Euarthropods, more specifically members of the clade Artiopoda, dominated diversity in Palaeozoic animal communities (Daley et al., 2018). This group, which includes trilobites and their non-biomineralizing relatives, are united by a morphology consisting of a headshield covering a pair of uniramous antennae and at least three pairs of biramous limbs, followed by a dorsoventrally flattened body with a series of biramous appendages (Stein and Selden, 2012; Giribet and Edgecombe, 2019). The origins of key morphological features facilitating the diversity of trilobites can be found in their non-biomineralizing artiopodan relatives (e.g. Du et al., 2019; Schmidt et al., 2022). The cephalon, as an informative and functionally specialized body region, affords pivotal data for resolving the phylogenetic relationships and evolutionary history of arthropods, artiopodans, and trilobites in particular (e.g. Stubblefield, 1936; Lieberman, 2002; Budd and Telford, 2009; Yang et al., 2013; Paterson et al., 2019; Strausfeld et al., 2022).

The presence, shape, and character of sutures in the trilobite cephalon have been central to classification schemes of the group for nearly 200 years (e.g. Emmrich, 1839; Salter, 1864; Beecher, 1897a; Beecher, 1897b; Stubblefield, 1936; Rasetti, 1945; Whittington et al., 1997), while the similarity in the cranidial outlines of Cambrian suture-bearing groups supported a single origin or strong evolutionary constraint on its origins (Foote, 1991; Hughes, 2007). These sutures provided lines of weakness along which the cephalon split during moulting, creating an anterior ecdysial gape through which the animal exited the old exoskeleton (e.g. Henningsmoen, 1975; Whittington, 1990; Daley and Drage, 2016). Sutures may have served additional purposes (Stubblefield, 1936) and were under additional functional demands, including feeding (Fortey and Owens, 1999) and burrowing behaviours (Esteve et al., 2021). Different trilobite clades display distinct suture patterns, with two sets, the circumocular and marginal sutures, relevant for the creation of this ecdysial gape (Henningsmoen, 1975). For the earliest diverging trilobites, olenellines (Palmer and Repina, 1993; Lieberman, 2002) which a have been recovered as a paraphyletic grade rather than a monophyletic group (e.g. Paterson et al., 2019) circumocular sutures facilitated shedding of the cornea during ecdysis (e.g. Ramsköld and Edgecombe, 1991) while a marginal suture (Figure 1e) facilitated anterior egression through separation of the lateral cephalic doublure from the dorsal cephalon (e.g. Stubblefield, 1936; Henningsmoen, 1975). In later diverging trilobites the circumocular and marginal sutures combined into a single system (Figure 1d). Here, the cornea is fused to the free cheek (e.g. Stubblefield, 1936; Ramsköld and Edgecombe, 1991) and these facial sutures – the fused circumocular and marginal sutures - facilitated both ecdysis of the visual surface and anterior egression of the exoskeleton following withdrawal from the free cheeks (e.g. Henningsmoen, 1975).

Figure 1

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The origin of these fused dorsal ecdysial sutures – facial sutures – has traditionally been considered to fall within Trilobita (e.g. Fortey and Whittington, 1989; Edgecombe and Ramsköld, 1999; Lieberman, 2002). However, the presence of dorsal ecdysial sutures in earlier diverging artiopodans – the Protosutura (Du et al., 2019) and eye slits in Petalopleura (Chen et al., 2019) raised questions about the homology and origins within Artiopoda (e.g. Hou et al., 2017b; Du et al., 2019). An alternative hypothesis for the origins of the dorsal cephalic sutures emerged: that these had a deep root within Artiopoda and were subsequently lost in some groups including multiple times within Trilobita (Hou et al., 2017b; Du et al., 2019), rather than the traditional view that dorsal cephalic sutures in trilobites were derived within the clade, and thus these eye slits and facial sutures were acquired independently. Support for the deep root hypothesis comes from the variability of facial sutures in trilobites, and the recognition that convergence and loss of features is common when they have a function or allow adapation to a particular niche (e.g. Moore and Willmer, 1997). Further evidence for the variability of suture morphology comes from ontogenetic studies, such as the fusion of dalmanitinid facial sutures during ontogeny (Drage et al., 2018), and the loss of this feature in other trilobites such as ‘Cedaria’ woosteri (Hughes et al., 1997). This indicates that there is scope for a facial suture to have been lost repeatedly in artiopodan groups earlier diverging than Trilobita, should this feature prove to have a deeper origin within Artiopoda (Hou et al., 2017b; Du et al., 2019). A deep root for facial sutures within Artiopoda would have repercussions for the importance of the facial sutures in determining trilobite relationships (e.g. Jell, 2003) and the position of a grade of olenelline trilobites as the earliest diverging members of the group (e.g. Paterson et al., 2019). To date, a third possibility – that the ventral plates of Acanthomeridion are homologous to the doublure of olenellids, and thus Acanthomeridion and olenelline trilobites share the presence of an unfused marginal suture – has not received broad attention in the literature, but should also be considered in light of the similarities in a position of the ventral plates and olenelline doublure, and the marginal rather than dorsal position of the suture in Acanthomeridion.

Resolving the complex history of dorsal ecdysial sutures evolution within Artiopoda, requires the nature of cephalic sutures in artiopodans such as Acanthomeridion, protosuturans, and petalopleurans to be resolved, and their phylogenetic position confidently determined (Hou et al., 2017b; Du et al., 2019). However, the morphology of many earlier diverging artiopodans is not completely understood (Ortega-Hernández et al., 2013; Lerosey-Aubril et al., 2017), contributing a significant amount of missing data to character matrices used in phylogenetic analyses. Up to now, Acanthomeridion was arguably the least well-known of these early diverging artiopodans, with its ventral anatomy and non-biomineralized structures very poorly known (Hou and Bergström, 1997; Hou et al., 2017b).

Here, we describe new specimens of Acanthomeridion serratum collected from Jiucun town, Chengjiang (Figure 2), using micro-CT to reveal details of the ventral anatomy, eyes, and appendages for the first time, and reconstruct the changing shape of the body during ontogeny. We use these new data to show that A. anacanthus is a junior synonym of A. serratum. We then assess the phylogenetic position of the species considering two hypotheses relating to the dorsal ecdysial sutures – as homologous to the dorsal ecdysial sutures (facial sutures combined of a fused marginal and circumocular suture) of trilobites (Figure 1d), that they are homologous to marginal sutures of olenellines (Figure 1e), and that these structures are unique to Acanthomeridion (Figure 1f), to evaluate the different hypothesis of origination within Artiopoda.

Figure 2

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All consensus trees support multiple origins of cephalic sutures in Artiopoda (Figure 10, Figure 10—figure supplements 1–3). When the sutures of Acanthomeridion and trilobites are considered as homologous (e.g. Hou et al., 2017a), as well as to those of other artiopodans (e.g. Du et al., 2019), terminals with ecdysial sutures in the cephalon are found in multiple groups in Artiopoda (Figure 10a), although Acanthomeridion is recovered sister to trilobites. If instead the sutures in Acanthomeridion are considered marginal, and homologous to those of olenelline trilobites, Acanthomeridion, and trilobites do not form a clade (Figure 10b, Table 1), and trees occupy a different region of the treespace (Figure 11). Indeed, the posterior samples of analyses treating the ventral plates as not homologous to any other artiopodan feature (Figure 10c) are extremely similar to those treating them as homologous to the doublure of trilobites, as illustrated by their broad overlap in treespace (Figure 11) and similar support for nodes in the consensus trees (Figure 10—figure supplements 1–3). Thus, two of the three coding strategies receive some support from the phylogenetic results. Either Acanthomeridion shares the presence of facial sutures with trilobites, and these form a clade, but the sutures in petalopleurans and Zhiwenia are distinct (multiple origins of dorsal ecdysial sutures) or the ventral lateral plates of Acanthomeridion are a distinct feature to the free cheeks of trilobites (and petalopeluran and Zhiwenia features are also distinct; multiple origins of sutures). The position of Acanthomeridion as not sister to trilobites in the analyses treating the ventral plates as homologous to doublure means that this hypothesis did not gain support, given the assumptions and character matrices of this study.

Support for Acanthomeridion as sister to trilobites comes from morphological similarity between the spinose termination of the ventral plates of Acanthomeridion (similar to trilobite genal spines) and the suture that follows an oblique line and passes near the eye both for Acanthomeridion (similar to trilobite opisthoparian facial sutures; Whittington et al., 1997). Taken together with trilobate exoskeleton of large Acanthomeridion specimens, the non-biomineralised Acanthomeridion bears many morphological similarities to biomineralised trilobites that further support a sister relationship. However, this sister relationship rests on the treatment of the ventral plates as free cheeks. If this interpretation is not favored, then these similarities can be considered convergent, and the position of Acanthomeridion within Artiopoda is not well resolved by these analyses. For this interpretation of the ventral plates, the path of the suture between the ventral plates and cephalon passing close to the eye may represent a way to minimise damage to the eye during ecdysis, which is comparable to what is observed in trilobites with facial sutures.

Other features observed in Acanthomeridion for the first time – uniramous deutocerebral appendages, an axe-shaped conterminant hypostome, two librigena-like plates, and three additional head appendages – are consistent with a phylogenetic position for the species as an early diverging artiopodan, or as sister to trilobites. Multipodomerous uniramous deutocerebral antennae are known in nearly all other members of the clade (Zhang et al., 2007; Du et al., 2019; Zhai et al., 2019), and thus this feature is not very informative. Similarly, a conterminant hypostome was already known in numerous other artiopodans (Zhang et al., 2004; Stein and Selden, 2012). As this type of hypostomal attachment is considered ancestral within Trilobita (Fortey, 1990), its presence in the possible sister to trilobites (if dorsal sutures are considered homologous) is consistent with these expectations from trilobite anatomy. If Acanthomeridion is instead an early diverging artiopodan, the presence of natant hypostomes in both trilobitomorphs and vicissicaudates (Fortey, 1990; Chen et al., 2019) demonstrates that a natant hypostome has most likely evolved repeatedly within Artiopoda.

In summary, new fossils and CT-scan data reveal the ventral anatomy and appendages of Acanthomeridion serratum for the first time, and additional specimens demonstrate that A. ‘anacanthus’ represents a junior synonym of A. serratum, with some features thought to distinguish the two instead ontogenetic in origin. The presence of a posteriorly orientated spine on the cephalic ventral plates of Acanthomeridion might be homologous to the genal spine of trilobite librigenae, providing additional support for a.sister group relationship between these taxa. Phylogenetic analyses interrogating this hypothesis recover a sister relationship between Acanthomeridion and trilobites, but still require multiple origins of cephalic sutures within the group. If the ventral plates are instead treated as homologous to the doublure of trilobites, or as not homologous to any trilobite feature, then no close relationship is recovered, and the morphological similarities between Acanthomeridion and trilobites such as the path of the suture separating the ventral plates from the cephalon, and details of the ventral plates, should instead be treated as convergent. Thus, regardless of how the ventral plates of Acanthomeridion are interpreted, multiple origins of cephalic sutures in artiopodans are recovered by phylogenetic analyses.

New specimens of Acanthomeridion were collected from Jiucun town, Chengjiang (Figure 2) and accessioned at the Management Committee of the Chengjiang Fossil Site World Heritage (CJHMD 00052–00062), and Research Center of Paleobiology, Yuxi Normal University (YRCP 0016–0020). New specimens of Wutingaspis tingi (YRCP 0021) and Xandarella spectaculum (YRCP 0022) were collected from Haikou town, Kunming (Figure 2) and housed at Research Center of Paleobiology, Yuxi Normal University. The holotype of Zhiwenia coronata (YKLP 12370) is deposited at the Key Laboratory for Palaeobiology, Yunnan University which was excavated from Xiaoshiba area, Kunming (Figure 2). The specimens were photographed by a LEICA DFC 550 digital camera mounted to a Stereoscope LEICA M205 C, and scanned with a Zeiss Xradia 520 Versa X-ray Microscope and Nikon XTH 225 ST micro-focus X-ray tomography machine. All micro-CT images were processed with the software Drishti (Zhai et al., 2019). Figures were processed with CorelDRAW X8, Inkscape 1.0, and Illustrator.

Given that specimens with thorax longitudinally curved and specimens with flattened thorax are considered as two species of Acanthomeridion, despite having the same exoskeletal morphology, we measured their axial length and attempted to determine their true maximum width in all specimens, including those described in this study and previously documented measurable specimens. The data were initially analyzed using a bivariate regression fitting function in Microsoft Office Excel. Subsequently, the software Past (Hammer et al., 2001) was employed for testing and validation, obtaining the same function and conducting a rational analysis of the function.

The character matrix used for the phylogenetic analyses was adapted from Schmidt et al., 2022, itself adapted from Du et al., 2019 and Chen et al., 2019. Olenellus getzi was added, to facilitate comparison with olenelline trilobites, while Anacheirurus adserai, Cryptolithus tesselatus, and Olenoides serratus were added to provide more trilobite terminals. Olenellus getzi was chosen as it represents the olenelline with best known soft tissues (antennae only; Dunbar, 1925). Trunk appendages are known in A. adserai, C. tesselatus, and O. serratus (Walcott, 1911; Whittington, 1975; Perez-Peris et al., 2021), with further descriptions by Bicknell et al., 2021; Losso and Ortega-Hernández, 2022 for coding the matrix. Our updated dataset includes 70 taxa (Acanthomeridion anacanthus was removed, four trilobites were added). Three matrices were used to test the alternative hypotheses of sutures, one with 100 characters considering the ventral plates unique, and not homologous to trilobite librigenae (‘ventral plates under head’ as a new character in the dataset) and two with 99 characters. The first of these considered the ventral plates homologous to trilobite librigenae, and the second to the doublure of olenellines (see Morphobank project). We added the character state ‘ringed distribution lamellae’ to Ch. 15 in all matrices (Chen et al., 2019; Du et al., 2019), and altered the characters relating to the suture pattern (Ch. 31, 32) to include character states suitable for Olenellus getzi. Given the difficulties of identifying circumocular sutures in non-biomineralizing artiopodans, Ch. 31 indicated the presence or absence of ecdysial sutures in the cephalon, while Ch. 32 indicated the type of suture. This could either be marginal and/or circumocular, but unfused (state 0) or marginal and circumocular fused into a facial suture (state 1). This facilitated differentiation between the state in Olenellus getzi and the other trilobites in the dataset, and thus allowed the two hypotheses relating to homology between the ventral plates and the doublure or free cheeks of trilobites to be tested. Eight additional characters relating to the trilobite glabellar region were added, taken from Paterson et al., 2019, in order to provide data to distinguish internal trilobite relationships. These were Ch. 5, 6, 7, 10, 11, 12, 14, and 15 in Paterson et al., 2019, which are Ch. 37–44 in all three matrices used for this study. A further character (calcified thorax separated into prothorax and opisthothorax) was also added (Ch. 99 or 100 in matrices used for this study). Matrices used in phylogenetic analyses are provided through Morphobank Project 4290 (http://dx.doi.org/10.7934/P4290).

Phylogenetic analyses were performed with MrBayes (Bayesian Inference) (Ronquist et al., 2012). Both the ‘maximum information’ and ‘minimum assumptions’ strategies of ref (Bapst et al., 2018) were utilized on an earlier version of the matrix, in order to confirm that model choice was not a major driver in differences for the results (Du et al., 2023). For the final version, the ‘maximum information’ strategy was used, as model choice did not play a large influence on the topologies recovered. Bayesian analyses were run for 20 million generations using four chains, every 1000^th sample stored and 25% burn-in (Lewis, 2001; Ronquist et al., 2012). Convergence was diagnosed using Tracer (Rambaut et al., 2018).

For each coding strategy, two sets of Bayesian analyses were run. The first was unconstrained, whereas for the second all trilobites except for Olenellus getzi were constrained as a monophyletic group to recover the relationships of trilobites (Olenellus as the earliest diverging member) comparable to the comprehensive trilobite phylogenetic analysis of Paterson et al., 2019. Resulting posterior tree samples were compared by comparisons in the consensus trees, and in multidimensional treespace (using R code and method from Pates et al., 2022). The number of trees where Acanthomeridion serratum was sister to trilobites was quantified for each coding strategy and for constrained and unconstrained analyses, and the area of treespace occupied by the posterior samples – and trees where A. serratum was sister to triobites within those samples – were visualized.

Datasets for the phylogenetic analyses are uploaded to Morphobank, Project 4290.

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Author details

1. Kun-sheng Du

   1. Research Center of Paleobiology, Yuxi Normal University, Yuxi, China
   2. Key Laboratory for Palaeobiology and MEC International Joint Laboratory for Palaeoenvironment, Institute of Palaeontology, Yunnan University, Kunming, China

   Contribution

   Conceptualization, Data curation, Investigation, Visualization, Methodology, Writing – original draft, Writing – review and editing

   Contributed equally with

   Jin Guo

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-3009-8323

2. Jin Guo

   1. Key Laboratory for Palaeobiology and MEC International Joint Laboratory for Palaeoenvironment, Institute of Palaeontology, Yunnan University, Kunming, China
   2. Management Committee of the Chengjiang Fossil Site World Heritage, Chengjiang, China

   Contribution

   Resources, Data curation, Validation

   Contributed equally with

   Kun-sheng Du

   Competing interests

   No competing interests declared

3. Sarah R Losso

   Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, United States

   Contribution

   Conceptualization, Data curation, Formal analysis, Investigation, Visualization, Writing – original draft, Writing – review and editing

   For correspondence

   sarahlosso@g.harvard.edu

   Competing interests

   No competing interests declared

4. Stephen Pates

   1. Centre for Ecology and Conservation, University of Exeter, Penryn, United Kingdom
   2. Department of Zoology, University of Cambridge, Downing Street, Cambridge, United Kingdom

   Contribution

   Conceptualization, Data curation, Formal analysis, Supervision, Validation, Visualization, Methodology, Writing – original draft, Project administration, Writing – review and editing

   For correspondence

   s.pates@exeter.ac.uk

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-8063-9469

5. Ming Li

   Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China

   Contribution

   Data curation, Methodology

   Competing interests

   No competing interests declared

6. Ai-lin Chen

   1. Research Center of Paleobiology, Yuxi Normal University, Yuxi, China
   2. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Nanjing, China

   Contribution

   Resources, Supervision, Funding acquisition, Visualization, Project administration

   For correspondence

   ailinchen@yxnu.edu.cn

   Competing interests

   No competing interests declared

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