Abstract It is now well established that seawater chemistry, as well as influencing non-skeletal marine precipitation (‘calcite’ and ‘aragonite seas’), has affected skeletal mineral secretion in some algal and marine invertebrate groups. Skeletal mineralogy has had a yet more profound consequence on fossil preservation. The realization that the fossil record of marine organisms with an aragonite shell is widely depleted in some shelf settings through early, effectively syn-depositional, dissolution (‘missing molluscs’ effect) has led to a re-evaluation of the composition, diversity, ecological and trophic structure of marine benthic communities. Comparisons of molluscan lagerstätten from ‘calcite’ and ‘aragonite seas’ show a similar pattern of skeletal mineralogical loss, that is, no differences are discernibly linked to changed seawater geochemistry. It is notable that the rare mollusc-rich skeletal lagerstätten faunas in the fossil record include many small individuals. Micromolluscs are quantitatively important among modern shell assemblages, yet small size is a major source of taphonomic and biodiversity loss in the fossil record. In skeletal lagerstätten faunas, micromolluscs contribute variably to mollusc biodiversity but appear particularly significant through at least to Triassic times. They highlight a further ‘missing molluscs’ effect of taphonomic loss through early dissolution.

Abstract The short-lived end-Ordovician Hirnantian glaciation allied to marine mass extinction is variously considered as a short-lived event or as the peak of long-drawn-out climatic cooling through at least late Ordovician–early Silurian times. Evidence from Early Palaeozoic facies, faunas and stable isotope excursions used to interpret climatic cooling events ranges farther, from late Mid-Cambrian to late Silurian times. Glacigenic sediments, structures and geomorphology provide direct evidence of glacial episodes. Cool-water carbonate deposition, which is particularly widespread during the late Ordovician Boda event in high-latitude peri-Gondwana–Gondwana, and beyond into mid–low palaeolatitudes, is interpreted as indicating global cooling, not warming as has been proposed. Such carbonates also characterize mid-latitude continents widely at horizons earlier in the Ordovician, and more locally in the mid-Silurian in high-latitude Gondwana. Cool-water carbonate mounds have distinctive facies-controlled mound faunas across palaeocontinents. Other facies evidence for palaeoclimates includes black shale deposition, including deglacial organic-rich ‘hot shales’, which indicate transgression in epeiric seas, and sea-level curves interpreted from facies and faunal successions. Correlation is shown between facies evidence and positive C isotope excursions, from which cyclicities are apparent. The possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography, and changes in global sea level and in p CO 2 . Facies and faunal evidence from peri-Gondwanan terranes (Armorica, Central Europe, Alborz) is assessed with that from Gondwana (mostly North Africa, South America) and correlatives in Avalonia, Baltica and Laurentia to establish a wider picture of early Palaeozoic cooling events.

Abstract Syndepositional aragonite dissolution at very shallow depths, above the lysocline, is a major process that affects carbonate deposition and skews the composition of carbonate sediments. Such dissolution is capable of altering sediment composition in many settings, and during microfacies analysis it is critical to be aware of this early, selective filtering by identifying taphonomic signatures. These effects are also capable of distorting the trophic composition of fossil biotas, potentially restricting the ability to identify nutrient levels and other controls. The evidence from widespread diagenetic limestones in shale (marl)-limestone rhythms supports the dissolution model, but the source of the precursor aragonite is unresolved; allochthonous aragonite mud is one possible source, but, especially during calcite- sea intervals, another possibility is from the autochthonous aragonitic fauna. Forward models for carbonate sedimentation will need to compensate for aragonite dissolution if realistic models are to be developed, but our knowledge of the environmental distribution and magnitude of aragonite dissolution is still woefully incomplete. Another major consequence of early aragonite loss is that the diagenetic potentials of many carbonate sediments have been changed, drastically reducing secondary porosity potentials long before they are affected by meteoric or burial processes.