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Trilobites were notably flexible in the moulting behaviours they employed, producing a variety of moult configurations preserved in the fossil record. Investigations seeking to explain this moulting variability and its potential impacts are few, despite abundant material being available for study. We present the first quantitative study on moulting in a single trilobite species using a dataset of almost 500 moult specimens of Estaingia bilobata from the Cambrian (Series 2, Stage 4) Emu Bay Shale, South Australia. Specimens were categorized by moulting mode (Salter's or Sutural Gape) and their associated configurations, and their body proportions measured from both a museum collection (including a bycatch sample) and a randomly-collected field sample. This enabled analysis of the proportion of E. bilobata specimens displaying the Sutural Gape and Salter's modes of moulting and their different configurations, and tests for association between moulting behaviour and body proportions. The results show a wide range of E. bilobata moulting configurations in all samples, suggesting that configurations represent definable instances in a largely continuous spectrum of variation. Analyses comparing body proportions of specimens showing the two modes of moulting were non-significant, suggesting there is no true association between moulting behaviour and body proportion, except for a single significant result for body length. All results were relatively consistent between the museum and field samples. However, removing accessioned specimens from the museum sample brought results even further in line with the field sample, supporting the need for consideration of museum collection bias in palaeontological analyses.

The Cambrian explosion represents the rapid emergence of complex marine ecosystems on Earth. The propagation of predator-prey interactions within these systems was almost certainly one of the major drivers of this evolutionary event, sparking an arms race that promoted the proliferation of biomineralised exoskeletons and shells, and the evolution of the first durophagous (shell-crushing) predators. The most commonly documented evidence of Cambrian durophagous predation comes from injured trilobites. However, quantitative analysis based on multiple specimens from single localities is lacking. Such studies are required to reveal the dynamics of ancient predator-prey systems at fine ecological scales (e.g. at the population or community level). This study documents injured specimens of two trilobite species, Redlichia takooensis and Redlichia rex, from the Emu Bay Shale Konservat-Lagersta center dot tte (Cambrian Series 2, Stage 4) on Kangaroo Island, South Australia. A total of 38 injured specimens exhibiting various healed cephalic and thoracic injuries are documented, in addition to the mangled remains of two individuals that probably resulted from the activities of a durophagous predator or scavenger. Specimens of both species show that most injuries are located on the posterior portion of the thorax, indicating that predators preferentially attacked from behind and/or prey individuals presented the posterior of the trunk towards the predator when threatened or fleeing. The larger sample of injured R. takooensis shows that while unilateral injuries are more common than bilateral ones, there is no evidence for a left-or right-side bias, contrasting with previous suggestions that Cambrian trilobites exhibit right-sided injury stereotypy. Comparing the position of injured and non-injured R. takooensis and R. rex in bivariate space, we illustrate that injured specimens of both species typically represent some of the largest individuals of these taxa. This suggests that