Trilobite moulting behaviour variability had little association with morphometry

Trilobite moult assemblages preserved in the fossil record show high variability in moulting behaviour and their resulting moult configurations. The reasons for this variability, and the impacts it might have had on their evolutionary trajectories, are unknown and have rarely been investigated quantitatively. A large dataset of trilobite moult morphometric measurements is presented and statistically analysed for associations between moulting behaviour and morphometry. Results indicate little significant statistical association between the two; only between moulting behaviour (usually generalised moult configuration) and the variances and means of thoracic tergite number, thorax length, and pygidium width. Anterior cranidium width, cranidium length, cephalothoracic joint width, thorax width, pygidium length, and total body length all have non-significant associations with moulting behaviour. Moult specimens showing inversion of the librigenae generally have more thoracic tergites, a correspondingly longer thorax, and a narrower pygidium. Thoracic tergite count and pygidium measurements may have multimodal distributions. Principal Components Analyses and Non-Metric Multidimensional Scaling analyses suggest minor differences in the extent of morphometric variation for specimens showing different moulting behaviours, but little difference in the region of morphospace they occupy. This may indicate that trilobite species using Salter’s mode of moulting had more constrained morphologies, potentially related to facial suture fusion in some groups. Overall, these results do not suggest a strong association between moulting behaviour variation and morphometry in trilobites, leaving open for further study the mystery of why trilobites were so variable in their moulting, and whether this contributed to their long evolutionary reign or ultimate extinction. PLAIN LANGUAGE SUMMARY Trilobites were an important and globally abundant group of arthropods (animals with an exoskeleton and jointed limbs) that lived ~521-251 million years ago. The exoskeletons of arthropods are crucial because they provide protection against predators and parasites, but also restrict their growth. All living and extinct arthropods must therefore periodically moult (shed) their exoskeletons; an incredibly risky event during which many individuals die. Due to its importance, it is presumed that exoskeleton moulting impacted the broad-scale evolution of arthropod morphology (their physical characteristics), behaviour, and ecology. Trilobite moults are preserved in great number in the fossil record, and this can tell us much about their moulting behaviour. Additionally, trilobites appear to be unique in showing many different moulting behaviours. However, we do not know why trilobites were so variable in their moulting behaviour, or what impact this had on their evolution. In this study, a large dataset of trilobite moulting behaviours and their body proportion (morphometry) measurements is presented and analysed to answer: ‘Was variability in trilobite moulting behaviour related to differences in their morphometry?’ The results suggest that there was little association between the moulting behaviours shown by trilobites and their morphometry. Species showing the different moulting behaviours had overall similar morphologies, although for one moulting behaviour this seemed more limited. Only thorax length and segmentation (the central part of the body), and pygidium (‘tail’) width, significantly differed between species showing the different moulting behaviours. This study does not indicate a strong relationship between moulting behaviour and morphology in trilobites. This is unexpected, and leaves open the mystery of trilobite moulting variability.