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Table 1 Main diagnostic characters for clades inclusive of or related to habeliidans, and remarks on their significance. Potentially important characters with ambiguous optimization on the tree are italicized

From: Mandibulate convergence in an armoured Cambrian stem chelicerate

Clade Character Remark
Arachnomorpha All cephalic endopods fully developed (char. 81) “Panchelicerates” and artiopodans are characterized by having well-developed endopods—based on a heptopodomeran ground pattern [27]—in their head tagmata. In mandibulates, at least one of these endopods is usually strongly modified; in leanchoiliids, the first post-frontal endopod is likely reduced [27, 67], but the condition is not well known in other megacheirans.
Third cephalic appendage gnathobasic (char. 107) We use here the term gnathobasic for a basipod with well-developed gnathal (usually dentate) edge on its proximal margin, without presence of a coxa. This is a possible ground pattern of Arachnomorpha.
Presence of gnathobase(s) (char. 177) By extension, the presence of a masticatory gnathobase on any body limb is another possible synapomorphy of arachnomorphs. This would not support the placement of Marrella [103] at the base of Artiopoda, but the proximal limb morphology in marellomorphs needs to be investigated in more detail.
Trunk endopods ending in set of three claws (“apotele”) (char. 202) Although the arrangement of the three terminal claws may vary, the tripartite apotele has already been presented as a potential synapomorphy of Arachnomorpha [104]. The claw complex seen in the thoracic endopods of habeliidans is consistent with this view.
Posteriormost trunk tergites fused into single plate (char. 212) Given our topology (Fig. 5), the thoracetron of xiphosurids and the pygidia of trilobites or other trilobitomorphs are not directly inherited from a common ancestor. The fact, however, that these structures are only found in arachnomorph arthropods suggests that the corresponding genetic pathways are shared and a possible case of parallelism.
“Panchelicerata” Ground pattern of a seven-segmented prosoma (chars. 32) We construe that in habeliidans, as in other xiphosurans [62, 66], the seventh appendage pair in the head is homologous to the chilaria. We also co-opt here the hypothesis that the “antennular” appendages of habeliidans are modified exopods of the head limbs, as previously interpreted in Sanctacaris [26, 32].
Trunk appendages with reduced or vestigial endopods (char. 183) In this study, we propose that the absence of endopods on the posterior trunk appendages of habeliidans is an ancestral condition related to the reduction of biramous trunk appendages in chelicerates. In many cases, trunk appendages are still present among euchelicerates in vestigial form, such as spinnerets, ventral sacs, gonopods or genital acertabula [105].
Labrum (char. 58) The presence and homology of a “labrum” remains ambiguous in higher nodes of euarthropods, but remains diagnostic of “panchelicerate” (as shown herein) and mandibulate taxa. We propose here that the soft dorsal structure observed in habeliidans is equivalent to the soft elements identified underneath the frontal sclerite of protocaridids [9].
Differentiation of the seventh prosomal appendage (char. 149) The value of this character depends on the semantic boundary assigned to “differentiated.” We did not consider here that the seventh pair of appendages in habeliidans or Weinbergina was already differentiated compared to other trunk limbs. Ideally, this character will be refined using a more precise statement of differentiation, for the diagnosis of either Panchelicerata or Euchelicerata.
Chelicerata Chelicerae (char. 73) The chelate condition of the reduced frontalmost endopods of habeliidans is uncertain. However, contrarily to other characters evaluated here, the presence of chelicerae is the defining condition of Chelicerata, and therefore this clade could be enlarged in the future.
Fused post-oral ganglia (char. 47) Whether this character can be coded in pycnogonids is not clear [106]. A single post-oral nerve mass has been interpreted in a leanchoiliid from China [29], but it appears to us that the central nervous system cannot be clearly isolated from other tissues in their specimen (such as cephalic shield and appendages), and thus the origin of this condition remains uncertain.
Euchelicerata Opercula on ventral surface of trunk (opisthosoma) (char. 151) The presence of ventral opisthosomal plates called opercula has been shown to be a likely apomorphy of euchelicerates [65], which is supported herein. No evidence of elements possibly homologous to opercula have been found in habeliidans, although we did not have access to a clear ventral view of the trunk.
Post-frontal appendage with chelate or subchelate termination (char. 93) Given the basal phylogenetic position of Offacolus, Dibasterium and xiphosurids, a chelate or subchelate pedipalp (or walking leg in xiphosurans) may be considered a groundplan character of Euchelicerata. However, this condition is clearly highly convergent in euchelicerates overall, and whether it represents broad parallelism bears on the resolution and morphology of synziphosurines at the base of euchelicerates.
Endosternum (char. 55) The euchelicerate endosternum is of course difficult to document in fossils, which hampers an assessment of its origin.