A new Devonian euthycarcinoid reveals the use of different respiratory strategies during the marine-to-terrestrial transition in the myriapod lineage

Myriapods were, together with arachnids, the earliest animals to occupy terrestrial ecosystems, by at least the Silurian. The origin of myriapods and their land colonization have long remained puzzling until euthycarcinoids, an extinct group of aquatic arthropods considered amphibious, were shown to be stem-group myriapods, extending the lineage to the Cambrian and evidencing a marine-to-terrestrial transition. Although possible respiratory structures comparable to the air-breathing tracheal system of myriapods are visible in several euthycarcinoids, little is known about the mechanism by which they respired. Here, we describe a new euthycarcinoid from Upper Devonian alluvio-lagoonal deposits of Belgium. Synchrotron-based elemental X-ray analyses were used to extract all available information from the only known specimen. Sulfur X-ray fluorescence (XRF) mapping and spectroscopy unveil sulfate evaporation stains, spread over the entire slab, suggestive of a very shallow-water to the terrestrial environment prior to burial consistent with an amphibious lifestyle. Trace metal XRF mapping reveals a pair of ventral spherical cavities or chambers on the second post-abdominal segment that do not compare to any known feature in aquatic arthropods, but might well play a part in air-breathing. Our data provide additional support for amphibious lifestyle in euthycarcinoids and show that different respiratory strategies were used during the marine-to-terrestrial transition in the myriapod lineage.

Lines 126-127 and 286-287: The "tube-like structures" in euthycarcinoids are not precisely associated with the paired sternal pores. Rather than say the tube-like structures resemble the tracheal respiratory system in "myriapods" (implicitly Myriapoda as a whole) it would be better to nuance this. The tube-like structures of euthycarcinoids resemble the sternal apodemes of Progoneata (there is a great drawing of them in each of the three progoneate groups in Dohle (1965, "Über die Stellung der Diplopoden im System", Zool. Anz., Suppl. 28), but they are not known in Chilopoda. In Symphyla they are attachments sites for extrinsic leg muscles and longitudinal muscles and do not have an association with the respiratory system (as the spiracles are only on the head). It is only within Dignatha that they are apodemes forming the "tracheal pouch" (in millipedes and hexamerocerate pauropods; see Dohle 1997, fig. 23.4, "trt", in the Fortey and Thomas book).
Line 294: Reference 44 is consistent with two origins of the tracheal system in Chilopoda but not reference 45.
Line 294: "Symphylan" rather than "symphilan", and delete "pseudocentipede", as it is a misnomer and is rarely used. The term "garden centipede" sometimes gets used for the agricultural pests within Symphyla, but no need to perpetuate "centipede" references for this group.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
This manuscript describes structures interpreted as adaptations for air breathing in a poorly preserved, poorly understood group of arthropods -euthycarcinoids. The novel aspect of the study is that the structures are delineated by geochemical imaging, particularly mapping of trace elements (As and Mn are shown in Fig. 2). These are compared to air breathing structures in other groups of arthropods.
The method assumes that the distribution of trace elements is controlled by and reflects the original anatomical structures of the organism. The diagenetic processes and history are not discussed at all. The authors should provide evidence that the structures that they are attributing to air breathing are not the nonbigenic artifacts of some diagenetic process. This is not the journal for a systematic description of a new genus. The section on systematics should be expanded and published in a journal focused on systematics. Removing systematics from the manuscript would allow space for better explanation of the processes (and products) of preservation.
The fact that the paper glues together --rather than fully integrates --euthycarcinoid systematics, arthropod anatomy, taxonomy, and history, and cutting-edge geochemical methods suggests that the authors have not identified a target audience on which to focus. The extensive and detailed discussion of air-breathing adaptations in different groups of arthropods is sufficiently crammed with anatomical and taxonomic jargon that its meaning and importance would elude all but arthropod specialists. Similarly, the geochemical methods and supplemental data would be opaque to non-geochemists.
The paper needs a thorough rewriting and reorganization to explain and highlight its contributions and significance to scientists with diverse perspectives.
If the following questions could be answered clearly, the paper would be excellent.
--How and when in the taphonomic/diagenetic history were the trace elements incorporated? --How does the diagenetic history relate to the (barely discussed) paleoenvironmental history? Significance of limonite?, sulfate? --What do the structures suggest about air-breathing by eucycarcinoids in the Devonian? (simple diagram would help) --How does this discovery about the group fit in with the environmental distributions (marine, fluvial, lacustrine, semi-aquatic, terrestrial) of euthycarcinoids later in their history, such as recorded in the Permian of Antarctica? --How do these adaptations -explained without jargon or illustrated with simple diagramsrelate to air breathing adaptations in other groups of arthropods, and what does this imply for the history of arthropods?
Decision letter (RSOS-201037.R0) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Dr Gueriau,
The editors assigned to your paper ("A new Devonian euthycarcinoid reveals the use of different respiratory strategies during the marine-to-terrestrial transition in the myriapod lineage") have now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Associate Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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• Authors' contributions All submissions, other than those with a single author, must include an Authors' Contributions section which individually lists the specific contribution of each author. The list of Authors should meet all of the following criteria; 1) substantial contributions to conception and design, or acquisition of data, or analysis and interpretation of data; 2) drafting the article or revising it critically for important intellectual content; and 3) final approval of the version to be published.
All contributors who do not meet all of these criteria should be included in the acknowledgements.
We suggest the following format: AB carried out the molecular lab work, participated in data analysis, carried out sequence alignments, participated in the design of the study and drafted the manuscript; CD carried out the statistical analyses; EF collected field data; GH conceived of the study, designed the study, coordinated the study and helped draft the manuscript. All authors gave final approval for publication.
• Acknowledgements Please acknowledge anyone who contributed to the study but did not meet the authorship criteria.
• Funding statement Please list the source of funding for each author.  (reference 54) said that part of the scorpion book lung (the operculum) is of telopodal origin and contradicted homology with epipodites. This does not contradict a book lung / book gill homology unless one subscribes to book gills as epipodites (which has, admittedly, been endorsed by some morphologists, e.g., Suzuki and Bergström 2008).
Line 84: Springtails are said to respire by cuticular diffusion but this is the case only in some members of the group (poduromorphs and entomobryomorphs). Symphypleona have a pair of spiracles.
Lines 90-92: Pneumodesmus newmani has been redated as Early Devonian rather than Silurian Lines 126-127 and 286-287: The "tube-like structures" in euthycarcinoids are not precisely associated with the paired sternal pores. Rather than say the tube-like structures resemble the tracheal respiratory system in "myriapods" (implicitly Myriapoda as a whole) it would be better to nuance this. The tube-like structures of euthycarcinoids resemble the sternal apodemes of Progoneata (there is a great drawing of them in each of the three progoneate groups in Dohle (1965, "Über die Stellung der Diplopoden im System", Zool. Anz., Suppl. 28), but they are not known in Chilopoda. In Symphyla they are attachments sites for extrinsic leg muscles and longitudinal muscles and do not have an association with the respiratory system (as the spiracles are only on the head). It is only within Dignatha that they are apodemes forming the "tracheal pouch" (in millipedes and hexamerocerate pauropods; see Dohle 1997, fig. 23.4, "trt", in the Fortey and Thomas book).
Line 294: Reference 44 is consistent with two origins of the tracheal system in Chilopoda but not reference 45.
Line 294: "Symphylan" rather than "symphilan", and delete "pseudocentipede", as it is a misnomer and is rarely used. The term "garden centipede" sometimes gets used for the agricultural pests within Symphyla, but no need to perpetuate "centipede" references for this group.

