PONE-D-20-10764

Using tooth enamel microstructure to identify mammalian fossils at an Eocene Arctic forest

PLOS ONE

Dear Dr. Eberle,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

This is an excellent study as noted by all three reviewers and although the system does not permit a category for 'very minor revisions' that is what these are. If the authors can make the suggested changes, which are primarily along the lines of clarifying a few things. I will be happy to accept the manuscript in very short order.

Please submit your revised manuscript by Aug 08 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Adam Csank, Ph. D.

Academic Editor

PLOS ONE

Journal Requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Please add the permit details to the Material and Methods section of your manuscript.

3. We note that Figure 1 in your submission contain [map/satellite] images which may be copyrighted. All PLOS content is published under the Creative Commons Attribution License (CC BY 4.0), which means that the manuscript, images, and Supporting Information files will be freely available online, and any third party is permitted to access, download, copy, distribute, and use these materials in any way, even commercially, with proper attribution. For these reasons, we cannot publish previously copyrighted maps or satellite images created using proprietary data, such as Google software (Google Maps, Street View, and Earth). For more information, see our copyright guidelines: http://journals.plos.org/plosone/s/licenses-and-copyright.

We require you to either (1) present written permission from the copyright holder to publish these figures specifically under the CC BY 4.0 license, or (2) remove the figures from your submission:

1.    You may seek permission from the original copyright holder of Figure 1 to publish the content specifically under the CC BY 4.0 license.

We recommend that you contact the original copyright holder with the Content Permission Form (http://journals.plos.org/plosone/s/file?id=7c09/content-permission-form.pdf) and the following text:

“I request permission for the open-access journal PLOS ONE to publish XXX under the Creative Commons Attribution License (CCAL) CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). Please be aware that this license allows unrestricted use and distribution, even commercially, by third parties. Please reply and provide explicit written permission to publish XXX under a CC BY license and complete the attached form.”

Please upload the completed Content Permission Form or other proof of granted permissions as an "Other" file with your submission.

In the figure caption of the copyrighted figure, please include the following text: “Reprinted from [ref] under a CC BY license, with permission from [name of publisher], original copyright [original copyright year].”

2.    If you are unable to obtain permission from the original copyright holder to publish these figures under the CC BY 4.0 license or if the copyright holder’s requirements are incompatible with the CC BY 4.0 license, please either i) remove the figure or ii) supply a replacement figure that complies with the CC BY 4.0 license. Please check copyright information on all replacement figures and update the figure caption with source information. If applicable, please specify in the figure caption text when a figure is similar but not identical to the original image and is therefore for illustrative purposes only.

The following resources for replacing copyrighted map figures may be helpful:

USGS National Map Viewer (public domain): http://viewer.nationalmap.gov/viewer/

The Gateway to Astronaut Photography of Earth (public domain): http://eol.jsc.nasa.gov/sseop/clickmap/

Maps at the CIA (public domain): https://www.cia.gov/library/publications/the-world-factbook/index.html and https://www.cia.gov/library/publications/cia-maps-publications/index.html

NASA Earth Observatory (public domain): http://earthobservatory.nasa.gov/

Landsat: http://landsat.visibleearth.nasa.gov/

USGS EROS (Earth Resources Observatory and Science (EROS) Center) (public domain): http://eros.usgs.gov/#

Natural Earth (public domain): http://www.naturalearthdata.com/

4. Please include captions for your Supporting Information files at the end of your manuscript, and update any in-text citations to match accordingly. Please see our Supporting Information guidelines for more information: http://journals.plos.org/plosone/s/supporting-information.

Additional Editor Comments (if provided):

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: N/A

Reviewer #2: N/A

Reviewer #3: N/A

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I preface my remarks by acknowledging that some of my comments may be irrelevant or inaccurate, because I have not been able to check some details in pertinent publications or by examining actual specimens (I do not have access to my library or collections at present, since my university remains closed due to covid-19). Some relevant articles (e.g., book chapters) are not available on line.

