SHIFTING MICROFOSSIL FOOD WEBS: ARE ACRITARCHS THE CATALYSTS TO MAJOR DIVERSITY AND ECOLOGICAL SHIFTS AT THE CAMBRIAN-PRECAMBRIAN BOUNDARY?

Acritarchs are a polyphyletic group of organic-walled microfossils, representing the remains of phytoplankton and various prokaryotic and eukaryotic organisms, which are useful biostratigraphic markers in Ediacaran and Cambrian strata of South China. Acritarch diversity trends also coincide with important ecological and environmental shifts, including the rise and fall of Ediacaran fauna, ocean oxygenation, and the diversification of modern animal phyla. The comparison of acritarch biodiversity to paleontological and geochemical records could yield a relationship between acritarch diversity and environmental and ecological changes at the Ediacaran-Cambrian boundary.

In the present study, we have investigated occurrences of late Ediacaran-early Cambrian microfossils of the Yanjiahe and Liuchapo formations at six localities in the Yangtze Gorges region and northern Hunan Province. Ediacaran strata are dominated by large leiospheres, the cyanobacterium Siphonophycus sp., and microbial fragments. Crossing into lower Cambrian strata, diversity increases considerably with occurrences of Asteridium sp., Comasphaeridium annulare, Heliosphaeridium ampliatum (ACH), Megathrix longus, Siphonophycus, small shelly fossils (SSF), including Zhijingites sp. and Kaiyangites sp., and hexactinellid sponge spicules. Fossils occur in chert layers in measured Ediacaran-Cambrian sections; small acanthomorphic acritarchs (e.g., Heliosphaeridium and Comasphaeridium) are commonly associated with peloidal phosphatic grains that could be fossilized fecal pellets. The distribution of acanthomorphic acritarchs is consistent with a planktonic lifestyle and their association with possible fecal pellets may indicate their significant role at the base of developing Cambrian food webs. In understanding the relationship of acritarch diversity to food web development, it is possible to better understand ecological interactions through the Phanerozoic Period.