الملخص الإنجليزي
Abstract The second half of the Ediacaran period began with a large impact - the Acraman impact in South Australia, which was accompanied by a negative δ13Ccarb anomaly and an extinction-radiation event involving acritarchs. A few million years later (∼570 Ma?) there was a second, deeper and longer-lived world-wide δ13Ccarb anomaly (the Shuram anomaly) which coincides with extinction of the acanthomorphic acritarchs. Wide distribution of the Shuram event is exemplified by stratigraphic sections from South Australia, Oman, southern California and South China. The widespread anomaly has been tentatively attributed to a marine impact. During recovery from the Shuram event the enigmatic Ediacaran biota achieved its zenith, only to be extirpated and replaced by a polyphyletic assemblage of shelly animals in what is known as the Cambrian "explosion". This extinction-radiation cycle was preceded by glaciation, another δ13Ccarb excursion and the highest 87Sr/86Sr values known from marine carbonates. These high Sr ratios have been linked to weathering of extensive tracts of continental crust that were elevated during amalgamation of the supercontinent Gondwana. Introduction of essential nutrients to the oceans would have promoted biological production of oxygen and provided P and Ca for the important skeletonization that characterizes the Cambrian "explosion" and caused a quantum leap in the preservation potential of animal remains. Turbulent events of the last 50 million years of Precambrian time include three glaciations, two large impacts and a massive orogenic episode. These dramatic environmental upheavals are held responsible for three consecutive extinction-radiation cycles that culminated in the appearance of a diverse array of shelly fossils. Various lines of evidence suggest that the metazoans have deep roots so that they too may have been subjected to the environmental pressures of the late Ediacaran period clearly illustrated by acritarchs and the Ediacaran biota but the long-lived diversity of the metazoan population was "suddenly" revealed by the acquisition of biomineralization.
