FOSSIL INVERTEBRATES/Arthropods 277 is difficult to determine from the fossil forms, extant members of the ancient lineages can lend clues as to what these pathways might have been For example, osmotic concentrations of body fluids in myriapods suggest that this group terrestrialized straight from the sea without an intermediate step in freshwater One particular locality which has yielded a disproportionately large amount of data on early terrestrial ecosystems is the Devonian Rhynie Chert Lagerstaătte in Aberdeenshire, Scotland Here, the silica-mineralizing action of hot spring and geyser fluids captured, in three-dimensional detail, early terrestrial arthropods, such as arachnids, insects, and centipedes, as well as aquatic forms, such as freshwater crustaceans and euthycarcinoids The remarkable morphological stasis in early terrestrial arthropods, in relation to extant forms, suggests that arthropod-dominated terrestrial ecosystems stabilized early on in the Earth’s history and that soil and litter habitats and inhabitants have changed very little ever since One of the novel innovations of arthropods was the evolution of muscle-powered flight in the insects (see Fossil Invertebrates: Insects) The earliest insects or apterygotes (insects without wings) first appear in sediments of Devonian age in the form of collembolans and thysanurans In earliest Carboniferous times, the major expansion and evolution of land-based flora provided new ecological niches available to this highly adaptable group The advent of insects and the development of flight led to the colonization of an entirely new ecospace Interestingly, the earliest non-scorpion arachnids appear to have relied solely on poisoned fangs to capture and subdue prey items Orb web-weaving using spider silk could only have developed in response to the presence of aerial insects By Lower Carboniferous times, the first winged insects had developed and an explosion in the diversity of insects occurred due to coal swamp forest ecosystems Preservation and discovery are aided by the presence of siderite concretions in the roof shales to many coal seams which, in turn, are commercially exploitable During the Middle Carboniferous, insects became diverse and widespread for the first time in their fossil record This trend continued through the Carboniferous and into the Permian period, whereupon an explosion in the total diversity of fossil insects took place The depositional environments in which these fossils are found not differ significantly, suggesting a real increase in diversity During the Jurassic and Cretaceous, favourable shallow (epieric) sea conditions saw the emergence and dominance of decapod crustaceans as a major player in nearshore marine ecosystems This development within the Crustacea continues to the present day, with arthropods still developing new and successive waves of colonization, particularly in terms of the terrestrialization of some decapod crustaceans, such as the land crabs Respiration From the same basic body plans, different solutions to the challenges of respiration in aquatic and terrestrial environments have evolved In the smallest of microarthropods, cutaneous respiration, by which the skin acts as a gas exchange surface, takes place The ratio of surface area to internal volume limits the maximum size of arthropod which can utilise this method In larger aquatic arthropods, respiratory gas exchange is through structures termed ‘book gills’ These thin lamellate cuticular structures are bathed in oxygen-containing water, either wholly external to the body, as in the gill branches of trilobites, or in a specialized cavity, as in the horseshoe crabs (Figure 2) Terrestrial arthropods show adaptation to the particular challenges of living in an environment in which the constant danger of desiccation means that Figure The xiphosuran, Rolfeia sp., from the Lower Carboniferous of Mumbie Quarry, Glencartholm, Dumfries and Galloway, Scotland (Â1.1) ß The Trustees of the National Museums of Scotland, Edinburgh, UK (NMS G.1998.35.3)