A Subsurface Stepping Stone Hypothesis for the Conquest of Land by Arthropods
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIt has been long believed that arthropods conquered the land from the marine environment directly, but an alternative way has not been proposed in order to address the challenges that the arthropods might had been facing during the process. In this study, Frumkin and Chipman proposed a model that explains the ancient arthropods might had been gradually colonize the land by adapting their lives through a subsurface cave or tunnel, that allow them to evolve and adapt to the terrestrial life styles.
I overall like with the idea and hypothesis that the authors have proposed. Although there is no direct evidence currently to support the hypothesis, e.g. fossil records and cave designs, as the authors have mentioned, which makes the hypothesis less sound, the authors have tried their best to provide indirect evidence observed from the extant basal arthropods and relate them to the history of land colonization.
The only figure in the manuscript has provide an overall idea of the proposed hypothesis, yet I believe the figure can be more self explained with more details. For instance, the effect of rainfall labelled in the figure has not been mentioned by the authors. Besides, what is the effect of streams as labelled in the figure? How the gradual change of water salinity provide a better ground for the colonization? And how about other challenges, including food availability? I would love to see an improved figure that can summarized more of the text.
Besides, there are some referencing issues identified in the text, please kindly address them.
1. Line 79: (Little, 1990)
2. Line 81: (Retallack, 2022)
Author Response
Reply to reviewer 1
-The only figure in the manuscript has provide an overall idea of the proposed hypothesis, yet I believe the figure can be more self explained with more details. For instance, the effect of rainfall labelled in the figure has not been mentioned by the authors. Besides, what is the effect of streams as labelled in the figure? How the gradual change of water salinity provide a better ground for the colonization? And how about other challenges, including food availability? I would love to see an improved figure that can summarized more of the text.
Reply: The figure is improved accordingly with additional explanation in the caption and text.
The newly written paragraph:
Caves provide gradual transitions from saltwater through anchihaline brackish to freshwater, and from water-inundated voids through hygropetric environment at the water margins, to dry habitats (Fig. 1). This is and has been promoted by the natural water cycle: Rainfall can infiltrate to the subsurface and reach caves, and can also be drained in subaerial catchments as runoff, collecting into streams that can form caves and become subsurface rivers. Any form of this groundwater, while flowing into the sea, may create freshwater to anchihaline brackish environments. These can provide transitional routes allowing arthropods to gradually adapt and move from salt water to freshwater. The subsurface rivers also carry food, such as fungus, from land into caves. Thus cave arthropods could benefit from several food sources, including sea-sourced materials, chemosynthetic microorganisms, and terrestrial materials.
-Besides, there are some referencing issues identified in the text, please kindly address them.
1. Line 79: (Little, 1990)
- Line 81: (Retallack, 2022)
Reply: these are now corrected
Reviewer 2 Report
Comments and Suggestions for AuthorsIn my opinion, although this is an interesting but erroneous hypothesis based on the analogy of modernity and the Paleozoic. However, the Paleozoic (especially the early) was fundamentally different from the modern world. The weak presence of terrestrial plants created a situation of constantly changing coastline. Caves, if they existed, which is very doubtful, must have been constantly flooded and destroyed. Soil, for example, is a more promising place for the evolution of terrestrial arthropods, and it is surprising that the authors do not mention the work of M. Ghilarov who has studied this issue.
However, I believe that this can be published because even incorrect hypotheses can be useful for the development of science.
Comments for author File: 
Comments.pdf
Author Response
Reply to reviewer 2
. The weak presence of terrestrial plants created a situation of constantly changing coastline.
Reply: This, if true, does not interfere with the hypothesis
Caves, if they existed, which is very doubtful, must have been constantly flooded and destroyed.
Reply: We cite references for evidences of Paleozoic caves
Soil, for example, is a more promising place for the evolution of terrestrial arthropods, and it is surprising that the authors do not mention the work of M. Ghilarov who has studied this issue.
We agree with the reviewer that soils are an option, but the hardly shield UV radiation.
We now cite the study of M. Ghilarov
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for Authors
Dear authors,
it was a pleasure for me to read up on your interesting hypothesis. You presented an intriguing scenario for the colonization of terrestrial habitats by arthropods. The text is overall very well written and does not make use of uninformative Linnean classifications. Please let me explain my decision to suggest a major revision despite I strongly suggest publishing the manuscript in the journal Diversity. In my opinion, there are some weak points that should be discussed before your ideas are communicated to the public, so that it does not appear as if you had not thought of these aspects.
