Trilobite moulting behaviour variability had little association with morphometry

Abstract

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 averages 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.

Associations between trilobite intraspecific moulting variability and body proportions: Estaingia bilobata from the Cambrian Emu Bay Shale, Australia

“…2002; RStudioTeam 2020) showing the proportion of Estaingia bilobata specimens that used Salter's or the Sutural Gape mode of moulting, in the hybrid Henningsmoen's configuration, and preserved in each of the moulting configurations (Drage et al . 2018; Drage 2019 a , 2019 b ). A, field sample.…”

“…The preserved moulting configurations, depending on the disarticulation and orientation of specific sclerites of the exoskeleton, include the axial shield, Harrington's, Henningsmoen's, McNamara's, Nutcracker, Somersault, Salter's and Zombie configurations, as described by Drage et al . (2018) and Drage (2019 a , 2019 b ). The Zombie configuration was not originally documented in E. bilobata by Drage et al .…”

“…The Zombie configuration was not originally documented in E. bilobata by Drage et al . (2018), but was later described and named by Drage (2019 b , ch. 1); this configuration comprises an intact cephalon disarticulated from the thoracopygon, with no inversion of the cephalon.…”

“…See Drage et al . (2018) and Drage (2019 a ) for full descriptions of moulting configurations, and Drage (2019 b ) for details on the Zombie configuration. All scale bars represent 5 mm.…”

Distinct causes underlie double-peaked trilobite morphological disparity

“…While morphometric analyses on trilobite data have been used to address a range of macroevolutionary (e.g., Foote 1989Foote , 1991Foote , 1993aHopkins 2014;Suárez & Esteve 2021), behavioural (e.g., Drage, 2022;Drage et al, 2023;Suárez and Esteve, 2021), developmental (e.g., Crônier et al, 1998;Hopkins and Pearson, 2016;Kim et al, 2002), systematic (e.g., Holmes et al, 2020;Martin et al, 2023;Paterson, 2005;Żylińska et al, 2013), and taphonomic (e.g., Hopkins and Pearson, 2016;Webster and Hughes, 1999) questions, several crucial macroevolutionary questions remain.…”