Lower Ordovician synziphosurine reveals early euchelicerate diversity and evolution

Domain: earth_science
Published: 2024-05-07
Editorial outcome: accept

Abstract
Euchelicerata is a clade of arthropods comprising horseshoe crabs, scorpions, spiders, mites and ticks, as well as the extinct eurypterids (sea scorpions) and chasmataspidids. The understanding of the ground plans and relationships between these crown-group euchelicerates has benefited from the discovery of numerous fossils. However, little is known regarding the origin and early evolution of the euchelicerate body plan because the relationships between their Cambrian sister taxa and synziphosurines, a group of Silurian to Carboniferous stem euchelicerates with chelicerae and an unfused opisthosoma, remain poorly understood owing to the scarce fossil record of appendages. Here we describe a synziphosurine from the Lower Ordovician (ca. 478 Ma) Fezouata Shale of Morocco. This species possesses five biramous appendages with stenopodous exopods bearing setae in the prosoma and a fully expressed first tergite in the opisthosoma illuminating the ancestral anatomy of the group. Phylogenetic analyses recover this fossil as a member of the stem euchelicerate family Offacolidae, which is characterized by biramous prosomal appendages. Moreover, it also shares anatomical features with the Cambrian euarthropod Habelia optata, filling the anatomical gap between euchelicerates and Cambrian stem taxa, while also contributing to our understanding of the evolution of euchelicerate uniramous prosomal appendages and tagmosis.

Introduction
Euchelicerata is a vast clade of mostly predatory arthropods comprising extant forms such as arachnids (the group including scorpions, spiders, mites and ticks) and their closest relatives, the xiphosurans (horseshoecrabs), as well as the extinct eurypterids (sea scorpions), chasmataspidids and the synziphosurines. While phylogenomic studies1 and fossil evidence2,3 assisted our understanding of the relationships between the crown-group euchelicerates, the origin and early evolution of euchelicerates remain poorly documented. Euchelicerates have traditionally been united by the presence of specialized frontal-most appendages, the so-called chelicerae, and by the tagmatization of the body into a prosoma, grouping the sensory organs and the walking limbs, and an opisthosoma, bearing book gill opercula4,5, but some researchers emphasize this last anatomical feature of book gill opercula may be the only diagnostic characteristic defining Euchelicerata6, a hypothesis that has been debated7–9. There is also no consensus on either the sister group to Euchelicerata or their relationships with Cambrian stem euarthropods. Several Cambrian sister taxa have been proposed for euchelicerates. Particularly: (i) Megacheira, with their subchelate “great appendages” suggested to be homologous structures from which chelicerae were derived10; (ii) Vicissicaudata, based on phylogenetic analyses11; and (iii) Habeliida12, owing to the head tagmosis and pseudotagmosis of Habelia optata12. Moreover, the anatomy of the synziphosurines13, a Silurian to Carboniferous paraphyletic14 or polyphyletic4 group of euchelicerates, is often only incompletely known, particularly with regard to their ventral anatomy, strongly limiting comparisons with the proposed possible sister taxa of stem chelicerates. Two notable exceptions are the basal-most taxa Offacolus kingi15,16 and Dibasterium durgae17 from the Silurian of Herefordshire. These two species possess elongate chelicerae and a peculiar limb arrangement in the prosoma with biramous appendages comprising stenopodous exopods. Since their description, they have been consistently retrieved as basal euchelicerates in phylogenetic analyses3,4,6,12,18,19.

Here we describe Setapedites abundantis gen. et sp. nov., a minute synziphosurine from the Lower Ordovician (late Tremadocian, ~478 million years ago) Fezouata Shale of Morocco20,21. The Fezouata Shale is a major marine fossil site with exceptional preservation of labile parts that is transitional between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE) and has provided critical insights into our understanding of the early evolution of metazoans and the structure of the Early Paleozoic marine biosphere21–25. Setapedites abundantis is one of the most abundant components of this unique assemblage, with hundreds of specimens housed in two main institutions: the Musée cantonal de géologie Lausanne, Switzerland, (MGL) and the Yale Peabody Museum, New Haven, CT, USA (YPM). Phylogenetic analyses recover Setapedites abundantis in a clade of stem euchelicerates together with Offacolus kingi and Dibasterium durgae. It shares several homologies with the Cambrian euarthropod Habelia optata, including bipartite opisthosomal tergites, proximal anatomy of the opisthosomal appendages, and possibly an anal pouch. Setapedites abundantis documents how euchelicerate uniramous prosomal appendages were derived from the appendages of a habeliid ancestor12 and illuminates the evolution of early tagmosis in euchelicerates.

Results
Systematic Paleontology
Euarthropoda Lankester, 190426
Arachnomorpha Størmer, 194427
Euchelicerata Weygoldt & Paulus, 19795
Offacolidae Sutton, Briggs, Siveter, Siveter & Orr, 200216

Emended diagnosis. Euchelicerates with elongate chelicerae, at least second to fifth prosomal appendage pairs being biramous and with reduced gnathobasipods and composed of stenopodous exopods of six podomeres bearing a brush-like group of long and radially arranged setae on the distal most podomere. Seventh pair of appendages (sixth post-cheliceral appendage pair) uniramous and lobate paddle-like, fringed by lateral spines. Tip of the telson bifurcate. Diagnosis modified from Sutton, et al16.

Included taxa. Dibasterium durgae17, Offacolus kingi15, Setapedites abundantis

Setapedites abundantis gen. et sp. nov. (Figs. 1–5)

Etymology. The genus name derives from the Latin “seta” and “pedes” referring to its brush-like group of setae on the distal most podomere of the prosomal stenopodous exopods. The species name derives from the Latin word for abundant “abundans” referring to the abundancy of the species in the Fezouata Shale.

Diagnosis. Offacolid with prosomal appendages 2 to 6 homonomous. Fully expressed first opisthosomal segment, and a developed axis from the second tergite to the sixth, bearing sub-axial nodes. Axis of the pre-abdominal tergites transversely bipartite in a less sclerotized anterior part and a more sclerotized posterior part. 11th segment bearing an ovoid ventral process, and ending posteriorly into two pairs of spines flanking a long needle-like telson with a bifurcate tip.

Material. Holotype MGL.107741 (Fig. 3A–D, see Supplementary Fig. 2A for normal light photo); paratypes MGL.102899 (part; Figs. 1A and 1B, see Supplementary Fig. 4A for counterpart and Supplementary Fig. 4C and 4D for anatomical details), MGL.102828 (part; Figs. 1C and 1D), MGL.102872 (part; Figs. 1E and 1F), YPM IP 517932c (counterpart; Fig. 2A–D, see Supplementary Fig. 3E and F for anatomical details) YPM IP 517932 (part; Supplementary Fig. 2B and 2C), MGL.102934 (part; Figs. 2E and 2F, see Supplementary Fig. 2G for full specimen and Supplementary Fig. 2I for detail of biramous appendages), MGL.102634 (counterpart; Figs. 2G and 2H, see Supplementary Fig. 2E for full counterpart specimen and 2F for full part specimen, see Supplementary Fig. 3G and 3H for anatomical details), MGL.102800a (part; Figs. 2I and 2J, see Supplementary Fig. 2H for full specimen and Supplementary Fig. 3A and 3B for anatomical details), MGL.102637 (part; Fig. 4A–E), MGL.102952 (part; Supplementary Fig. 1A and 1B), MGL.102800b (part; Supplementary Fig. 1C and 1D), MGL.102902 (part; Supplementary Fig. 2E and 2F), MGL.102469 (part; Supplementary Fig. 2G and 2H), MGL.102247a (part; Supplementary Fig. 2D, Supplementary Fig. 3C and 3D), MGL.102690 (part; Supplementary Fig. 4B and 4E), MGL.102841 (part; Fig. 4F–K).

Locality and age. Fezouata Shale, Zagora province, Morocco (detailed locality information is curated with the specimens). Early Lower Ordovician, late Tremadocian, Araneograptus murrayi bio-zonation.

Remarks. Setapedites abundantis gen. et sp. nov. (Figs. 1, 2, 3, 4 and 5) is superficially similar to Offacolus kingi Orr et al.15 and Dibasterium durgae Briggs et al.17 from the Silurian Herefordshire Konservat-Lagerstätte16,17, yet it differs from both by its fully expressed first opisthosomal tergite and its bipartite tergal axis. Furthermore, the prosomal shield anatomy of Setapedites abundantis strictly resembles that of Offacolus kingi (with the presence of a sunken region and a median ridge), while Dibasterium durgae is devoid of any dorsally expressed characters on the prosoma. The opisthosoma of Setapedites abundantis, on the other hand, more closely resembles that of Dibasterium durgae, with the same number of tergites and the same overall dorsal anatomy (see Supplementary information for discussion about anatomical comparison with synziphosurines other than offacolids). Among Cambrian arthropods, the dorsal morphology of Jianshania furcatus Luo et al28. from the Chengjiang biota appears most similar to that of Setapedites abundantis28,29 (see Supplementary information for additional discussion about a possible relationship linking Jianshania furcatus with Setapedites abundantis and its implications).

Description of the Fezouata Shale synziphosurine
Setapedites abundantis possesses an elongate, dorsoventrally flattened body, divided into an anterior prosoma bearing a fused dorsal head-shield, and an unfused opisthosoma clearly differentiated into (medially) a pre-abdomen and (posteriorly) an abdomen (Figs. 1A and 1B). Its total length varies between 4.33 and 6.5 mm (excluding appendages and telson), its maximum width (prosoma) between 2.23 and 2.9 mm (see Supplementary Table 1 for list of measurements).

The head shield is semi-circular in outline and wider than long (Figs. 1C and 1D). It projects anteromedially into a minute prosomal spine (ps in Supplementary Fig. 1C and 1D) and latero-posteriorly into a pair of small, ventrally directed genal spines (gs in Supplementary Fig. 1C and 1D) often partially overlapping the tergopleura of the first opisthosomal segment in dorsal view. Specimens preserved in lateral view show that the head shield is not flat, but domed (Figs. 1E and 1F, Supplementary Fig. 1A and 1B). The prosoma bears a narrow semi-circular rim along the anterior margin preserved as a thickened cuticular structure (pr in Fig. 1E and 1F) and a median ridge extending from the posterior to the anterior prosomal margins (mr in Figs. 1E and 1F). The median ridge is flanked on each side by slightly concave regions (sunken region) (sr in Fig. 1E and 1F, Supplementary Fig. 1A and 1B). The head shield ventrally folds into a doublure that widens antero-medially (db in Fig. 2B, Supplementary Fig. 1E and 1F). On the ventral side, a centrally-positioned structure composed of a small, rounded plate aligned antero-posteriorly can be identified as a labrum (lb in Figs. 2A–D, 2I and 2J, Supplementary Fig. 2B and 2C, see Supplementary information for additional discussion about the labrum in Setapedites abundantis), presumably covering the mouth. No eyes or ophthalmic region are visible.

The prosoma bears six pairs of appendages, which insert around the labrum (Figs. 2A and 2B, 2E–2J, Figs. 3A and 3B, Fig. 4). The first pair of appendages are uniramous, elongate chelicerae, anteroventrally oriented (gray highlighted appendages in interpretative drawings; Fig. 2A–D, Fig. 2I–2J, Figs. 3A and 3B, Fig. 4, Supplementary Fig. 1E–1H, Supplementary Fig. 2B and 2C). Evidence for a chelate last podomere in the chelicerae is provided by a single specimen (gray highlighted appendages in interpretative drawings; Fig. 2A–D, Supplementary Fig. 2B and 2C), otherwise they are preserved retracted and bent under themselves (Figs. 2I and 2J, Figs. 3A and 3B). Appendages 2 to 6 are biramous and lateroventrally oriented (Fig. 2E–K, Figures 3A and 3B, Fig. 4). The endopods are often preserved retracted and bent back onto themselves (blue appendages in Figs. 2I and 2J, Figs. 3A and 3B and Fig. 4); nevertheless, their detailed anatomy is preserved. At least one specimen shows evidence for a chelate last podomere in the post-cheliceral endopods (che in Figs. 2I and 2J, Supplementary Fig. 3A and 3B). No count of the podomeres is possible for the endopods. The exopods are divided into six podomeres (1–6 in Figs. 2G and 2H, Supplementary Fig. 3G and 3H). The sixth podomere bears brush-like setae (bst in Figs. 2G and 2H, Supplementary Fig. 2D–F, Supplementary Fig. 3C and 3D and Supplementary Fig. 3G and 3H). The fifth podomere bears a couple of setae ventrally (st in Figs. 2G and 2H, Supplementary Fig. 3G and 3H) and the fourth one bears one single seta on the opposite side (ss in Figs. 2G and 2H, Supplementary Fig. 3G and 3H). The rami of the biramous appendage attach to a protopodite (basipodite) of rectangular shape (bs in Figs. 2E and 2F, Supplementary Fig. 2G and 2I).

The opisthosoma, 1.5 times longer than the prosoma, tapers posteriorly and consists of eleven somites (each opisthosomal somite bearing a tergite; t1–t11 in Figs. 1B, 1D and 1F and Supplementary Fig. 1G and 1H dorsally), is divided into a pre-abdomen comprised of the first eight somites and bearing pleural spines (pl in Fig. 1A–D), and an abdomen made up of the last three somites plus a needle-like telson (of which only the last somite preserves modified pleural spines) (Figs. 1A and 1B, Supplementary Fig. 1A–D and Supplementary Fig. 4). The first tergite (tergite 1) is fully developed but partially overlapped by the prosomal shield and does not present a clear distinction between tergopleura and axis (Fig. 1A–F). In some specimens tergite 2 is slightly hypertrophic (Figs. 1A and 1B). Tergites 2–6 have an inflated axis, which constitutes the most prominent part of the body (Fig. 1A–F). The axis has squared lateral margins and antero-posterior bipartite sclerotization, the anterior part being less sclerotized (btg in Fig. 1C–F). A transverse margin divides the anterior and posterior portions of this structure. However, the anterior portion is often less well-preserved than the posterior portion due to its lower degree of sclerotization. Tergites 2–8 bear a couple of sub-axial nodes on the lateral edges of the axis, which connect the axis with the tergopleura (saxn in Figs. 1E and 1F). The width of the tergites decreases slightly from tergites 3 to 8. Tergites 1 to 8 bear leaf-shaped tergopleura, starting with a wide attachment to the axis and ending in a pointed spine directed ventro-proximally (pl in Fig. 1A–D, Supplementary Fig. 1C and 1D).

In the pre-abdomen, somites VII to XIII bear a pair of appendages, while somite XIV is devoid of appendages (oa1-7 in Fig. 3 and Supplementary Fig. 1G and 1H). Opisthosomal appendages possess a distinct proximal part (px in Figs. 3A and 3B). Appendages of pre-abdominal somite VII are uniramous and paddle-like (Fig. 3 and Supplementary Fig. 1G and 1H) and insert medially (api in Figs. 3A and 3B). Appendages 2–6 are rarely preserved but are uniramous and have similar insertions as appendage 1 (Fig. 3 and Supplementary Fig. 1G and 1H). The abdomen, which is half the length of the pre-abdomen, is composed of three somites (t9–11 in Figs. 1A and 1B) without appendages. The first two abdominal somites lack pleural spines. The pretelson is twice as long as the other opisthosomal somites and carries posteriorly-directed tergopleura modified into two pairs of spines flanking the telson (ls in Supplementary Fig. 1C and 1D). Each abdominal somite is encased by a fused tergite and a sternite forming a ring-shaped structure (Fig. 1A and 1B, Supplementary Fig. 1). A small symmetrical rounded process (pretelsonic process) is often preserved under tergite 11, in some cases appearing longitudinally bisected and ovoid in shape (ptp in Figs. 2A and 2B, Figs. 3A and 3B, Supplementary Fig. 1A–F, Supplementary Fig. 3E and 3F) which could represent an anal pouch.

The terminal telson, as long as the pre-abdomen, is narrow and straight, and ends in two spines directed posteriorly (bt in Supplementary Fig. 4). It is slightly triangular in section, and some specimens show evidence for the presence of a median keel (tk in Figs. 1C and 1D). It is internally articulated with the last abdomen segment by an enlarged head (Fig. 1A and 1B).

The Fezouata Shale synziphsurine is a stem euchelicerate
Four different phylogenetic analyses were conducted to determine the affinity of Setapedites abundantis. We first coded it into the matrix from Aria & Caron6, to assess its broad position among euarthropods. Bayesian phylogenetic analyses were performed and Setapedites abundantis is found alongside Offacolus kingi and Dibasterium durgae, as part of the family Offacolidae, in a sister group position relative to all other euchelicerates (Supplementary Fig. 5, 9 and 10: see “Methods” and Supplementary Information for full phylogenetic methods). We then performed a second analysis focusing on Panchelicerata (= Chelicerata + Habeliida), using the matrix from Lamsdell4 that includes numerous synziphosurines and euchelicerates, and to which we added Habelia and Mollisonia plenovenatrix. Parsimony (implied and equal weighting) and Bayesian phylogenetic analyses were both performed, and Setapedites abundantis is found again in a clade with Offacolus kingi and Dibasterium durgae, inside Euchelicerata and as a sister group to Prosomapoda (sensu Lamsdell4) (Fig. 6 and Supplementary Fig. 6–8, 11–15: see “Methods” and Supplementary Information for full phylogenetic methods). In the first analysis, Offacolidae is united in possessing the following synapomorphies: short and stout stenopodous prosomal exopods ending in a setal brush, and post-antennular appendages chelate or sub-chelate. The synapomorphies supporting Offacolidae in the second set of analyses are: sixth post-antennular appendage flap-like, seventh to thirteenth post-antennular appendages lacking the endopods, and bifurcated telson tip. Diagnosis modified from Sutton, et al16.

