EXTINCTION OF CLOUDINA AND NAMACALATHUS AT THE PRECAMBRIAN-CAMBRIAN BOUNDARY IN OMAN: AN INITIAL CONDITION FOR EARLY CAMBRIAN RADIATION

New data regarding the timing and significance of key events at the Precambrian-Cambrian boundary have been obtained from the terminal Proterozoic-Cambrian Ara Group of Oman. The Ara Group represents at least 5 third-order sequences (A1-A5) of carbonate to evaporite sedimentation. Cloudina and Namacalathus body fossils occur only in the A1-A3 carbonate units despite favorable facies in the A4-A5 carbonates. Carbonates of the basal Ara Formation (unit A1) show d¹³C values of about +2 to +3 ‰, which is displayed through all of the overlying carbonate units (A2-A5), with the exception of unit A4 which shows an abrupt decrease to d¹³C values of approximately -5 ‰ . Values gradually increase to about -2 ‰ through the remainder of unit A4. This sharp negative anomaly in the A4 carbonates is interpreted as being correlative with an excursion of nearly identical magnitude observed globally near the Precambrian-Cambrian boundary. A volcanic ash bed occurs within basal strata of the A4 carbonate unit which marks the abrupt excursion in d¹³C from +2‰ to -5‰, interpreted as the Precambrian-Cambrian boundary. Its age thus provides the first direct age estimate for the onset of this globally significant biogeochemical event while also constraining the age for the upper range limit for Cloudina and Namacalathus. The best estimate for the age of these zircons and inferentially the ash bed is 542.0 Ma ± <0.5 Ma, which agrees well with previous constraints from Siberia and Namibia. A second ash bed occurs in the stratigraphically lower, fossiliferous, A3 carbonate. The best estimate for the age of these zircons and inferentially the ash bed is 543.2 ± 0.5.

The simplest interpretation of these data is that extinction occurred as a result of a global biogeochemical process represented by this carbon isotope anomaly, essentially at the time of the Precambrian-Cambrian boundary. With the global negative excursion as a rigid temporal benchmark, it appears that the terminal Proterozoic organisms followed a more abrupt rather than spaced sequence of extinctions. As in the case of younger extinctions, this reorganization of ecosystems likely contributed significantly to the innovation of novel new faunas that constitute the Cambrian explosion.