The context and reasons for the study of ostracodes have changed greatly over the past half-century, as well as how fossil ostracodes are viewed. Their use for the reconstruction of major changes in paleogeography and paleoceanography has been amply demonstrated. Some of the increased importance of ostracodes is due to new exploration via drilling and the discovery of new forms in the floors of sea and ocean basins. Some of it has been made possible by the development of the scanning electron microscope and attention to details in carapace morphology, particularly in mapping patterns of reticulation. The discovery that the shape of the carapace subscribes to architectural restraints of a membrane static-frame used in dome construction has introduced the possibility to describe it in functional and adaptive terms, and those principles are set forth here. [see Chapter 6 for details on the latter]

The Ostracoda are one of the most diverse arthropod groups alive today; they also have a tremendous fossil record. Because of their widespread environmental distributions, small size and carbonate shell, they have become extremely useful biostratigraphic and paleoenvironmental proxy indicators, particularly in nonmarine environments. Despite this utility, little is known about the phylogenetic history of this important group. We reconstructed a phylogenetic history of the major orders and suborders of Ostracoda in order to test the legitimacy of current classification schemes, determine if it is possible for ostracodes to have a Precambrian origin, and test the fidelity of some of the major morphological characters that have documented trends of either increased complexity, such as the hinge and marginal pore canals, or reduction in segments, such as the adductor muscle scar.

In our phylogenetic analysis to test taxonomic fidelity, we coded seven morphological hard part characters for nine taxa from the orders Archaeocopida, Leperditicopida, Palaeocopida, Podocopida, and Myodocopida. A parsimony analysis was performed using PAUP (v. 4.0) yielding 4 trees of 17 steps with low levels of homoplasy and a strong phylogenetic signal. A majority rule consensus tree indicates there is not complete agreement between the standard classification scheme and the phylogeny produced by the characters used to establish the classification. In our complete analysis of Ostracoda, we coded 28 morphological characters that included 14 hard part and 14 soft part characters for twelve taxa that include the Archaeocopida, Leperditicopida, Podocopida, and Myodocopida. A parsimony analysis was completed using PAUP (v. 4.0) yielding 1 tree of 125 steps with low levels of homoplasy and a strong phylogenetic signal. An unrooted analysis of this character set has the Cambrian Archaeopodocopida and the Ordovician-Devonian Leperditicopida in an unresolved polytomy with much younger groups such as the Myodocopina, suggesting a much deeper split in the lineage and a possible Precambrian origin for the Ostracoda. Testing the various character state acquisitions over the tree indicates that the hinge does not show an increase in complexity within a phylogenetic context, while the adductor muscle scars do show a significant trend of decrease in complexity across the tree topology. The marginal pore canals, which are functionally tied to osmoregulation as well as carapace secretion, are extremely homoplastic, indicating that this character, which is related to nonmarine invasions and tolerances, was acquired many times throughout the evolutionary history of Ostracoda.

By creating an evolutionary framework for the Ostracoda such as is presented here, we can further assess character state acquisition, and how it functionally and evolutionarily relates to ostracode paleoenvironmental tolerances. The framework will not only allow us to understand the overall evolution ofthis group but will also allow us to compare the history of the ostracode clade with other groups that also have a history ofmarine and nonmarine transitions.

A broad array of the highly variable morphological characters available in Ostracoda were used (1) to construct a new tabular key to the families of the subclass Myodocopa (and to differentiate the Myodocopa from the Podocopa), and (2) to explore the evolution of bioluminescence in the myodocopid family Cypridinidae. Results of a cladistic analysis of the Cypridinidae strongly support a single origin of bioluminescence within the family; a single clade contains at least 64 species known to luminesce (not all described). Furthermore, within that large clade, complex bioluminescent mating displays were confined by the analysis to a single clade of exclusively Caribbean cypridinids, though that result was unresolved by bootstrap analysis.

Chemically different bioluminescence occurs convergently in two ostracode groups; it is produced within carapace glands of certain Halocypridina, but it is produced and extruded from the upper lips of many cypridinid Myodocopida and serves as an antipredatory behavior in both groups. Additionally at least 60 cypridinid species also produce spectacular and complex species-specific male mating displays nightly in the Caribbean Sea. The cladistic analysis using 58 morphological characters of 44 taxonomic units (24 described genera of Cypridinidae, 11 individual species assigned to the genus Vargula, six groups of bioluminescent signalling Caribbean species (many undescribed), the Cylindroleberididae, and the Philomedidae) was performed using PAUP with the parsimony criterion. Twenty-five of the characters were previously underutilized characters of the complex upper lip and large male copulatory limbs discovered through dissection and SEM. These proved especially valuable, but characters of all other limbs were also included.

