Ontogenetic moulting behavior of the Cambrian oryctocephalid trilobite Arthricocephalites xinzhaiheensis.

Yifan Wang,Hui Zhang,Jin Peng,Dezhi Wang,Xiuchun Luo,Quanyi Sun,Chenchen Ling,Qiujun Wang,Yunbin Shao

doi:10.7717/peerj.12217

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.

Highlights

The periodic shedding of the old exoskeleton during development is called ecdysis, or moulting, and this is a key feature of Ecdysozoa, which includes three well-supported clades, namely Scalidophora (Loricifera, Priapulida and Kinorhyncha), Nematoida (Nematoda and Nematomorpha) and Panarthropoda (Arthropoda, Tardigrada and Onychophora) (Aguinaldo et al, 1997; Giribet & Edgecombe, 2017; Wang et al, 2019)
The major fossil record of moulting comes from the phylum Arthropoda and the literature published on ecdysis in the fossil record mostly concerns the moulting behavior in trilobites (Daley & Drage, 2016)
Previous studies showed that trilobite exoskeleton moulting behaviors are both highly inter-and intraspecifically variable in comparison with what has been observed for other extinct groups and for modern groups (Wang et al, 2019; Daley & Drage, 2016; García-Bellido & Collins, 2004; Yang et al, 2019; Topper et al, 2013; Phlippen et al, 2000; Ayali, 2009; Brandt, 2002)

Summary

Introduction

The periodic shedding of the old exoskeleton during development is called ecdysis, or moulting, and this is a key feature of Ecdysozoa, which includes three well-supported clades, namely Scalidophora (Loricifera, Priapulida and Kinorhyncha), Nematoida (Nematoda and Nematomorpha) and Panarthropoda (Arthropoda, Tardigrada and Onychophora) (Aguinaldo et al, 1997; Giribet & Edgecombe, 2017; Wang et al, 2019). Henningsmoen (1975) and Drage et al (2018) named a number of moult configurations and described possible behaviors associated with these These works contribute to standardizing and better enabling further research into trilobite moulting. A concentration of well-preserved disarticulated configurations of the early Cambrian oryctocephalid trilobite Ar. xinzhaiheensis provides an unparalleled opportunity to allow the distinction between carcasses and exuviae to be made with confidence (Wang et al, 2020), and explore moulting behavior during development

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Discussion

Conclusion