Andrey Yu. Zhuravlev and Robert Riding
THE CAMBRIAN RADIATION, which commenced around 550 million years ago, arguably ranks as the single most important episode in the development of Earth's marine biota. Diverse benthic communities with complex tiering, trophic webs, and niche partitioning, together with an elaborate pelagic realm, were established soon after the beginning of the Cambrian period. This key event in the history of life changed the marine biosphere and its associated sediments forever.
At first glance, abiotic factors such us climate change, transgressive-regressive sea level cycles, plate movements, tectonic processes, and the type and intensity of volcanism appear very significant in the shaping of biotic evolution. We can see how rapid rates of subsidence, as expressed in transgressive system tracts on the Australian craton, selectively affected the diversity of organisms such as trace fossil producers, archaeocyath sponges, and trilobites (Gravestock and Shergold—chapter 6); how globally increased rates of subsidence and uplift accompanied dramatic biotic radiation by increasing habitat size and allowing phosphorus- and silica-rich waters to invade platform interiors (Brasier and Lindsay—chapter 4); how climatic effects, coupled with intensive calc-alkaline volcanism, at the end of the Middle Cambrian may have caused a shift from aragonite- to calcite-precipitating seas, providing suitable conditions for development of the hardground biota (Seslavinsky and Maidanskaya— chapter 3; Eerola—chapter 5; Guensburg and Sprinkle—chapter 19); how the reorganization of plate boundaries (Smith—chapter 2; Seslavinsky and Maidanskaya) created conditions for current upwelling, which may in turn have been responsible for the appearance and proliferation of acritarch phytoplankton and many Early Cambrian benthic organisms (Brasier and Lindsay; Ushatinskaya—chapter 16; Moldowan et al.—chapter 21).
However, biotic factors themselves played a remarkable role in the environmental changes that formed the background to the Cambrian radiation. We see how, by means of biomineralization, shell beds and calcite debris contributed to the appearance of hardground communities (Droser and Li—chapter 7; Rozhnov—chapter 11); how