The Cypridinidae was confirmed as a monophyletic taxon with several well-supported subclades in addition to the large bioluminescent one. Skogsbergia, which apparently uses an antennular fan of iridescent blue filaments in courtship (Parker, 1997) belongs to a clade with five other cypridinid genera, including the four that have a similar fan. The genus Vargula was determined to be a polyphyletic assemblage. New genera will be established for former Vargula species including: “V.” tubulata, “V.” hilgendorfii, “V.” tsujii and all bioluminescent signaling species. Similar problems may exist for other large cypridinid genera, and clearer resolution of generic definitions might possibly reveal more significant patterns of geographic distribution within the family.

Most characters of the carapace were found to be too convergent to be useful. Though carapace characters are diagnostic for some ostracode taxa, most should not be used by themselves to identify ostracodes with certainty. General aspects of reproduction are also reviewed.

Molecular work on ostracodes has thus far either used allozyme-based or DNA-based techniques. The application of allozyme-based methods has provided numerous data on population genetics and reproductive modes in ostracodes. With this technique, it has also been possible to construct phylogenies, although these have been restricted to distance-based methods. With the usage of DNA-based methods, a new era in ostracode research has begun. It is now possible to obtain accurate estimates for genetic diversities at very fine scales, even within individuals. The DNA-based approach is also very useful for reconstructing evolutionary histories at different taxonomic levels. Within species, for example, phylogenies reveal past episodes of dispersal and genetic exchange. Together with morphological data, DNA sequence data can test for congruence of molecular and morphological evolution and variability. At high taxonomic levels, DNA sequence data provide information on mechanisms of evolution and speciation. This allows for testing whether a certain, morphological feature has evolved once or several times independently. By applying the principle of the molecular clock, DNA sequence data provide relative age estimates that can be calibrated against fossil data and be linked to past climatic or geological events.

Of the 50 thousand or more species descriptions that exist in my reference library, there is very little change over 130 years in the basic vocabulary used to describe the shape or the ornament of the ostracode valves. Although there have been some modifications (Sylvester-Bradley and Benson, 1971) following the availability of the SEM, arguably the most important change in ostracodology will come when we are able to describe the outside form of the carapace in functional terms, as we do the inside. It is here where the future of learning about their adaptive history lies (Benson, 1974, 1975, 1981, 1983).

Morphometric techniques for the analysis of shape change in organisms have experienced a noteworthy development in the last decade. But despite the significant contributions that ostracodologists made to the field, their use in standard ostracode research is far from common. This contribution stresses the usefulness of morphometric methods to describe ostracode valve outlines and to summarize shape changes cued by environmental factors. Focus is on nonmarine ostracodes which are generally poorly ornamented so that their carapaces offer few landmarks for characterization of morphological change. Out of several alternatives three techniques for shape analysis are applied here: the B-splines method for approximative description of ostracode contours, Elliptic Fourier Analysis (EFA) and a Generalized (Resistant Fit) Procrustes Analysis. B-splines method is presented here for the first time within a biological framework and both its mathematical basis and practical usage are discussed. Additionally a computer program, Morphomatica, developed for performing B-splines analyses of ostracod outlines is briefly documented.

Three case studies exemplify here how morphometric analysis might help either to detect environmental influences in ostracode shape or to show how morphological diversity of ostracode valve reflects environmental change. First, morphological variability within a clonal lineage of Heterocypris barbara (Gauthier and Brehm) is shown to be related to environmental variables (mainly temperature) when raised under controlled conditions in the lab. Second, carapace variability at the population level is explored in a widely distributed species (Limnocythere inopinata Baird) sampled from distant localities. Morphometric analyses illustrate how such variability is not related to geographic distance but to environmental conditions. Finally, patterns of temporal change in morphological diversity of a widely distributed ostracode group, the Candoninae, are elucidated by using the B-splines method combined with multivariate statistical analysis.