Methods
Experimental model and subject details
All material of Setapedites abundantis used in this study comes from the Fezouata Shale (Early Lower Ordovician, late Tremadocian) in the Zagora province, (Morocco) and was collected by authorized and academically recognized avocational Moroccan collector Mohamed Ben Moula and his family over the period of 2015 to 2016 (MGL collection), and between 2009 to 2014 (YPM collection); ACD together with paleontologist Peter Van Roy worked in collaboration with them to collect the metadata associated with the collected fossils. Mohamed Ben Moula has a long-standing working relationship with several academics, has received the Mary Anning Award from the Paleontological Association, and has a radiodont fossil named after him by the same two authors (Van Roy et al. 2015b) in honor of his great contribution to the field of paleontology. The MGL fossil collection was purchased with funds from the University of Lausanne and the Swiss National Science Foundation, following all regulations for purchases. The fossil collection was transported to Casablanca and subjected to export approval by the Ministry of Energy, Mines and the Environment of the Kingdom of Morocco and approved for shipment to Switzerland on 11.05.2017 (export permits curated with the collection). Fossils were shipped by sea and land to the University of Lausanne, where they are curated as part of the collection of the Musée Cantonal de Géologie (MGL). The collections of the Yale Peabody Museum of Natural History were obtained both through a collection of specimens by Peter Van Roy during fieldwork and through purchase using dedicated museum funds for the acquisition of scientific collections. Export permits were obtained through the Moroccan Ministry of Energy, Mines, and the Environment, with specimens being transported from Casablanca by sea.

This published work and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the International Code of Zoological Nomenclature (ICZN). The ZooBank LSIDs (Life Science Identifiers) can be resolved, and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSIDs for this publication are: Setapedites abundantis Lustri, Gueriau & Daley, In Press, LSID urn:lsid:zoobank.org:act:A122B462-E673-4F8C-8F03-74C8905F7D63.

Fossil photography
Specimens of Setapedites abundantis were initially examined with a light microscope (Wild Heerbrugg M8) under full-spectrum light. Subsequently, selected specimens were photographed with a digital SLR camera (Canon EOS 800D equipped with CANON MACRO LENS MP-E 65mm 1:2.8 1-5X) mounted on a stand and connected to a z-stacking system (STACKSHOT 3X), using different lighting conditions: normal or polarized light, dry or covered in alcohol. Z-stacks were rendered using Helicon Focus software and the depth map function. All specimens were analysed in the Optical laboratory at Lausanne University (GEO-3439).

Interpretative drawings
Interpretative drawings were made in Adobe Illustrator, using a Wacom ver. 6.3.38-3 graphic table and a pen tool on the photos.

Synchrotron tomography and 3D rendering
Two specimens (MGL.102637a and MGL.102841) were imaged using synchrotron X-ray microtomography at the X02DA TOMCAT beamline of the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. Measurements were performed using a monochromatic beam of 35 and 18 keV respectively, a single propagation distance of 250 mm, a 100 µm LuAg:Ce scintillator, and a 4× objective, yielding reconstructed tomographic data with a voxel size of 1.75 µm. 1501 projections were recorded over 180o with exposure of 400 and 1000 ms, respectively. Reconstruction was performed on a 60-core Linux PC farm using a Fourier transform routine and a regridding procedure48. Virtual sections presented in Fig. 4 were processed using ImageJ, and segmentation and three-dimensional rendering was performed using the software MIMICS Innovation Suite 19.0 (Materialize) at the IPANEMA laboratory (Gif-sur-Yvette, France).

Chronogram
Chronological data for the 39 taxa included in the parsimony analyses were collected from PBDB (paleobiodb.org)49 and from data available in4, and are available in Supplementary Table 3. The tree resulting from Bayesian phylogenetic analyses was plotted against the geological timescale using the strap R package50 in RStudio51 in order to obtain the chronograms presented in Fig. 6 and Supplementary Fig. 6. Tree was rescaled with command “mbl”.

Phylogenetic analyses
The first Bayesian phylogenetic analyses (Supplementary Fig. 5, 9 and 10) were performed following methods in Aria & Caron6, using mrBayes ver. 3.2.7a52,53, through the Cipres science gateway web site (www.phylo.org)54 (modifications and character coding for S. abundantis are available in Supplementary Information). Tree searches followed an Mkv+Γ model (Lewis 2001) with four chains sampling during four runs for 10,000,000 Markov chain Monte Carlo generations, a tree sampled every 1000 generations and burn-in of 20%. Analyses were constrained with a partial backbone. The character matrix was based on that found in Aria & Caron6, with our final data matrix including 102 taxa and 267 discrete characters. Two versions of this matrix have been analysed to test two different character codings for Mollisonia plenovenatrix (see Supplementary Notes for a list of modified codings and Supplementary Data 1 and 2 for phylogenetic characters and matrix). A constraint for the Arachnopulmonata has also been used for the analysis resulting in Supplementary Fig. 9 (data available in Supplementary Data 3 for phylogenetic characters and matrix).

The second set of Bayesian phylogenetic analyses (Fig. 6 and Supplementary Fig. 6 and 11) were performed on the data matrix from Lamsdell4, modified by removing Fuxianhuia protensa and Willwerathia laticeps from the data matrix (given its uncertain status as basal euarthropod in the first case55 and as euchelicerates in the second56,57), using Yohoia tenuis as outgroup, and adding Setapedites abundantis, Dibasterium durgae (based on 3,17), Habelia optata (based on 12), and Mollisonia plenovenatrix (based on 6). Two versions of this matrix have been analysed to test different characters codings for Mollisonia plenovenatrix (see Supplementary Notes for list of modified codings and Supplementary Data 4 and 5 for phylogenetic characters and matrix). Modifications and character coding for Setapedites abundantis, Dibasterium durgae, Habelia optata and Mollisonia plenovenatrix are available in Supplementary Information and Data. The final data matrix includes 40 taxa and 114 discrete characters. We followed the same methods of the previous Bayesian analyses but without partial backbone constraint.

The first parsimony analyses (Supplementary Fig. 7, Supplementary Data 6) were performed on the same data matrix modified from Lamsdell4 and following its methods using TNT ver. 1.558 (random addition sequences followed by branch swapping with 100,000 repetitions, all characters unordered and of equal weight, followed by jacknife (33% deletion, 1000 repetitions) and bootstrap (50% deletion, 1000 repetitions)).

