Palaeobiology and Chemostratigraphy of the Precambrian - Cambrian Transitional Beds on the Siberian Platform
Sammanfattning: Biological innovations during the Precambrian-Cambrian transition opened evolutionary gates towards the modem biosphere. Two interrelated aspects of this process are investigated. The first one is the ecological radiation and beginning of calcareous biomineralization in mollusc-like organisms (shelled molluscs, coeloscleritophorans, and hyoliths). The second one is the carbon isotopic record as a long-scale environmental archive and stratigraphical tool. The early Cambrian diversification of mollusc-like forms was a result of their adaptations to various habitats. This ecological radiation led to the appearance of major trophic groups: deposit-feeders, scrapers and grazers, suspension feeders, predators and scavengers. General size increase of molluscan shells during the Cambrian was related to the invasion of shallow-water high-energy environments and seems to have accompanied changes in life cycles and the appearance of planktonic veliger larva. Shells of the Early Cambrian mollusc-like organisms grew by marginal accretion of new growth lamellae or sclerites. The former type of shells had an outer layer of possibly mineralized prism-like units and an inner layer of lamellar units consisting of aragonitic fibres. Nacreous and crossed-lamellar aragonitic microstructures probably evolved in the Cambrian from such lamellar aragonitic microstructures in different groups of molluscs. Shells composed of sclerites belong to the coeloscleritophorans, a group confined to the Cambrian. The juvenile part of some composite shells was assembled from needle-like sclerites; these were substituted by hollow broader sclerites at later stages of development. Palaeozoic group of hyoliths has been regarded as a separate phylum close to molluscs and sipunculans or been ascribed to one of these. Their exoskeleton, penetrated by numerous pores, consisted of organic filaments mineralized by aragonite and assembled as bundles forming a characteristic orthogonal network. Their alleged crossed-lamellar microstructure of mollusc type is, therefore, reinterpreted. Extremely well preserved fossils give an unexpected insight into early metazoan diversification. Phosphatised minute globules from the Precabmrian-Cambrian transitional strata of Siberia are interpreted as fossilized tetrameric embryos with features of modem cnidarian larva-actinula. Found in association with anabaritids (problematic early Cambrian organisms, characterized by triradially symmetrical tubular skeletons) they may represent their early developmental stages. However, until more material and new evidence is gathered, this connection is speculative. Carbon isotopic oscillations enable a novel angle of view on Vendian-Cambrian stratigraphy, evolutionary rates at the Precambrian-Cambrian boundary, and biogeochemical events accompanying this transition. New data from northern Siberia elucidate the correlation of the Lower Cambrian and Vendian-Cambrian transitional beds across the Siberian Platform. Several pronounced isotopic oscillations have been revealed in the lower beds of the Emyaksin Formation. They are absent at the base of the Tommotian Stage in its type section, owing to a depositional hiatus, and can be accommodated between the pre-Tommotian excursion I and Tommotian excursion II of the Siberian isotopic reference scale. These facts provide additional chemostratigraphical support for a pre-Tommotian Cambrian Stage in Siberia, the biostratigraphical framework for which has been elaborated earlier, and support suggestions of a gradual, rather than abrupt history of first appearances of the earliest skeletal groups.
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