My main point of criticism against your subterranean terrestrialization hypothesis lies in the necessity for it. In the introduction, you laid out why it is necessary to think about alternative routes for the colonization of land by arthropods. Your main point there is the claimed absence of abundant land plants. For this you use the age of the first undisputed land plant fossils and the estimated ages of clades for which the most recent common ancestor can be assumed to be terrestrial. By this you are applying two different measures. If you used the same method to date the emergence of land plants you would reach much older dates (see discussion in Lozano-Fernandez et al. 2020), which would weaken the necessity of an alternative terrestrialization route. Another point that I discovered in Lozano-Fernandez et al. (2020) is that there seems to be a very high rate of molecular evolution in the early history of arachnids (figure 2). However, for cave environments one would expect longer generation times than in more nutrient-rich and warmer environments. It would be good if you could include this apparent inconsistency in your discussion and provide some explanations why this does not refute your proposed scenario. It would also be good if you could provide some references for the UV (in-) tolerance of different arthropods, which is another point that you argue proves the necessity of a stepping-stone hypothesis.
The geological stability of the proposed suitable cave systems, especially those that offer some light to still reach the the habitats, seems questionable to me. Without much doubt, there probably were suitable cave systems present throughout the time needed to bridge the gap between the molecular dates of arthropod terrestrialization and the earliest land plants, however, these cave systems would have been continuous throughout the time period as well. Otherwise the arthropods would have needed to go back into the Ocean again before entering a new cave system.
The figure you provided does not include a route through freshwater but instead depicts the “direct” route to be uphill of along a steep relief, making it appear very unlikely. I think it would be best to at least draw the relief much flatter and to include a river for the popular hypothesis that terrestrialization happened through limnic environments. In the conclusions you briefly mentioned that future Palaeozoic fossils could substantiate your hypothesis. I think it would be great if you could include a small section in the discussion that explains what kind of fossils (what age and morphological features) could best substantiate it. On a more technical note, I would suggest to reduce the amount of very short paragraphs (two or three sentences), as it disrupts the reading flow, while not adding value by separating topic-wise drastically different parts of the text.
I think it should be well possible to discuss the above mentioned points in your manuscript without completely restructuring the text. Please note that I provided minor suggestions in form of an annotated PDF file. I a m happy to read a revised version of this manuscript soon and hope that you will find my suggestions and comments helpful. This manuscript has the potential to provide an important reference for many future articles that deal with terrestrialization events.
Best wishes,
Comments for author File: Comments.pdf
There are some "the"s missing and there were a few cases of odd sentence constructions, but otherwise the text was well written and does not require intensive editing.
Author Response
We thank the 3rd reviewer for the thorough reading and important remarks.
All the annotation/remarks in the pdf were accepted and modified accordingly.
We now added additional information regarding most of the remarks. These are included in the following paragraphs The unchanged paragraphs are not included. For additional remarks, please see below:
The terrestrialization of plants and arthropods represents a major milestone in the diversity, evolution, and adaptations of Earth’s ecosystems. Conquering land required numerous morphological and physiological adaptations including aerial respiration and gas exchange, osmoregulation, terrestrial locomotion without the benefit of buoyancy, aerial sensory structures, reproduction outside of water, increased exposure to ultraviolet radiation, new food sources, and protection against environmental stresses such as desiccation and rapid temperature fluctuations (Little, 1990; Lozano-Fernandez et al., 2016; Sharma, 2017). However, details of the journey from water to land remain obscure, with the precise timing of the water-to-land transition being disputed for different taxa. Molecular clocks may reliably date evolutionary timescales only if ground-truthed by fossils, which are rare for early terrestrial organisms (Rota-Stabelli et al., 2013; Wolfe et al., 2016; Howard et al., 2019).
The oldest undisputed land plant fossils are Ordovician cryptospores (Steemans et al., 2009; Lüttge, 2020) and spore-bearing plant fragments (Wellman et al., 2003; Buschmann and Holzinger, 2020), although signs of fungus-like fossils are known in paleo-caves since the Ediacaran (Gan et al., 2021;2022). Molecular clocks date crown group plants to the mid-Cambrian to early Ordovician (Clarke et al. 2011; Donoghue et al. 2021;Morris et al. 2018). However, molecular clocks suggest arthropods may have colonized land as early as the mid-Cambrian ~510 Ma (Lozano-Fernandez 2020; Wheat and Wahlberg, 2013), probably earlier than macroscopic terrestrial plants that produce significant amounts of oxygen and provide adequate shelter.
Arthropods underwent at least three independent terrestrialization events in the Paleozoic (Rota-Stabelli et al., 2013), but relevant fossils of the first stages of terrestrialization are almost nonexistent. Molecular phylogenies show myriapods – an entirely terrestrial group – diverged from other mandibulates and started diversifying in the Cambrian (Rota-Stabelli et al. 2013; but see Lozano-Fernandez et al., 2016). Some late Cambrian trace fossils may have myriapod or euthycarcinoid (a stem-group lineage of myriapods) origin, likely made by a semi-terrestrial arthropod (e.g. MacNaughton et al., 2002; Lozano-Fernandez et al., 2016). The earliest undisputed myriapod body fossils are much younger, from the Silurian (Lozano-Fernandez et al., 2016).