Reviewer: 2 Comments to the Author(s)
This manuscript describes structures interpreted as adaptations for air breathing in a poorly preserved, poorly understood group of arthropods -euthycarcinoids. The novel aspect of the study is that the structures are delineated by geochemical imaging, particularly mapping of trace elements (As and Mn are shown in Fig. 2). These are compared to air breathing structures in other groups of arthropods.
The method assumes that the distribution of trace elements is controlled by and reflects the original anatomical structures of the organism. The diagenetic processes and history are not discussed at all. The authors should provide evidence that the structures that they are attributing to air breathing are not the nonbigenic artifacts of some diagenetic process. This is not the journal for a systematic description of a new genus. The section on systematics should be expanded and published in a journal focused on systematics. Removing systematics from the manuscript would allow space for better explanation of the processes (and products) of preservation.
The fact that the paper glues together --rather than fully integrates --euthycarcinoid systematics, arthropod anatomy, taxonomy, and history, and cutting-edge geochemical methods suggests that the authors have not identified a target audience on which to focus. The extensive and detailed discussion of air-breathing adaptations in different groups of arthropods is sufficiently crammed with anatomical and taxonomic jargon that its meaning and importance would elude all but arthropod specialists. Similarly, the geochemical methods and supplemental data would be opaque to non-geochemists.
The paper needs a thorough rewriting and reorganization to explain and highlight its contributions and significance to scientists with diverse perspectives.
If the following questions could be answered clearly, the paper would be excellent.