The second set of parsimony analyses (Supplementary Fig. 8, Supplementary Data 6) was performed on the same data matrix modified from Lamsdell4 and following its methods using TNT ver. 1.5 except an implied weight of 12K (data available in Supplementary Data 2 for phylogenetic characters and matrix). The exclusion of Fuxianhuia protensa from these analyses and the usage of Yohoia tenuis instead, lead to a paraphyly of the megacheiran. To test if this negatively affected our results in the other reiteration, we ran another set of phylogenetic analyses excluding the artipodans (Olenoides serratus, Emeraldella brocki, and Sidneyia inexpectans; Supplementary Fig. 12–15, Supplementary Data 7–9, see also Supplementary Table 2 and supplementary discussions regarding the different phylogenetic approach).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is a welcome contribution featuring new fossil material from the Fezouata Lagerstätte which\nindeed builds onto our knowledge of early euchelicerate evolution. For those who are a bit familiar\nwith its history, the collection of Fezouata material and its study is a major undertaking, faced with\nvarious challenges, and such a publication is much, much more than the sum of its words and figures.\nThe piece is well presented, organized and written, the figures are overall of good quality (but please\ncheck your panel labelling before submission!) and the phylogenetic analyses were executed\nproperly—although there are some small issues. However, I also see a number of shortcomings and\nomissions, for instance with the systematics and the presentation of the evidence.\nThe title is a bit over-reaching. As I develop below, the fossil morphotype arguably cements a taxon\nincluding a couple of horseshoe-crab-like genera from the Herefordshire biota, adding to morpho-\nanatomical diversity in this group, but does not exactly “reveal” the origin of euchelicerate\nappendages—also because there are Cambrian fossils that also contributed to build the picture of\neuchelicerate origins. A much more accurate (and no less impactful) title could be “An Ordovician fossil\ndocuments early euchelicerate diversity”, or something of the sort. In any case, quotation marks are\nunnecessary for synziphosurines because it’s a vernacular name (“Synziphosura” would require them).\nIn the Arachnopulmonata context, some colleagues question whether the choice of topological\nconstraints in my matrix influence the placement of fossils in a certain way. I obviously support the\nbasal placement of synziphosurines, and incidentally that of scorpions, but it would be good if you ran\nthe analysis with an Arachnopulmonata constraint, just to see what you obtain—and whatever the\nresult, this would warrant but a couple lines of discussion.\nI am quite puzzled as to why you are not erecting a new family. Taxonomy requires it at this point. You\nwould propose a new clade with no Linnean rank if you were describing a putative clade at the supra-\nfamilial or ordinal level maybe, but here, even if your clade is not very well supported, the grouping is\njustified. Accordingly, the family name should be Offacoliidae.\nThere are some issues with the phylogenetic treatment:\n- Knowing that Bayesian analyses generally outperform parsimony, why use a Bayesian approach for\none matrix but parsimony for the other? If you defend the value of parsimony, you should then run\nparsimony in both cases, although it might be tedious to implement the backbones. Run at least your\nsecond analysis with MrBayes.\n- What is your outgroup in your analysis of the Lamsdell matrix? You did well to include Habelia and\nDibasterium, but Mollisonia is crucially missing, and rooting on Fuxianhuia is ill-advised, given that its\nstatus as basal euarthropod is contested from several fronts. I tested a parsimony run with Yohoia as\noutgroup (your best candidate here) and Fuxianhuia resolves as sister-group to Panchelicerata.\nSomething’s off. You might as well remove it entirely.\n- For the sake of reproducibility, please indicate whether you used sectorial search, ratchet, drift and\ntree fusing, and with what settings.\nThe absence of Mollisonia throughout is also something I find very strange, especially since you say\nyou reused the data from that paper. If you disagree with the interpretations, state it somewhere, but\nI don’t see why, because it wholly integrates the evolutionary picture you’re describing.\nTaxonomy: Besides the family issue, I find the name wanting. Any stem taxon to an extent could be a\nchimaera of what it stands between, so the genus name doesn’t say much. My recommendation would\nbe to take the opportunity to put forward the Fezouata locality and its people and culture. As for the\nspecies name, “robustus, a, um” and “axis, is” are indeed Latin words (and should be referred to), so\nit works technically, but “axis, is” being a masculine substantive (3rd declension), you are in fact\nputting the species name as a masculine apposition of a feminine... It’s odd. “robusta” could be a\nspecies name while transferring the “axis” idea to the genus. In any case, there’s a lot of room to be\nmore creative here!\nThe diagnosis of “Setopoda”.... Biramous prosomal appendages with “detached” stenopodous exopods\nare plesiomorphic since they are present in habeliidans already, so this shouldn’t be part of your\ndiagnosis. A “brush” tip is also present in Mollisonia, and I draw your attention to the fact that there is\na confounding ramus with the same termination in Sanctacaris (Aria and Caron 2017 Add. File 8)—\nwhich makes “Setopoda” not very fitting, but see my comment about erecting a family instead\nanyway. As unambiguous diagnostic traits, when we consider Offacolus and Dibasterium for now, this\nleaves the bifurcating telson tip, the lobate and setose seventh prosomal appendage, but also the\nelongate chelicerae and, importantly, the extensive reduction of gnathobasipods.\nNow, these are nice specimens, but they are not very detailed either, and the preservation is\nchallenging:\n- Given the arrangement of the matrix, I am skeptical about a clear bifurcation of the telson tip in\nMGL.102690. I expect it’s very likely to be the case, but we have to be honest with the evidence at\nhand. Try to make a better case for it by adding a close-up in Supplementary.\n- Setation is not preserved, and seeing the identity of the seventh prosomal appendage compared to\nopisthosomal ones is frankly a leap of faith. Provide close-ups of these relevant specimens in Figure 2\nfor the proso-opisthosoma transition.\n- Yes, the axial area is well defined, but is it so different from what is known in Offacolus and\nDibasterium? Remember that we are comparing compression fossils with fully 3D nodular ones... The\ntergite-pleurae demarcation is lesser in dorso-oblique preserved specimens.\n- You interpret your opisthosomal appendages as vaguely “paddle-like”, implying they are not\noperculate book-gills, and therefore that your taxon is not a euchelicerate (contradicting your sys pal),\nand also that this diagnostic feature might have originated independently in Offacolus + Dibasterium\nand other euchelicerates... This begs caution (see also my comment below). The fact is that indeed\nyour opisthosomal appendages are not well preserved, and it is entirely possible that you have book\ngills like in the sister taxa. If, after coding these states as “?” in your matrix, you still retrieve your\n“Setopoda” clade, then you can invoke secondary homology and justify assuming operculate book\ngills. Otherwise, your taxa should more likely be paraphyletic without this feature.\n- I do not entirely agree with your assessment of the first opisthosomal segment. It appears to me\ndifferentiated from posterior segments, larger notably, and overlap by the cephalon can be extensive.\nIn that sense, this segment is not so different from what is observed in Dibasterium.\nThis really leaves for the diagnosis of Offacoliidae;\n- The elongate chelicerae: In your morphotype they are shorter than in Offacolus or Dibasterium\n(which might support their grouping), but elongate also compared to stem euchelicerates or synziph.\nThere’s of course variability in eurypterids and arachnids but local apomorphies are just a Cambrian\nreality (and why Bayesian phylogenetic treatments are better);\n- The small size of the gnathobasipods;\n- The morphology of the seventh prosomal appendage;\n- Maybe the telson tip.\nIt is therefore essential to me that you be more circumspect with this fossil evidence and that you\ntighten up the taxonomic treatment, as well as a number of other aspects, as detailed below.\nSee also below about making your main figures more useful.\nOther comments:\n41 – The text here and there could do with more punctuation. “and their closest relatives, the\nxiphosurans...”\n43 – This sentence is not accurate. Euchelicerata is diagnosed by the one synapomorphy that is the\npresence of book gill opercula. Both chelicerae and tagmatization are present in the stem in the strict\nsense (see Aria and Caron 2019 Nature).\n48 – This presentation is somewhat outdated. Since the publication pertaining to habeliidans and\nMollisonia, there haven’t been counter-arguments as to how megacheirans or some artiopodans could\nbe closer to Euchelicerata. The rooting of Euchelicerata by Mollisonia and habeliidans was retrieved\nindependently by Zeng et al. 2020 (Nature). The issue I think is more about how megacheirans or\ncertain artiopodans articulate, if at all, the Panchelicerata clade that would start with habeliidans.\n53 – It is “habeliidans” and not “habeliids” – “habeliids” would refer to the family Habeliidae, but here,\nit is the order Habeliida. This was coined by Aria and Caron 2017.\n54 – I think you mean the exopods of xenopodans (Emeraldella, Sidneyia). The trunk appendages of\nSanctacaris like that of Habelia are broad and lobate and their proximalmost portion is not well known.\nThe argument of Legg was more a comparison between the head of Sanctacaris and that of\nxenopodans and then with the exopods of xenopodans and those of xiphosurans. However, the\ninterpretation of the head was, like the original one, not entirely correct.\nAlso, please choose to use exite or exopod throughout your work, for consistency, and make sure to\nexplain your choice if you choose exite.\n58 – Given how Offacolus and Dibasterium are the central, most relevant taxa to your study, you\nshould introduce them a bit more, and especially say how they have consensually been retrieved as\nbasalmost euchelicerates across studies. Say more about their main morpho-anatomy in introduction.\n69 – “This species displays...” This last part repeats the abstract and is not necessary.\n135 – That’s indeed intriguing.\n140 – “Description” should be continuous with the systematic palaeontology (italicized).\n243 – I don’t think “doubts” can be “enhanced”...\n245 – Again, there wasn’t exactly a debate... The matrix by Legg hasn’t been published with updated\ninformation about habeliidans and Mollisonia recently, as far as I’m aware, so this is not a matter of\nwhether, in this specific case, there were two sets of data with opposite signals. Like I wrote before, it\nis more about 1) a historical reference to the open question whether megacheirans, “vicissicaudates”\nor another group represented the closest sister group to Chelicerata, which translated into 2) how\nMegacheira and Artiopoda articulate with Panchelicerata. In that sense, the significance of your taxon,\nin my view, is more that it *finally bridges the gap with these Silurian horseshoe-crab-like forms and\nstem euchelicerates from the Cambrian explosion*.\n248 – I don’t understand what you mean here. And yes, be careful that the phylogenetic resolution of\nsea spiders is a long-standing problem, and unless you’re extremely confident about your branch\nsupport, I’d refrain from making such a statement.\n252 – No. The gill operculum is the unique synapomorphy of Euchelicerata (as per Weygolt and\nPaulus; see also e.g. Dunlop 1997, Dunlop and Lamsdell 2017, Aria and Caron 2019), and unless\nyou’re willing to amend the taxonomy (which I would *not* recommend doing because there is no\nneed to), you have to remove this statement.\n253 – 254 – Dibasterium and Offacolus were considered euchelicerates before, and were already\nforming a clade basal to “Prosomapoda.” And obviously Euchelicerata was already monophyletic. You\nshould focus on the novelties.\n255 – As explained above, you cannot be certain.\n257 – Again, refer to the actual diagnosis of Euchelicerata.\n260 – A number of these characters are actually shared with Sanctacaris. Why is it absent?\n269 – To be clear, you construe that the megacheiran-like exopods of the seventh pair of cephalic\nappendages in Habelia are homologous to the seventh pair of appendages in “setopods”, implying the\nreduction of the endopod.\n275 – I think it’s fair to cite Aria and Caron 2019 Nature here.\n285 – Again, see Mollisonia, specifically ROMIP 65262. The taxon has the same “brush” terminations\nin the prosoma. The absence of the feature in habeliids just has to do with the fact that the exopods\nare very slender (like exopod #6 in Offacolus), with small tips, but a bunch of setae is present too.\n290 – You identify what possibly is the basipod of an appendage (as was done with Offacolus and\nDibasterium already), but nowhere I see that you can demonstrate where exopods attach. Given the\nresolution of your material compared to the Burgess Shale or the Herefordshire, I think this will be\ndifficult.\n291 – Proximalmost appendage morphology in Habeliidans as well as Mollisonia is no less clear than in\nOffacolus and Dibasterium: in all cases, the semi-independence of the exopod is based on the absence\nof evidence for the point of attachment and the observation that they do not seem constrained by the\nmovement of the endopods. Which means that this condition started off with Panchelicerata. Thus the\npolarization is quite clear. Did this lead to the loss of prosomal exopods? Yes, most likely – this was\ndiscussed before (Aria and Caron 2017, 2019, Aria 2021 PaleorXiv).\n294 – What about exite instead of exopod?\n306 – Mmmm... but 1) nothing here is new, because this scenario was discussed in the context of\nMollisonia, and 2) the morphology of opisthosomal limbs in your fossil is unclear beyond a lobate\naspect (that could very well be opercula!).\n307 – Whether the name should define the group of taxa or the reverse is always a tautological\nargument. If you say “Should Euchelicerata represent the least inclusive clade containing XXX”, then I\nunderstand, but this is explicitly at odds with the current formal diagnosis. I do see the issue that the\nabsence of opercula in your taxon could render Euchelicerata polyphyletic, but 1) like I wrote above,\nthe presence of opercula remains entirely possible, and 2) this is dependent on the topology and the\nrobustness of the results. As a matter of fact, if you support the monophyly of “setopods” and its\npolytomy, the presumed absence of opercula in your morphotype would parsimoniously optimize as a\nreversal (ACCTRAN or DELTRAN), contradicting your statement. Why don’t you optimize important\ncharacter state changes on your topology? This would make it very clear.\nFigures: Some of your Supp. figure panels are much more interesting to put as main figure panels,\ne.g. 2C or E (but not 2F) (that synchrotron effort should have more visibility!), 3A-B (for the\nchelicerae and putative bipartite epistome-labrum), a larger version of 3J. In your main figures 1 and\n2, many panels are redundant or don’t show much, so they can easily be replaced.\nFigure 1. L panel should be K.\nFigure 2. In E, please correct tone – this is much too yellow and hampers visibility of features.\nFigure 2C. If the labrum is the posterior lobe of the structure you divide in 2, what’s the anterior\nportion? The epistome? This is a bit of a rhetoric question, as you’ve probably reached the limit of\ninformative resolution here. I think you’re better off calling the entire area putative epistome-labrum\n(if you want to keep with chelicerate terminology, or any other term as long as it’s justified).\nSupplementary Figure 3. Drawings G and O are overinterpretations. You draw all boundaries as if\nthose were genuine, but it is evident that the structures are severely altered and that you cannot\nmake the case for these extra long spines, etc.\nSupplementary Figure 4. Put the PPs everywhere, not just for the Chelicerata clade.\nSupp. discussion – Prosoma ground pattern: Habeliidans and mollisoniids, as well as some other\nunpublished Burgess Shale taxa, have seven pairs of appendages under the cephalic shield. Should\nthese taxa form a clade, this could be indeed autapomorphic. But it is also not unfathomable to think\nthat this tergite was secondarily released—this is commonplace among crustacean cephalothoraxes!\nI don’t think there is a need for you to oppose the evidence so strongly, especially when it’s otherwise\nvery clear in many specimens and several taxa.\nI hope these comments were fair and constructive. I will be glad to see a revised version of this work\nand, if satisfying, recommend it for publication.\nCédric Aria", "reviewer_2": "This is a nice manuscript describing a very common synziphosurine from the Fezouata biota. The\nrecord is important for a number of reasons in terms of both it's age and the quality of preservation.\nThe manuscript is well written and well presented. I am not a specialist on chelicerates so I decided to\nsimply read the manuscript and see if the presentation was sound and whether I could identify the\ncharacters in question.\nThe specimens are really small. Hence the appendages are faint infill of iron oxides and clays after\npyrite. I bet some referees may say they can't see the podomere boundaries and while the images are\nnot always the best ones I can see them and believe the authors' interpretation. So, in case you get\nsome obstinate referees saying that they can't see a thing, tell them that its because they are blind or\nat least not in the possession of trained eyes to look at exceptional fossils and hence should abstain\nfrom commenting on such issues again.\nI can see the biramous prosomal appendages and the chelicerae. I am struggling a bit more with the\nopisthosomal appendages and their morphology, but agree that there is some structures that clearly\nderives from exposure of structures below the tergal skeleton and which is appendicular in nature. The\nsupplemental figures are more helpful.\nToo bad these couldn't have been revealed with the synchrotron. I surely would have taken 20-40\nindividuals to the machine after having cut them down to size and scanned the hell out of them. Then\nI would have manually segmented them out with a nice bottle of Swizz white wine by my side rather\nthan thresholding them which often miss more faintly preserved features. Surely a lot of cool stuff\nwould be revealed that way and some specimens could reveal more appendicular details, but given the\napproaches done here, this manuscript is sufficient for the description to be useful for other\nresearchers, so I will not linger on this thought or suggestion for longer.\nI notice that the material is supposedly all based on collections made in 2015 and 2016 and kept in\nboth Laussanne and Yale. Now, I believe a lot of the material at Yale was collected in 2009 and up to\n2011/2012. I was involved in collecting some of this material with PVR and I remember specimens of\nsimilar quality as the ones shown here and I even have a couple of specimens in Bristol preserving\nnice limbs. I wonder why this material was not included or mentioned?\nMinor additional comments.\nFigure 2 labels are confused. I was looking for the opisthosoma appendagel panels (Fig. 2E,F), but the\nlabels indicating those panels are missing and then the subsequent panel labels follows a much higher\nset of alphabetical letters. I guess some editing between journals led to this omission.\nI don't see any reason to hold this manuscript up from what I can gather. Unless there is an issue with\nthe taxonomic work, which someone like David Legg, Greg Edgecombe, Jason Dunlop, James Lamsdell\nor such noticed I think it is fine to publish with a few tidbits to be clarified. I don't need to see the\nmanuscript again.\nBest\nJakob Vinther", "reviewer_3": "What are the noteworthy results?\nThis is the first formal description of this common element of the Fezouata biota, a very important\nOrdovician Fossil-Lagerstätte. Compariosn of other early Palaeozoic euchelierates with appendages\nrevealed that the new genus provides important information concerning the evolution of euchelicerate\nappendages.\nWill the work be of significance to the field and related fields? How does it compare to the established\nliterature? If the work is not original, please provide relevant references.\nThe work is significant because it adds to our knowledge of early euchelicerates.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nIn general, but the new genus has only been compared, at least in the text (the supplementary\nmaterial is simply NEXUS files), to other synziphosurines with appendages.\nAre there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication\nor require revision?\nThe main problem with this work is that, at first sight, the new genus looks remarkably like other\nsynziphosurines, such as Pasternakevia Selden & Drygant, 1987 (poorly redescribed by Krzeminski et\nal. 2010) and aglaspidids such as Tremaglaspis Rushton & Fortey (see, for example, Lerosey-Aubril et al. 2013),\nand others. Most of these do not show appendages, but this is no reason to exclude them\nfrom the analysis. How do we know they are unrelated to the new genus? I would like to see\ndiscussion of these genera, which greatly resemble the new form, at least in dorsal morphology.\nIs the methodology sound? Does the work meet the expected standards in your field?\nSee comments above re comparison with other synziphosurines and aglaspidids.\nIs there enough detail provided in the methods for the work to be reproduced?\nYes.