The ecological ranges of Archaeobacteria and Eubacteria are constrained by a requirement for liquid water and the physico-chemical stability limits of biomolecules, but within this broad envelope, prokaryotes have evolved adaptations that permit them to tolerate a remarkable spectrum of habitats. Laboratory experiments indicate that prokaryotes can adapt rapidly to novel environmental conditions, yet geological studies suggest early diversification and long-term stasis within the prokaryotic kingdoms. These apparently contradictory perspectives can be reconciled by understanding that, in general, rates and patterns of prokaryotic evolution reflect the developmental history of the Earth's surface environments. Our understanding of modern microbial ecology provides a lens through which our accumulating knowledge of physiology, molecular phylogeny and the Earth's history can be integrated and focussed on the phenomenon of prokaryotic evolution.

When compared with sea water, most marine invertebrates are isosmotic but show varying degrees of ionic regulation, from accumulation of potassium and calcium to reductions of magnesium and sulphate. A reduction of sulphate in pelagic animals such as jellyfish and salps, and an accumulation of ammonium ions in arrow-worms (Sagitta) contribute to their near-neutral buoyancy. Marked reductions in salinity exclude mahy marine invertebrates. In the Baltic, polychaetes, bivalve molluscs and decapod crustaceans are reduced from 193, ninety-two and sixty-four species in the Belt Sea (salinity S 10-30‰) to three, four and two in the Gulf of Finland (salinity S 5-9‰). The chelicerate Limulus can stand a wide range in salinity, a few insect larvae can tolerate concentrations of 200% sea water, while the branchiopod shrimp Artemia can stand crystallising brine (36-37% NaCl). Very few species can tolerate temperatures of 40°C. One such species is the polychaete Alvinella pompejana, a hydrothermal vent animal at East Pacific Ridge (to the W of S America). The harpacticid copepod Tigriopus living in evaporating rockpools has a lethal temperature of 42°C at a salinity of S 90‰, but at S 8‰ that temperature is 34°C. Lack of oxygen and presence of hydrogen sulphide limit the distribution of animals in certain areas. Most active animals have respiratory pigments in their principal body fluid. Burrowing invertebrates such as Arenicola and Lingula have respectively haemoglobin in the blood and haemerythrin in the coelomic fluid, with mean oxygen capacities of 6 ml O2 per 100 ml in each case, compared to 0·6 and 0·5 ml in sea water at 10° and 20°C.

The nature of the fauna of brackish-water environments is reviewed. It is concluded that: (a) a specific brackish-water macrofauna does not exist; (b) in salinities of >c. 5-8‰ the fauna is one that also occurs in soft sediments under fully marine conditions when circumstances (possibly the absence of competing species) permit; (c) in salinities of <c. 5‰ the fauna is essentially a freshwater one; and (d) inability to cope physiologically with brackish water is not a factor of major importance in limiting species diversity in these habitats, except in the vicinity of 5‰ salinity. Caution is therefore advised in assigning brackish status to past environments on the basis of their preserved fauna.

