Episodic Bio-Radiations and Extinctions during the Precambrian-Cambrian Transition: Insight from South China  

Earth experienced major biological and environmental revolutions during the Precambrian-Cambrian transition.  Most notable was the advent and radiation of animals following the termination of the extreme glaciations around 635 Ma.  The sedimentary successions of South China preserve a wide range of facies and exquisitely preserved fossil biotas (e.g. the Weng’an and Chengjiang biotas). These well-exposed sequences offer an unsurpassed opportunity to study these major bio- and geo-events, and their interactions and causal relationships.  Accumulating data from many disciplines has demonstrated four bio-radiations and four mass extinctions across the Precambrian-Cambrian transitional interval in South China, which may be applicable on a global scale.  The first bio-radiation is evidenced by the rapid appearance of the large acanthomorph acritarchs shortly after the extensive glaciations of the Cryogenian period.   These organic microfossils are unique to the early Ediacaran period; they probably represent diapause egg cysts of the earliest metazoans.   They became extinct before the advent of an extensive negative carbon isotope excursion (the Dounce, Shuram or Wonoka event), with a nadir of −12‰. The second bioradiation produced the megascopic Ediacara-type biotas widely distributed later in the Ediacaran period.   These became extinct near the Precambrian-Cambrian boundary (542 Ma), coinciding with the base of a major negative carbon isotope excursion (the Bace event). It may have taken 20–40 million years for life to recover from the end-Ediacaran mass extinction.  The abrupt appearance and diversification of the small shelly fossils (SSF) represents the first episode of the Cambrian explosion.  However, the SSF experienced a mass extinction before the first appearance of trilobites, and the onset of the second episode of the Cambrian explosion – which is amplified by the global abundance of fossil lagerstätten between 521–515 Ma.   A major mass extinction of the Cambrian fauna, notably the archaeocyathid and Redlichiid-Olenellid trilobites, took place around 515–510 Ma. 

During the Precambrian-Cambrian transition, positive carbon isotope excursions tend to coincide with times of bioradiations, while negative excursions seem to correspond to the major mass extinctions.