Synchronized moulting behaviour in trilobites from the Cambrian Series 2 of South China

Abstract

The study of moulting behaviour in the fossil record is relatively well known in arthropods and this is especially true for trilobites. Nevertheless, while studies focusing on the style of moulting in social and semi-social groups of modern animals (e.g. arthropods) are common, very few works investigate moulting adaptations in deep time. Here we report a trilobite assemblage from the Cambrian Series 2 “Tsinghsutung” Formation of South China. Around 850 specimens were used for this study from three different levels across one section near Balang (SE Guizhou Province, South China). These levels preserve numerous trilobite clusters in some cases containing around 400 individual specimens. Up to four species have been found in these clusters, but two species are more common. Trilobite clusters bear a high percentage of disarticulated specimens that we interpret as moults. Additionally, measurements of bioclast orientation and the dorsoventral attitude suggests very quiet water conditions followed by rapid burial events, prior to scavenger disturbance. Together, this indicates that the fossil assemblages were a result of a biological phenomenon rather than mechanical processes, allowing us to interpret the position of the fossil parts as different moulting configurations. Since the trilobite assemblage seems to be in situ, the large number of exuviae suggests a local place of migration. This was triggered by the need for group protection while moulting, which is suggestive of gregarious behaviour, possibly synchronized. These trilobites from the Cambrian Epoch 2, Age 4 constitute one of the earliest known gregarious community of trilobites and has important implications for understanding the ecology of this group during their emergence in the Cambrian.

Cephalic biomechanics underpins the evolutionary success of trilobites

“…Corrales‐García et al . 2020). Likewise, ollenellids had a circumocular suture in the genal field which appears in some taxa during late meraspid or early holaspid stages (Palmer 1957; Webster 2015).…”

“…On the other hand, despite the consistent body plan in trilobites, moulting behaviours described in trilobites have been highly variable throughout their evolutionary history and especially diverse during the Cambrian (Daley & Drage 2016; Drage 2019; Corrales‐García et al . 2020). This high disparity in moulting behaviour can be partially explained by a shift in the position of ecdysis sutures from the edge in ollenelids to the dorsal surface in other trilobites.…”

“…Although the facial sutures were only functional during moulting, their traces are seen in entire trilobites and constitute preferential lines of weakness (Daley & Drage 2016; Corrales‐García et al . 2020). It is unclear what factors drive the trade‐off between the advantages of having a facilitator for moulting, and the disadvantage of a potentially weak zone.…”

Trilobite moulting behaviour variability had little association with morphometry

“…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.…”

Allopatric molting of Devonian trilobites

“…For example, exuviae found in the shells or burrows of other organisms suggest that molting trilobites would seek shelter 2 – 5 . Large collections of exuviae are interpreted as evidence that some trilobites molted in aggregations, indicating collective behavior 6 , 7 . There is also evidence that some would burrow into the sediment to molt, reflecting an adaptive evolution to predation and high-stress ecological environments 8 .…”