Over the last several decades a small group of intrepid vertebrate paleontologists (including two of the three authors) have been exploring the Canadian High Arctic for fossil vertebrates. Though fossils are relatively scarce, their efforts have resulted in discovery of a sizable vertebrate fauna. These fossils are especially important for understanding Eocene paleoclimate and paleoenvironments, climate change, and mammalian dispersal patterns. In this article the authors report the first fossil mammal remains from an Eocene Arctic forest known as Strathcona Fiord Fossil Forest, in the Margaret Fm. of Ellesmere Island. Underscoring the rarity of vertebrate fossils from this fossil forest, the remains consist only of a couple of enamel chips that the authors deem undiagnostic from their external morphology. They provide compelling evidence from study of enamel microstructure (at several magnification levels) that these fragments belong to the well-known Holarctic pantodont Coryphodon (based on previous documentation of Coryphodon enamel). To my knowledge this is the first time anyone has employed enamel microstructure to identify a fossil mammal to genus. It therefore demonstrates the potential of enamel microstructure as a tool for identifying otherwise indeterminate mammal tooth fragments.

The manuscript is clear, well written, and well illustrated. I have no mandatory changes and only a few observations the authors may wish to consider in their final manuscript.

Regarding the described enamel fragments, their size and relatively flat or slightly curved surface alone (indicating derivation from a much larger tooth) already eliminates most early Eocene mammal taxa, including tapiroids, Phenacodus. Size would not necessarily rule out rhinocerotoids (which have somewhat similar enamel), but unless the authors are implying that the fossils are contaminants or that the early Eocene age of the Margaret Fm. is erroneous, rhino is a very unlikely ID. Brontotheres known from the Margaret Fm. have U-shaped HSB, eliminating them. I do not know if the enamel of late early Eocene Lambdotherium (usually considered a basal brontothere) has been investigated, but these fragments may be too big to represent Lambdotherium. Several other taxa that have somewhat similar enamel structure (noted in the text) can be discounted for reasons of geologic age or biogeography.

The authors state (lines 79-81) that “the tooth fragments from the Strathcona Fiord Fossil Forest are so incomplete as to be undiagnostic by using their external morphology,” and this appears to be true from their very fragmentary condition. However, the surface texture visible in Fig. 4 is unusual and characteristic of Coryphodon (as observed by Koenigswald & Rose, 2005); it is quite possibly diagnostic. (If the authors know of any other early Eocene taxon that shows the same surface texture, a figure illustrating this would be useful to demonstrate that these fragments cannot be identified by surface texture alone.) So everything points to Coryphodon—the size, enamel surface features, and previous documentation from the Margaret Fm. all would have suggested this without analysis of enamel microstructure. But the enamel study confirms this ID and shows that enamel microstructure can be a valuable tool for taxonomic determination. The identification of these fragments as Coryphodon is convincing.

To make their conclusions even more compelling, I suggest the following additions—if possible. Could sections of enamel from undoubted Margaret Fm. Coryphodon be examined and compared with the tooth fragments described here? The only other large mammals present, or possibly present, in the Margaret Fm., whose enamel might be confused with that of Coryphodon are oxyaenid creodonts and mesonychids, which have somewhat similar zigzag enamel (Stefen, 1997). The authors state that both have a different pattern, with undulating HSB in the lower part of the tooth and zigzag enamel above. Would it be possible to include an illustration showing how they differ from the Coryphodon tooth fragments? (The large oxyaenid Oxyaena is a common early Eocene mammal that could potentially be present in the Margaret Fm., though not yet recorded—it should be eliminated as well.)