The modes of life and environments of the extant agnathans (cyclostomes) are discussed in relation to their adaptations to temperature, light, oxygen and salinity. As their antitropical distribution indicates, both hagfishes and lampreys are cold water groups. Since hagfishes live in deeper waters than lampreys, they are not exposed to the marked seasonal changes in temperature and light which influence major events in the lamprey life cycle. Both groups tend to be nocturnally active, either burrowing during daylight as in the case of larval lampreys (ammocoetes) and most hagfishes, or showing cryptic behaviour as in the case of adult lampreys. Olfaction plays a major part in the location of prey, presumably aided in adult lampreys by their eyes and sensitive electrosensory system. Rates of standard oxygen consumption, ventilatory frequency and heart rate of adult lampreys increase at night. Standard oxygen consumption is relatively low in ammocoetes (as it also is in hagfishes) but increases markedly during metamorphosis into the adult lamprey. Ammocoetes and hagfishes, and to a lesser extent adult lampreys, are resistant to reduced environmental oxygen tensions. Differences in the oxygen dissociation curves of ammocoetes, adult lampreys and hagfishes can be related to differences in the characteristics of their monomeric haemoglobins and their environments and modes of life. The extraordinary tolerance of the hagfish heart to hypoxia is a reflection of a robust capacity for glycolysis, an LDH isozyme geared towards anaerobic functioning and a low work output. The hagfishes, which are restricted to marine waters, are osmoconformers. The osmolality of their blood, which is almost wholly attributable to inorganic ions, is virtually identical to that of full strength sea water (c. 1000 mOsmkg−1). By contrast, the osmolality of the blood of larval and adult lampreys when in fresh water is only 205-260 mOsm kg−1, i.e. about one quarter to one fifth of those of hagfish, and these rise only to 240-270 mOsm kg−1 in the adults of anadromous lampreys in sea water. The regulation of ions by adult lampreys is achieved by mechanisms similar to those adopted by teleosts. The implications of the contrasting ionic and osmotic physiology of the two living groups of agnathans are discussed in relation to their possible environmental history and against the background of their Carboniferous fossil representatives.

Ash-free-dry-weight determinations for a representative range of living brachiopod genera have revealed that a consistently high proportion of total organic mass is contained within the shell, partly as the organic matrix for biomineralisation and partly as minute extensions of the mantle tissues (caeca) housed within hollow endopunctae permeating the shell. On average 40% to 50% of the total organic mass of both articulate and inarticulate brachiopods is situated within the shell. This is true even for a rhynchonellid brachiopod which does not possess endopunctae, but which has a more dense protein matrix in its shell. The effectively hidden constituent of brachiopod tissue mass which is included in this component has often been overlooked, and as a result total metabolic tissue mass has been underestimated. This throws into question some previous interpretations of brachiopod respiratory and metabolic data.

The oxygen consumption rates of several living brachiopods have been measured, and when respiring tissue in caeca in the shell is taken into consideration, it is clear that brachiopod metabolic rates are low when compared with other marine invertebrates (e.g. between 10% and 50% of the oxygen uptake of comparable gastropods and bivalve molluscs held in similar conditions). This low rate cannot be attributed to a slower pumping rate by the brachiopod lophophore, as has been suggested, because the rate of water movement is comparable to that across the bivalve gill.

Nitrogen excretion rates have also been measured for a few living brachiopods, allowing a comparison with rates of oxygen consumption and providing an indication of the metabolic substrates used. These data on oxygen: nitrogen ratios suggest that one Antarctic brachiopod utilises exclusively protein as a metabolic substrate, while a temperate latitude species uses mainly protein during winter but lipids and carbohydrates during summer months. Histological observations, particularly of Terebratulina retusa from temperate waters, show that a specialised tissue layer in the brachiopod outer mantle epithelium proximal to the shell may be the site of storage of the protein that is metabolised during winter, and of carbohydrate mobilised during gonadal development in summer. The caeca have also been suggested as sites of storage of metabolites, and the possible relationships between these areas of mantle are discussed. It seems that the ability to store nutrients in the mantle, combined with flexibility of substrate utilisation and an inherently low metabolic rate, have been important factors in brachiopod evolution.

Present-day organisms have colonised two distinct high pressure environments: the deep sea and oil well and other crustal fluids. In the former, pressures attain 100 MPa and temperatures are generally less than 4°C. In the latter, the temperatures are high, up to 150°C, and occur in combination with pressures of up to 50 MPa. The high temperature is close to the limit of thermal stability of the macromolecules essential for life. The adaptation of present-day marine organisms to high pressure is known to involve modifications to their cell membrane lipids and subtle changes in both structural proteins and enzymes. There is no reason to suppose that they are close to the maximum pressure to which they could adapt and higher pressures could have been colonised in the geological past. The existence of “marker” compounds, characteristic of high pressure organisms, is discussed, and the possibility that isotope ratios are distorted by metabolic processes at high pressure is raised.