\nReferences\nKrzemiński, W., Krzemińska, E., and Wojciechowski, D. 2010. Silurian synziphosurine horseshoe crab\nPasternakevia revisited. Acta Palaeontologica Polonica 55 (1): 133–139.\nLerosey-Aubril, R., et al. 2013. \"Occurrence of the Ordovician-type aglaspidid Tremaglaspis in the\nCambrian Weeks Formation (Utah, USA).\" Geological Magazine 150(05): 945-951.\nRushton, A. W. A. and R. A. Fortey 2003. \"A new aglaspidid arthropod from the Lower Ordovician of\nWales.\" Palaeontology 46(5): 1031-1038.\nSelden, P. and Drygant, D.M. 1987. A new Silurian xiphosuran from Podolia, Ukraine, USSR.\nPalaeontology 30: 537–542."}, "author_response": "We thank all three reviewers for their highly beneficial and constructive comments, which we think\nimproved the quality of our manuscript as a whole. In order to match the reviewer’s suggestions we have\nrenamed our fossil and we have improved our phylogenetic analyses, including a new taxon (Mollisonia)\ninto the matrix, changed the outgroup and performed an analysis with Bayesian methodology for both\nthe matrices. We have also restructured most of our figures in order to increase the quality of the\npictures shown and we have added a new whole figure in the main text showing two different specimens\nthat have been scanned with synchrotron X-ray microtomography and manually segmented. Edits to the\ndiscussion have been made in order to acknowledge properly different hypotheses regarding the early\nevolution the euchelicerates. We also include in the supplemental information two new specimens to\nbetter illustrate critical characters such as the bifurcate telson and the opisthosomal appendages as well\nas two whole new paragraphs treating the relationship of our new taxa with other synziphosurines and\nAglaspids and the interpretation of its labrum.\n\nReviewer #1 (Remarks to the Author):\nThis is a welcome contribution featuring new fossil material from the Fezouata Lagerstätte which\nindeed builds onto our knowledge of early euchelicerate evolution. For those who are a bit familiar\nwith its history, the collection of Fezouata material and its study is a major undertaking, faced with\nvarious challenges, and such a publication is much, much more than the sum of its words and figures.\nThe piece is well presented, organized and written, the figures are overall of good quality (but please\ncheck your panel labelling before submission!) and the phylogenetic analyses were executed\nproperly—although there are some small issues. However, I also see a number of shortcomings and\nomissions, for instance with the systematics and the presentation of the evidence.\n-We thank Cedric Aria for his supporting words and encouraging comments about this manuscript. It was\nindeed a major undertaking involving examining hundreds of fossils, and we are happy to hear that this\nwork is appreciate by this reviewer. We have read his comments with great attention to detail, and have\namended the manuscript to account for all of Cedric’s excellent and constructive suggestions.\nThe title is a bit over-reaching. As I develop below, the fossil morphotype arguably cements a taxon\nincluding a couple of horseshoe-crab-like genera from the Herefordshire biota, adding to morpho-\\anatomical diversity in this group, but does not exactly “reveal” the origin of euchelicerate appendages—\nalso because there are Cambrian fossils that also contributed to build the picture of euchelicerate origins. A\nmuch more accurate (and no less impactful) title could be “An Ordovician fossil documents early\neuchelicerate diversity”, or something of the sort. In any case, quotation marks are unnecessary for\nsynziphosurines because it’s a vernacular name (“Synziphosura” would require them).\n-Amended. The title is now “Ordovician synziphosurine reveals early euchelicerate diversity and provides\ninsight into euchelicerate appendage evolution”.\nIn the Arachnopulmonata context, some colleagues question whether the choice of topological\nconstraints in my matrix influence the placement of fossils in a certain way. I obviously support the\nbasal placement of synziphosurines, and incidentally that of scorpions, but it would be good if you ran\nthe analysis with an Arachnopulmonata constraint, just to see what you obtain—and whatever the\nresult, this would warrant but a couple lines of discussion.\n-Amended. We tested the phylogeny using the Arachnopulmonata constraint and the result is attached in\nthe material for the reviewer. However, we find this to be unrelated to the main story of our paper, so\nwe decide to exclude it from the MS.\nI am quite puzzled as to why you are not erecting a new family. Taxonomy requires it at this point. You\nwould propose a new clade with no Linnean rank if you were describing a putative clade at the supra-\nfamilial or ordinal level maybe, but here, even if your clade is not very well supported, the grouping is\njustified. Accordingly, the family name should be Offacoliidae.\n-Amended. We include Dibasterium durgae and Setapedites in the Offacolidae.\nThere are some issues with the phylogenetic treatment:\n- Knowing that Bayesian analyses generally outperform parsimony, why use a Bayesian approach for one\nmatrix but parsimony for the other? If you defend the value of parsimony, you should then run parsimony\nin both cases, although it might be tedious to implement the backbones. Run at least your second analysis\nwith MrBayes.\n- We promote neither the specific merits of parsimony nor Bayesian analyses. We consider the most\nunbiased way to test phylogenetic hypotheses is to use both approaches since neither of them can be\nunequivocal in its results. We accepted the suggestion of using the Bayesian approach on the matrix from\nLamsdell 2013, and as such, we replace the tree shown in figure 6 with the one obtained with the\nBayesian analyses performed on it. We moved the tree obtained from the matrix from Lamsdell 2013\nwith maximum parsimony to the supplementary material.\n- What is your outgroup in your analysis of the Lamsdell matrix? You did well to include Habelia and\nDibasterium, but Mollisonia is crucially missing, and rooting on Fuxianhuia is ill-advised, given that its status\nas basal euarthropod is contested from several fronts. I tested a parsimony run with Yohoia as\noutgroup (your best candidate here) and Fuxianhuia resolves as sister-group to Panchelicerata. Something’s off.\nYou might as well remove it entirely.\n-Amended. Thank you for the suggestions. We removed Fuxianhuia, and we now use Yohoia as outgroup.\nAlso, we now include Mollisonia even in the analyses preformed on the matrix from Lamsdell 2013.\n- For the sake of reproducibility, please indicate whether you used sectorial search, ratchet, drift and\ntree fusing, and with what settings.\n-Amended. We included the script used for the parsimony analyses on TNT command line version as a\nnew supplemental file. This indicates the settings requested.\nThe absence of Mollisonia throughout is also something I find very strange, especially since you say you\nreused the data from that paper. If you disagree with the interpretations, state it somewhere, but I don’t\nsee why, because it wholly integrates the evolutionary picture you’re describing.\n-Amended. While we find the relationship of Habelia with euchelicerates strongly supported, we find the\ninterpretation of Mollisonia as a euchelicerate with chelicera and proto-book gills less compelling. The\npreservation state of the chelicera is not optimal and could be interpreted differently (Budd, 2021,\nArthropod structure & development, 62, 101048.), as well as the interpretation of the proto-book gills.\nAlso, other papers such as Lerosey-Aubril et al. (2020, PeerJ, 8, e8879), and more recently description of\nthe nervous system (Ortega-Hernández et al. 2022, Nature Communications, 13(1), 1-11.) suggested\ndifferent affinities for Mollisonia. We did update the MS with an addition of this interpretation of\nMollisonia in the first paragraph of the Discussion, but we did not move the focus of our comparison\nfrom Habelia. The addition of Mollisonia to all the phylogenetic analyses has been beneficial and it\nsolidifies the status of Offacoliidae as a group including Offacolus, Dibasterium and Setapedites.\nTaxonomy: Besides the family issue, I find the name wanting. Any stem taxon to an extent could be a\nchimaera of what it stands between, so the genus name doesn’t say much. My recommendation would be\nto take the opportunity to put forward the Fezouata locality and its people and culture. As for the\nspecies name, “robustus, a, um” and “axis, is” are indeed Latin words (and should be referred to), so\nit works technically, but “axis, is” being a masculine substantive (3rd declension), you are in fact\nputting the species name as a masculine apposition of a feminine... It’s odd. “robusta” could be a\nspecies name while transferring the “axis” idea to the genus. In any case, there’s a lot of room to be\nmore creative here!\n-Amended. We have changed the name to Setapedites abundantis.\nThe diagnosis of “Setopoda”.... Biramous prosomal appendages with “detached” stenopodous exopods are\nplesiomorphic since they are present in habeliidans already, so this shouldn’t be part of your\ndiagnosis. A “brush” tip is also present in Mollisonia, and I draw your attention to the fact that there is\na confounding ramus with the same termination in Sanctacaris (Aria and Caron 2017 Add. File 8)—\nwhich makes “Setopoda” not very fitting, but see my comment about erecting a family instead\nanyway. As unambiguous diagnostic traits, when we consider Offacolus and Dibasterium for now, this\nleaves the bifurcating telson tip, the lobate and setose seventh prosomal appendage, but also the\nelongate chelicerae and, importantly, the extensive reduction of gnathobasipods.\n- Amended. In our interpretation of the phylogenetic results, Euchelicerata is a valid clade which does not\ninclude (at the present time) unequivocally Mollisonia, Sanctacaris or Habelia. As such, “euchelicerates\nwith at least second to fifth prosomal appendage pairs being biramous and composed of stenopodous\nexopods of six podomeres bearing a brush-like group of setae on the distalmost podomere” is unique\ninside the monophyletic group of euchelicerata. Also, while it is certainly true these characters (biramous\nprosomal appendages with detached stenopodous exopods) are shared with habeliids (and we do\nacknowledge it, focusing much of our discussion on it) the presence of biramous prosomal appendages\nwith detached stenopodous exopods associated with six podomeres bearing a brush-like group of setae\non the distalmost podomere is unique of Offacoliidae. We are skeptical about the presence of\nhomologous brush tips in the exopods of Mollisonia. However, we do now include elongate chelicerae\nand reduced gnathobasipods as a valuable addition to the amended diagnosis for Offacoliidae.\nNow, these are nice specimens, but they are not very detailed either, and the preservation is challenging:\n- Given the arrangement of the matrix, I am skeptical about a clear bifurcation of the telson tip in\nMGL.102690. I expect it’s very likely to be the case, but we have to be honest with the evidence at hand.\nTry to make a better case for it by adding a close-up in Supplementary.\n-We added images to show the bifurcation of the telson tip more clearly. We add images of a new\nspecimen 102899 counterpart, with close-up available in supplementary material (supp. Fig. 4).\n- Setation is not preserved, and seeing the identity of the seventh prosomal appendage compared to\nopisthosomal ones is frankly a leap of faith. Provide close-ups of these relevant specimens in Figure 2 for\nthe proso-opisthosoma transition.\n- We add a new specimen 102469 in the supplementary discussion and refine the contrast of the\nspecimen in figure 2 (now figure 3), to show this anatomical feature more clearly.\n- Yes, the axial area is well defined, but is it so different from what is known in Offacolus and Dibasterium?\nRemember that we are comparing compression fossils with fully 3D nodular ones... The tergite-pleurae\ndemarcation is lesser in dorso-oblique preserved specimens.\n-Amended. We remove from the description “strongly developed axis” and let only “developed axis”.\nEven if a certain degree of compression affects the preservation of the described fossils flattening them,\nthey are not completely flat. The axis area is in all the specimens the most raised part of the body.\n- You interpret your opisthosomal appendages as vaguely “paddle-like”, implying they are not operculate\nbook-gills, and therefore that your taxon is not a euchelicerate (contradicting your sys pal), and also that\nthis diagnostic feature might have originated independently in Offacolus + Dibasterium and other\neuchelicerates... This begs caution (see also my comment below). The fact is that indeed your opisthosomal\nappendages are not well preserved, and it is entirely possible that you have book gills like in the sister taxa.\nIf, after coding these states as “?” in your matrix, you still retrieve your “Setopoda” clade, then you can\ninvoke secondary homology and justify assuming operculate book gills. Otherwise, your taxa should more\nlikely be paraphyletic without this feature.\nWe acknowledge that the presence of Habelia-like exopods in Setapedites implies a convergent evolution\nof lamellate gills in Offacolidae and crown-Euchelicerata. We insert a mention to this in the discussion.\nHowever, we found paddle-like appendages and no trace of operculate book gills, so we state what we\nhave and we have not written any statement about what we could have. We did not contradict ourselves\nbecause we state “This suggests that euchelicerate book gills evolved from Habelia- or Setapedites-like\nbilobate paddle-like exopods. Consequently, gill opercula have been acquired during euchelicerate\nevolution, and as such are not an apomorphy for Euchelicerata, unlike the chelicera and body\ntagmatization that are characteristic of the entire clade, including its stem members.”\n- I do not entirely agree with your assessment of the first opisthosomal segment. It appears to me\ndifferentiated from posterior segments, larger notably, and overlap by the cephalon can be extensive. In\nthat sense, this segment is not so different from what is observed in Dibasterium.\nFirst opisthosomal segment in Dibasterium is described as “narrow transversely and lacks pleurae”(Briggs\net al. 2012). The fully expressed first tergite in Setapedites is not narrow transversely, in fact even though\nit can be extensively overlapped by the prosomal shield it is comparable in width and length to the\nfollowing tergites.\nThis really leaves for the diagnosis of Offacoliidae;\n- The elongate chelicerae: In your morphotype they are shorter than in Offacolus or Dibasterium\n(which might support their grouping), but elongate also compared to stem euchelicerates or synziph. There’s\nof course variability in eurypterids and arachnids but local apomorphies are just a Cambrian\nreality (and why Bayesian phylogenetic treatments are better);\n- The small size of the gnathobasipods;\n- The morphology of the seventh prosomal appendage;\n- Maybe the telson tip.\n-We thank the reviewer for these comments that have helped us refine and clarify the diagnosis for\nOffacoliidae, for which we are grateful. The diagnosis now focuses on these features suggested by the\nreviewer and reads as follows:\nAmended diagnosis : Euchelicerates with elongate chelicera, at least second to fifth prosomal appendage\npairs being biramous and with reduced gnathobasipods and composed of stenopodous exopods of six\npodomeres bearing a brush-like group of setae on the distalmost podomere. Seventh pair of appendages\n(sixth post-cheliceral appendage pair) uniramous and lobate paddle-like, fringed by lateral spines. Tip of\nthe telson bifurcate.\nIt is therefore essential to me that you be more circumspect with this fossil evidence and that you\ntighten up the taxonomic treatment, as well as a number of other aspects, as detailed below.\nWe thank the reviewer again for these detailed comments, which greatly improved the taxonomic\ntreatment of our new taxon.\nSee also below about making your main figures more useful.\nOther comments:\n41 – The text here and there could do with more punctuation. “and their closest relatives, the\nxiphosurans...”\n-Amended.\n43 – This sentence is not accurate. Euchelicerata is diagnosed by the one synapomorphy that is the\npresence of book gill opercula. Both chelicerae and tagmatization are present in the stem in the strict sense\n(see Aria and Caron 2019 Nature).\n-The suggested single diagnostic character as used in Aria & Caron (2019) is in contrast to the more long-\nstanding and generally accepted list of characters accepted by the community to define Euchelicerata.\nThe count of body segments and tagmatization as well as the true nature of the supposed chelicerae of\nMollisonia have already been questioned (Lerosey-Aubril et al. 2020. PeerJ, 8, e8879), with their\ninterpretation being further weakened by recent information on the nervous system of Mollisonia\n(Ortega-Hernández et al. 2022. Nature Communications, 13(1), 1-11.) The formula we are suggesting is\nthe least controversial and better represents our own views, as well as the more widely held views in the\ncommunity, which is more reasonable for an introduction. We have however added reference to this\nhypothesis of one synapomorphy, along with the debate surrounding the hypothesis, to the manuscript,\nfor example in the first paragraph of the Introduction.\n48 – This presentation is somewhat outdated. Since the publication pertaining to habeliidans and\nMollisonia, there haven’t been counter-arguments as to how megacheirans or some artiopodans could\nbe closer to Euchelicerata. The rooting of Euchelicerata by Mollisonia and habeliidans was retrieved\nindependently by Zeng et al. 2020 (Nature). The issue I think is more about how megacheirans or certain\nartiopodans articulate, if at all, the Panchelicerata clade that would start with habeliidans.\n-The different hypotheses regarding euchelicerate closest relatives cannot be solved by one single paper.\nWe inserted a mention to Mollisonia later in the manuscript, in the first paragraph of the Discussion, to\npoint out that it is one of the hypotheses, but we do not favor the interpretation of Mollisonia given in\nAria and Caron (2019). As for the above Mollisonia interpretation as a stem-euchelicerates, this is not\nwidely accepted and has been questioned already in the two recent publications mentioned in the\nprevious paragraph.\n53 – It is “habeliidans” and not “habeliids” – “habeliids” would refer to the family Habeliidae, but here, it is\nthe order Habeliida. This was coined by Aria and Caron 2017.\n-Amended. To avoid ambiguity, we amended the text to use the formal name of Order Habeliida\nthroughout the manuscript.\n54 – I think you mean the exopods of xenopodans (Emeraldella, Sidneyia). The trunk appendages of\nSanctacaris like that of Habelia are broad and lobate and their proximalmost portion is not well known. The\nargument of Legg was more a comparison between the head of Sanctacaris and that of xenopodans and\nthen with the exopods of xenopodans and those of xiphosurans. However, the interpretation of the head\nwas, like the original one, not entirely correct.\n-Amended. We deleted the reference to Sanctacaris.\nAlso, please choose to use exite or exopod throughout your work, for consistency, and make sure to\nexplain your choice if you choose exite.\n-Amended. Exopods is now used consistently throughout.\n58 – Given how Offacolus and Dibasterium are the central, most relevant taxa to your study, you should\nintroduce them a bit more, and especially say how they have consensually been retrieved as basalmost\neuchelicerates across studies. Say more about their main morpho-anatomy in introduction.\n-Amended. A brief introduction to their morphology and their basal-euchelicerate affinity was added to\nthe end of the first paragraph of the introduction.\n69 – “This species displays...” This last part repeats the abstract and is not necessary.\n-Amended. Repetitive text removed.\n135 – That’s indeed intriguing.\nWe agree!\n140 – “Description” should be continuous with the systematic palaeontology (italicized).\n-Amended.\n243 – I don’t think “doubts” can be “enhanced”...\n-Amended.\n245 – Again, there wasn’t exactly a debate... The matrix by Legg hasn’t been published with updated\ninformation about habeliidans and Mollisonia recently, as far as I’m aware, so this is not a matter of\nwhether, in this specific case, there were two sets of data with opposite signals. Like I wrote before, it is\nmore about 1) a historical reference to the open question whether megacheirans, “vicissicaudates” or\nanother group represented the closest sister group to Chelicerata, which translated into 2) how Megacheira\nand Artiopoda articulate with Panchelicerata. In that sense, the significance of your taxon, in my view, is\nmore that it *finally bridges the gap with these Silurian horseshoe-crab-like forms and stem euchelicerates\nfrom the Cambrian explosion*.\nAnswered above.\n248 – I don’t understand what you mean here. And yes, be careful that the phylogenetic resolution of\nsea spiders is a long-standing problem, and unless you’re extremely confident about your branch support,\nI’d refrain from making such a statement.\n-Amended. We delete the phrase” The biramous prosomal appendages shared between Habelia,\nDibasterium, Offacolus and now C. robustaxis support the hypothesis of a convergent origin for chelifores\nand chelicerae” being a controversial argument not at the center of our discussion.\n252 – No. The gill operculum is the unique synapomorphy of Euchelicerata (as per Weygolt and Paulus; see\nalso e.g. Dunlop 1997, Dunlop and Lamsdell 2017, Aria and Caron 2019), and unless you’re willing to amend\nthe taxonomy (which I would *not* recommend doing because there is no need to), you have to remove\nthis statement.\n- As stated above, our research does not suggest or support the unique synapomorphy of Euchelicerata.\nWe kept the statement, however we rephrase the sentence and acknowledge the hypotheses supported\nin Dunlop 1997, Dunlop and Lamsdell 2017, Aria and Caron 2019.