Geologic age of the enamel fragments (early Eocene) is one piece of evidence supporting their identification as Coryphodon, but it is circular reasoning to then cite this as evidence that the Margaret Fm. is early Eocene in age. Is there independent evidence of the age of the Margaret Fm.? “Lithologic correlation” is cited but not further explained. Are any volcanics or datable minerals present, any paleomag or isotope stratigraphy, any age-diagnostic pollen present, or nannoplankton in the underlying marine beds? Or is the early Eocene age based mainly on the mammalian fossils, including the identification of these fragments as Coryphodon? The end of the second paragraph of the conclusions lists presence of Coryphodon as a reason to (re)interpret the Margaret Fm. as early Eocene rather than late Paleocene as originally mapped by Harrison et al. (2009); but as the authors note, Coryphodon has been reported from earliest Bridgerian (Br1a; Gunnell et al., 2009), and it is also known from the latest Paleocene (Clarkforkian, Rose, 1981). If other age-diagnostic mammals are known from the formation, it would be worth listing them here.

At the end of the Conclusions, the authors suggest that its enamel pattern preadapted Coryphodon for eating a tough diet of wood and evergreens during the Arctic winter, and that this might explain its abundance in the Arctic while equoids and artiodactyls are absent there. Maybe so, but if Coryphodon evolved in the mid-latitudes where it is abundant, why did it develop this bizarre enamel pattern in the first place? Or are the authors speculating that it evolved in the Arctic? Given the abundance of equoids and artiodactyls in the early Eocene of both North America and Europe (and the likelihood that intercontinental dispersal occurred during the PETM), it seems that both had to have been present in the Arctic but have not yet been discovered.

Minor issues:

Please define NUFV, YG, and repository (if different).

Line 316—should read (Fig. 4B and 8)

Line 320—‘area above the EDJ’: ‘above’ is ambiguous; use ‘external to’ or add (i.e., toward the OES

Line 404—replace “creodont” with either oxyaenid (preferable) or oxyaenid creodont

Is it possible to locate Dawson’s previous samples relative to the Strathcone Fiord Fossil Forrest in Fig. 3 (or in the text following line 107)? Is there significance to the colors in Fig. 3?

Fig. 6B, label IPM

Why is there a Supplemental File? It appears to be identical with Fig. 5.

Reviewer #2: This is a clearly written and straightforward contribution that illuminates a novel method for identifying fragmentary remains of fossil mammals for paleobiological research. I have only a few minor edits to propose/suggest:

line 245: "astropotheres" should read "astrapotheres"

lines 278 & 337: "parallel" really isn't a verb, so maybe "run parallel to one another" is better here

line 401: I don't know whether you want to conform to the convention proposed by J. Flynn, F. Sole and others, but these stem members of the carnivoran clade are called Carnivoramorpha by them, i.e., not "true" or crown-clade carnivorans

line 404: There is a current debate about the monophyly of "creodonts" (oxyaenids and hyaenodontids). So, again, it might be safer to refer to Palaeonictis as an oxyaenid just to be safe.

I like your hypothesis about Coryphodon being able to withstand Arctic winters while equids and artiodactyls were not. Nevertheless, most biogeographic models for Laurasian mammals have promoted a Thulean land bridge between Greenland and NW Europe as the most likely pathway for dispersal of equids and artiodactyls between North America and Europe, meaning these clades probably occupied Ellesmere or adjacent areas sometime during the early Eocene. Care to comment on this here?

Reviewer #3: Th manuscript is a good example that fossil fragments are also useful to identify taxa. Enamel microstructure is species specific. The use of several techniques and scales of observation is fruitful, even if some samples are "destroyed" by the preparative processes (polishing, etching). The authors have made the good choice in trying to identify the samples. There are so many fragments in museums that are not known from this point of view because they have to be intact!

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Kenneth D. Rose

Reviewer #2: Yes: K. Christopher Beard

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

Using tooth enamel microstructure to identify mammalian fossils at an Eocene Arctic forest

PONE-D-20-10764R1

Dear Dr. Eberle,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Adam Csank, Ph. D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments: