a1 Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, U.K.
The late Precambrian and Cambrian world experienced explosive evolution of the biosphere, including the development of biomineral skeletons, and notably of phosphate and siliceous skeletons in the initial stages of the adaptive radiation. Ongoing research indicates profound changes in climate and atmospheric carbon dioxide over this span of time. Glacial conditions of the Varangian epoch occur enigmatically at low latitudes, associated with carbonate rocks. Later changes in palaeogeography, sea level rise and salinity stratification encouraged prolonged ‘greenhouse’ conditions in both latest Precambrian and Cambrian times, with indications of relatively low primary production in the oceans. The Precambrian–Cambrian boundary interval punctuated this trend with evaporites, phosphogenic events and carbon isotope excursions; these suggest widespread eutrophication and conjectured removal of carbon dioxide from the atmosphere. Whatever the cause, nutrient–enriched conditions appear to have coincided with the development of phosphatic and siliceous skeletons among the earliest biomineralized invertebrates.
(Received November 05 1991)
(Accepted December 02 1991)