Early Ordovician Seleneceme (Trilobita) in Ziyang Fauna, South China

Abstract

ABSTRACT Ziyang Fauna is a significant early Ordovician Floian fossil Lagerstätte, recovered from blackish-grey mud limestone and calcareous mudstone of possible slope facies in the Yangtze platform in Ziyang County, Shaanxi Province, China, distinguished by predominate trilobites, accompanied with graptolites, brachiopods, arthropods and some trace fossils. It is marked by high abundance of exceptionally preserved Seleneceme (Trilobita). Two trilobite species named Caputrotundum bashanense and C. ziyangensis, are reassigned here to Seleneceme based on a detailed study of more than one hundred well-preserved specimens. The taxonomic generic and ontogeny characters of Seleneceme are as follows: cephalon semicircular in outline, with axial spine and genal spines; without eye lobes; glabella subcircular in outline, with deep longitudinal glabellar furrows; thorax and pygidium with much parallel segments. The morphological variation during ontogeny of Seleneceme is characterised by the gradual increase of the length/width ratio (L/W) of the body, but with a constant width/length ratio (W/L) of the cephalon, and also a faster growth rate in the pygidium than in the thorax.