Decision letter (RSOS-201037.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Dr Gueriau,
It is a pleasure to accept your manuscript entitled "A new Devonian euthycarcinoid reveals the use of different respiratory strategies during the marine-to-terrestrial transition in the myriapod lineage" in its current form for publication in Royal Society Open Science.
You can expect to receive a proof of your article in the near future. Please contact the editorial office (openscience_proofs@royalsociety.org) and the production office (openscience@royalsociety.org) to let us know if you are likely to be away from e-mail contact --if you are going to be away, please nominate a co-author (if available) to manage the proofing process, and ensure they are copied into your email to the journal. Due to rapid publication and an extremely tight schedule, if comments are not received, your paper may experience a delay in publication.
Please see the Royal Society Publishing guidance on how you may share your accepted author manuscript at https://royalsociety.org/journals/ethics-policies/media-embargo/. We hereby submit a revised version of our manuscript describing a new arthropod from the Devonian of Belgium, providing new insights into for the marine-to-terrestrial transition in the myriapod lineage.
We have carefully considered all reviewers' comments and have modified our manuscript accordingly (see our point-by-point responses to the reviewers' comments below). As requested, we paid particular attention to Reviewer 2's comments on "what level of detail is appropriate to include in a RSOS paper and whether the paper will be accessible to readers outside of a particular specialist area". Note that, however, while Reviewer 2 called for a simplification of the level of details, Reviewer 1 asked for highly detailed terminology corrections. We rather concur with Reviewer 1 and advocate that multidisciplinary studies such as the one performed in our paper require being scientifically correct in all involved fields (using proper scientific terminology). Nonetheless, we agree with Reviewer 2 that some reorganization would make our work clearer, which we have done to some extent. He/She also requested more details about the preservation and palaeoenvironment associated with the fossil, which we have provided. Finally, Reviewer 2 also commented on two other points that we find very questionable and have therefore decided not to address: (i) that RSOS "is not the journal for a systematic description of a new genus", which we do not think is true and prefer to leave up to you; (ii) He/She requested comparison with later euthycarcinoids and particularly a fossil from the Permian of Antarctica, for which age is actually poorly constrained and that some of us are convinced is not even an arthropod but a vertebrate.
We thank you for considering this revised version, and both reviewers for their comments that helped improving and clarifying our manuscript.

Point-by-point responses to the Reviewers' comments and suggestions.
We present the reviewers' comments in bold italics and our responses in normal typeface.

Line 54: Di et al (reference 54) said that part of the scorpion book lung (the operculum) is of telopodal origin and contradicted homology with epipodites. This does not contradict a book lung / book gill homology unless one subscribes to book gills as epipodites (which has, admittedly, been endorsed by some morphologists, e.g., Suzuki and Bergström 2008).
Instead of "Line 54: Di et al (reference 54)", the reviewer actually refers to Line 99: Di et al (reference 9)". We clarified this point by modifying the sentence as follows: " […] suggesting that arachnid book lungs are derived from internalized book gills [10]. Gene expression data and embryology of modern scorpions however showed that part of the book lung (the operculum) is derived from the walking legs of abdominal appendages (telopodites), rather than from epipods (the outermost ramus of appendages) as for book gills [11]."

Line 84: Springtails are said to respire by cuticular diffusion but this is the case only in some members of the group (poduromorphs and entomobryomorphs). Symphypleona have a pair of spiracles.
We clarified this point by modifying the sentence as follows: "[…] except for some springtails […]".

Lines 90-92: Pneumodesmus newmani has been redated as Early Devonian rather than Silurian (Suárez et al., 2017; Brookfield et al. 2020, Historical Biology).
The reviewer is absolutely right. This mistake on our part has been corrected, and the two mentioned references cited and added to the references list.

Line 124: Johnson et al (reference 30) is cited in the context of terrestrial excursions but note that Shillito and Davies (2018) reinterpreted these trackways as subaqueous rather subaerial.
The reviewer is absolutely right. In order to keep the message concise, we have chosen to drop the reference to Johnson et al.

