coprolites with particulate content, and includes three ichnospecies, distinguished on morphology, L. simplex, L. divisa and L. moniliforma.

• • Rotundafaex encompasses rotund coprolites with particulate content, and includes a single ichnospecies, R. aggregata.

• • Bacillafaex encompasses rod-shaped coprolites with amorphous content, and includes two ichnospecies, distinguished on size, B. constipatus and B. mina.

The first fossil opilioacariform mite (Acari: Opilioacariformes), Paracarus pristinus sp. n., is described from Baltic amber (Tertiary: Eocene). This well-preserved inclusion has six eyes and, thus, can be excluded from the derived, four-eyed genera. Based on characters such as the extremely long leg 4, it is assigned to the extant central Asian genus Paracarus Chamberlin & Mulaik, 1942. As with many Baltic amber inclusions, we lack an exact provenance for the type specimen, but the Baltic region is further north than the present day range of Recent opilioacariform species and suggests that the range of this group was once more extensive within Eurasia. The first solifuge (Solifugae) from Baltic amber is described as Palaeoblossia groehni gen. et sp. n. Based on its size and general appearance, it can be assigned to Daesiidae, and again represents an arachnid group which previously had a much wider geographical range. If adult, it may represent the smallest solifuge known.

Thirteen specimens of type and/or figured Hexapoda from the Rhynie chert are housed at The Natural History Museum, London, UK. They are listed along with a complete bibliography of where they have been cited and figured. Two taxa are present: Rhyniella praecursor Hirst & Maulik 1926 (Collembola); and Rhyniognatha hirsti Tillyard 1928 (Insecta)

The currently accepted cladogram of spider phylogeny and palaeontological data are used to evaluate spider family richness through geological time. A significantly more diverse spider fossil record is predicted than observed. The predicted rate of spider family diversification is considered more accurate because of its close similarity at 0 Ma to the number of extant families. Predicted spider family palaeodiversity is compared with insect family palaeodiversity to investigate whether spiders track insects through geological time. At the family level, the insects, and observed and predicted spider fossil records show an exponential increase over time, the pattern typical of a radiating group. No significant differences are observed in the rates of change in the slopes, and hence rate of diversification of spiders and insects over time. This suggests that spiders probably co-radiated alongside the insects, with the major radiations of both groups occurring at least 100 Ma before the origin of angiosperms.

Brief descriptions and comments on relationships are given for the seven embryophytic sporophytes in the cherts at Rhynie, Aberdeenshire, Scotland. They are Rhynia gwynnevaughanii Kidston & Lang, Aglaophyton major D. S. Edwards, Horneophyton lignieri Barghoorn & Darrah, Asteroxylon mackiei Kidston & Lang, Nothia aphylla Lyon ex Høeg, Trichopherophyton teuchansii Lyon & Edwards and Ventarura lyonii Powell, Edwards & Trewin. The superb preservation of the silica permineralisations produced in the hot spring environment provides remarkable insights into the anatomy of early land plants which are not available from compression fossils and other modes of permineralisation. They include soft tissues, such as those surrounding stomata, rhizoids, apical and lateral meristems, and diversity in conducting cells, with inferences for palaeoecophysiology, including water use efficiency, transport and absorption, and for growth processes and patterns.

Several new gametophytes are described from the Early Devonian Rhynie chert. The new monotypic genus Remyophyton, being the gametophyte of Rhynia, is represented by a dense stand of in situ preserved gametophytes with antheridia- and archegonia-bearing axes. The gametophytes are remarkably small in comparison to those of other Rhynie chert plants. The generic diagnoses of Lyonophyton and Langiophyton are emended to include archegonia- and antheridiabearing axes. All essential stages of the reproductive cycle, i.e. sporophytes, and male and female gametophytes, can now be demonstrated for three of the six land plants from the Rhynie chert, i.e. Rhynia gwynne-vaughanii, Aglaophyton major and Horneophyton lignieri.

Detailed studies have been undertaken on in situ spores of the plant Horneophyton lignieri (Kidston & Lang) Barghoorn & Darrah, 1938 preserved in the Rhynie chert. It is concluded that mature spores are trilete, with proximal radial ribbing, and a distal ornament of coni. These spores are identical to Emphanisporites decoratus Allen, 1965, a widely reported and morphologically distinctive dispersed spore taxon. Emphanisporites decoratus is a common element of dispersed spore assemblages from throughout the stratigraphical sequence of the Rhynie outlier, and it is concluded that the dispersed spore taxon E. decoratus was produced by H. lignieri. The nature (reproductive biology, ecology and distribution) of H. lignieri is interpreted on the basis of evidence from the dispersed spore record from the Rhynie outlier and elsewhere. It is concluded that H. lignieri was a common and widespread plant of the Lower Devonian. It flourished in the unusual hot springs environment of the Rhynie chert because it was preadapted to do so, not because it was a highly specialized and adapted plant. Integrated evidence from the in situ and dispersed spore records permits a detailed consideration of the biology of the plant H. lignieri.

The discovery of new specimens of Palaeonitella cranii (Kidston & Lang) Pia in the Early Devonian Rhynie chert permits the classification of this charophyte alga. This relatively small charophyte is composed of multicellular furcating branchlets with antheridia attached to the parent plant by a pedicel. The non-calcified gyrogonites, which are not seen in life position, are composed of six sinistrally spiralling cells and have six coronula cells arranged in a single layer around an apical pore.

Because the oogonia of P. cranii have six sinistrally spiralling cells and an apical pore, Palaeonitella has been assigned to the Palaeocharaceae. The shape and form of the oogonia is reminiscent of the extant Chareae, but the morphology of the thalli is indicative of the Nitelleae. This suggests that P. cranii, although retaining some characteristics of the Nitelleae, has some of the characteristics of the Chareae, supporting existing molecular studies which place the Nitelleae at the base of the Characeae.

Palaeonitella cranii inhabited the freshwater alkaline pools and streams which formed on sinter aprons created by hot spring activity. Extant charophytes exist in a similar environment (pH 8·66–8·9, temperature 25–30°C in summer) in pools and channels fed by hot springs in Yellowstone National Park, Wyoming, USA.

The exquisite preservation of organisms in the Early Devonian Rhynie chert ecosystem has permitted the documentation of the morphology and life history biology of fungi belonging to several major taxonomic groups (e.g., Chytridiomycota, Ascomycota, Glomeromycota). The Rhynie chert also provides the first unequivocal evidence in the fossil record of fungal interactions that can in turn be compared with those in modern ecosystems. These interactions in the Rhynie chert involve both green algae and macroplants, with examples of saprophytism, parasitism, and mutualism, including the earliest mycorrhizal associations and lichen symbiosis known to date in the fossil record. Especially significant are several types of specific host responses to fungal infection that indicate that these plants had already evolved methods of defence similar and perhaps analogous to those of extant plants. This suggests that mechanisms underlying the establishment and sustenance of associations of fungi with land plants were well in place prior to the Early Devonian. In addition, a more complete understanding of the microbial organisms involved in this complex ecosystem can also provide calibration points for phylogenies based on molecular data analysis. The richness of the microbial community in the Rhynie chert holds tremendous potential for documenting additional fungal groups, which permits speculation about further interactions with abiotic and biotic components of the environment.

Modern, silica-precipitating hot springs, like those found in the Taupo Volcanic Zone (TVZ) on the North Island of New Zealand, are natural laboratories for assessing microbial silicification. Many of the silicified microbes found in the siliceous sinters of these spring systems seem to be life-like replicas of the original microbes. Such preservation reflects the fact that many of the microbes are replaced and encrusted by opal-A before they are destroyed by desiccation and decay. The taxonomic fidelity of these silicified microbes depends on the preservation potential of those features which are needed to identify them. For example, identification of extant cyanobacteria, relies on as many as 37 different features, most of which are not preserved by silicification.

In the hot-spring systems of the TVZ, characterisation of cyanobacteria which have been replaced and encrusted by opal-A is typically restricted to colony morphology, the length, diameter and morphology of the filament, and the presence/absence of septa, branching or a sheath. In many cases, description is limited to a subset of these parameters. Such a limited set of morphological characteristics severely impedes identifications in terms of extant taxa. The physical changes which accompany the stepwise diagenetic progression from opal-A to opal-CT ± opal-C to microcrystalline quartz may lead to further degradation of the silicified microbes and the loss of more taxonomically important features. Clearly, considerable care must be taken when trying to name silicified microorganisms and make palaeoenvironmental inferences.

The Tahunaatara sinter, Taupo Volcanic Zone, New Zealand, is a ∼17–20-kyr-old hot-spring deposit of opal-A mineralogy. It is interbedded with fluvial, lacustrine and volcaniclastic sediments, some silicified by infusing thermal waters. The exposed sinter (∼4 m thick, 90 m long) was truncated at its southern margin by a landslide, which deposited a conglomerate (up to 2 m thick, 56 m long) of sinter blocks and associated strata nearby. Kaolinite-rich cobbles at the base of the conglomerate indicate a change in the thermal regime and its probable trigger: acid steamcondensate produced alteration. Clasts in the landslide are oriented SW, the same direction as flattened plant reeds entombed in sinter, and as intercalated fluvial beds. Thus, thermal waters, stream flow and the landslide all likely followed the same palaeo-valley, which is similar in terrain and stratigraphy to the Devonian Rhynie hydrothermal system. The plant-rich, layered, in situ sinter contains fossilised microbes and rare stromatolites, and was deposited on mid- to distal slopes adjacent to marshes. Ash falls, fluvial activity and ponding occurred during and after the thermal activity. Unsilicified tephric Ohakea loess (∼26–17 kyr BP) and Taupo Tephra (1·86 kyr BP) blanket both sinter and landslide. Today, the deposits form resistant remnants in a topographically inverted landscape.

During experiments conducted within the vent pool of Medusa Geyser, Norris Geyser Basin, Yellowstone National Park, USA, amorphous opaline silica (opal-A) was deposited on/within plant tissues within 30 days of immersion. Initially, deposition created inter/intra-cellular films which lined cell walls plus intercellular colloid suspensions (sols) of opal-A nano/microspheres. By 330 days, opal-A deposition created a robust external and internal matrix that stabilised tissues against collapse and replicated plant structure. Opal-A films increased to micron-order thicknesses and intracellular sols were created. Systematic variation of opal-A fabric between tissues comprising living/dead cells at the time of deposition indicate that cell function, architecture and shape influence fabric development. Heterogeneity of opal-A fabric within adjacent cells of similar structure/function indicates spatially/temporally fluctuating physicochemical conditions and the presence of intraorganic microenvironments. Early deposition of opal-A films suggests a period of low silica supersaturation and slow opal-A deposition. In contrast, intracellular sols suggest high levels of supersaturation, and rapid opal-A deposition. Shell-like microsphere growth suggests cyclic variation of silica supersaturation, and alternations between rapid and slower opal-A deposition. Microsphere growth to the upper limit of colloidal stability and colloidal crystal structures indicate prolonged sol stability, whilst floc-like microsphere networks indicate localised sol instabi