Ontogenetic moulting behavior of the Cambrian oryctocephalid trilobiteArthricocephalites xinzhaiheensis

Abstract

Moulting behaviors in trilobites are a crucial strategy during development. Previous studies have demonstrated inter-and intraspecific variability of moulting behavior in trilobites. Currently, ecdysial motifs for trilobites are considered not stable even within species and fewer detailed studies dealt with moulting behaviors in a single species of trilobite during development. Here a large sample of meraspid to holaspid exuviae of Arthricocephalites xinzhaiheensis (131 specimens) from the Cambrian Balang Formation of South China has allowed description of the reasonably complete ontogenic moulting sequence. Both ontogenetic stage and body size reveal gradual transition of configuration from Somersault configuration to Henningsmoen’s configuration during development. Somersault configuration is exclusive till meraspid degree five and exists in subsequent growth stages. This suggests that opening of the facial and rostral sutures allowing the emergence forward of the post-ecdysial trilobite was prevalent in early growth stages. In later development, Henningsmoen’s configuration (showing disarticulation of the cranidium) became more dominant. This study indicates that gradual transition of ontogenetic moulting behavior occurred in oryctocephalid trilobites in the early Cambrian.

Trilobite moulting behaviour variability had little association with morphometry

“…Inversion has been suggested to occur through dorsal flexure or partial enrolment of the individual during moulting (Whittington, 1990; McNamara and Rudkin, 1984; Daley and Drage, 2016; Drage, 2019a,b), for example in Estaingia bilobata (Drage et al, 2018a), and so its rarity here suggests many trilobites may have remained unflexed during moulting. Although, changes in moulting behaviour through ontogeny described by Wang et al (2021) may suggest that certain trilobite species did regularly employ flexure for moulting but that this was linked to their development. Alternatively, this may reflect a description bias of trilobite moult specimens.…”

“…Specimens that might have been of a late juvenile developmental stage, based on thoracic tergite number and relative size, were removed from the dataset. This is because the inclusion of juveniles could bias results as immature individuals have been shown to moult differently to their adult counterparts in some species (Drage et al, 2018b; Wang et al, 2021), and ontogenetic studies show that morphology can differ extensively between developmental stages (Park and Choi, 2011). It can be difficult to distinguish late-stage meraspides, which can have a full complement of thoracic tergites, from holaspides, as many trilobite species are not known from a sufficient number of juvenile specimens to describe their ontogenies, and some species also had variable numbers of tergites during functional adulthood (Hughes, 2007; Park and Choi, 2011).…”

“…Although, changes in moulting behaviour through ontogeny described by Wang et al (2021) may suggest that certain trilobite species did regularly employ flexure for moulting but that this was linked to their development. Alternatively, this may reflect a description bias of trilobite moult specimens.…”

“…Several early works emphasised the potential important impacts of trilobite moulting behaviour variability on their evolutionary trajectories (Henningsmoen, 1975;Whittington, 1990;Budil and Bruthansova, 2005) and the potential impact of this behaviour on their extensive success and ultimate decline (Brandt, 2002). Recent studies have described moulting in specific trilobite groups, such as Zong (2020) on moults of Ovalocephalus tetrasulcatus arranged end-to-end, synchronised moulting behaviour of several oryctocephalid species preserving diverse moulting configurations by Corrales-García et al (2020), and ontogenetic moulting sequence trends in Arthricocephalites xinzhaiheensis by Wang et al (2021). These studies have further increased support for inter-and intraspecific variability in moulting across Trilobita and its links with other aspects of morphology and behaviour.…”

Associations between trilobite intraspecific moulting variability and body proportions: Estaingia bilobata from the Cambrian Emu Bay Shale, Australia

“…2018; Drage 2019 a ; Wang et al . 2021). They also demonstrate a variety of morphological adaptations to different moulting behaviours, such as the facial sutures used specifically to create an ecdysial gape for moulting in some of the earliest trilobites and across all major clades (Stubblefield 1959; Whittington et al .…”