Morphology and trunk development of the trilobite Arthricocephalus chauveaui from the Cambrian series 2 of Guizhou, South China

The morphology and trunk development of the trilobite Arthricocephalus chauveaui described herein are based on 216 articulated specimens collected from the Lazizhai section of the Balang Formation (Stage 4, Series 2 of the Cambrian) in Jianhe County, Guizhou Province, South China. The new specimens exhibit a relatively complete ontogenetic sequence ranging from the protaspid to holaspid periods and reveal more morphological changes of the species in detail. For example, from degrees 0 to 4, the facial sutures are proparian and then gradually move backward from degree 5, until in the holaspid period, the suture becomes gonatoparian. Based on both linear measurements, we investigate here the morphological variation of some cranial traits throughout ontogeny. Our results demonstrate that the allometric growth pattern shows continuous variations, some of which are related to trunk development. By exploring the trunk segmentation in A.chauveaui, the developmental mode shows that somitogenesis and tagmosis may have developed independently at each meraspid degree beyond degree 0. The holaspid period is consistent with the beginning of the epimorphic phase.