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

Abstract

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 Emu Bay Shale: A unique early Cambrian Lagerstätte from a tectonically active basin

“…We infer that the trilobite/petalloid prodelta assemblage may have been preserved in situ within the Konservat-Lagerstätte interval (Fig. 5), as supported by paleoecologic and biostratinomic data, including common molt ensembles of both trilobite species (62,63), the full range of E. bilobata growth stages (64,68), and articulated specimens of petalloids. The endobenthic palaeoscolecid worm Wronascolex antiquus is also common (table S3) (69).…”

“…3 and 4, and table S3) reveals that the biota was strongly influenced by these environmental factors. We hypothesize that most taxa were transported to the prodelta setting, although evidence of trilobite moult assemblages (62,63) suggests periodic colonization of the benthic environment by a low diversity community. The most abundant taxon is the trilobite Estaingia bilobata (Fig.…”

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 1989, 1991, 1993a, b; Hopkins 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. What are the extents of trilobite disparity in cephalic morphometry? How does cephalic morphometry vary with order-level taxonomy, and can cephalon shape be used to predict taxonomic assignment? How does cephalic morphometry vary across the Palaeozoic, in terms of morphospace volume occupation and movement through this space, and can cephalon shape be used to predict the geological Period occupied? To address these questions, we analyse a dataset of nearly 1000 trilobite cephala, digitised as 2D outlines.…”

“…For example, Vargas-Parra and Hopkins (2022) tested patterns of modularity in the trilobite cephalon, and hypotheses relating to the developmental origins of the eye. Drage (2022) used traditional multivariate morphometric analyses to test for an association between body proportions and exoskeleton moulting behaviour across Trilobita, while Drage et al (2023) tested the same hypothesis on an intraspecific scale, using a large dataset of Estaingia bilobata Pocock, 1964.…”

“…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.…”