中国科学院南京地质古生物研究所机构知识库

The Ediacaran-Cambrian transition, which incorporates the radiation of animals, lacks a robust global temporal and spatial framework, resulting in major uncertainty in the evolutionary dynamics of this critical radiation and its relationship to changes in palaeoenvironmental geochemistry. We first present a new delta C-13(carb) composite reference curve for the Ediacaran Nama Group of southern Namibia, and we then outline four new possible global age models (A to D) for the interval 551-517 million years ago (Ma). These models comprise composite carbonate-carbon isotope (delta C-13(carb)) curves, which are anchored to radiometric ages and consistent with stron-tium isotope chemostratigraphy, and are used to calibrate metazoan distribution in space and time. These models differ most prominently in the temporal position of the basal Cambrian negative delta(13)C(carb & nbsp;)excursion (BACE). Regions that host the most complete records show that the BACE nadir always predates the Ediacaran-Cambrian boundary as defined by the first appearance datum (FAD) of the ichnospecies Treptichnus pedum. Whilst trep-tichnid traces are present in the late Ediacaran fossil record, the FAD of the ichnospecies T. pedum appears to post-date the last appearance datums (LADs) of in situ representatives of the skeletal organisms Cloudina and Namacalathus in all environments with high-resolution delta C-13(carb & nbsp;) data. Two age models (A and B) place the BACE within the Ediacaran, and yield an age of ~538.8 Ma for the Ediacaran-Cambrian boundary; however models C and D appear to be the most parsimonious and may support a recalibration of the boundary age by up to 3 Myr younger. All age models reveal a previously underappreciated degree of variability in the terminal Ediacaran, incorporating notable positive and negative excursions that precede the BACE. Nothwithstanding remaining uncertainties in chemostratigraphic correlation, all models support a pre-BACE first appearance of Cambrian-type shelly fossils in Siberia and possibly South China, and show that the Ediacaran-Cambrian transition was a protracted interval represented by a series of successive radiations.

Calibrating the temporal and spatial dynamics of the Ediacaran - Cambrian radiation of animals

Calibrating the temporal and spatial dynamics of the Ediacaran - Cambrian radiation of animals

Calibrating the temporal and spatial dynamics of the Ediacaran - Cambrian radiation of animals