Discoveries, most of them recently, in more than thirty Lower and Middle Cambrian horizons with soft-bodied fossils have shown that forty-one of the genera occur also in the celebrated Burgess Shale (Middle Cambrian). Significantly, they tend to have lengthy stratigraphic durations which together encompass an interval from the early Lower Cambrian (Tommotian) to near the end of the Middle Cambrian. At least some genera have also wide geographical ranges, with occurrences around much of the Laurentian (N America) craton, and also in N and S China, Australia, Siberia, Spain and Poland. Although a few genera, e.g. Isoxys, may have been pelagic, for the most part these distributions are explained in terms of a deeper-water biota with an evolutionarily conservative aspect. Both the origins and further recruitment to this biota may have been from shallower water, with more limited in situ diversification. It is speculated that this distinctive Cambrian biota was gradually driven to extinction with the arrival of Ordovician competitors, although some relics may have survived until at least the Devonian. This history has implications for our understanding of deeper-water faunas throughout the Phanerozoic, and supports the notion that archaic forms may take refuge in this environment.

Examples of the mechanisms involved in body fluid regulation by present-day crustaceans inhabiting variable salinity habitats are described using amphipod gammarids and the isopod Mesidotea (Saduria) entomon as models. Appropriately the species inhabiting the most demanding habitats have the greatest range and most sophisticated regulatory responses. Behaviour, micromorphology and physiology are all involved to a variable degree and, in the examples discussed, responses to salinity change seem finely tuned to countering the problems generated by particular environments. This applies both in the rapid responses to sudden alteration in salinity and to the longer term changes associated with acclimation to a new steady state condition. The isolation of populations and features of derived freshwater races are considered and the implications for the presumed physiological mechanisms of fossil forms discussed.

Shrimp-like malacostracan crustaceans first appeared in the late Devonian and underwent a substantial adaptive radiation in the Carboniferous. They are rarely found in rocks of fully marine origin but are well represented in sediments laid down in brackish water and marginal marine conditions; such transitional environments provide the exceptional circumstances required for the preservation of unmineralised shrimps. The best examples are in the Dinantian of Scotland, the Namurian of Montana, and the Westphalian of Illinois. It is probable that shrimps were widespread in contemporaneous marine environments, but are not preserved.

Any approach to understanding the physiology of fossil organisms is necessarily indirect. The diversity of crustacean communities, and the nature of associated taxa and trace fossils, are the most useful biotic factors for interpreting the habitat and tolerance of fossil examples.

All known Carboniferous crustacean communities lived in brackish conditions; none is fully marine. Malacostracan assemblages in the Dinantian of Britain show a general trend of increasing diversity with salinity, from a single taxon at Gullane (stratified freshwater lake or brackish lagoon) to ten at Glencartholm (approaching normal marine). Tealliocaris is associated with low salinities. Crangopsis socialis is confined to the brackish water interdistributary bay environment, but Bairdops and Belotelson display a broader environmental tolerance. Crangopsis eskdalensis, Sairocaris, and Perimecturus occur only at Glencartholm, indicating a requirement for a strong marine influence. Those taxa confined to a limited environmental range all occur where a marine influence is pronounced; none occurs solely in areas of lower salinity. While salinity was apparently the dominant influence on distribution, a complex of independently varying environmental factors was involved. The range of habitats colonised early in the Carboniferous indicates that the preserved taxa had already developed advanced osmoregulatory mechanisms.