It is concluded that morphometric methods deserve to be included in the methodological toolbox of practicing ostracodologists as they can provide useful information in ecological and paleoecological research.

The shell chemistry of ostracodes is a useful indicator of past environmental conditions especially when the chemistry data are considered along with other proxy data. The complexities involved with the chemical and isotopic changes accompanying hydrologic change, solute evolution, and the autoecology of ostracodes all point to the need to exercise caution when interpreting shell chemistry. Nevertheless, the stable-isotope values and cation ratios (e.g., Mg/Ca, Sr/Ca) as well as the species assemblage of ostracodes can provide powerful tools for the reconstruction of paleoclimate and paleohydrology. In particular, the changes in Mg/Ca and Sr/Ca of well-calcified ostracodes shells record the qualitative changes in solute composition, and when the dissolved Mg/Ca remains relatively constant, the Mg/Ca in the ostracode shell is proportional to water temperature.

Over recent decades ostracods have become established indicators of ecosystem health, biodiversity and environmental change. With applications ranging across the earth sciences (from modern pollution studies to sea-level change, basin evolution, plate tectonics, palaeoceanography) and related disciplines such as archaeology, ecology and genetics, their utility extends to almost every aquatic and semi-aquatic habitat, from the deep ocean to high mountain springs. Their temporal range is now known to cover the last 500 million years of earth history.

The study of fossil ostracod assemblages follows traditional palaeontological lines of investigation, including taphonomy, morphometries and diversity, but there are a number of methodological approaches, specific to the ostracods, that render them potentially one of the most versatile organisms in the fossil record. Ostracods have been employed on a range oftemporal and spatial scales to reconstruct past environments, from world-wide, geological-scale global events in the deep-sea through to smaller-scale studies of lakes and their archives of local environmental change over recent centuries.

Much information can be obtained from ostracod assemblages but it is particularly through recent advances in the chemical and physical study of single shells or carapaces that the utility of these organisms has been brought to the fore. In this paper the potential palaeoenvironmental information derived from an ostracod assemblage, a single species, or an individual shell is reviewed. The main applications for ostracods are outlined for marine and non-marine ecosystems. Finally, the role of the ostracods in detailing the recent history of the Aral Sea is outlined.

Ostracodes are ecologically diverse at the present day, inhabiting marine, nonmarine and (semi)terrestrial environments. Modern benthic faunas are dominated by Podocopa (marine and nonmarine Podocopida, marine Platycopida, and extremely rare marine Palaeocopida), while the Myodocopa (Myodocopida and Halocyprida) are diverse in the marine pelagic realm, as well as having many nektobenthic taxa. Their excellent fossil record facilitates reconstructions of their phylogenetic relationships and ecological adaptations throughout their Phanerozoic history. The earliest known ostracodes are of Ordovician age, when representatives of the extant orders Podocopida, Platycopida and Palaeocopida were already present, together with (possible) early Myodocopa and extinct orders such as the Leperditicopida. Cambrian bivalved arthropods such as bradoriids and phosphatocopids are no longer regarded as Ostracoda.

Ordovician ostracodes were predominantly marine meiobenthos, diversifying into depth-related assemblages dominated by palaeocopids. The beginnings of podocopan radiations in marginal marine environments (brackish and hypersaline waters) are seen in the Silurian, as is an ecological shift of nektobenthic myodocopans to form the first pelagic ostracode faunas. Of the diverse marine Paleozoic palaeocopids, only a single lineage, the puncioids, survived beyond the Permian and today live interstitially in high-energy shallow marine environments. Post-Paleozoic marine benthic ostracode faunas are dominated by cytheroidean podocopids which gave rise to several radiations in the Mesozoic and Cenozoic. The healdiid metacopines (podocopans), of Devonian origins, enjoyed a marine radiation in the Triassic and Early Jurassic and then became extinct. Marine platycopids were also significant components of Mesozoic marine faunas and are relatively diverse in warm, shallow carbonate environments today.

