Early Cretaceous ostracods living within the rift lakes of Central Gondwana reflect, through their morphology, abundance and species diversity, depositional environments and changes in water chemistry that occurred within the lakes at that time. By studying the indigenous ostracod assemblages it is possible to interpret past environmental changes with respect to both water depth and water chemistry, and to utilize this information to estimate the probability of source and reservoir sediment deposition. The alternation of siliclastic and carbonate sequences throughout rift lacustrine sediments can be explained by changes in the depositional environments and related water-level movements. Such changes being brought about as a direct result of tectonism leading to increases or decreases in precipitation as a consequence of river capture or regional changes in climate. Two cycles of sedimentary infill are recognized, each initiated by an onlapping, fining-upwards transgressive systems tract within which a maximum flooding surface is developed. This is followed by a coarsening-upwards highstand systems tract in which a prograding sequence retains the diverse ostracod fauna of the transgressive systems tract below. A coarsening-upwards lowstand systems tract, having a low diversity ostracod fauna, brings each cycle to a close. An understanding of the cyclic sequence stratigraphy of these basins suggests that the break-up of Gondwana commenced as early as the late Barremian. Commercially, these rift basin lakes contain excellent hydrocarbon source rocks, but it is the interpretation of the lacustrine faunas that provides the essential information as to lake size and hydrocarbon potential.

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The carapace ultrastructure of three Recent [Heterocypris incongruens (Ramdohr), Cypridopsis uidua (Müller), and Conchoecia belgica Müller] and one Cretaceous ostracode species [Cypridea propunctata Sylvester‐Bradley] has been studied by means of standard electron microscope techniques. The carapace is shown to be an organic structure divisible into an outer epicuticle, a median exocuticle and an inner endocuticle in the three cypridacean ostracodes examined. In Conchoecia belgica only an outer epicuticle and an inner endocuticle were observed, but this may be due to preservation. The chitin structure of the carapace of calcareous ostracodes is shown to be essentially an interlocking lattice, more coarsely developed in the exocuticle; layered (lamellar) chitin is only present in the selvage and in the connective tissue joining the valves at the hinge. The non‐calcareous Conchoecia belgica possesses the layered chitin structure common to other crustaceans and to insects. The relationship of the carapace structure to that present in the decapod and insect exoskeleton is discussed. The technical problems met with during this study are considered in order to outline the difficulties associated with the study of organisms of this size.

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