\n253 – 254 – Dibasterium and Offacolus were considered euchelicerates before, and were already forming a\nclade basal to “Prosomapoda.” And obviously Euchelicerata was already monophyletic. You should focus on\nthe novelties.\nWe feel that keeping this short text is justified and ensures a clear understanding of the implications of\nour research, so we left this text in with only slight modification. As clarified earlier in our comments ,\nour results show that Setapedites, even lacking the book gills opercula and being part of Offacolidae, is\npart of Euchelicerata according to our phylogenetic analyses.\n255 – As explained above, you cannot be certain.\nAmended. Added new specimen.\n257 – Again, refer to the actual diagnosis of Euchelicerata.\nSee answer to comments above, also there is not an actual diagnosis only proposed once.\n260 – A number of these characters are actually shared with Sanctacaris. Why is it absent?\nIt does not have chelicera, which in our argumentation is defining euchelicerates.\n269 – To be clear, you construe that the megacheiran-like exopods of the seventh pair of cephalic\nappendages in Habelia are homologous to the seventh pair of appendages in “setopods”, implying the\nreduction of the endopod.\nCorrect, a secondary loss of the endopods of the seventh appendages in Offacolidae is implied in this\nview.\n275 – I think it’s fair to cite Aria and Caron 2019 Nature here.\n-Amended. Citation to this reference has been added.\n285 – Again, see Mollisonia, specifically ROMIP 65262. The taxon has the same “brush” terminations in the\nprosoma. The absence of the feature in habeliids just has to do with the fact that the exopods are very\nslender (like exopod #6 in Offacolus), with small tips, but a bunch of setae is present too.\n-Amended. We checked the proposed material and considering the generally excellent preservational\nstate of the Mollisonia material we consider the presence of this feature questionable and the same is\ntrue for Habelia, however, we added an acknowledgement of this interpretation in the text.\n290 – You identify what possibly is the basipod of an appendage (as was done with Offacolus and\nDibasterium already), but nowhere I see that you can demonstrate where exopods attach. Given the\nresolution of your material compared to the Burgess Shale or the Herefordshire, I think this will be\ndifficult.\n-In order to show the basipod more clearly, we have inserted a new 3D rendering of synchrotron µCT\ndata to show it.\n291 – Proximalmost appendage morphology in Habeliidans as well as Mollisonia is no less clear than in\nOffacolus and Dibasterium: in all cases, the semi-independence of the exopod is based on the absence of\nevidence for the point of attachment and the observation that they do not seem constrained by the\nmovement of the endopods. Which means that this condition started off with Panchelicerata. Thus the\npolarization is quite clear. Did this lead to the loss of prosomal exopods? Yes, most likely – this was\ndiscussed before (Aria and Caron 2017, 2019, Aria 2021 PaleorXiv).\n-Amended. The uncertainty about Habelia is now omitted from the paragraph. The suggested\nevolutionary scenario and references to Aria & Caron 2017 and 2019 were already in the manuscript text\nat this point.\n294 – What about exite instead of exopod?\nPaddle-like appendages in both Dibasterium and Offacolus are considered to be the exopodal in origin.\nWe do agree with this interpretation and follow it.\n306 – Mmmm... but 1) nothing here is new, because this scenario was discussed in the context of\nMollisonia, and 2) the morphology of opisthosomal limbs in your fossil is unclear beyond a lobate aspect\n(that could very well be opercula!).\n-We acknowledge this scenario to have been discussed in Aria & Caron 2019 adding the citation into the\ntext. We answered the doubts regarding the interpretation of the opisthosomal appendages in other\ncomments above and with new figures and specimens (see S1 G and H and new figure 3)\n307 – Whether the name should define the group of taxa or the reverse is always a tautological argument.\nIf you say “Should Euchelicerata represent the least inclusive clade containing XXX”, then I understand, but\nthis is explicitly at odds with the current formal diagnosis. I do see the issue that the absence of opercula in\nyour taxon could render Euchelicerata polyphyletic, but 1) like I wrote above, the presence of opercula\nremains entirely possible, and 2) this is dependent on the topology and the robustness of the results. As a\nmatter of fact, if you support the monophyly of “setopods” and its polytomy, the presumed absence of\nopercula in your morphotype would parsimoniously optimize as a reversal (ACCTRAN or DELTRAN),\ncontradicting your statement. Why don’t you optimize important character state changes on your\ntopology? This would make it very clear.\n-Amended or answered above. The phylogenetic analyses we have performed, even including Mollisonia\nand using different methodology as suggested by the reviewer support Offacolidae as a monophyletic\ngroup at the base of Euchelicerata (sensus Weygoldt & Paulus, 1979). As such, the absence of opercula in\nSetapedites is considered a retained feature. We also add a sentence in the MS exploring the possibility\nof it being a reversal, hypothesis that we do not support but we cannot exclude.\nFigures: Some of your Supp. figure panels are much more interesting to put as main figure panels, e.g. 2C or\nE (but not 2F) (that synchrotron effort should have more visibility!), 3A-B (for the chelicerae and putative\nbipartite epistome-labrum), a larger version of 3J. In your main figures 1 and 2, many panels are redundant\nor don’t show much, so they can easily be replaced.\n-Amended. Thank you for the suggestion. We have added the requested images from the supplementary\nfiles into the main manuscript Figures 2 and 3. 3D renderings and tomographs from the synchrotron data\nhave now been included in a figure in the main manuscript, Figure 4.\nFigure 1. L panel should be K.\n-This has been corrected.\nFigure 2. In E, please correct tone – this is much too yellow and hampers visibility of features.\n-Colour tone has been balanced in this figure.\nFigure 2C. If the labrum is the posterior lobe of the structure you divide in 2, what’s the anterior portion?\nThe epistome? This is a bit of a rhetoric question, as you’ve probably reached the limit of informative\nresolution here. I think you’re better off calling the entire area putative epistome-labrum (if you want to\nkeep with chelicerate terminology, or any other term as long as it’s justified).\n- A detailed discussion of the labrum region has been added to the supplementary information. Possible\ninterpretations for both structures mentioned in the reviewers comment are now discussed.\nSupplementary Figure 3. Drawings G and O are overinterpretations. You draw all boundaries as if\nthose were genuine, but it is evident that the structures are severely altered and that you cannot\nmake the case for these extra long spines, etc.\n-In order to show the presence of these anatomical structures, we have provided better-quality photos\nthat better support our interpretive drawings.\nSupplementary Figure 4. Put the PPs everywhere, not just for the Chelicerata clade.\n-Amended.\nSupp. discussion – Prosoma ground pattern: Habeliidans and mollisoniids, as well as some other\nunpublished Burgess Shale taxa, have seven pairs of appendages under the cephalic shield. Should these\ntaxa form a clade, this could be indeed autapomorphic. But it is also not unfathomable to think that this\ntergite was secondarily released—this is commonplace among crustacean cephalothoraxes!\nI don’t think there is a need for you to oppose the evidence so strongly, especially when it’s otherwise\nvery clear in many specimens and several taxa.\n-We amended the discussion to tone down our opposition to this hypothesis.\nI hope these comments were fair and constructive. I will be glad to see a revised version of this work and,\nif satisfying, recommend it for publication.\nWe thank Cédric for these helpful comments, which we believe have greatly improved the manuscript\nand clarified our arguments. We addressed all comments as fairly and extensively as possible, while also\nstill maintaining some of our original line of thinking, in the few places where we have to respectfully\nagree to disagree with the reviewer on one or two points. We find our manuscript now presents a better\nbalance between our point of view and that of Cédric. We hope he finds our revised manuscript to be\nsatisfactory.\nCédric Aria\nReviewer #2 (Remarks to the Author):\nThis is a nice manuscript describing a very common synziphosurine from the Fezouata biota. The\nrecord is important for a number of reasons in terms of both it's age and the quality of preservation.\nThe manuscript is well written and well presented. I am not a specialist on chelicerates so I decided to\nsimply read the manuscript and see if the presentation was sound and whether I could identify the\ncharacters in question.\nWe thank Jakob for his supportive comments, and we appreciate hearing his suggestions on the overall\npresentation and argumentation in the manuscript. We believe the manuscript is even stronger after\nhaving incorporated his suggested changes.\nThe specimens are really small. Hence the appendages are faint infill of iron oxides and clays after\npyrite. I bet some referees may say they can't see the podomere boundaries and while the images are\nnot always the best ones I can see them and believe the authors' interpretation. So, in case you get\nsome obstinate referees saying that they can't see a thing, tell them that its because they are blind or\nat least not in the possession of trained eyes to look at exceptional fossils and hence should abstain\nfrom commenting on such issues again.\nMany thanks for our words of support here. Fortunately, we did not receive major opposition to most of\nour anatomical features. Some comments from other reviewers made suggestions for specific\nimprovements to the photographs, and so we have hopefully generally increased the quality of our\nphotographs and figures. A few anatomical features were challenged by other reviewers (e.g. bifurcate\ntelson tip) in response to which we added some new photos or new specimens. New 3D rendering from\nsynchrotron data have also been added.\nI can see the biramous prosomal appendages and the chelicerae. I am struggling a bit more with the\nopisthosomal appendages and their morphology, but agree that there is some structures that clearly\nderives from exposure of structures below the tergal skeleton and which is appendicular in nature. The\nsupplemental figures are more helpful.\nWe are happy that the supplemental figures clarify our interpretation of the opisthosomal appendages.\nIn this regard, we add a new specimen to clarify even more this anatomical feature(Figure S1 G and H).\nToo bad these couldn't have been revealed with the synchrotron. I surely would have taken 20-40\nindividuals to the machine after having cut them down to size and scanned the hell out of them. Then\nI would have manually segmented them out with a nice bottle of Swizz white wine by my side rather\nthan thresholding them which often miss more faintly preserved features. Surely a lot of cool stuff would be\nrevealed that way and some specimens could reveal more appendicular details, but given the\napproaches done here, this manuscript is sufficient for the description to be useful for other\nresearchers, so I will not linger on this thought or suggestion for longer.\n-Amended. In order to better illustrate the anatomical features identified by the reviewer as unclear, and\nfollowing his suggestion, we have deeply reworked the segmentation of our synchrotron µCT scan, and\nhave also obtained additional scans for other specimens. Among the new scans, which exhibit a strong\nheterogeneity in terms of the amount of details visible for the cephalic appendages, we have manually\nsegmented a second specimen, preserved in dorso-lateral view. We have dedicated a whole new figure\n(Figure 4) to this aspect, including several 3D renderings and tomographs from these datasets. This\nrevealed the opisthosomal appendages and other structures in better detail (see also our replies to the\ncomments of the Cedric Aria).\nI notice that the material is supposedly all based on collections made in 2015 and 2016 and kept in both\nLaussanne and Yale. Now, I believe a lot of the material at Yale was collected in 2009 and up to 2011/2012.\nI was involved in collecting some of this material with PVR and I remember specimens of similar quality as\nthe ones shown here and I even have a couple of specimens in Bristol preserving nice limbs. I wonder why\nthis material was not included or mentioned?\nWe included all material that was available to us based on our study of the Lausanne and Yale\nspecimens. This includes all the YPM material collected by Ou Said Ben Moula, his family, Peter Van Roy\nand others prior to 2015 (that is accessioned at Yale) and we have amended the text to better reflect that\nthe entire collection in Yale was examined. We were not aware of material accessioned in collections at\nBristol. Generally speaking, the major focus of this work was on the Lausanne collection, and while we\ndid make sure to include all relevant and important specimens from the Yale collection, we did\npreferentially choose Lausanne specimens to illustrate in the manuscript when material of equivalent\nquality was available from both collections.\nMinor additional comments.\nFigure 2 labels are confused. I was looking for the opisthosoma appendagel panels (Fig. 2E,F), but the labels\nindicating those panels are missing and then the subsequent panel labels follows a much higher set of\nalphabetical letters. I guess some editing between journals led to this omission.\n-We have corrected the labelling in this, and double checked labels in all figures. Thank you for pointing\nout this error.\nI don't see any reason to hold this manuscript up from what I can gather. Unless there is an issue with\nthe taxonomic work, which someone like David Legg, Greg Edgecombe, Jason Dunlop, James Lamsdell\nor such noticed I think it is fine to publish with a few tidbits to be clarified. I don't need to see the\nmanuscript again.\nThank you again for your comments and for your support of this manuscript.\nBest\nJakob Vinther\nReviewer #3 (Remarks to the Author):\nWhat are the noteworthy results?\nThis is the first formal description of this common element of the Fezouata biota, a very important\nOrdovician Fossil-Lagerstätte. Compariosn of other early Palaeozoic euchelierates with appendages\nrevealed that the new genus provides important information concerning the evolution of euchelicerate\nappendages.\nWill the work be of significance to the field and related fields? How does it compare to the established\nliterature? If the work is not original, please provide relevant references.\nThe work is significant because it adds to our knowledge of early euchelicerates.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nIn general, but the new genus has only been compared, at least in the text (the supplementary\nmaterial is simply NEXUS files), to other synziphosurines with appendages.\n- A supplementary file had previously been provided showing additional figures and with additional text,\nsome of which extended the discussion to taxa before the other synziphosurines. We regret to hear that\nthe reviewer did not have access to the supplementary file and we hope that it will be provided to the\nreviewer, if necessary, in its revised version. Based on comments from other reviewers, we have revised\nand in some places expanded our discussion of the wider significance of this taxon.\nAre there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or\nrequire revision?\nThe main problem with this work is that, at first sight, the new genus looks remarkably like other\nsynziphosurines, such as Pasternakevia Selden & Drygant, 1987 (poorly redescribed by Krzeminski et al.\n2010) and aglaspidids such as Tremaglaspis Rushton & Fortey (see, for example, Lerosey-Aubril et al. 2013),\nand others. Most of these do not show appendages, but this is no reason to exclude them from the\nanalysis. How do we know they are unrelated to the new genus? I would like to see discussion of these\ngenera, which greatly resemble the new form, at least in dorsal morphology.\n-We welcome the reviewer’s suggestion here to broader our discussion of other synziphosurines and\ntheir similar dorsal morphology. A new section has been added to the Supplementary Information\ndirectly addressing the taxa listed above and comparing them to our new Fezouata taxon.\nIs the methodology sound? Does the work meet the expected standards in your field?\nSee comments above re comparison with other synziphosurines and aglaspidids.\nThe requested comparisons have been added to the supplementary text.\nIs there enough detail provided in the methods for the work to be reproduced?\nYes.\nReferences\nKrzemiński, W., Krzemińska, E., and Wojciechowski, D. 2010. Silurian synziphosurine horseshoe crab\nPasternakevia revisited. Acta Palaeontologica Polonica 55 (1): 133–139.\nLerosey-Aubril, R., et al. 2013. \"Occurrence of the Ordovician-type aglaspidid Tremaglaspis in the Cambrian\nWeeks Formation (Utah, USA).\" Geological Magazine 150(05): 945-951.\nRushton, A. W. A. and R. A. Fortey 2003. \"A new aglaspidid arthropod from the Lower Ordovician of Wales.\"\nPalaeontology 46(5): 1031-1038.\nSelden, P. and Drygant, D.M. 1987. A new Silurian xiphosuran from Podolia, Ukraine, USSR. Palaeontology\n30: 537–542.\nWe thank the reviewers for their comments and for providing the necessary references, which we are\nsure to include in the revised version of the manuscript and its supplementary material.\n\nReviewers' Comments:\nReviewer #1:\nRemarks to the Author:\nThe authors have been respectful with my comments and have generally taken care to address them,\nalthough some critical points have been left wanting. There remains, in my view, substantial work to\ndo to make the study sound and tight. In particular, looking now more attentively into the fossils,\ncritically, some interpretations have to be reconsidered, and most likely revised. Unfortunately, this\nalso means the cladistic analyses will have to be rerun. Camera lucida drawings also need to be\nimproved, because as it stands, they are not accurate enough.\nAs preamble, and especially considering the preservation of chelicerae documented in this very paper,\nI regret that the authors doubt the well-preserved chelicerae of Mollisonia but not the “evidence” for\nnervous tissues in specimens shown in Ortega-Hernández et al. 2022, Nat Comm, which is thoroughly\nnon-existent. As the authors are aware, circum-intestinal carbon films with branching ventral\ntonguelets are ubiquitous among BST arthropods and have already been documented extensively\n(since before Whittington…)—the specimens in question fall within that taphonomic range, not to\nmention other inconsistencies and misinterpretations in this publication. Nevertheless, it is the current\nauthors’ prerogative to follow these views, and it is by all means presented fairly. In places, however,\nthe authors need to be more explicit about the argumentative ground for their disagreement, in the\nstead of broad dismissal – see below.\n> That might be seen as peripheral, but to me it is important. It does not serve you to inflate your\ntitle. If we did not know Offacolus and Dibasterium already, perhaps you would be in your right to say\nthat your fossil was “revealing” early euchelicerate diversity. Besides, you do not provide more\ndetailed insight than the Herefordshire taxa. Expands, illustrates, even “evinces”, all of those are fine.\nAgain, this does not change anything to the importance of your findings, and really improves the rigor\nof your output.\n> I would not discard the Arachnopulmonata result. This is a very current (and sensitive!) debate, and\nthis issue will pop in the mind of many interested readers (especially neontologists). It is not much\nwork to add another tree to your supplementary and strengthens your paper by showing that your\nresults are robust to the position of Scorpiones.\n> An exopod with brush-like termination is shown in Aria and Caron 2019, Nature, Fig. 2g, ED Fig. 3f.\nThis is certainly as well preserved as in your specimens. What is there to be skeptical about? What is\nyour alternative interpretation?\nIn certain cases of high variability of traits, it is indeed warranted to diagnose by character state\ncombinations, but clearly this is not the case of exopodial setal brushes, present in habeliidans and\nvery much arguably so in mollisoniids. Considering the habeliidan setal brush as analogous based on\nsize difference (?) is simply untenable and even ad hoc justification. It is also the case for biramy and\nthese exopods being stenopodous—these states are plesiomorphic, clearly present in the common\neuchelicerate ancestor (emphasizing the euchelicerate assignment does not change that fact).\nNext to this, the six-podomerous exopod character needs revision. Briggs et al. counted 5 in\nDibasterium, the fifth being a separately branching podomere, not counting setal tip, but exact\npodomere boundaries (especially pod. 3) were not entirely unambiguous. Interestingly, Offacolus’\nprosomal exopods purportedly (but convincingly) had six podomeres, without setal termination, but\nthe designated pod. 5 is identical morphoanatomically to pod. 4 of Dibasterium, being the one\nproducing the branching out distal podomere. As a result, both indeed likely had 6-podomerous\nexopods *excluding termination*—however, in Aria and Caron 2017 BMCEB, limb terminations (claws\nor setal brushes) were included in the podomere count as has been done traditionally. There is\ntherefore ground for heptapodomerous exopods being plesiomorphic too.\nThen the problem, as I explain below, is that you interpret exopods in Fig. 2G, H when they seem in\nfact to be endopods.