| Abstract: | The extraordinarily preserved, diverse arthropod fauna from the Lower Cambrian Maotianshan shale, central Yunnan (southwest China), represents different evolutionary stages stepping from stem lineages towards crown arthropods (also called euarthropods), which makes this fauna extremely significant for discussion of the origin and early diversification of the arthropods. Anatomical analyses of the Maotianshan shale arthropods strongly indicate that the origin of crown arthropods involved three major evolutionary events, arthrodisation, arthropodisation and cephali-zation. We try to explore possible evolutionary changes of the developmental mechanism that may have underlain origins of euarthropod appendage and head. Fossil evidence suggests that the formation of a jointed limb known as arthropodisation and formation of multi-segmented head (called cephali-zation), which characterize euarthropods, is an event after arthrodisation characterized with the formation of seg-mented-exoskeleton and the joint membrane between tergites. We propose that the Hox complex was already operating at least as early as in the Early Cambrian and is responsible for the formation of the joint membrane between two semgents through Hox gene regulation along the D-V and P-D axis. Fossil evidence indicates that the head in ground state of arthropods consists only of two segments, an ocular and an antennal one. The formation of multiple segmented, euarthropod head (called syncephalon) from the two-segmented head was a separate event, which is called cephali-zation. Presence of the Hox gene head expression domain and change of developmental mechanism in head segments might be responsible for the formation of the syncephalon and this event has been broadly finished in the Early Cambrian arthropods. The post-oral limbs in the early syncephalons as evidenced from the Lower Cambrian Maotianshan shale arthropods however were almost identical to those in trunk. Therefore we proposed that the Hox genes specified-expression in individual segment was a late evolutionary step postdating the formation of syncephalon. |
