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 homonymously 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 compatible with the variation observed.

Automated summary

Abundant articulated specimens of the oryctocarine trilobite Oryctocarella duyunensis from the lower Cambrian Balang Formation in western Hunan Province, South China, allow the description of all meraspid degrees. The study shows progressive and gradual change in overall form during meraspid growth, with variation in the number of pygidial segments. A model based on degree-based ontogenetic staging is more likely to explain the growth pattern observed, compared to a model based on the number of thoracic and pygidial segments.