中国科学院南京地质古生物研究所机构知识库

The ontogenies are described of four species of the Early Cambrian oryctocephalid trilobite Arthricocephalus from the Balang Formation, eastern Guizhou in Southwest China. This represents not only the first descriptions of the ontogenies of these species, but also the first of any Early Cambrian trilobites from the Order Corynexochida. The protaspid to holaspid ontogenies are described for Arthricocephalus chauveaui Bergeron; while meraspid to holaspid ontogenies are described for A. pulchellus Zhang and Chien, A. xinzhaieensis Chien and Lin and A. balangensis Qian and Lin. In addition to descriptions of the ontogenies, the familial emplacement of the genus is reviewed. The subgenus A. (Arthricoccphalites) is considered to be a junior synonym of A. (Arthricocephalus). The extent of morphological change during the ontogenetic development of these species is shown to be very variable, as is the degree of variation within the ontogenies of some particular species, in particular A. chauveaui. The ontogenetic variation within the four species of Arthricocephalus is analysed and shows an evolutionary trend in the form of a paedomorphocline. It is proposed that the sequential reduction in number of thoracic segments and concomitant increase in number of pygidial segments has occur-red due to the effect of changes to the Hox gene domains that are likely to have controlled thoracic/pygidial architecture.