Stable isotopic techniques in geology illuminate not only variations in past climates and oceans, but also the life-histories of extinct animals, plants and protistans. This volume focuses on the ways that stable isotopes can be used as tracers of the fossil biology and ecology of long-dead organisms and ecosystems. Here, we introduce relevant aspects of stable isotope systematics and provide a summary of the papers collected in this volume. The nine contributions collected here, from some of the most eminent workers in their respective fields, explore aspects of the ecology, evolution and biology of organisms from planktonic foraminifera to dinosaurs.

Application of planktonic foraminifera to micropaleontological, paleoceanographic and paleoclimatic research has enjoyed more than 150 years of activity. During the first century, foraminifera were used primarily for biostratigraphic analysis. Although fossil shells were recognized from beach sands and deep sea sediments as early as 1826 (d'Orbigny, 1826; Parker and Jones, 1865), it wasn't until Owen (1867) and the scientific results of the Challenger expedition (Brady, 1884) that the planktonic life habitat of these marine protozoans was clearly established. By the early 20th century, researchers were studying the biology of planktonic foraminifera at the cellular level (Rhumbler, 1901; Le Calvez, 1936), and linking their distributional patterns to regions of the ocean surface (Lohmann, 1920; Schott, 1935).

The isotopic composition of fossil invertebrates contains a wealth of information about the physical and chemical environments of the ancient past. The exploitation of this biogeochemical archive by paleontologists began about 50 years ago with the realization that the ratios of oxygen isotopes in the shells and skeletons of marine organisms offered the potential to accurately reconstruct paleoenvironmental conditions, particularly temperature (Urey, 1947; Urey et al., 1951). With the introduction of the oxygen isotope paleotemperature methodology (Epstein et al., 1951, 1953; Epstein and Lowenstam, 1953), the field of “isotope paleontology” was born (Wefer and Berger, 1991).

Symbiotic relationships are common between algae and diverse lineages of corals, planktic foraminifera, benthic foraminifera, and bivalves as well as in many other groups that do not leave an extensive fossil record. Ancient photosymbioses can be inferred using various lines of inference including the morphology of the host, its habitat, rates of calcification, (which tend to be high in many photosymbiotic taxa), and the geochemistry of the host's shell. This survey shows that stable isotopes are reliable means of identification of photosymbioses in corals and planktic foraminifera, but are far less reliable in benthic foraminifera and bivalves. Organisms that increase their symbiont density proportionate to their own growth display a steep rise in shell δ13C, while organisms whose own growth exceeds that of their symbiont population display a far smaller effect on shell δ13C.

Isotopic studies of vertebrate material have a short history, while isotopic analyses of invertebrates originated in the 1940's. Interestingly, the driving force behind Harold Urey's desire to derive a carbonate paleotemperature scale in the 1940's and 1950's was the hope that it would solve the mystery of dinosaur extinction by demonstrating temperature changes at the K/T boundary. The most useful and commonly investigated stable isotopes for paleobiologic studies of vertebrates are carbon, nitrogen and oxygen. Oxygen is available from the inorganic bone or tooth apatite phase. Carbon is most often derived from tooth enamel carbonate or organic collagen, and nitrogen is derived from collagen. Each of these stable isotopes provides information on different aspects of an animal's biology and when combined, provide powerful analyses for ecological and evolutionary reconstructions. In the 1970's, much work was done describing the carbon and nitrogen variations in plants. This period was followed in the late 1970's and early 1980's by research on these isotopic variables in mammals (e.g., DeNiro and Epstein, 1978, 1981; Vogel, 1978; Van der Merwe, N.J., 1982). The utility of these isotopes for dietary recognition led to their extensive investigation in archeological studies. Not until the mid to late 1980's and 1990's have these isotopes been utilized in both the inorganic component of teeth and bones as well as the organic component of bones in Pleistocene and older paleobiologic studies. The 1980's also saw the emergence of research on the oxygen isotopic variations in mammals. However, the focus of isotopic studies on vertebrates was not for paleobiologic purposes, but rather, for attempts to derive paleohydrologic or paleoclimatic information from them (e.g., Longinelli, 1984; Luz et al., 1984).

The study of evolution by natural selection is difficult because, by definition, it occurs under uncontrolled conditions. All biological organisms are highly complex entities and their interactions with the environment and each other are unpredictable, except in the most general terms. Countless chance events impact cumulatively on the genetic composition of a descendant lineage, resulting in substantial evolutionary change over very long periods of time. Understanding how evolution works in practice is a matter for basic science, but it is attended by unique and difficult problems.

Carbon isotopic studies have documented the effects of mass extinctions and biological recoveries on the global carbon cycle. Stable carbon isotopes can also be used to document the disappearance and re-appearance of specific ecological strategies (such as life span, seasonality of growth, relative depth of habitat, and photosymbiont reliance) during mass extinctions and evolutionary recoveries. Nitrogen isotopes have never been used to study ecological collapse and recovery from mass extinctions. However, they have strong potential for testing the effects of mass extinctions and biological recoveries on the biologically mediated global nitrogen cycle. They may also be very helpful for documenting the effect of mass extinctions and taxonomic radiations on the trophic structure of terrestrial and marine ecosystems.

Long thought to be inaccessible to empirical inquiry, Earth's early biosphere has in recent decades become a central focus of evolutionary and paleobiological research. Knowledge of Precambrian ecosystems comes from three principal sources. The conventional fossil record consists of the compressed and permineralized remains of cyanobacteria, protists and other microorganisms (e.g., Knoll, 1996), complemented by stromatolites and oncolites, the accretionary trace fossils of microbial mat communities (Walter, 1976). Independent inferences about early evolution can be drawn from molecular phylogenies (Pace, 1997). The third principal source of information comprises biogeochemical signatures encrypted in the chemistry of ancient sedimentary rocks. Biomarker molecular fossils and distinctive isotopic compositions record the metabolic activities of organisms not necessarily preserved morphologically (Summons and Walter, 1990). In this paper, we review the inferences about early life and environments that can be drawn from the isotopic records of carbon and sulfur.

The oxygen isotope method is probably the most widely used proxy of paleotemperature determination in the fossil record. The relationship as first proposed by Urey (1947) suggests that the ratio of 18O to 16O in the calcitic shells of fossils is proportional to temperature. This was subsequently confirmed by empirical studies (Epstein et al, 1951, Emiliani, 1954; 1955). However, Shackleton (1967), suggested on the basis of co-variance of benthonic and planktonic foraminifera, that the δ18O composition of seawater varied only as a function of glacial ice growth and decay. However, more recent studies have shown that there is still a residual temperature component in the δ18O variability of deep waters.

The exploration of chemosynthetic communities in the geologic record over the last ten years has generated a series of sedimentological, tectonic and geochemical criteria that help define a continuum of environments from hot hydrothermal vents to nearshore geothermal deposits. Many of these studies have used stable isotope geochemistry to uncover a depleted carbon signature that characterizes most fossil chemosynthetically derived deposits. Isotope geochemistry (carbon, nitrogen and sulfur) as been an important thread in the story of the discovery of modern chemosynthetic communities, as well, adding to understanding of the biogeochemistry of these ecosystems. With increasing awareness of the prominence of these communities, not just as a biological novelty, but as a fundamental component of life on Earth (and perhaps elsewhere), the drive to develop geochemical proxies for chemosynthetic taxa in the fossil record intensifies. Increased ability to recognize these communities provides access to a second tier of paleobiological questions, including ideas of evolutionary history and selective advantage.