Rise and fall of the phacopids: the morphological history of a successful trilobite family

Abstract

Phacopidae were a successful family of the Silurian–Devonian period. Although their diversity trends are well identified, their shape evolution is unknown; their morphology often considered to be conservative. We have quantified these morphologies using geometric morphometrics (landmarks) and investigated their evolution using morphological disparity indices. Results identified morphological variations between the genera, and through time. Phacopids differ from each other by the position of the facial suture linked to the size of the visual complex, the shape of the genal angle and the elongation of both cephalon and pygidium. The morphological disparity of cephala was high from the Silurian, contrary to that of pygidia. Subsequently, the morphological disparity increased in the Early Devonian with the development of narrow cephala and triangular pygidia. Morphological disparity was greater in the Emsian for both cephala and pygidia, more than 50 myr after the origination of phacopids. It constituted a perfect example illustrating that a peak of biodiversity does not necessarily happen in the early history of a clade. Subsequently, a strong decrease of morphological disparity occurred in the Middle Devonian, in conjunction with sea‐level changes and anoxic events. Taxonomic richness and morphological disparity declined strongly in the Givetian, in a non‐random extinction affecting particularly blind genera. The morphological disparity remained low in the Frasnian despite progressive eye reduction influenced by environmental changes. An extensive recovery occurred in the Famennian with an important increase of both taxonomic and morphological diversity. The Hangenberg event caused the final extinction of phacopids.

Distinct causes underlie double-peaked trilobite morphological disparity

“…A large part of trilobite diversity and disparity of forms is manifested in their cephalic morphometries; the shapes and sizes of the cephala of different groups. The variation in functional morphology of the cephalon is intrinsically linked to the varied life modes of trilobites; different cephalic shapes and structures have been suggested to be adaptations to different feeding modes (e.g., Fatka and Szabad, 2011; Fortey and Gutiérrez-Marco, 2022; Fortey and Owens, 1999; Hegna, 2010; Hughes, 2000; Pearson, 2017), life modes (e.g., Bault et al, 2023b; Cherns et al, 2006; Esteve et al, 2021; Fortey, 2014), or specific behaviours (e.g., Drage, 2019; Henningsmoen, 1975; Suárez and Esteve, 2021). However, the evolution and extent of disparity of the trilobite cephalon remains unclear, with uncertainty around the unstable high-level trilobite taxonomy (Adrain, 2013, 2011; Paterson, 2019), the potential homology of cephalic structures (Du et al, 2023; Hughes, 2003; Park and Kihm, 2017), and the adaptation of cephalic shape to hypothetical life mode.…”

“…These analyses have been used to assess shape change during ontogeny, such as for Cryptolithus tesselatus Green, 1832 (Hopkins and Pearson, 2016), to better understand development trajectories. Several studies explored shape evolution within specific clades; for example, Hopkins (2017) coupled cranidial shape change with exoskeleton morphological characters in pterocephaliids, and Bault et al (2023b) explored shape evolution in Phacopidae and the associations between disparity and external forcing events. Many studies have used morphometric analyses to aid in taxonomic work (e.g., Álvaro et al 2018; Zhao et al 2020).…”

A dynamic and collaborative database for morphogeometric information of trilobites

“…For example, our method is able to capture the cephalon outline, but also suture and glabellar outline and eye morphology, as well as key pygidial features, all of which have been interpreted as important traits when describing the morphological variability among trilobites 97 . Results using this protocol demonstrated its ability to distinguish main morphological variations across all orders and families present during the Devonian (see Usage notes), as well as at regional and local scales during the lower Ordovician 77 , and within a single family (Phacopidae) along its whole evolutionary history 44 .

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“… 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.…”

“…The landmark template defined here is the result of a collaborative work among many trilobite specialists in our group (e.g 44 , 48 , 51 , 65 – 86 ) and is also based on a long historical research of landmark-based protocols in trilobites (e.g 42 , 84 , 87 – 95 ). It is designed to maximise trilobite shape description across all major trilobite orders.…”