Lines 126-127 and 286-287: The "tube-like structures" in euthycarcinoids are not precisely associated with the paired sternal pores. Rather than say the tube-like structures resemble the tracheal respiratory system in "myriapods" (implicitly Myriapoda as a whole) it would be better to nuance this. The tube-like structures of euthycarcinoids resemble the sternal apodemes of Progoneata (there is a great drawing of them in each of the three progoneate groups in Dohle (1965, "Über die Stellung der Diplopoden im System", Zool. Anz., Suppl. 28), but they are not known in Chilopoda. In Symphyla they are attachments sites for extrinsic leg muscles and longitudinal muscles and do not have an association with the respiratory system (as the spiracles are only on the head). It is only within Dignatha that they are apodemes forming the "tracheal pouch" (in millipedes and hexamerocerate pauropods; see Dohle 1997, fig. 23.4, "trt", in the Fortey and Thomas book).
We thank the reviewer for these precisions. We have nuanced the sentences as follows: Introduction: "Several euthycarcinoids display a pair of ventral pores per pre-abdominal somite, located anteriorly to internal tube-like structures [25,32,33] resembling the sternal apodemes of progoneate myriapods." Discussion: "No pore or opening structure is visible in E. potii gen. et sp. nov., whereas some other euthycarcinoids exhibit ventral pores located anteriorly to internal tube-like structures [25,32,33] resembling the sternal apodemes that form the tracheal respiratory system within progoneate dignathan myriapods (these pores have alternatively been interpreted as vesicles minimizing water losses [25])." Lines 214-223, 281-283 (several instances): avoid "somite" when referring to euthycarcinoid tergites, as they cover multiple segments (somites). Likewise, "segments" in lines 232-234. Use "tergites".
We used "somite" throughout our text because the specimen shows a ventral, internal view of the tergites for the cephalon and pre-abdomen, but an external view of the sternites (covering the rest of the somites) for the post-abdomen and telson. Following the reviewer's comment, we made this clear in §3c, and accordingly replaced "somite" and "segment" throughout the text with "tergites" when referring to tergites.
Line 307: Change "myriapods" to "diplopods" or "progoneate myriapods". Some myriapods have an unpaired posterior gonopore that does not have an appendicular association (Chilopoda). Fig. 2). These are compared to air breathing structures in other groups of arthropods. The method assumes that the distribution of trace elements is controlled by and reflects the original anatomical structures of the organism. The diagenetic processes and history are not discussed at all. The authors should provide evidence that the structures that they are attributing to air breathing are not the nonbigenic artifacts of some diagenetic process.

This manuscript describes structures interpreted as adaptations for air breathing in a poorly preserved, poorly understood group of arthropods -euthycarcinoids. The novel aspect of the study is that the structures are delineated by geochemical imaging, particularly mapping of trace elements (As and Mn are shown in
The issue raised by the reviewer that the "structures attributed to air breathing" indeed reflect 'true' anatomical features and not the result of diagenetic processes is indeed central here, and we apologize for not having been clear on this point. No one will contest the fact that cuticular remains of a largely articulated euthycarcinoid are preserved on the slab investigated herein. The fact that elemental distributions strictly match these cuticular remains (or their outline) clearly indicates that the distribution of trace elements has been controlled by and reflects original anatomical structures of the organism. But admittedly not the entire anatomy, as softtissues and most appendages are not preserved; only cuticular remains. In contrast, elemental distributions that do NOT match the fossil anatomy are clearly associated with the sedimentary matrix where they do not exhibit organized morphologies and seem randomly distributed. Regarding the pair of 2mm-large spherical air-breathing cavities revealed by elemental mapping, it is therefore very unlikely that diagenetic processes would produce, only there, such spherical and symmetrical morphologies. In order to make this point clearer in our manuscript, we have included these arguments within a new "Preservation and palaeoenvironment" section ( §3.3), see below.

This is not the journal for a systematic description of a new genus. The section on systematics should be expanded and published in a journal focused on systematics. Removing systematics from the manuscript would allow space for better explanation of the processes (and products) of preservation.
Several much longer papers published in RSOS have described new genus, and to our knowledge there is no space issue here. Therefore, unless requested by the Editors, we prefer to keep the systematic description of the fossil in our manuscript. We have also expanded our section relating to fossilization processes, see below.

The fact that the paper glues together --rather than fully integrates --euthycarcinoid systematics, arthropod anatomy, taxonomy, and history, and cutting-edge geochemical methods suggests that the authors have not identified a target audience on which to focus. The extensive and detailed discussion of air-breathing adaptations in different groups of arthropods is sufficiently crammed with anatomical and taxonomic jargon that its meaning and importance would elude all but arthropod specialists. Similarly, the geochemical methods and supplemental data would be opaque to non-geochemists.
We recognize that our work is highly multidisciplinary, which requires being scientifically correct in different fields (using proper scientific terminology). Just as "geochemical methods would be opaque to non-geochemists", oversimplifying the geochemical methods and data will frustrate geochemists. Likewise, simplifying anatomical and taxonomic terminology will frustrate palaeontologists and systematicists, as illustrated by several comments from reviewer #1. Regarding the "target audience on which to focus", it appears clearly in our title and introduction, and it is the only subject of our discussion: air-breathing in euthycarcinoids. Technical details about the geochemical analyses performed appear only in the Material and Methods section, and very minimally in the description of the results.