\nTraits that hold for Offacoliidae: elongate chelicera, reduced prosomal gnathobasipods, seventh\nprosomal appendage pair reduced to setose paddle exopod, bifurcate telson tip.\nFor Setapedites: See my comments below. You make a case for the raised tergum, and segmental\nanatomy as well as the shape of cephalon (closer a priori to Dibasterium) should be emphasized here.\nThe shape of the 11th opisthosomal tergite indeed also counts in. It is unclear to me how you\ndifferentiate the short triangular pleurae from said sub-axial nodes.\n> The interpretative drawings of Figure 1 are inaccurate—See my annotated pictures attached. There\nare three regular metasomal segments followed by a larger plate, as well as possibly intercalary\nelements between this plate and the telson (likely including an additional segment and a telson head).\nOf the mesosomal segments, 6 are well exposed, while an anterior one is mostly covered by the\noccipital margin of the head shield. However, MGL102872 also shows an additional element of what\nseems to be a tergo-pleura anterior to that segment (thick arrow). This very sensibly can make the\nlink between the habeliidan body plan and the offacolid one, insofar as you seem to have 12 trunk\nsegments preceded by a separate segment integrated into the cephalon, but apparently still expressed\nhere. This is in fact a solid and interesting find! On my image, db is doublure - it is nicely preserved,\nyou should mark it.\n> Overall, morphoanatomy seems close to Dibasterium, including the ventral side. It appears indeed\nthat all mesosomal segments bear appendages, so 7 pairs. Here again, I believe your interpretative\ndrawings are inaccurate, or at least incomplete (see annotated image). Given the disposition of\nsegments, it seems very likely that the 7th appendage pair, borne by a very reduced segment, is\ntucked mesially, as in Dibasterium. Alternatively, you could be right about this first large pair being the\n7th prosomal pair, implying that the other pairs are attached a bit more anteriorly, thus leaving you\nwith an anatomy comparable to Dibasterium (the last mesosomal segment lacking appendages). It\nalso appears to me that the triangular shaped formed by the juxtaposition of reduced endopods\nmesially (as in limulids) is visible—you drew one of them, but not the others…\n> In MGL102634 (Fig. G, H), I think that what you see are the endopods, not the exopods. The\npodomeres are thick and the distalmost podomere looks like a stout claw, bordered by setae on a\nsmall platform. This is similar to what is known is habeliidans. Most importantly, these appendages are\nattached very mesially, close to the central axis—as is expected of endopods seen from ventral view.\nIn Fig. 3A, B, these indentations would correspond to the tougher masticatory margins of the\nbasipods, which tend to be more three-dimensionally preserved in compression fossils. I reckon you’re\ninterpreting these as folded endopods, based on Offacolus and perhaps extant horseshoe crabs, but\nconsider the differential size—to me, that doesn’t match. This is also consistent with what you get\nfrom X-ray tomography, in Fig. 4B-E: These sclerotized wedges are the gnathobasipods. In Fig. 2I, J,\nyou draw exopods and endopods sharing a basis. Given the evidence, this seems a conjecture,\nespecially in the context of odd relationship between these rami. Your CT data, however, are better in\nthat regard; I don’t see exopods attaching to a gnathobasipod, but it is fair to say that the\nproximalmost part of the exopods is very close to that of the endopods.\nI’m attaching a collage to compare the limbs in MGL102634 with the exopod of Offacolus and the\nendopods of Habelia. Please carefully consider.\n> I am sorry to say, but calling the chelicerae in Mollisonia “poorly-preserved” while taking for granted\nall the fossil evidence provided in this very paper is unfair to me. As a matter of fact, the outline of\nchelicerae, including chelate elements in dorsal view, is better preserved in Mollisonia than in the fossil\ndescribed here. Budd’s arguments were responded to in Aria 2022, Biological Reviews. If you still think\nthese elements could be something like non-appendicular protocerebral protrusions, say so, but at\nleast give an argument.\nRegarding the pygidium, I am not sure what the argument is exactly, but Mollisonia’s pygidium is four-\nsegmented (not three-segmented as written here), as shown by the number of appendage pairs and\nstructure (Aria and Caron 2019, Nature, Figs. 1d,e, 2i, ED Fig. 4). It is exactly the same as in any\nother arthropod with a pygidium…\n> Back to book gills and Euchelicerata… First of all, Euchelicerata is taxonomically diagnosed by the\npresence of opercula, so you cannot write that your fossil would demonstrate this is not the case—this\nmakes no sense. This is not Aria and Caron (2019) who suggested it, it was formally defined by\nWeygoldt and Paulus (1979: Untersuchungen zur Morphologie, Taxonomie und Phylogenie der\nChelicerata. Z. Zool. Syst. Evol-forsch. 17 (85-115), 177-200.). This is a rule of systematics that you\nhave to follow.\nNow, if you were to find that gill opercula arose convergently, what you could say is that Euchelicerata,\nas currently diagnosed, is polyphyletic (I already specified that in my first review…). You may also\nemend the diagnosis to add or replace diagnostic traits. However, and again, you need to be more\ncircumspect here, and admit that your evidence is not good enough to be unambiguous. In\nMGL107741, those well-defined paddle-shaped elements appressed onto the ventral side could very\nwell be opercula—which is simply defined as a semi-rigid lobate sheet covering lamellae. What you\nidentify as a setose margin is quite tenuous, and would not in fact rule out an operculum in itself.\nNevertheless, and again I am repeating what I wrote in my first review, even if you coded with\ncertainty the absence of operculum in your species, based on parsimony this optimizes as homoplasy!\nI tried to show that using your Lamsdell dataset and tree result… only to find out that the operculum is\nnot even present in this matrix. How could you test anything related to the evolution of the operculum\nif you don’t even code it in your dataset?\nSo I used the other dataset, from Aria and Caron 2019, and looked at the optimization of opercula for\nboth parsimony and likelihood on the Bayesian result. See attached figure. Likelihood optimization is\nextremely strict and does not achieve more than ambiguity, but a parsimony-based optimization\nclearly places the presence of opercula at the Euchelicerata node as retrieved previously in Aria and\nCaron 2017, 2019, with the ambiguity in Setapedites being a reversal.\nSimilar story for the peculiar exopod attachment condition. This most parsimoniously optimizes as\nancestral for panchelicerates—sea spiders notwithstanding—despite you recoding uncertainty in\nMollisonia. This evolutionary pattern was put forward as the illustration of the possible mechanism for\nthe loss of the outer branch in the euchelicerate prosoma in Aria and Caron 2017, 2019, an idea that\nyou misleadingly claim to be new here.\nAdditionally, if Mollisonia’s trunk appendages are not multi-lamellate structures akin to book gills,\nwhat are they? I am defender of a good science based on open refutation, but refutation should be\nexplicit, fact-based.\n> There are issues with the use of “stem” and “crown” terminology throughout, which I think has\nmore to do in fact with the taxonomic issue of what Euchelicerata means as explained above. First, in\nyour Fig. 6, Synziphosurines are not part of the crown, since they left no extant descendants. Second,\nas I demonstrate above, likelihood and parsimony optimization still places Euchelicerata at the node\njoining up offacolids and prosomapods. Third, considering that you’re leaving Chelicerata out of the\npicture, you need to include early panchelicerates Mollisonia and Habelia into the stem of\neuchelicerates.\nIn summary:\n- You need to be more explicit about your rejection of the evidence in Mollisonia.\n- The evidence is not clear enough to ascertain the absence of opercula. This should be left more\nopen.\n- If you actually optimize your character on your tree (which I suggested you to do in the first round\nbut you did not), you find that opercula are present in the common ancestor of Euchelicerata sensus\nWygoldt and Paulus and as retrieved by Aria and Caron 2019. This contradicts your statements.\n- Same for the prosomal exopod condition: Most parsimoniously this is present in the common\nancestor of panchelicerates, again as stated in Aria and Caron 2019.\n- Improve your interpretative drawings and reconsider segmental anatomy, as well as prosomal\ngnathobasipod and endopod identification.\nI refrained from examining the MS in finer details – language, references, etc. The authors must first\naddress these remaining salient problems, some of which being carried over from the first round of\nreviews. Should they do so, I expect that only one additional round will be necessary for said details.\nI want to make clear that I am not trying to impose my views, and I would also rather not spend a\nweek on a review, but I am trying my best to show the authors that the shortcomings I point out are\nnot simply a matter of opinion—as they should not be, on either side.\nOther specific comments:\nl.38: Offacolidae\nl.51: The statement that it is “obscure” washes away the work on both the Herefordshire and the\nBurgess Shale. It remains poorly documented, or something equivalent.\nl.51: Regarding the definition of Euchelicerata here and hereafter, see my comment and correct.\nl.56: You refer to phylogenetic results that are 10 years old—and in the case of Haug et al. 2012, this\nis not even a cladistic study. Why not acknowledge that these hypotheses have not been recovered\nagain? What recent phylogenetic study has disputed that Habeliida is the closest sister group to\nChelicerata? (Mollisonia notwithstanding). Even if you reject the evidence provided by Mollisonia,\nPanchelicerata as defined by Aria and Caron (2019) remains valid, yet there is no mention of it here.\nThis grouping was recovered in most recent cladistic work by Zeng et al., Moysiuk and Caron, etc.\nl.82: The anal pouch is completely speculative. Without redundancy of evidence, I don’t see how this\ncan be differentiated from an artefact. Consider it putative if you want, but just in the figure caption;\nno need to dwell on that.\nl.83: “Setapedites abundantis resolves the position of several problematic stem-euchelicerate taxa”:\nWhat do you mean? Setapedites has no phylogenetic impact on the placement of early panchelicerates\nor other offacolids based on recent phylogenetic results.\nSys. Pal.: See main comments.\nl.285: This makes no sense. It’s not because frogs have anterior members homologous with birds that\nit would play a role in supporting their bird affinity! Also, the statement above is contradictory with\nwhat you say elsewhere about gill opercula not being apomorphic of euchelicerates.\nl.290: Well, no, they are shared with Sanctacaris, and the enlarged gnathobasipods also shared with\nMollisonia. Or do you also contest the presence of large gnathobasipods in Mollisonia?\nl.301: Not sure what kind of bilobation you mean in Habelia.\nl.327: No, it does not; quite the opposite in fact. If you posit the clear attachment to a basipod in\nSetapedites, which I contest, it is another autapomorphy, and a reversal, (like the putative absence of\nopercula) amid the condition shared by offacolids, habeliids and mollisoniids.\nl.337: The seventh cephalic pair of early panchelicerates has already been argued to be ancestral to\nthese features in Aria and Caron 2017, 2019. This was one of the main findings in these papers…\nPhylogenetic figures: Are you sure you are displaying Maximum clade credibility consensuses? The\ndefault output from MrBayes is extended majority rule, and the topology coming out of my basic run is\nthe same as yours.\nAdditionally, happening to be, I think, the “someone like David Legg, Greg Edgecombe, Jason Dunlop,\nJames Lamsdell or such”, I was asked to check whether the authors had fully addressed the comments\nof reviewer #2, Jakob Vinther. I can’t say about the bottle of “Swizz” white wine, but it seems the\nauthors took care of providing a better rendition of the raw CT data, as well as adding another\nspecimen—which is, I think, what Dr. Vinther suggested. Clarification was also provided regarding the\nmaterial used, and labelling in Figure 2 was corrected.\nCédric Aria\nReviewer #3:\nNone\nPoint-by-point responses to the Referees’ comments and suggestions.\nWe present the referees' comments in normal typeface and our responses in green bold.\nReviewer #1 (Remarks to the Author):\nThe authors have been respectful with my comments and have generally taken care to\naddress them, although some critical points have been left wanting. There remains, in my\nview, substantial work to do to make the study sound and tight. In particular, looking now\nmore attentively into the fossils, critically, some interpretations have to be reconsidered, and\nmost likely revised. Unfortunately, this also means the cladistic analyses will have to be\nrerun. Camera lucida drawings also need to be improved, because as it stands, they are not\naccurate enough.\nWe are happy that our edited version generally pleased the reviewer. We have\nconsidered all his new comments and modified our text and figures accordingly\neverywhere we felt the demands were legitimate and not the reflection of his own\nopinion which we (and the majority of the community) do not endorse. Regarding this\nparticular point of the cladistic analyses, the new requested analyses are no longer\nneeded after we removed our statements about the presence/absence of opercula and\ngills anatomy following other demands by the reviewer (see below). However, we did\nrerun the analyses after recoding Mollisonia (detailed later in our response).\nAs preamble, and especially considering the preservation of chelicerae documented in this\nvery paper, I regret that the authors doubt the well-preserved chelicerae of Mollisonia but not\nthe “evidence” for nervous tissues in specimens shown in Ortega-Hernández et al. 2022, Nat\nComm, which is thoroughly non-existent. As the authors are aware, circum-intestinal carbon\nfilms with branching ventral tonguelets are ubiquitous among BST arthropods and have\nalready been documented extensively (since before Whittington…)—the specimens in\nquestion fall within that taphonomic range, not to mention other inconsistencies and\nmisinterpretations in this publication. Nevertheless, it is the current authors’ prerogative to\nfollow these views, and it is by all means presented fairly. In places, however, the authors\nneed to be more explicit about the argumentative ground for their disagreement, in the\nstead of broad dismissal – see below.\nIt should be noted here that:\n1) we added theses sentences (lines 260–266) about the affinities of Mollisonia in our\nrevised manuscript because (as stated in our cover letter) we believe that it was\nimportant to incorporate a mention of (and cite) the personal view of the reviewer on\nthe matter even though it does not reflect the majority view of the community,\nincluding ourselves;\n2) as shown by our revised cladistic analyses, the inclusion (or not) and coding of\nMollisonia have no impact on the conclusions of our work.\nThat said, we confirm that we are not convinced by the presence of chelicera in\nMollisonia, but we acknowledge that our wording may let the reviewer think that we\nfully support Ortega-Hernández et al. (2022)’s interpretation, which was not our\nintention. Nonetheless, the re-evaluation by Ortega-Hernández et al. (2022) is only one\npotential opposite view for the interpretation of Mollisonia as a chelicerate. Indeed, we\nalso refer to (and cite) a paper by Lerosey-Aubril et al. (2020) that highlights\ndifferences in the homology of the pygidium. Furthermore, Budd (2021) also cast\ndoubts about Aria & Caron (2019)’s interpretation: “it is difficult to understand how\nthe putative chelicerae are meant to have functioned, as they seem too far away from\nthe mouth to be used for manipulating food; and definitive evidence for the structures\nactually being chelate or having podomeres is lacking. The possibility of these\nstructures being pre-antennal outgrowths of the sort discussed by Ortega-Hernández\nand Budd (2016), is not considered. In addition, a central rounded structure between\nthe two supposed chelicerae is dismissed without evidence as being an artefact, even\nthough it is symmetrically placed in the organism and indeed appears to be present in\nmore than one specimen (i.e. MCZ, 1811 and ROMIP 62978). Thus, the phylogenetic\nposition of Mollisonia seems presently unclear”. We did not, however, refer to this\nopinion by Budd (2021) in our revised manuscript, and have now added a mention in\nour new revised manuscript (see below).\nAll three papers mentioned above cannot be considered irrelevant, and the\nambiguous affinities of Mollisonia remain a fact within the scientific community.\nConsidering that there are not enough consistent arguments to fully support either\nview, we feel (as stated in our cover letter) that both should be presented in our work.\nWe also included in our new revised manuscript a mention to a highly provocative\nand controversial paper published by the reviewer after the submission of his review\n(Aria et al. 2023, BioEssays), in which they provide a contrarian view on the\nfossilization of nervous systems.\nConsidering all these points, we have modified this section (l260–266) as follows:\n“Among Cambrian euarthropods, Mollisonia plenovenatrix has been described as\npossessing a pair of short chelicerae and proto-book gills composed of overlapping exopod\nflaps, and retrieved as a basal chelicerate (Aria & Caron 2019). Counter arguments include\nthe poor preservation of these features and functional considerations (Budd 2021), the\norganization of the central nervous system in Mollisonia symmetrica (Ortega-Hernandez et\nal. 2022) (but see Aria et al. 2023), as well as the origins and development of the last three\nsegments complex (pygidium) in the genus Thelxiope and Mollisonia (Lerosey-Aubril et al.\n2020). In any case, the inclusion (or not) and coding of Mollisonia have no impact on the\nconclusions of the present work.”\n> That might be seen as peripheral, but to me it is important. It does not serve you to inflate\nyour title. If we did not know Offacolus and Dibasterium already, perhaps you would be in\nyour right to say that your fossil was “revealing” early euchelicerate diversity. Besides, you\ndo not provide more detailed insight than the Herefordshire taxa. Expands, illustrates, even\n“evinces”, all of those are fine. Again, this does not change anything to the importance of\nyour findings, and really improves the rigor of your output.\nWe have modified our title accordingly: The oldest-known synziphosurine reveals\nearly euchelicerate diversity and evolution\n> I would not discard the Arachnopulmonata result. This is a very current (and sensitive!)\ndebate, and this issue will pop in the mind of many interested readers (especially\nneontologists). It is not much work to add another tree to your supplementary and\nstrengthens your paper by showing that your results are robust to the position of Scorpiones.\nWe have added the requested tree as a new Extended Data figure.\n> An exopod with brush-like termination is shown in Aria and Caron 2019, Nature, Fig. 2g,\nED Fig. 3f. This is certainly as well preserved as in your specimens. What is there to be\nskeptical about? What is your alternative interpretation?\nAlthough we indeed find it difficult to determine whether the feature in Aria & Caron\n2019 represents a subchelate, spinose, or trilobite-like termination, we have now\nincluded a sentence in the main text mentioning their presence and using the\ndescription of Aria & Caron 2019, as follows “Mollisonia also shows a single exopod in\none specimen with a termination made of setae 6, but their precise arrangement are\nunknown. Possessing exopodial setal brushes may be homologous amongst these taxa, but\nwe interpret the precise morphology of stenopodous exopods 2–5 ending in a brush-like\ngroup of long and radially arranged setae to be an autapomorphy of Offacolidae …”.\nWe have additionally rerun both matrices using the original coding of Aria & Caron\n2019 for Mollisonia, and we include these results in the supplementary information as\nSupplementary Fig. 10 and 11. So now we show his view, as well as our own view.\nOverall, the trees change very little when the original codings for Mollisonia from Aria\n& Caron are used. Therefore, the text in Supplementary Information describing the\ncharacter coding change for Mollisonia has been changed to indicate we use their\noriginal coding, as follows:\n“Character 123- Stenopodous exopod type coded as both ? and 1 (respectively\nSupplementary data 1 and 2)\nWe ran two Bayesian analyses, one with this character state coded as ?, resulting in the\ntrees of Supplementary Fig. 5-8 and another where this state is kept as 1 as originally coded\nin Aria & Caron (2019) and resulting in Supplementary Fig. 10 and 11. This is because, in\nour opinion, this structure (described from a single exopod in one specimen in Aria & Caron\n2019) is difficult to interpret in detail but seems to have a different structure to that seen in\nSetapedites abundatis.”\n“Character 69- Exopod on second post-antennular limb coded as both 2 and 3 (respectively\nSupplementary data 4 and 5).\nWe ran two Bayesian analyses, one with character state 3 (reduced state of prosomal\nexopod), resulting in the trees of Fig. 6 and Supplementary Fig. 6, 7 and 8, and another\nwhere this state is changed to 2 (pediform exopod), resulting in the trees of Supplementary\nFig. 11. These two different codings reflect different interpretations of Mollisonia cephalic\nexopods in comparison with offacolids. The two different codings however do not change the\nresult of the analyses for the target group.”