Setapedites abundantis: A stem-group Euchelicerate from the Ordovician Fezouata Shale of Morocco.

The Euchelicerates are a large group of Arthropods, comprising the living Arachnids and Xiphosurans (Horseshoe Crabs), as well as extinct groups such as the Eurypterids (Sea Scorpions), Chasmataspidids and the Synziphosurines. The closest living relatives of the Euchelicerates are the Pycnogonids (Sea Spiders), with the two groups together forming the Chelicerata. However, the closest outgroup to the Euchelicerata among the Arthropod groups of the Lower Palaeozoic is less certain, with groups such as the Megacheira, Artiopoda, Vicissicaudata, and Habeliida all suggested. Understanding the relationship between Euchelicerates and other Arthropods is dependent on understanding the order in which the group acquired its key features, i.e. the chelicerae (frontal appendages) from which the group gets its name, and the organisation of the body segments into two divisions, the prosoma, which hosts both the sensory organs and the walking limbs, and the opisthosoma, which hosts the book gills. The Synziphosurines, a (possibly paraphyletic or polyphyletic) stem group of Euchelicerates known from Silurian to Carboniferous deposits, are thought to offer a potential key to this, although very few complete specimens are known. Two noteworthy Synziphosurines (and the two earliest known species to date) are Offacolus kingi and Dibasterium durgae, both from the Silurian Herefordshire Lagerstätte of England, both of which have biramous (branching) limbs (a trait unknown in more derived Euchelicerates), and which have been consistently recovered as the two basalmost Euchelicerates in phylogenetic analyses.

In a paper published in the journal Nature Communications on 7 May 2024, Lorenzo Lustri of the Institute of Earth Sciences at the University of Lausanne, Pierre Gueriau, also of the  Institute of Earth Sciences at the University of Lausanne, and of the Université Paris-Saclay, and Allison Daley, again of the Institute of Earth Sciences at the University of Lausanne, describe a new species of Synziphosurine Euchelicerate from the Early Ordovician Fezouata Shale of Morocco, and discuss the implications of this species for the origin of the Euchelicerate clade.

The Fezouata Shale is noteworthy for the production of a large number of exceptionally preserved Arthropods, as well as Molluscs and Echinoderms dating to about 478 million years ago, during the early stages of the Great Ordovician Biodiversification Event, providing key insights into this interval in the history of life.

The new species is named Setapedites abundantis, where 'Setapedites' means 'hair-foot', in reference to the presence of a brush-like arrangement of hairs on its prosomal exopods (feet), and 'abundantis' refers to the super-abundant nature of the species, which is one of the most numerous fossils in the Fezouata Shale. The species is described from two large collections of specimens, each comprising hundreds of individuals, belonging to the Musée cantonal de géologie Lausanne, and the Yale Peabody Museum.

Dorsal anatomy of Setapedites abundantis. (A), (B) MGL.102899 and interpretative drawing, articulated specimen in dorsal view. (C(, (D) MGL.102828 and interpretative drawing, articulated specimen in dorsal view. (E), (F) MGL. 102872 and interpretative drawing, articulated specimen in dorsal view. Abbreviations: btg, bipartite tergites; mr, median ridge; pl, pleura; pr, prosomal rim; saxn, sub-axial node; sr, sunken region; t1–11, tergites 1–11; t, telson; tk, telson keel. Scale bars, (A)–(F) 1 mm. Lustri et al. (2024).

Setapedites abundantis possesses an elongate, dorsoventrally flattened body, divided into an anterior prosoma bearing a fused dorsal headshield, and an unfused opisthosoma clearly differentiated into (medially) a pre-abdomen and (posteriorly) an abdomen. Its total length varies between 4.33 and 6.5mm (excluding appendages and telson), its maximum width (prosoma) between 2.23 and 2.9mm.

Prosomal appendicular anatomy of Setapedites abundantis. (A), (B) YPMIP 517932c and interpretative drawing (counterpart), articulated specimen in ventral view. (C), (D) YPM IP 517932c and interpretative drawing, chelicerae, and labrum anatomy detail. (E), (F) Close-up of the prosoma ofMGL.102934 and interpretative drawing, in dorso-lateral view. (G), (H) Close-up of the prosoma of MGL.102634 and interpretative drawing, in ventral view. (I), (J) Close-up of the prosoma of MGL.102800a under alcohol and polarized lighting, and interpretative drawing, in ventral view. Abbreviations: 1–6, podomeres 1–6 of the exopod; ptp, pretelsonic process; bs, basipodite; bst, brush-like setae; che, chelate podomere; db, doublure; lb, labrum; ss, single setae; st, pair of setae. Chelicerae are highlighted in gray, endopods in blue, exopods in green, opisthosomal appendages in red, and the pretelsonic process in purple. Scale bars, (A), (B) 1mm; (C), (D) 100μm; (E)–(K) 500 μm. Lustri et al.  (2024).

A phylogenetic analysis including Setapedites abundantis found that it grouped with Offacolus kingi and Dibasterium durgae, together the family Offacolidae, togther forming the sister group to the Crown Euchelicerates, with the Habeliida forming the closest outgroup.

Phylogenetic position of Setapedites abundantis among Panchelicerates, showing early euchelicerate body plan evolution. Simplified extended majority rule tree of a Bayesian analysis chronogram of Euchelicerate relationships, based on amatrix of 39 taxa and 114 discrete characters, showing the position of Setapedites abundantis within Offacolidae. Lineages extending after the Silurian are indicatedwith arrowheads. Schematic models of the body organization in Habelia, Setapedites abundantis, Dibasterium, Offacolus, and Xiphosurida illustrate the origin and early evolution of Euchelicerate uniramous prosomal appendages and tagmosis. Roman numbers designate somites. Prosoma somites are highlighted in blue, pre-abdomen somites in yellow, abdomen somites in brown, and the possible anal pouch or post-ventral structure (pvs) in purple. Black dorsal lines indicate tergites and cephalotorax. Lustri et al. (2024).

The Pycnogonids (Sea Spiders) have long been seen as the sister group to the Euchelicerates, with the two groups together forming the clade Chelicerata. While they are clearly the closest living Animals to the Chelicerates genetically, the assumption that they are closer than many fossil groups has relied upon morphological similarities, notably the presence of a pair of limbs on the head called the chelifores, which have been assumed to be homologous with the chelicerae of the Euchelicerata, a head tagama made up of four segments, and uniramous limbs.  However, the presence of biramous limbs in both the Offacolidae, recovered as the basalmost stem group of the Euchelicerates and Habeliida, recovered as the closest outgroup, casts doubts upon this analysis. Modern Pycnogonids have a very specialised bodyplan, and the group has a very limited fossil record, making it hard to assess how Sea Spiders are related to Palaeozoic Arthropod groups.

Life reconstruction of Setapedites abundantis. Elissa Sorojsrisom in Lustri et al. (2024).

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