Review History
New insights into the Devonian sea spiders of the Hunsrück Slate (Arthropoda: Pycnogonida)

Thank you for addressing our suggestions. The updated information on localities, Palaeoisopus, stratigraphy and the taphonomic scenario makes the manuscript easier to follow and of even greater relevance. Thank you for explaining your choices in greater length in the rebuttal. I agree with reviewer 1 that your manuscript can be accepted for publication – pending you include an updated link to the raw data (repository) within the manuscript during the proofing phase and it becomes accessible at the latest publication. I look forward to seeing this published.

[# PeerJ Staff Note - this decision was reviewed and approved by Laura Wilson, a PeerJ Section Editor covering this Section #]

You provide a spectacular revision of the Hunsrück Slate pycnogonids supported by the latest imaging techniques. This will be of great relevance to the community, paleontology, and other disciplines more generally. I apologize for the delay in making my decision but given the length of the manuscript and difficulties in finding reviewers to review on short notice, it was beyond my control. It would love to see this published, but some crucial points need to be addressed before publication:

Use of Hunsrück Slate: many authors still informally use the Hunsrück slate to refer to slate unit containing stratigraphic levels of exceptionally preservation within the Central Hunsrück Basin (Bundenbach – Herrstein – Gemünden area) but this is not a stratigraphic unit (compare Wilkin 2023). Furthermore, it has generally been inconsistently used. Some use it to refer to all slates of Pragian to Emsian ages within the area, while other consistently use it to refer to the slates from Central Hunsrück basin. The slate unit in this area does not just contain such levels of exceptional preservation but also units of background sedimentation. The entire slate unit containing those levels of exceptional preservation is now assigned to the middle Kaub Formation (e.g., Schindler et al. 2002). Based on ammonoid and dacryoconarid biostratigraphy, these levels are now consistently assigned to the lower Emsian (De Baets et al. 2013). Given the preservation and the fact that at least some of the specimens can be confidently assigned to the Wingertshell member at the Obereschenbach quarry (compare Poschmann & Dunlop 2002; Kühl et al. 2013). I assume most if not all your specimens derived from middle Kaub Formation and have an early Emsian age. Please discuss these aspects and specify the stratigraphic unit and age of the units in the Central Hunsrück basin relevant to you. The use of Hunsrück Slate is ok but should be clarified. Given you mention the importance for calibrating the evolutionary history of sea spiders, appropriate age assignment of these specimens is even more important.

Palaeoisopus: Please verify your statement about fusion of telson to a preanal segment and discuss include a discussion of abdominal segmentation more generally (see reviewer 1 and compare Vilpoux & Waloszek 2003).

Ecology: Please consider pointing out that multiple modern sea spider seems to have a parasitic life stage (e.g., Brenneis et al. 2017; Dunlop 2021) and how its appearance could have impacted their preservation potential. Interestingly, it seems that already at the time of the Hunsrück Slate some pycnogonids may have had symbioses or repeated interactions with other taxa.

Taphonomic scenario: I feel your discussion on taphonomy would benefit from included information on the non-exceptional “background” faunal. There is a considerable background sedimentation which often yields ammonoids and shelled organisms more generally but even those “background” organisms are occasionally exceptionally preserved within those intervals yielding exceptional preservation such as preservation of fragile whorls or embryonic shell (compare Sutcliffe 1997; Sutcliffe et al. 1999; De Baets et al. 2013). The rarity of exceptional preserved ammonoids or lack of soft-tissue preservation maybe corroborate your scenario for the rarity of pelagic sea spiders. Other biases might have exacerbated these patterns. Klug et al. (2008) did not explicitly compare size between Hunsrück Slate and Moroccan lagerstätten of similar age and environments but when looking at common elements, they are usually larger in the Hunsrück slate and higher proportion of shelled organisms (e.g., mollusks) described in the Moroccan lagerstätten – likely related to more 3D-preservation and shells details mostly lacking from the Hunsrück Slate specimens. Please consider discussing in greater detail the issues with interpreting fossil structures X-ray analyses alone (e.g., Otto 1999, 2000 and references therein).

Provide locality information: I am a bit surprised about the missing locality information for all specimens. I assume most derive from the Central Hunsrück basin (Bundenbach-Gemünden area). From the preservation alone, I assume they derive from the middle Kaub Formation which is confirmed at least for some specimens by cross-referencing previously published one (e.g., Wingertshell member of the Obereschenbach quarry in Poschmann and Dunlop 2006; Kühl et al. 2013 – both of which you cited already). Also, Broili mentions Bundenbach or Gemünden for the provenance of his specimens. I assume for most that locality information will be available and should therefore be added in systematic paleontology section and (at minimum) in the supplementary table.

