Morphology and ontogeny of the eodiscoid trilobite Pagetia vinusta from the Cambrian (Miaolingian) of South China

Abstract

The ontogenetic series of the eodiscoid trilobite Pagetia vinusta is first documented, from the crack-out specimens including numerous articulated individuals and many disarticulated sclerites from the Cambrian (Wuliuan) Kaili Formation, Guizhou, southwestern China. Better descriptions of morphological changes during growth are presented, especially the large spine on the glabella and sixth axial pygidial segment, as well as the prominent pygidial larval notch in meraspid development which became progressively less distinct and disappeared in holaspides. An ontogenetic series is established based on the immature and mature exoskeletons from the protaspid to holaspid period. Two substages can be differentiated in the protaspides. Two holaspid instars are recognized with a new pygidial segment added, indicating that the start of the holaspid phase preceded the onset of the epimorphic phase and, accordingly, its developmental mode is attributed to the protarthrous pattern. On this basis, due to the broad palaeogeographic distribution of Pagetia, its valid species are also thoroughly examined and discussed.