Paper No. 3-2
Presentation Time: 8:15 AM
10 M.Y. EVOLUTIONARY STASIS OF EARLIEST CAMBRIAN (TERRENEUVIAN) MOLLUSK-RICH FAUNAS AND CAMBRIAN EVOLUTIONARY RADIATION CORRELATIONS
Mollusk-rich assemblages with Watsonella crosbyi with or without Aldanella spp. represent a temporally problematical, earliest Cambrian interval. Although frequently regarded as Tommotian-equivalents (upper Terreneuvian), new biotic and carbon isotope data demonstrate a pre-Tommotian (middle Terreneuvian) age for early W. crosbyi-A. spp. assemblages. Both W. crosbyi and A. spp. occur below the I’ carbon isotope excursion in the Siberian “Manykayan/Nemakit-Daldynian” Stage. Roughly coeval (pre-Tommotian) W. crosbyi-bearing faunas with A. attleborensis (=A. yanjiaheensis) occur below a distinctive, positive δ13C excursion in the middle Meishucunian Stage (China; L4, P3, ZHUCE excursions) and correlatives (middle Chapel Island Formation, Avalonia; upper Bayan Gol Formation, Mongolia). Heraultia Limestone (Gondwanan France) faunas are also pre-Tommotian-equivalents. The δ13C excursion above early W. crosbyi-bearing faunas is a reliable Stage 2 basal marker that is older (ca. 530 Ma) than earliest Tommotian (ca. 525.5 Ma). Benthic faunas with W. crosbyi and A. spp. record a ca. 10 m.y. “slow down” in the Cambrian Evolutionary Radiation. A Stage 2 base defined by W. crosbyi or A. attleborensis FADs is discouraged; these FADs are diachronous; W. crosbyi Zone taxa also appear with exceptionally early Cambrian acritarchs (Baltica), and their ranges are bracketed by lithofacies changes and unconformities (SE Siberia, South China, Avalonia, France).