Morphological evolution during the last hurrah of the trilobites: morphometric analysis of the Devonian asteropyginid trilobites

Abstract

The Asteropyginae Delo, 1935 is a group of phacopid trilobites in the family Acastidae Delo, 1935 that has served as the focus for several studies due to their distinctive morphologies and diversity. However, despite an interest in these characteristic morphologies, there have been no studies that have examined this group using morphometric techniques. Our investigation utilized both geometric morphometric and elliptical Fourier methods to quantify the morphology of cephalic sclerites of asteropyginid specimens representing wide taxonomic sampling of the clade. We constructed a phylomorphospace that shows temporal and spatial patterns of phenotypic evolution within the framework of a novel tip-dated phylogenetic tree generated using Bayesian inference. We recovered similar patterns in disparity regardless of the morphometric approach. Both analyses illustrated a marked expansion into morphospace throughout the temporal range of the clade, peaking in disparity in the Emsian and with European taxa exhibiting the highest disparity in glabellar morphospace. Additionally, glabellar shape showed low phylogenetic signal and no major patterns in phylomorphospace. This study highlights the utility of employing different methodologies to quantitatively explore the disparity of fossil taxa. It also illustrates some of the patterns of morphological change occurring during one of the final and major evolutionary radiations within Phacopida.

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.…”

“…Many studies have used morphometric analyses to aid in taxonomic work (e.g., Álvaro et al 2018; Zhao et al 2020). These analyses can also be informative for building or testing phylogenetic hypotheses; for example, Martin et al (2023) produced a phylomorphospace for asteropyginid glabellae. Still more research used trilobite morphometric data to explore specific morphological or behavioural evolutionary questions.…”

“…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.…”