Moulting behaviors in oryctocephalid trilobites reveal ontogenetic shifts in ecdysial strategies

Abstract

Similar to other arthropods, trilobite underwent ecdysis as a periodic stage, and this life history strategy had a profound effect on their ontogeny and morphological evolution. Previous paleontological studies of moulting behavior in trilobites are based on few exuviae within later meraspides and holaspid phases, and also too little is known about ontogenetic mouting behavior in trilobites. Here, we investigate abundant exuviae exhibiting relatively complete ontogenetic sequences of Arthricocephalus chauveaui, Arthricocephalites xinzhaiheensis, Duyunaspis duyunensis and Changaspis elongata from the Cambrian Stage 4 Balang Formation. The results indicate two clear trends of ontogenetic moulting behavior in oryctocephalid trilobites during development. A. chauveaui utilized a single Somersault moulting pattern, which deals with opening the cephalic sutures (facial and rostral sutures), with lower cephalic unit (LCU) somersaulting anteriorly and lying beneath the trunk in moult assemblages. However, a gradual transition of moulting behavior during the development, from Somersault to Henningsmoen's moulting pattern, i.e., from inverting the LCU to disarticulated cranidium, is confirmed in Ar. xinzhaiheensis, D. duyunensis and C. elongata. This shift during development might reflect an evolutionary experiment with different moulting modes throughout development, a significant transition with macroevolutionary implications. The moulting mode may be canalized by structures that develop or change with growth, meaning that the change in strategy is not optional, but rather a consequence of morphological constraints and developmental pathway.