Development and trunk segmentation of a redlichiid trilobite from Cambrian Series 2 of China

The ontogenetic sequence of an early Cambrian redlichiid trilobite Bathynotus kueichouensis is presented on the basis of numerous articulated specimens from Guizhou, China. The body regionalization of B. kueichouensis witnessed a wide variation in the morphology of trunk segments between different thoracic regions, referred to as the ‘thoracic region 1-4’. A greater size variation of these long pleural spines is recognized from the earlier meraspid degrees in a gradual appearance or disappearance until the holaspid period. The thoroughly inverse variation of these pleural spines in size and form is seen for the first time during the process of trilobite segment formation, which offers an opportunity to trace the evolutionary path of trilobite body structures, associated with their functional morphology and behavior. The ontogenetic strategy of B. kueichouensis is broadly similar to those of other redlichioid representatives where the full ontogenetic sequence exhibited a balanced rate in segment expression and liberation. Such a developmental mode may be the reason why most redlichiids possess a micropygous body patterning during ontogeny and provides insights into the controls of early arthropod developmental segmentation during evolutionary process.