Abstract:

The sudden appearance of small shelly fossils near the Ediacaran-Cambrian boundary represents one of the most conspicuous biomineralization events. It witnessed the explosive radiation of metazoans and signals the formation of the Cambrian ecosystem reigned by animals, especially bilaterians. The Yanjiahe Formation in the Three Gorges area represents one of the most important Cambrian stratigraphic sequences bridging the chronostratigraphic units between the Fortunian and Cambrian Stage 2. Here we summarize the recent progresses on small shelly fossils, particularly mollusks and cnidarians, from the Yanjiahe Formation to provide an important paleontological basis for defining the base of Cambrian Stage 2 and for better understanding the evolution and ontogeny of molluscs and medusozoans in early Cambrian oceans. The co-occurrence of Watsonella crosbyi and Aldanella attleborensis, both as the candidate GSSP markers for defining the base of Cambrian Stage 2, is documented for the first time at the base of Member 5 of the Yanjiahe Formation and assigned the age of Member 5 (Age 2). Watsonella crosbyi and A.attleborensis are widely distributed among Cambrian palaeocontinents (e.g., South China, Mongolia, Siberia, Avalonia) and range from low to high paleolatitudes in carbonate facies. The stratigraphic range is restricted to Cambrian Stage 2, which further supports the FAD of either W.crosbyi or A.attleborensis as a candidate GSSP marker for defining the base of this unit. Moreover, a single pair of cardinal processes below the apex of W.crosbyi most likely provide muscle attachment sites. This trait provides desiderative new data on the shell musculature of W.crosbyi, which suggests that this species is an untorted helcionelloid mollusc with an endogastrically coiled shell. The new hexangulaconulariid Septuconularia yanjiaheensis has a laterally compressed and biradially symmetrical deriderm, exhibiting fourteen gently tapered faces, thus representing a most specialized hexangulaconulariid taxon. Longitudinally, the periderm consists of three regions that probably correspond, respectively, to an embryonic stage, a transient juvenile stage, and a long adult stage. Septuconularia yanjiaheensis may have been derived from six-faced Hexaconularia(Fortunian Stage), which is mostly like a morphologically intermediate taxon between Arthrochites and Septuconularia. The new olivooid Octapyrgites elongatus is similar to the genera Olivooides and Quadrapyrgites from the Cambrian Fortunian Stage; its periderm consists of a quadrate apical region and a strongly corrugated reversed pagoda-like adapical region bearing folded apertural lobes. However, O.elongatus exhibits eight V-shaped apertural lobes while Quadrapyrgites has twelve. In contrast to the radiation-induced abundant medusozoans with diverse symmetry patterns during the Fortunian Age, the paucity of olivooids and absence of pentaradial cnidarians and carinachitids in Cambrian Stage 2 indicate a marked decline in the disparity of cnidarians during the Fortunian-Age 2 transition, when, by contrast, bilaterians underwent rapid diversification.

Key words: Yanjiahe Formation, small shelly fossils, Stage 2, Terreneuvian, Three Gorges area