\nIn certain cases of high variability of traits, it is indeed warranted to diagnose by character\nstate combinations, but clearly this is not the case of exopodial setal brushes, present in\nhabeliidans and very much arguably so in mollisoniids. Considering the habeliidan setal\nbrush as analogous based on size difference (?) is simply untenable and even ad hoc\njustification.\nRegarding the setae condition in Mollisonia, please see our reply to the previous\ncomment.\nRegarding the condition in habeliidans, we thank the reviewer for his comment that\nhelped us strengthen our manuscript. Setae in Habelia are definitely much shorter\n(shorter than the podomeres) than those in Offacolidae (longer than the podomeres).\nSutton et al. (2002) described them for Offacolus as “a brush-like splay of long setae”,\na terminology that we are happy to follow in our revised text. But, more importantly,\nanother significant difference between Habelia and Offacolidae is that setae are\nconfined to the inner margin of the exopod in Habelia whereas they are radially\narranged from the inner to the outer margins in Offacolidae. We have made this\nclearer in our manuscript as follows:\n- lines 96–97, “stenopodous exopods of six podomeres bearing a brush-like group of long\nand radially arranged setae on the distalmost podomere.”\n- lines 316–319, “Exopodial setal brushes are also present in Habelia12. However, they are\nshort and confined to the inner margin of the exopod in Habelia whereas they are long and\nradially arranged from the inner to the outer margins in Offacolidae, clearly differentiating\nthese features.”\n- lines 321–323, “We interpret the morphology of stenopodous exopods 2–5 ending in a\nbrush-like group of long and radially arranged setae to be an autapomorphy of Offacolidae,\na conclusion which is supported by our phylogenetic analyses (Fig. 6, Supplementary Fig. 5–\n8).”\nIt is also the case for biramy and these exopods being stenopodous—these states are\nplesiomorphic, clearly present in the common euchelicerate ancestor (emphasizing the\neuchelicerate assignment does not change that fact).\nWe fully concur with the reviewer and also recognize the stenopodous condition as\nlikely plesiomorphic of euchelicerates and shared by habeliidans and offacolids, and\npossibly Mollisonia. The manuscript text states this already in the discussion\n“Biramous prosomal appendages with specialized exopods (with respect to the\nancestral condition for stem euarthropods 43), together with a uniramous first pair of\nappendages (chelicerae) and uniramous appendages of somite VII made of bipartite\npaddle-like exopods, can therefore be considered as part of the euchelicerate ground\nplan.”\nNext to this, the six-podomerous exopod character needs revision. Briggs et al. counted 5 in\nDibasterium, the fifth being a separately branching podomere, not counting setal tip, but\nexact podomere boundaries (especially pod. 3) were not entirely unambiguous. Interestingly,\nOffacolus’ prosomal exopods purportedly (but convincingly) had six podomeres, without\nsetal termination, but the designated pod. 5 is identical morphoanatomically to pod. 4 of\nDibasterium, being the one producing the branching out distal podomere. As a result, both\nindeed likely had 6-podomerous exopods *excluding termination*—however, in Aria and\nCaron 2017 BMCEB, limb terminations (claws or setal brushes) were included in the\npodomere count as has been done traditionally. There is therefore ground for\nheptapodomerous exopods being plesiomorphic too.\nWe concur that the designated pod. 5 in Offacolus is identical morphoanatomically to\npod. 4 of Dibasterium. Looking carefully at the exopods of Dibasterium in Briggs et al.\n(2012), in particular the one depicted in fig. 1M, a 6th podomere could well be present\nbetween those labelled 4 and 6, corresponding to pod. 5 in Offacolus. Therefore, if\nfollowing the reviewer’s suggestion we count the setal tip as a separate podomere,\nwe find seven podomeres in Offacolidae, whereas Aria & Caron (2017) indicated that\nexopods in Habelia “are represented by five long and slender rami made of seven or\nmore podomeres”. Considering that “seven or more” may be seven, then matching\nthe number in Offacolidae, we deleted our statement l319–321 (“Other than the\ndistinctive brush- like podomeres, the number of podomeres is higher in Habelia\noptata (7 or more) than in Offacolidae.”), which had no impact on our conclusions.\nThen the problem, as I explain below, is that you interpret exopods in Fig. 2G, H when they\nseem in fact to be endopods.\nBased on the distinctive morphologies of the endopods and exopods in our fossil\n(which is visible in dozens of specimens), we are absolutely certain that at least the\nmain highlighted appendage branches are exopods. Their morphology perfectly\nmatches that described for Offacolus exopods. No change was made in response to\nthis comment.\nTraits that hold for Offacoliidae: elongate chelicera, reduced prosomal gnathobasipods,\nseventh prosomal appendage pair reduced to setose paddle exopod, bifurcate telson tip.\nWe agree with these traits. Following the previous answer to previous comments, we\npreserve the following diagnosis for the group: “Euchelicerates with elongate\nchelicerae, at least second to fifth prosomal appendage pairs being biramous and\nwith reduced gnathobasipods and composed of stenopodous exopods of six\npodomeres bearing a brush-like group of setae on the distalmost podomere. Seventh\npair of appendages (sixth post-cheliceral appendage pair) uniramous and lobate\npaddle-like, fringed by lateral spines. Tip of the telson bifurcate. Diagnosis modified\nfrom Sutton, et al. 2002.”\nFor Setapedites: See my comments below. You make a case for the raised tergum, and\nsegmental anatomy as well as the shape of cephalon (closer a priori to Dibasterium) should\nbe emphasized here. The shape of the 11th opisthosomal tergite indeed also counts in. It is\nunclear to me how you differentiate the short triangular pleurae from said sub-axial nodes.\nAs illustrated in Fig. 1 E-F, the sub-axial nodes are rounded and located at the\njunction between the bipartite tergites and the triangular pleurae. We have made the\n“saxn” label line longer to locate more clearly the sub-axial node in the drawing\nshown in Fig. 1F.\n> The interpretative drawings of Figure 1 are inaccurate—See my annotated pictures\nattached. There are three regular metasomal segments followed by a larger plate, as well as\npossibly intercalary elements between this plate and the telson (likely including an additional\nsegment and a telson head). Of the mesosomal segments, 6 are well exposed, while an\nanterior one is mostly covered by the occipital margin of the head shield. However,\nMGL102872 also shows an additional element of what seems to be a tergo-pleura anterior\nto that segment (thick arrow). This very sensibly can make the link between the habeliidan\nbody plan and the offacolid one, insofar as you seem to have 12 trunk segments preceded\nby a separate segment integrated into the cephalon, but apparently still expressed here.\nThis is in fact a solid and interesting find! On my image, db is doublure - it is nicely\npreserved, you should mark it.\nWe thank the reviewer for his enthusiastic re-interpretation of our fossil, which is in\nfact a dorsal view specimen. The feature that the reviewer identifies as a doublure\n(db) is instead the groove delimiting the so-called sunken region, which we describe\nclearly in our text and is a dorsal anatomical feature of offacolids homologous to the\nophthalmic ridge of other euchelicerates. It is true nevertheless that this feature\nshould be labelled in our Fig. 1 for clarity, which we have done in the revised version.\nOn the other hand, a doublure (which is a ventral and not a dorsal feature), is indeed\npresent in another ventral-view fossil, and we have added a label on Fig. 2B to\nhighlight it.\nRegarding the number of trunk segments (11 in our interpretation, but 12 for the\nreviewer), we strongly disagree with the interpretation of the reviewer. Our count of\nthe number of trunk segments in based on the observation of over 350 specimens in\ntotal. In the annotated pictures sent by the reviewer, we believe the count of 12 results\nfrom miscounting some of the segments or counting the anterior part of the telson as\na segment. We added new labels to Fig. 1 and 2 to show this better, but otherwise we\ndo not to change our interpretation of the number of trunk segments.\n> Overall, morphoanatomy seems close to Dibasterium, including the ventral side. It appears\nindeed that all mesosomal segments bear appendages, so 7 pairs. Here again, I believe\nyour interpretative drawings are inaccurate, or at least incomplete (see annotated image).\nGiven the disposition of segments, it seems very likely that the 7th appendage pair, borne by\na very reduced segment, is tucked mesially, as in Dibasterium. Alternatively, you could be\nright about this first large pair being the 7th prosomal pair, implying that the other pairs are\nattached a bit more anteriorly, thus leaving you with an anatomy comparable to Dibasterium\n(the last mesosomal segment lacking appendages). It also appears to me that the triangular\nshaped formed by the juxtaposition of reduced endopods mesially (as in limulids) is visible—\nyou drew one of them, but not the others…\nBased on our observation of hundreds of specimens, we favor the second hypothesis\nproposed by the reviewer, that our fossil is anatomically very similar to Dibasterium.\nRegarding the vestigial endopods, we indeed noticed the presence of what looks like\npotential endopods, but rather preferred not to over-interpret such small and faint\nanatomical features, which could also represent remains of the basipods. No change\nwas made associated with this comment.\n> In MGL102634 (Fig. G, H), I think that what you see are the endopods, not the exopods.\nThe podomeres are thick and the distalmost podomere looks like a stout claw, bordered by\nsetae on a small platform. This is similar to what is known is habeliidans. Most importantly,\nthese appendages are attached very mesially, close to the central axis—as is expected of\nendopods seen from ventral view.\nIn Fig. 3A, B, these indentations would correspond to the tougher masticatory margins of the\nbasipods, which tend to be more three-dimensionally preserved in compression fossils. I\nreckon you’re interpreting these as folded endopods, based on Offacolus and perhaps\nextant horseshoe crabs, but consider the differential size—to me, that doesn’t match. This is\nalso consistent with what you get from X-ray tomography, in Fig. 4B-E: These sclerotized\nwedges are the gnathobasipods. In Fig. 2I, J, you draw exopods and endopods sharing a\nbasis. Given the evidence, this seems a conjecture,\nespecially in the context of odd relationship between these rami. Your CT data, however, are\nbetter in that regard; I don’t see exopods attaching to a gnathobasipod, but it is fair to say\nthat the proximalmost part of the exopods is very close to that of the endopods.\nI’m attaching a collage to compare the limbs in MGL102634 with the exopod of Offacolus and the\nendopods of Habelia. Please carefully consider.\nWe do not agree with the suggestion of the reviewer that these are endopods, and we\nhave kept our interpretation of these structures being exopods. We understand that\nlooking only at Fig. 2G-H the appendages could appear as located ventrally and\nattached very mesially. However, when looking at the whole specimen (shown in\nSupplementary Fig. 2) there is no doubt that the specimen is actually preserved in\ndorsal view. We agree with the mesial position of the exopods, but do not see this as\nan argument for them to be endopods – as both endopods and exopods have their\nproximalmost parts very close to each other (as the reviewer agrees with based on\nour CT data). We maintain that these have a basipod to which they attach, and we\ninvite the reviewer to re-examine Figure 2E, F, Supplementary Figure 2G and Figure\n4J, where we have additionally labelled the structures we consider to be the basipods.\nRegarding the anatomy of the appendages, we find more similarities between the\nexopod of Offacolus and Setapedites, than to the endopods of Habelia. We draw the\nreviewers attention to Supplementary Figure 3G,H, which show the complete anatomy\nof the appendage in a new higher resolution photo provided after the first round of\nreview. The podomeres of Habelia endopods are elongated and flare outwards distally\nin a pronounced manner, as shown in Aria & Caron 2019 figures 2D, G, I, M, whereas\nthe podomeres of both Offacolus exopods (Sutton et al 2002, figure 4B) and\nSetapedites exopods are stouter and with a square shape. Also Setapedites does not\nshow a stout claw bordered by setae on a small platform, but rather is a series of\nradially arranged longer spines, as is now described in the new text we added\ncomparing the exopod setal brushes with Mollisonia and Habelia. The exopod of\nOffacolus also possesses a pair of thicker spines on the inner margin of the\npreterminal podomere, which is not seen in Habelia endopods (which have a platform\nof small spines) – Setapedites also bears this pair of thicker spines on the preterminal\npodomere (labelled “st” in Supplementary Figure 3H). We made a comparison collage\nand include it below. For these reasons, we keep our interpretation of these\nappendages as exopods in Setapedites.\n> I am sorry to say, but calling the chelicerae in Mollisonia “poorly-preserved” while taking for granted\nall the fossil evidence provided in this very paper is unfair to me. As a matter of fact, the outline of\nchelicerae, including chelate elements in dorsal view, is better preserved in Mollisonia than in the fossil\ndescribed here. Budd’s arguments were responded to in Aria 2022, Biological Reviews. If you still think\nthese elements could be something like non-appendicular protocerebral protrusions, say so, but at\nleast give an argument.\nOur scepticism regarding the state of preservation of the chelicera in Mollisonia (lines\n260–263) has been removed, and the sentence modified following a previous\ncomment of the reviewer (see above). The new sentence is more factual and devoid of\nscepticism: “Among Cambrian euarthropods, Mollisonia plenovenatrix has been described\nas possessing a pair of short chelicerae and proto-book gills composed of overlapping\nexopod flaps, and retrieved as a basal chelicerate (Aria & Caron 2019).” It is followed by a\nsentence describing the reasons put forward against this interpretation as currently\nfound in the literature : “Counter arguments put forward in the literature against Mollisonia\nbeing a basal chelicerate cite the poor preservation of these features and functional\nconsiderations (chelicerae too small and far from the mouth) 9, the organization of the central\nnervous system in Mollisonia symmetrica 7 (but see 35), as well as the origins and\ndevelopment of the last three segments complex (pygidium) in the genus Thelxiope and\nMollisonia 8.”\nRegarding the evidence for chelicera in our fossil, it must be noted that this is\ndefinitely much less controversial than the presence (or not) of chelicera in\nMollisonia. Indeed, the latter is the first and only description of this critical feature in a\nCambrian organism, potentially changing our view of the origin of the chelicerate\nbauplan. Conversely, the presence of chelicera in a synziphosurine, which is\nanatomically extremely similar to Dibasterium and Offacolus, which preserve\nchelicera in details, is expected. We argue that if extraordinary claims require\nextraordinary evidence (i.e. for the presence of chelicera in Mollisonia) ordinary\nclaims require ordinary evidence, which we provide in Fig. 2C-D and Suppl. Fig. 3A-B\n(and consistently for two different specimens).\nRegarding the pygidium, I am not sure what the argument is exactly, but Mollisonia’s\npygidium is four-segmented (not three-segmented as written here), as shown by the number of appendage pairs and\nstructure (Aria and Caron 2019, Nature, Figs. 1d,e, 2i, ED Fig. 4). It is exactly the same as in any\nother arthropod with a pygidium…\nWe acknowledge that the reviewer might well be correct here, but referred in our\nmanuscript to the work by Lerosey-Aubril et al. (2020) who expressed a different\nopinion. As this issue is absolutely peripheral to our work and we mention the\nsegmentation condition of the pygidium in Mollisonia while referring to Lerosey-\nAubril et al. (2020) we made no change to our manuscript.\n> Back to book gills and Euchelicerata… First of all, Euchelicerata is taxonomically\ndiagnosed by the presence of opercula, so you cannot write that your fossil would demonstrate this is not the case—this\nmakes no sense. This is not Aria and Caron (2019) who suggested it, it was formally defined by\nWeygoldt and Paulus (1979: Untersuchungen zur Morphologie, Taxonomie und Phylogenie der\nChelicerata. Z. Zool. Syst. Evol-forsch. 17 (85-115), 177-200.). This is a rule of systematics that you\nhave to follow.\nNow, if you were to find that gill opercula arose convergently, what you could say is that\nEuchelicerata, as currently diagnosed, is polyphyletic (I already specified that in my first\nreview…). You may also emend the diagnosis to add or replace diagnostic traits. However,\nand again, you need to be more circumspect here, and admit that your evidence is not good\nenough to be unambiguous. In MGL107741, those well-defined paddle-shaped elements\nappressed onto the ventral side could very well be opercula—which is simply defined as a\nsemi-rigid lobate sheet covering lamellae. What you identify as a setose margin is quite\ntenuous, and would not in fact rule out an operculum in itself.\nWe agree with the reviewer that considering the state of preservation of these\nfeatures, we cannot unambiguously confirm they are not opercula. We have\naccordingly tempered and limited our description and discussion of opisthosomal\nappendages to their presence and morphology. This is our modified text: “The more\nposterior opisthosomal appendages in Dibasterium durgae and Offacolus kingi are\nmodified into gill opercula, but this condition cannot be clearly stated for Setapedites\nabundantis (Fig. 3 and 6, Supplementary Fig. 1G and H), which exhibits a similar\nproximal portion of the opisthosomal appendages to that of Habelia optata (Fig. 3\nand 6).”\nNevertheless, and again I am repeating what I wrote in my first review, even if you coded with\ncertainty the absence of operculum in your species, based on parsimony this optimizes as homoplasy!\nI tried to show that using your Lamsdell dataset and tree result… only to find out that the operculum is\nnot even present in this matrix. How could you test anything related to the evolution of the operculum\nif you don’t even code it in your dataset? So we used the other dataset, from Aria and Caron 2019, and looked at the optimization of opercula for both parsimony and likelihood on the Bayesian result. See attached figure. Likelihood optimization is extremely strict and does not achieve more than ambiguity, but a parsimony-based optimization clearly places the presence of opercula at the Euchelicerata node as retrieved previously in Aria and Caron 2017, 2019, with the ambiguity in Setapedites being a reversal.\nNevertheless, and again I am repeating what I wrote in my first review, even if you\ncoded with certainty the absence of operculum in your species, based on parsimony\nthis optimizes as homoplasy!\nAs stated above, we have now reduced any statement regarding this character to the\nmaximum that can unambiguously been said about it.\nRegarding the phylogenetics matrices used, we indicate (and justify) clearly in our\nwork that we used a dual phylogenetic approach:\n- Firstly we assessed the position of our fossil within Arthropoda using Aria and\nCaron (2019)’s matrix, which as indicated here by the reviewer is well-adapted to look\nat the optimization of the opercula\n- Secondly, we used Lamsdell (2013)’s matrix to resolve relationships within\nChelicerata. And logically, as noticed by the reviewer, this matrix does not contain\nany character about the opercula as it will be of poor phylogenetic information in this\nmatrix. We never intended to use this matrix to constrain the evolution of the\nopercula, which we had already done by using the Aria & Caron (2019) matrix.\nThat said, we warmly thank the reviewer for running analyses on the optimization of\nthe opercula, and confirming the presence of opercula at the Euchelicerata node.\nSimilar story for the peculiar exopod attachment condition. This most parsimoniously\noptimizes as ancestral for panchelicerates—sea spiders notwithstanding—despite you\nrecoding uncertainty in Mollisonia. This evolutionary pattern was put forward as the\nillustration of the possible mechanism for the loss of the outer branch in the euchelicerate\nprosoma in Aria and Caron 2017, 2019, an idea that you misleadingly claim to be new here.\nThis has actually been first recognized by Briggs et al (2012), well before Aria and\nCaron (2017, 2019). We did not claim it to be a new idea, but just stated that our\nfindings support this condition. To make this clear, we added the citations to the end\nof every sentence discussing this topic.\nAdditionally, if Mollisonia’s trunk appendages are not multi-lamellate structures akin to book\ngills, what are they? I am defender of a good science based on open refutation, but\nrefutation should be explicit, fact-based.\nIt is true that due to the controversial opinions regarding the chelicerate affinities of\nMollisonia we do not mention its reported book gills in our discussion, but nowhere in\nthe text we express any scepticism regarding the interpretation of book gills in\nMollisonia, which we consider to be out of the scope of this work.\n> There are issues with the use of “stem” and “crown” terminology throughout, which I think\nhas more to do in fact with the taxonomic issue of what Euchelicerata means as explained\nabove. First, in your Fig. 6, Synziphosurines are not part of the crown, since they left no\nextant descendants. Second, as I demonstrate above, likelihood and parsimony\noptimization still places Euchelicerata at the node joining up offacolids and prosomapods.