The wide range of organs of respiration (book-gills, book-lungs, sieve- and tube-tracheae), reproduction, sensory perception, etc., among the chelicerates indicates that the major groups made the transition to land life independently. The fossil record is patchy for most chelicerate groups, certain intervals (e.g. Westphalian) being particularly rich in chelicerate bearing Lagerstatten while in others (e.g. Mesozoic) they are sparse. Due, apparently, to their unusual hyaline exocuticle, scorpions are better preserved than other arthropods, and show a fairly continuous record from fully aquatic forms in the Silurian, to both aquatic and terrestrial faunas in the Carboniferous. In particular, new and well-preserved material of the earliest demonstrably terrestrial scorpions from the Lower Carboniferous of East Kirkton, West Lothian, suggests that book-lungs, at least in the scorpions, developed directly from book-gills by suturing of the covering plate (Blattfuss of the related eurypterids) to leave stigmata for diffusion of air. This evidence supports the ideas of early authors that the scorpion mesosomal ‘sternites’ are fused plates, contra Kjellesvig-Waering (1986) who envisaged the plates being lost to reveal true sternites beneath. The fossil evidence also indicates that by the Triassic at least two scorpion lineages had evolved intra-‘sternite’ stigmata.

Flight—defined as the ability to produce useful aerodynamic forces by flapping wings—is one of the most demanding adaptations in vertebrates. The mechanical problems of flight ensure considerable external morphological homogeneity and behavioural similarity in extant fliers. Observations of the vortex wakes and wingbeat geometry of modern birds and bats confirm that the two groups are mechanically very similar, despite differences in phylogeny, anatomy and physiology. With this background it is possible to attack two problems: the evolution of flight in vertebrates, and the flight performance of extinct animals such as pterosaurs and Archaeopteryx.

The origin of flight has been surrounded by considerable controversy, due in part to terminological inconsistencies, in part to phylogenetic uncertainty over the relationships of birds, bats and pterosaurs, in part to disagreement over the interpretation of the available fossil evidence, and in part to argument over the relative importance of morphological, mechanical and ecological specialisations. The mechanical changes needed in the course of the evolution of flight favour a gliding origin of tetrapod flight, and on mechanical and ecological grounds the alternative cursorial hypothesis may be discounted. This argument is particularly strong in bats, but has been thought to be weaker in birds owing to apparent inconsistencies with the fossil evidence. However, fossils of the Jurassic theropod dinosaur Archaeopteryx also support a gliding origin for flight, and suggest that this animal was adapted for flapping flight at moderately high speeds associated with gliding; it could fly less well at the slow speeds which would have been required for incipient flight in a running cursor, and at which the wingbeat is aerodynamically and kinematically considerably more complex. Slow flight in birds and bats is the more derived condition, and vertebrate flapping flight apparently evolved through a gliding stage.

The pterosaurs have become the subject of much controversy over the nature of their stance, the wing surface, and the degree of involvement of the leg in the wing membrane. Reconstruction of their wings indicates proficient flying animals, and comparison with birds suggests that most pterosaurs probably occupied marine or coastal/estuarine habitat.

The spatially fixed sporophyte body of vascular land plants has to be adapted to both atmospheric and substrate environments. Potentially almost all stages in the life cycle of a land plant are fossilisable and the physiological adaptations to these environments are reflected in the morphology, anatomy, and chemistry of the plant. Although solutions to the plant's physiological problems have been refined through evolution, the basic responses to fundamental environmental parameters such as temperature, water availability, nutrient supply, gas exchange and light, evolved early in land plant history. These taxon-independent solutions can be used qualitatively, and sometimes quantitatively, to track changes in atmospheric and edaphic conditions throughout much of the Phanerozoic. Increase in body size and height in Middle and Late Devonian times was coupled with intense demand for light, nutrients and water supply and with elaborations of vascular systems, photosynthetic surfaces, organ abscission and ‘root’ organisation. During the Carboniferous extreme adaptations to substrate waterlogging evolved. Mycorrhizal associations and “phi” layers in Triassic roots represent early aspects of modern root physiology. From the mid-Cretaceous, angiosperms exhibit leaf architectural characteristics which in modern plants relate qualitatively to moisture and light, and quantitatively to temperature, while vessel size and distribution in trunk wood is related to water stress and susceptibility to freezing. The relative proportion of plants utilising C3, CAM, and C4 photosynthetic pathways varies with environment. Isotopic analysis of plant fossils may demonstrate changing relative frequencies of photosynthetic pathways through time in relation to atmospheric composition and temperature.