Contrasting patterns of disparity suggest differing constraints on the evolution of trilobite cephalic structures during the Cambrian ‘explosion’

Abstract

Trilobites are an abundant group of Palaeozoic marine euarthropods that appear abruptly in the fossil record c. 521 million years ago. Quantifying the development of morphological variation (or 'disparity') through time in fossil groups like trilobites is critical in understanding evolutionary radiations such as the Cambrian 'explosion'. Here, I use geometric morphometrics to quantify 'cumulative disparity' in functionally-important structures within the trilobite cephalon across their initial radiation during Cambrian Series 2. Overall cephalic disparity increased rapidly and attained a maximum within several million years. This pattern is dominated by the cephalic outline (in particular the genal spines), reflecting rapid, convergent expansion to the extremes of morphospace in a few early families. In contrast,

Distinct causes underlie double-peaked trilobite morphological disparity

“…While cephalic outline does not represent the whole (Holmes, 2023), the lack of homologous morphological points otherwise shared between all trilobite groups makes this a way to utilise the broadest taxonomic dataset. Furthermore, while 3D data might be expected to be more representative of form, previous studies comparing the use of 2D and 3D morphometric data on comparable scales suggest 2D data is sufficient for evolutionary hypothesis testing, as it reconstructs the same patterns as 3D data (Hopkins and Pearson, 2016).…”

“…Cephalic outline shape, alongside the presence and morphology of other cephalic features such as facial sutures, eye ridges, and glabellar furrows, are key for taxonomic descriptions and assignments within Trilobita (Beecher, 1893a(Beecher, , 1893bEmmrich, 1839;Holmes, 2023;Park and Kihm, 2017;Rasetti, 1948;Salter, 1864;Stubblefield, 1936;Whittington et al, 1997). Indeed, many original species descriptions are based solely on cephala.…”

“…Holmes (2023) assessed cephalic disparity during the Cambrian, with different cephalic structures suggesting differing patterns in morphospace expansion. Cephalic outline showed rapid diversification in form, while interior structures showed a more gradual increase in disparity (Holmes, 2023). Hopkins et al (2023) analysed diversity, disparity, and geological range in Permian trilobites and compared this to the other Palaeozoic Periods.…”

“…used 400 cephalon outlines (including agnostids) to assess trends in cephalic morphometry with the prevalence of different enrolment strategies through the Palaeozoic Bault et al (2023a). used a large dataset of cephala and pygidia across trilobite groups to determine whether disparity trends followed diversity across the Devonian Holmes (2023). assessed cephalic disparity during the Cambrian, with different cephalic structures suggesting differing patterns in morphospace expansion.…”

A dynamic and collaborative database for morphogeometric information of trilobites

“… 38 – 40 ). Regarding trilobites, there has been an increase in taxonomically comprehensive morphological databases 41 , such as outline data describing the shape of the cranidium 31 , semilandmarks describing the shape of the cephalon 42 , landmark and semilandmarks configuration on cephalic outline, glabellar and eye ridges 43 , landmarks and semilandmark data on cephala and pygidia 44 , as well as trilobite moult morphometric measurements 45 . Here, we develop a database for morphological information of trilobites that is also quantified by means of geometric morphometrics state-of-the-art methods.…”