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The conundrum is how and whether early arthropod terrestrialization could occur before suitable plant and ozone-based habitats developed. If molecular clocks are reliable, myriapod and possibly other arthropod diversification began before plants significantly populated land, and while ozone levels were apparently too low to adequately shield against damaging UV radiation outside of water. The amount of damage caused by UV radiation to extant arthropods is highly variable and context dependent (reviewed in Ben-Yakir and Fereres 2016; Ben-Yakir 2020), and it is difficult to assess how it would have affected Cambrian-Ordovician arthropods. However, it is likely that the level of UV radiation they would have sustained before the establishment of an adequate ozone layer would have reached levels that are lethal to extant arthropods.
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2 Discussion
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3 Conclusion
The development of complex terrestrial arthropod life on Earth required overcoming several major hurdles. First, the challenges of transitioning from an aquatic environment onto dry land were immense. Arthropods needed to evolve adaptations to overcome UV radiation, prevent desiccation, move on solid substrates, obtain oxygen, and find new food sources outside of water when little or no vegetation was available.
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In conclusion, the proposed terrestrialization route would have allowed one or more arthropod taxa to adapt to new frontiers through via a stepwise and gradual dispersion from sea into anchihaline cave waters, hygropetric margins of water bodies in caves, and finally to the surface. This hypothesis is consistent with modern cave observations. The recovery of early Paleozoic fossils representing crown group or upper-stem group terrestrial arthropod taxa, with identifiable adaptation to cave life or to bacterial mat feeding would help to substantiate this hypothesis.
We added the following New References:
Clarke, J.T., Warnock, R.C.M., Donoghue, P.C.J., 2011. Establishing a time-scale for plant evolution. New Phytol. 192, 266-301.
Morris, J.L., Puttick, M.N., Clark, J.W., Edwards, D., Kenrick, P., Pressel, S., Wellman, C.H., Yang, Z.H., Schneider, H., Donoghue, P.C.J., 2018. The timescale of early land plant evolution. Proc. Natl. Acad. Sci. U. S. A. 115, E2274-E2283.
Donoghue, P.C.J., Harrison, C.J., Paps, J., Schneider, H., 2021. The evolutionary emergence of land plants. Curr. Biol. 31, R1281-R1298.
Ben-Yakir, D., 2020. Direct and indirect effects of UV radiation in: Ben-Yakir, D. (Ed.), Optical Manipulation of Arthropod Pests and Beneficials. CABI DIgital Library, pp. 49-59.
Ben-Yakir, D., Fereres, A., 2016. The effects of UV radiation on arthropods: a review of recent publications (2010-2015). Acta Horticulturae, 335-342.
About this remark:
“Another point that I discovered in Lozano-Fernandez et al. (2020) is that there seems to be a very high rate of molecular evolution in the early history of arachnids (figure 2). However, for cave environments one would expect longer generation times than in more nutrient-rich and warmer environments. It would be good if you could include this apparent inconsistency in your discussion and provide some explanations why this does not refute your proposed scenario.”
Addressing this point is beyond the scope of the current contribution. It would need a highly speculative discussion of the generation time of early arachnids, which is not applicable in such an article.
About this remark:
‘The geological stability of the proposed suitable cave systems, especially those that offer some light to still reach the habitats, seems questionable to me.”
We agree with the reviewer that Without much doubt, there probably were suitable cave systems present throughout the time needed to bridge the gap between the molecular dates of arthropod terrestrialization and the earliest land plants. We also agree that these cave systems would have been continuous throughout the time period as well. Deep caves prove to be very continuous throughout the time, as they are inherently below surface, and thus they are protected from surface processes such ad denudation, erosion and even glaciation.
About this remark:
“The figure you provided does not include a route through freshwater but instead depicts the “direct” route to be uphill of along a steep relief, making it appear very unlikely. I think it would be best to at least draw the relief much flatter and to include a river for the popular hypothesis that terrestrialization happened through limnic environments.”
We now modified the figure to show a possible river route (A). the topography is now flatter, but not too flat, otherwise we cannot show the cave – caves need some relief to develop, with gradients towards base level.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsDear authors,
thank you for taking the time to diligently go through the points I addressed in the first round of review. In my opinion, by adding additional references and further explanations you have significantly improved the quality of the contribution and made the text more accessible. I very much appreciate the modified figure that you provided. However, for the figure I still have a small recommendation. As it is now, the order of the routes is A (subaerial streams), B (direct) and C (subterranean). By this, the order is a bit odd (starting not with the simplest route. I would suggest (without modifying the drawing to change the order to A (direct), B (subaerial streams) and C (subterranean). But this is just a recommendation, feel free to disagree on this matter. Please note that I uploaded an annotated version of the manuscript PDF that included a few suggestions, which should all be in your interest to adopt. Therefore, I recommend to "Accept in present form", leaving the adoption of my suggestions at your judgement.
Best wishes!
Comments for author File: Comments.pdf
