Development of the early Cambrian oryctocephalid trilobite Oryctocarella duyunensis from western Hunan, China

Abundant articulated specimens of the oryctocarine trilobite Oryctocarella duyunensis from the lower Cambrian (Stage 4, Series 2) Balang Formation at the Bulin section in western Hunan Province, South China, permit the description of all meraspid degrees. The maximum number of thoracic segments observed in this collection is 11. Meraspid growth was accompanied by progressive and gradual change in overall form, and this animal showed an homonomously segmented trunk with variation in the number of pygidial segments during ontogeny. Such variation permits a variety of plausible explanations, but a model of successive instars defined by the number of thoracic segments and in suborder by the number of pygidial segments is highly unlikely to explain the growth pattern because it would result in the loss of trunk segments between some instars. Degree-based ontogenetic staging is also compatible with the variation observed.