Biodiversity of Late Ordovician cephalopods from the Pagoda Formation in Northern Guizhou

The Upper Ordovician Pagoda Formation (Sandbian-Katian), a stratigraphic marker unit paleogeographically widespread in South China, is exemplified by its pervasive limestone with network structures throughout the formation and its abundant cephalopod fossils. The Pagoda Formation, with a thickness of around 25 meters, crops out extensively in the Yaolongshan area of northern Guizhou. It is perfectly exposed at the Jiudianya, Jizhen quarry and Tianyujidi sections in the study area. Here we report on a study of a relatively productive cephalopod fossil assemblage from which 11 species in eight genera and several taxa incertae sedis are documented. These cephalopod fossils, with ortho- or gyro-ceracones and small siphuncles, have long been cited as evidence for a pelagic migrant lifestyle and are consistent with the deep-water shelf Sinoceras–Michelinoceras–Disoceras cephalopoda biofacies in the Yangtze Platform during the Late Ordovician. Principal Component Analysis (PCA) based on the parametric variations including rate of conch tapering, diameter ratio of shell to siphuncle, ratio of camerae height to septal neck width, concavity of septa in camerae and ratio of conch diameter to camerae height shows that Sinoceras and Michelinoceras can be readily distinguished by the rate of conch tapering, and that species of Michelinoceras differ notably from each other in their camerae height and septal neck ratio. Other parametric variations, however, are limited in taxonomic classification. Quantitative analysis shows that the Pagoda cephalopod assemblage in Yaolong-shan area has a relative higher biodiversity and is dominated in richness and abundance by large orthoconic genera, i.e., Sinoceras and Michelinoceras. Potentially, a nutrient-rich deeper neritic environment and an increasing ecosystem complexity may give rise to the recovery phase of the cephalopods of the Yangtze Platform and their diversity climax during the Ordovician Katian age.

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