Primicaris Larvaformis (Arthropoda)from The Early Cambrian Chengjiang Biota Revisited

Primicaris larvaformis is a small, larva-like arthropod from the Early Cambrian Chengjiang Biota (Zhang et al.2003). It was originally described as the larva form of naraoiids from the same biota, based on the small size, and the protaspid-like morphology including 11 pairs of spines along the margins of the single-piece shield (Hou et al.1991). According to the overall size, the morphology of the appendages, and the heterochronic process of its ontogeny, a proposal of P. larvaformis being a new species and a new genus was made (Zhang et al.2003). The taxonomic position of Primicaris at a higher level, however, remains to be discussed.This thesis focuses on the re-description of the morphology of P. larvaformis on the basis of 162 well-preserved specimens. New findings include:(1) the antenna consists of 13 annuli each bearing several small setae; (2) each biramous limb consists of a multi-segmented exopod and an endopod, while only the flagelliform outline of the latter can be seen due to the poor preservation of the specimens, the former is clearly observed to be composed of more than 10 podomeres each carrying several setae; (3) indications of 11 segments are observed in the axial region after the hypostome (i.e. axial lobe, sensu Hou et al.1991); and (4) the one-to-one match between appendages and segments in the axial lobe. I also discuss the morphological characters of P. larvaformis, including tagmosis, single shield, and appendage morphology. Based on this study, I propose that the previous comparison between P. larvaformis and the Edicaran Parvoncorina minchami was inaccurate as the poor preservation of the latter doesn’t allow convincing evidence showing the morphology of the appendages. I hereby suggest to assign P. larvaformis to Lamellipedia based on the fact that the exopod of this species carries flattened setae. Most likely, P. larvaformis occupies a basal phylogenetic position within Lamellipedia.