The Ediacaran-Early Cambrian Fossil Record in Southwest Gondwana

Abstract

A review of the Ediacaran-Early Cambrian fossil record in SW Gondwana is presented. Organic-walled microfossils (acritarchs) reported from the region can be assigned for the most part to the Late Ediacaran Leiosphere Palynoflora (LELP), with only a few occurrences of lower Ediacaran acritarchs. So far, complex acanthomorphs (ECAP assemblage) are absent in fossil-bearing units of lower to mid-Ediacaran age, which may be the result of an adverse paleoclimate. The Ediacara soft-bodied biota is best recorded in the Nama Group, with occurrences recently reported in South America. Biostratigraphically, soft-bodied fossils are best assigned to the latest Ediacaran Nama Association. Ediacaran shelly fossils are widespread in SW Gondwana, including biostratigraphically relevant genera such as Cloudina, Namacalathus, Corumbella and Titanotheca. The only group with a record across the Ediacaran–Cambrian boundary are trace fossils. Whereas simple, mostly bedding-parallel burrows and biomat structures occur in Ediacaran “matgrounds,” diverse vertical bioturbations characterize Early Cambrian “mixgrounds.” Ediacaran fossils can be useful for biostratigraphy. The early and Mid-Ediacaran are best zoned using acritarchs, the Late Ediacaran with shelly fossils and the Ediacaran–Cambrian boundary can be determined using trace fossils. Biostratigraphy, in turn, enables the correlation of distant sedimentary successions, which may have paleogeographic implications. Ediacaran paleobiogeography, on the other hand, is hampered by the cosmopolitan nature of most taxa found in SW Gondwana.