Restructure systematic descriptions: in addition to adding geographic and stratigraphic information, I agree with reviewer 2 that the descriptions contain information that should be moved into the discussion or a remarks section. Descriptions should include only the observed morphology, while interpretations and explanations relying on the literature should be moved to the remarks section (or alternatively the discussion). This would make the paper easier to read. Please make sure all museum abbreviations are included – also those listed in supplementary materials (compare reviewer 1). Please consider providing an amended diagnosis for some of the re-described species (compare reviewer 2).

Homogenise synonymy lists: I agree with reviewer 1, that there are some inconsistencies in the synonymy list which need to be resolved. Make sure, they are consistent throughout.

Add more labels and figure calls in the description: I agree with reviewer 2 that the manuscript would benefit from referring more often and more precisely to figures as adding more labels.

Formatting and terminology issues: there are some phrases which could rephrased for clarity (see suggestions by reviewer 1 and 2); also, there are some typographical issues ( Bundenbach and Bundenbachia instead of Budenbach and Budenbachia; see annotated pdf) as well as potential issues which could lead to confusion (e.g., line 155: use of plate; compare reviewer 1).

Raw data availability: the raw image stack and ideally also the finished models in readable formats should be made available in appropriate repositories and explicitly state with digital object identifiers in the finished manuscript at the latest upon publication (e.g., Davies et al. 2017). Currently you only mention a link in the reviewing materials, but I was unable to access its content.

Please make sure these points as well as all other points raised including those in annotated pdfs are addressed. I look forward to receiving the revised manuscript and seeing it published.


Suggested references:

Brenneis, G., Bogomolova, E. V., Arango, C. P., & Krapp, F. (2017). From egg to “no-body”: an overview and revision of developmental pathways in the ancient arthropod lineage Pycnogonida. Frontiers in Zoology, 14, 1-22.

De Baets, K., Klug, C., Korn, D., Bartels, C., & Poschmann, M. (2013). Emsian Ammonoidea and the age of the Hunsrück Slate (Rhenish Mountains, Western Germany). Palaeontographica A, 299(1-6), 1-113.

Dunlop, J. A. (2021). Chelicerates as parasites. In The Evolution and Fossil Record of Parasitism: Identification and Macroevolution of Parasites (pp. 315-346). Cham: Springer International Publishing.

Klug, C., Kröger, B., Korn, D., Ruecklin, M., Schemm-Gregory, M., De Baets, K., & Mapes, R. H. (2008). Ecological change during the early Emsian (Devonian) in the Tafilalt (Morocco), the origin of the Ammonoidea, and the first African pyrgocystid edrioasteroids, machaerids and phyllocarids. Palaeontographica abteilung a-Palaozoologie-Stratigraphie, 283(4-6), 83-U58.

Otto, M. (1999). The problem of fossil interpretation. Lethaia, 32(2), 140-142.

Otto, M. (2000). Supposed soft tissue preservation in the Hunsrückschiefer (Lower Devonian, Rheinisches Schiefergebirge): the example of brachiopods. Paläontologische Zeitschrift, 74(1), 79-89.

Schindler, T., Sutcliffe, O.E., Bartels, C., Poschmann, M. & Wuttke, M. (2002): Lithostratigraphic subdivision and chronostratigraphic position of the middle Kaub Formation (Lower Emsian, Lower Devonian) of the Bundenbach area (Hunsrück, SW Germany).– Metalla 9 (2): 7388.

Sutcliffe, O.E. (1997): The sedimentology and ichnofauna of the Lower Devonian Hunsrück Slate, Germany: taphonomy and palaeobiological significance.– Unpubl. PhD thesis, Univ. Bristol, 297 pp.

Sutcliffe, O. E., Briggs, D. E., & Bartels, C. (1999). Ichnological evidence for the environmental setting of the Fossil-Lagerstatten in the Devonian Hunsruck Slate, Germany. Geology, 27(3), 275-278.

Vilpoux, K. & Waloszek, D. (2003). Larval development and morphogenesis of the sea spider Pycnogonum litorale (Ström, 1762) and the tagmosis of the body of Pantopoda. Arthropod Structure & Development, 32(4), 349-383.

Wilkin, J. T. (2023). The Hunsrück Slate Konservat‐Lagerstätte. Geology Today, 39(2), 72-76.


[# PeerJ Staff Note: It is PeerJ policy that additional references suggested during the peer-review process should only be included if the authors are in agreement that they are relevant and useful #]