\nWe agree. In our modified Fig.6 synziphosurines are now clearly indicated as stem\neuchelicerates and not crown euchelicerates, and Euchelicerata is clearly placed at\njoining up offacolids and prosomapods.\nThird, considering that you’re leaving Chelicerata out of the picture, you need to include\nearly panchelicerates Mollisonia and Habelia into the stem of euchelicerates.\nWe agree. We modified Fig. 6 to differentiate lower and upper stem euchelicerates,\nand removed the non-Euchelicerate from the legend of supplementary Fig. 5.\nIn summary:\n- You need to be more explicit about your rejection of the evidence in Mollisonia.\n- The evidence is not clear enough to ascertain the absence of opercula. This should be left\nmore open.\n- If you actually optimize your character on your tree (which I suggested you to do in the first\nround but you did not), you find that opercula are present in the common ancestor of\nEuchelicerata sensus Wygoldt and Paulus and as retrieved by Aria and Caron 2019. This\ncontradicts your statements.\n- Same for the prosomal exopod condition: Most parsimoniously this is present in the\ncommon ancestor of panchelicerates, again as stated in Aria and Caron 2019.\n- Improve your interpretative drawings and reconsider segmental anatomy, as well as\nprosomal gnathobasipod and endopod identification.\nAll these points have been addressed above.\nI refrained from examining the MS in finer details – language, references, etc. The authors\nmust first address these remaining salient problems, some of which being carried over from\nthe first round of reviews. Should they do so, I expect that only one additional round will be\nnecessary for said details. I want to make clear that I am not trying to impose my views, and\nI would also rather not spend a week on a review, but I am trying my best to show the\nauthors that the shortcomings I point out are not simply a matter of opinion—as they should\nnot be, on either side.\nAs stated in our cover letter, we believe that we have substantially revised our\nmanuscript according to most of the reviewer’s new comments, notably by giving up\non our interpretation of the opisthosomal appendages. However, unlike what the\nreviewer is claiming here, his suggested changes in several places of this review\namount to forcing us to accept his own views. We cannot compromise our own\nscientific views of this highly debated topic by agreeing to every change he has\nsuggested, but we have tried to find a middle ground and temper our interpretations\nby serious and careful consideration of his comments. We thank Cedric for investing\nso much of his time into these in-depth reviews of our work.\nOther specific comments:\nl.38: Offacolidae\nAmended.\nl.51: The statement that it is “obscure” washes away the work on both the Herefordshire and\nthe Burgess Shale. It remains poorly documented, or something equivalent.\nAmended.\nl.51: Regarding the definition of Euchelicerata here and hereafter, see my comment and\ncorrect.\nAmended.\nl.56: You refer to phylogenetic results that are 10 years old—and in the case of Haug et al.\n2012, this is not even a cladistic study. Why not acknowledge that these hypotheses have\nnot been recovered again? What recent phylogenetic study has disputed that Habeliida is\nthe closest sister group to Chelicerata? (Mollisonia notwithstanding). Even if you reject the\nevidence provided by Mollisonia, Panchelicerata as defined by Aria and Caron (2019)\nremains valid, yet there is no mention of it here. This grouping was recovered in most recent\ncladistic work by Zeng et al., Moysiuk and Caron, etc.\nIn this section of the introduction we give an overview of the current hypotheses (and\ndivergent opinions) on the origin of chelicerates, and as such we find it totally\nappropriate to mention views arising from different teams and lines of evidence (even\nif they are 10 years old). Moreover, the latter mentioned studies by Zeng et al. and\nMoysiuk & Caron all based their analyses on the same matrix (Aria and Caron 2019) to\nassess the position of stem arthropods with no impact on Chelicerata, and as such\nhave recovered similar groupings, but cannot be considered as independent\nconfirming lines of evidence.\nWe agree with the use of Panchelicerata and we have now used this term in the\ncaption to Figure 6 and text we added in the presentation of phylogenetic results that\nreads: “We then performed a second analysis focusing on Panchelicerata\n(=Chelicerata+Habeliida), using the matrix from Lamsdell (2013) that includes numerous\nsynziphosurines and euchelicerates, and to which we add Habelia and Mollisonia.”\nl.82: The anal pouch is completely speculative. Without redundancy of evidence, I don’t see\nhow this can be differentiated from an artefact. Consider it putative if you want, but just in the\nfigure caption; no need to dwell on that.\nIn the caption for Fig. 2 (l649) we previously indicated “and the anal pouch in purple”,\nwhich we have now modified into “and the possible anal pouch or post-ventral\nstructure (pvs) in purple” to make it clearer that the interpretation of this feature is not\nunambiguous.\nAll mentions of this feature in the text are very tempered:\n- l82-83, “and possibly an anal pouch”\n- l226–229, “A small symmetrical rounded process (pretelsonic process) is often\npreserved under tergite XI, in some cases appearing longitudinally bisected and ovoid\nin shape (ptp in Fig. 2A and 2B, Fig. 229 3A and 3B, Supplementary Fig. 1A–F,\nSupplementary Fig. 3E and 3F)”.\nAnd as such we already meet the request of the reviewer.\nConsidering that, according to Aria and Caron (2017), Habelia possesses an anal\npouch of similar morphology, we believe that it is worth keeping our (short)\ndiscussion about this feature in Supplementary information.\nl.83: “Setapedites abundantis resolves the position of several problematic stem-\neuchelicerate taxa”: What do you mean? Setapedites has no phylogenetic impact on the\nplacement of early panchelicerates or other offacolids based on recent phylogenetic results.\nSys. Pal.: See main comments.\nWe agree this was unclear. We have changed the sentence to: “Setapedites\nabundantis documents how euchelicerate uniramous prosomal appendages were\nderived from the appendages of a habeliid ancestor 12 and illuminates the evolution of\nearly tagmosis in euchelicerates.”\nl.285: This makes no sense. It’s not because frogs have anterior members homologous with\nbirds that it would play a role in supporting their bird affinity! Also, the statement above is\ncontradictory with what you say elsewhere about gill opercula not being apomorphic of\neuchelicerates.\nWe agree that this was not clear. This sentence has been entirely removed. Based on\nother comments from this reviewer, we no longer discuss the gills in the manuscript.\nl.290: Well, no, they are shared with Sanctacaris, and the enlarged gnathobasipods also\nshared with Mollisonia. Or do you also contest the presence of large gnathobasipods in\nMollisonia?\nWe do not contest the large gnathobasipods in Mollisonia. We rephrased our\nsentence as follows to make it clearer: from “possesses its own derived character not\nshared with Offacolidae” to “possesses characters not shared with Offacolidae”.\nl.301: Not sure what kind of bilobation you mean in Habelia.\nWe refer to the fact that is made of two elements “paddle connected to an attachment\npodomere”. We have made this clearer by modifying the sentence as follows:\n“appendages of somite VII made of bipartite paddle-like exopods”.\nl.327: No, it does not; quite the opposite in fact. If you posit the clear attachment to a\nbasipod in Setapedites, which I contest, it is another autapomorphy, and a reversal, (like the\nputative absence of opercula) amid the condition shared by offacolids, habeliids and\nmollisoniids.\nAria and Caron (2019) indicated that the “frontal appendages in Sanctacaris and\nHabelia are in fact quite distinct morphologically from the exopods of Offacolus and\nDibasterium, with long and slender podomeres, giving them indeed a more antennular\naspect. Similar to at least Dibasterium (but also likely Offacolus), however, these rami\nin Habelia are preserved separately from the endopod bundle in non dorso-ventrally-\npreserved specimens, suggesting that they do not attach to the limb basis like regular\nexopods”.\nThis suggestion has, however, not been unequivocally supported by any fossil\nevidence. In turn, our fossil shows real evidence for a common basis (if not a true\nbasipod as contested by the reviewer), while this has only been suggested for\nHabelia. Nevertheless, we slightly modified our sentence as follows to temper the\nidentification of a true basipod: “The evidence for the attachment of prosomal\nbiramous limbs onto a common basipodite (or at least a common basis) in\nSetapedites abundantis […]”.\nl.337: The seventh cephalic pair of early panchelicerates has already been argued to be\nancestral to these features in Aria and Caron 2017, 2019. This was one of the main findings\nin these papers…\nAcknowledged. Citations added.\nPhylogenetic figures: Are you sure you are displaying Maximum clade credibility\nconsensuses? The default output from MrBayes is extended majority rule, and the topology\ncoming out of my basic run is the same as yours.\nAmended. Thanks for pointing this out. We changed the text to indicate we are using\nmajority rule.\nAdditionally, happening to be, I think, the “someone like David Legg, Greg Edgecombe,\nJason Dunlop, James Lamsdell or such”, I was asked to check whether the authors had fully\naddressed the comments of reviewer #2, Jakob Vinther. I can’t say about the bottle of\n“Swizz” white wine, but it seems the authors took care of providing a better rendition of the\nraw CT data, as well as adding another specimen—which is, I think, what Dr. Vinther\nsuggested. Clarification was also provided regarding the material used, and labelling in\nFigure 2 was corrected.\nWe thank the reviewer for taking the time to confirm that our revised manuscript\nsatisfied reviewer #2 (Jakob Vinther)’s comments and suggestions.\n\nReviewers' Comments:\nReviewer #1:\nRemarks to the Author:\nI am glad to have received this revised manuscript and that the authors have further considered my\ncomments. While the authors have made interesting points regarding their morpho-anatomical\ninterpretations, and several sections of this work are really excellent, I identified a few strong\nremaining issues. The problems (one in particular) with the phylogenetic optimization of characters\nare non-interpretative, and should really be resolved before publication.\nI therefore recommend “minor” revisions but these are minor only in number; some of them are still\nmajor scientifically speaking. I am attaching an annotated Word file for a number of comments made\ndirectly on the manuscript.\nI will otherwise not engage in peripheral discussions about what the “majority of the community”\nsupposedly represents or thinks, or debates related to my own work. Note however that the\nchelicerate affinity of Mollisonia was accepted by experienced figures in our field such as Edgecombe\n(2020 Ann Rev) and that no colleague has so far proposed to reexamine the published material.\nAnatomy:\n- Obviously, the endopod / exopod interpretation is a challenging issue. The authors have made their\npoints with solid arguments and this can serve as ground for further discussions. Nevertheless, 1) I\nthink it would be nice to report some of what you wrote in your response to the main text, as this can\nbe of interest for other colleagues, and 2) you should explain the inconsistency in length between\nwhat you label as exopods in 2H and then in 3B and 4I, J. The short and stout rami you interpret as\nexopods in MGL.102634 are somehow jutting out extensively beyond the cephalic margin in some\nother specimens…\n- Regarding the body segment counts, it is fine that you disagree with my reinterpretations, but you\nare still leaving these areas unexplained. For the element anterior to the telson especially, please\nexplain why you think this is part of the telson yet remains physically separated from it in three of\nyour main specimens.\nPhylogeny and evolutionary implications:\nThere is still no optimization of characters on any of the trees. I suppose the diagrams in Fig. 6\npalliate this somewhat, but, as shown hereafter, this continues to hamper a correct assessment (for\nauthors and readers) of the evolution of characters.\n- About the seventh prosomal appendage... Your paragraph is extremely confusing and doesn’t hold\nphylogenetically, contrary to what you claim. First of all, both Dibasterium and Offacolus have been\ndescribed with a seventh prosomal pair of appendages, yet your diagram in Fig. 6 represents\nDibasterium’s 7th pair as part of the opisthosoma. Although in Briggs et al. 2012 PNAS the ventral-dorsal alignment was\ndiscussed, this should be put in the broader perspective of the chilaria and the 7th pair of Offacolus,\nalongside the synziphosurine microtergite. You also seem to be forgetting about\nWeinbergina. In all your topologies, the plesiomorphic 7th prosomal pair continues to be both the most\nparsimonious and likely scenario. Like I’ve suggested during 2 rounds of review, just open\nMesquite if you don’t want to go through the trouble of reconstructing ancestral states with R…\n- Illustrating the origin of uniramy in euchelicerates… Setapedites fuels discussions about uniramy in\nother offacolids as well as the question of prosomal limb attachment. In itself, however, I don’t\nsee how it illustrates the origin of uniramous appendages in all euchelicerates, as stated in your MS.\nYou should also comb through the bibliography as I’ve seen a number of issues (e.g., date missing,\nextra characters, R Team entry…)\nCédric Aria\nReviewer #4:\nRemarks to the Author:\nThe manuscript describes a new genus and species of synziphosurine chelicerate, named Setapedites\nabundantis, from the Lower Ordovician Fezouata Formation of Morocco and explores its phylogenetic\nrelationships and implications for the early evolution of chelicerates. The manuscript comprises two\nmain components; the description of the fossil material, and the phylogenetic analyses.\nThe available fossil material supports their interpretations, which are in line with my own thoughts\nabout the material. Particularly of interest are the structure of the prosomal appendages - the\nelongated chelicera and biramous appendages II-V are particularly important, as this is the first time\nthey have been unequivocally documented in a taxon outside of the Herefordshire Lagerstatte. I do\nthink it would be worthwhile drawing particular attention to the form of the chelicerae given some of\nthe previous suggestions regarding the composition of the appendages (see the list of suggestions\nbelow for more detail).\nThe phylogenies are well-executed, although I do have some concerns about the Lamsdell 2013 matrix\n- specifically, by rooting the analysis on Yohoia the authors essentially constrain the clade comprising\nEmeraldella, Sidneyia, and Olenoides to be the sister group of chelicerates and force megacheirans to\nbe paraphyletic. These are obviously all hypotheses that have been suggested at one time or another,\nalthough I do think that a trilobitomorph affinity for chelicerates is not particularly well supported in\nthe current literature and it may be best to remove those three taxa from the analysis as they may be\nexerting an undue influence on character polarity. The authors should also remove Willwerathia from\nthe analysis, as the taxon was recently excluded from Chelicerata (see Lamsdell\n2020;https://doi.org/10.1007/s12542-019-00493-8).\nFinally, in the comparison of Setapedites to other taxa, the authors may want to note that the telson\nof Bunodes lacks a birfurcate termination and is quite unlike that of offacolids. The authors should\nprobably also discuss Sarotrocercus, which Legg (2014; 10.1007/s00114-014-1245-4) retrieved as\nthe sister taxon to Offacolus and Jago et al. (2016; 10.1111/pala.12243) considered to be a\nchelicerate. While I do not find the chelicerate affinities of Satrocercus particularly compelling, the fact\nthat it has implicitly been considered an offacolid in the past means it should be discussed at least in\npassing.\nOverall this is a good manuscript, and I am glad to see the Fezouata synziphosurine finally get a\nformal description. The figures are clear and the photographs excellent given the very small size of the\nmaterial. I just have a handful of suggestions that I hope the authors will take into account, which can\nbe summarized as:\n1. Make specific note of the fact that the elongated chelicera are composed of a few elongated articles\nand are not in fact antenna-like, which has been suggested by previous authors (e.g. Legg 2014).\n2. Remove Willwerathia, Olenoides, Emeraldella, and Sidneyia from the phylogenetic analysis building\nupon the Lamsdell 2013 matrix.\n3. Note the form of the telson of Bunodes.\n4. Discuss the affinities of Satrocercus given its previously suggested relationship to Offacolus.\nPoint-by-point responses to the Referees’ comments and suggestions.\nWe present the referees' comments in normal typeface and our responses in blue.\nReviewer #1 (Remarks to the Author):\nI am glad to have received this revised manuscript and that the authors have further considered my\ncomments. While the authors have made interesting points regarding their morpho-anatomical\ninterpretations, and several sections of this work are really excellent, I identified a few strong\nremaining issues. The problems (one in particular) with the phylogenetic optimization of characters\nare non-interpretative, and should really be resolved before publication.\nI therefore recommend “minor” revisions but these are minor only in number; some of them are still\nmajor scientifically speaking. I am attaching an annotated Word file for a number of comments made\ndirectly on the manuscript.\nI will otherwise not engage in peripheral discussions about what the “majority of the community”\nsupposedly represents or thinks, or debates related to my own work. Note however that the\nchelicerate affinity of Mollisonia was accepted by experienced figures in our field such as Edgecombe\n(2020 Ann Rev) and that no colleague has so far proposed to reexamine the published material.\nWe are happy that our new reviewer evaluated our manuscript positively. Accordingly, with the suggestion\nreceived, we have now incorporated a specific note of the fact that the elongated chelicera are composed\nof a few elongated articles in Setapedites, we have provided new phylogenetic analyses and modified old on\naccording with suggestions, discussed the telson of Bunodes and finally discussed Satrocercus.\n1. Make specific note of the fact that the elongated chelicera are composed of a few elongated articles\nand are not in fact antenna-like, which has been suggested by previous authors (e.g. Legg 2014).\nWe have now included a statement on the anatomy of the chelicera:” The first pair of uniramous\nappendages in Setapedites abundantis, although rarely preserved, appear to be composed of few elongated\narticles, confirming their elongated chelicera interpretation in other offacolids and discharging the antenna-\nlike hypothesis (Fig. 2C-D).”\n2. Remove Willwerathia, Olenoides, Emeraldella, and Sidneyia from the phylogenetic analysis building\nupon the Lamsdell 2013 matrix.\nWillwerathia has been removed from all phylogenies. Regarding the outgroups, we agree that using Yohoia\nas an outgroup creates this problem and we were advised in a previous round of review to remove\nFuxianhuia protensa as outgroup. Our original intent was to leave the outgroups unchanged. We have\nincluded a new set of phylogenetic analyses in the supplementary material to show our result of a\nphylogenetic analysis without the Artiopoda as suggested by the reviewer. However, we did not use this as\nthe main tree in our paper. This is because to remove from our matrix 5 out of 40 taxa, of which 4 are\noutgroups will have a negative effect on the stability of the analyses (SFig.14). So even though we agree\nwith the reviewer, we find that leaving only the megacheirans as an outgroup of the chelicerates is not\nenough to polarize the analyses correctly, and we proposed a tree without artiopodans in the\nsupplementary material (SFig. 12-15) to show that this has no major effects on our conclusions.\n3. Note the form of the telson of Bunodes.\nWe have now included in the supplementary discussion on the other Synziphosurines this statement to\nunderline the diverse telson morphology of offacolids and Bunodes: “However, it is differentiated by the\npresence of a reduced first tergite (microtergite) and the total number of tergites and a thicker, more\nconical telson devoid of bifurcation.”\n4. Discuss the affinities of Satrocercus given its previously suggested relationship to Offacolus.\nWe have also implemented in the same section of the supplementary discussions a comparison with\nSarotrocercus: ” Finally, we consider Sarotrocercus oblitus, which has been recovered in some phylogenies as\npossible a chelicerate 8,25. According to its most recent redescription26, even though this taxon resembles the\nOffacolidae anatomy dorsally, especially Offacolus kingi, its ventral anatomy differs from this group in many\naspects such us: a lower head tagmatization, reduced exopods, and absence of a division of the trunk into\npre-abdomen and abdomen. However, the trunk exopods of Sarotrocercus oblitus may represent an early\nevolutionary step towards the opercula with the possible loss or strong reduction of the endopods.\nConfirmation of its chelicerates affinities is subject to further study.”\nReviewers' Comments:\nReviewer #4:\nRemarks to the Author:\nThe manuscript is an important contribution to our understanding of early chelicerate evolution. The\nrevisions address all of the comments from my previous review, and I have no further suggested\nrevisions or additions."}]

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