Suggestions that the first freshwater ostracodes were Devonian leperditicopids are controversial; undoubted nonmarine / freshwater radiations developed during the Early Carboniferous, including darwinuloidean and carbonitoidean podocopids and possibly some platycopids, together with cytheroidean podocopids (limnocytherids) in the Late Carboniferous. Of these only the darwinuloideans and limnocytherids survived the end-Permian extinctions and are still found in modern nonmarine waters; however, the dominant freshwater ostracodes today are the cypridoidean podocopids, whose radiation began in the Triassic and attained explosive proportions in the Late Jurassic - Early Cretaceous (although there are controversial suggestions of Paleozoic origins for this group). In addition to the limnocytherids there have been several other, separate invasions of nonmarine waters by cytheroidean podocopids, notably the cytherideids and the commensal entocytherids. Radiations in damp terrestrial environments have been initiated by both marine and nonmarine groups, but such invasions lack a recognized fossil record; (semi)terrestrial cypridoideans and darwinuloideans may represent Late Mesozoic radiations, while the Terrestricytheroidea, with marine affinities, may be much older, possibly Late Paleozoic in origin.

Although the majority of publications on extant nonmarine ostracode species in North America are concerned with lacustrine settings, many species that are potentially valuable as indicators of water quality changes live in non-lacustrine settings. Ostracode distributions in 157 springs, wetlands and streams in the United States are examined here in order to assess 1) species richness, 2) association with physical and chemical parameters of their habitats and 3) the presence of potentially useful biomonitors and environmental sentinels. The 157 non-lacustrine sites are a subset of a large database (North American Non-marine Ostracode Database: NANODe version 1) consisting of 611 mostly lacustrine sites with ostracode species, presence-absence data, hydrochemistry and climate data (Forester et al., in review). Of the 89 species represented in NANODe version 1, 51 species are found in springs, 59 species are found in wetlands and only 15 species are found in streams. Many species are found in at least two of these habitats and some in all three. Principal Components Analysis of these 157 sites indicates that 71% of the variance is explained by salinity (total ionic concentration), alkalinity and temperature, a result consistent with previously published analyses of natural water. Cluster analysis shows that spring species are most strongly tied to temperature, whereas wetlands and streams are most strongly tied to ionic composition. Three species are found to be potentially valuable biomonitors: Cavernocypris wardi in springs, Fabaeformiscandona rawsoni in wetlands and Physocypria globula in streams.

Climatic effects on aquatic environments are an important factor in the ecology of ostracodes judging from their biogeography especially with respect to moisture balance and ecoregions (vegetation zones). The importance of climate in controlling ostracode distribution is underscored by major ecological changes indicated by co-stratigraphic changes in ostracode and pollen biozones throughout the Quaternary. Climatic interpretation of ostracode records are complicated, however, by several non-climatic factors, including basin shoaling, changes in water chemistry due to abrupt changes in the flux of groundwater, rivers, and streams to lakes and wetlands, and increasingly shortened water residence time in areas where precipitation exceeds evaporation due to erosion by basin overflow.

Ostracodes are bivalved Crustacea whose fossil shells constitute the most abundant and diverse metazoan group preserved in sediment cores from deep and intermediate ocean water depths. The ecology, zoogeography, and shell chemistry of many ostracode taxa makes them useful for paleoceanographic research on topics ranging from deep ocean circulation, bottom-water temperature, ecological response to global climate change and many others. However, the application of ostracodes to the study of climate change has been hampered by a number of factors, including the misconception that they are rare or absent in deep-sea sediments and the lack of taxonomic and zoogeographic data. In recent years studies from the Atlantic, Pacific, and Arctic Oceans show that ostracodes are abundant enough for quantitative assemblage analysis and that the geochemistry of their shells can be a valuable tool for paleotemperature reconstruction. This paper presents practical guidelines for using ostracodes in investigations of climate-driven ocean variability and the ecological and evolutionary impacts of these changes.

Information abounds on the Internet. Information about the Ostracoda is no exception. Here we assess 40 web sites that deal fully or in part with a wide variety of aspects of ostracode biology, paleontology, and ecology.

In recent decades, research on ostracodes has grown dramatically. While many aspects of the group have been investigated, this review focuses on the paleoenvironmental applications of ostracodes from marine and nonmarine environments. It is argued that while ostracodes have great potential as paleoenvironmental tools, much of that potential has not yet been fully realized because of our imperfect understanding of ostracode biology, taxonomy, systematics and ecology. Future developments will be sure to result from additional studies in these areas, and will also be effected by exchange of ideas with scientists working on related proxies. However, the flow of ideas should not be one-way; workers in other disciplines can also learn from the study of ostracodes.