Recognising moulting behaviour in trilobites by examining morphology, development and preservation: Comment on Błażejowski et al. 2015

Abstract

A 365 million year‐old trilobite moult‐carcass assemblage was described by Błażejowski et al. (2015) as the oldest direct evidence of moulting in the arthropod fossil record. Unfortunately, their suppositions are insufficiently supported by the data provided. Instead, the morphology, configuration and preservational context of the highly fossiliferous locality (Kowala Quarry, Poland) suggest that the specimen consists of two overlapping, queued carcasses. The wider fossil record of moulting actually extends back 520 million years, providing an unparalleled opportunity to study behaviour, ecology and development in early animals. Taking cues from modern analogues, it is possible to quantify precise details about moulting behaviour to determine broad‐scale evolutionary trends, ontogenetic sequences and morphological selection pressures. In this review, we argue that this rich source of data has been underused in evolutionary studies, though has great potential for investigating the life history and evolution of arthropods in deep time.

Ecdysis in a stem-group euarthropod from the early Cambrian of China

“…The Open Moult Position would appear to represent the common mechanism for performing ecdysis without the aid of specialized exoskeletal adaptations (e.g. suture lines), which are common in more phylogenetically-derived forms such as trilobites 7–9 , and appear to may have evolved multiple times amongst various groups of Cambrian euarthropods 37,38 . The occurrence of the Open Moult Position seems constrained by the overall construction of the exoskeleton, particularly in taxa characterized by the presence of an extensive head shield or carapace covering the anterior region.…”

“…However, most fossil evidence of moulting is biased in favour of organisms with heavily biomineralised exoskeletons (e.g. trilobites, decapod crustaceans) 7–10 , or those whose behaviour may lead to the burial of numerous exuviae simultaneously (e.g. eurypterid mass moulting events) 6,11 .…”

Quantifying intra- and interspecific variability in trilobite moulting behaviour across the Palaeozoic

“…Trilobites, which appear earliest in the fossil record amongst crown-group arthropods (Daley et al, 2018), are therefore unique in their moulting. Previous studies of trilobite moulting, despite their clear importance, have been limited in scope or entirely qualitative observations (e.g., Henningsmoen, 1975;Whittington, 1980Whittington, , 1990McNamara and Rudkin, 1984;Busch and Swartz, 1985;Speyer, 1985;Speyer and Brett, 1985;McNamara, 1986;Brandt, 2002;Bruthansová, 2003;Clarkson et al, 2003;Budil and Bruthansová, 2005;Hunda et al, 2006;Paterson et al, 2007;Cederström et al, 2010;Rustán et al, 2011;Drage and Daley, 2016;Drage et al, 2018a), leaving many unanswered questions about the evolution of this behaviour. How flexible were trilobites in their moulting behaviour compared to other arthropods?…”

“…This study presents the first broad-scale quantitative exploration of trilobite exoskeleton moulting behaviours. Previous qualitative studies named moult configurations and reconstructed behaviours, while noting that despite the overall similarity of body forms within Trilobita, their moulting behaviour is seemingly highly variable (see Henningsmoen, 1975;Whittington, 1990;Brandt, 2002;Hughes, 2003;Daley and Drage, 2016;Drage and Daley, 2016;Drage et al, 2018a). This is in comparison to groups like chelicerates or decapod crustaceans, which have several body forms, and each has one specialised mode of moulting (Daley and Drage, 2016, and references therein).…”

Phyllocarid crustaceans from the Late Devonian of the Kowala quarry (Holy Cross Mountains, central Poland)

“…Błażejowski et al . () described an in situ specimen of Trimerocephalus that may represent a freshly moulted individual (see also Drage & Daley ).…”