Basic reporting

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Experimental design

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Validity of the findings

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Comments for the author

This is an excellent and extremely thorough attempt to revise the taxonomy of an important and iconic group of dinosaurs, as well as providing a detailed description of a new species. This paper will provide a benchmark for future studies examining all aspects of diplodocid evolution, but will also play an important role in discussions of genus versus species determination in paleontology, and teasing out whether differences are taxonomically significant or represent individual variation. The detailed discussion of each phylogenetic character is also extremely welcome, as is the illustration of each state – on both accounts this raises the bar for papers focused on phylogenetic relationships of fossil groups.

Although I have made numerous comments throughout the MS (annotated on the PDF), these are predominantly of a minor nature (including typos and grammar), but I have summarized several broader issues below.

1. The systematic section at the start of your paper, where you name Galeamopus, is a little confusing as presently written. You need to make it clear that you're providing a new generic name for Diplodocus hayi in this section: this isn't remotely clear at present. Also, the type species of your new genus, G. hayi, needs a diagnosis, and again should be provided in this section. I realise that it is provided in your Discussion, but it makes much more sense to go here.

2. There are several references that have accidentally been left in from this work’s origin as a thesis (e.g. “see next chapter” on line 298). I think I’ve flagged them all up, but it’s possible that I’ve missed some.

3. The opening section of the Discussion (lines 4455–4544) feels much more like it belongs either in the Background/Materials and Methods section, or that it should come after the subsequent sections whereby you go through all of the different clade supports. Also – are these support sections not really Results? In general I think it would be good to try and make the sections that would more normally end in an appendix separate from true discussion, which will be the parts that most readers will want to read.

4. I think it would be extremely useful if the affinities of each specimen were summarized in a table, so that a reader can easily see what belongs to what, without having to wade through a huge amount of text.

5. The section ‘Biostratigraphic and paleobiogeographical implications” should be moved to the end of the Discussion – bigger picture information should follow on from all the diagnoses, not be buried amongst them. This should basically be the final part of the paper prior to your Conclusions. This also seems a disproportionately short section of the paper – perhaps there are other aspects worth discussing of diplodocid evolution? Or you could explicitly outline what work still needs to be done? The lack of actual discussion almost seems to sell short the enormous amount of work undertaken.

Best wishes,

Phil Mannion

Basic reporting

No Comments

Experimental design

No Comments

Validity of the findings

No comments

Comments for the author

Dear authors,

As I expressed in my review of the original submission, this is an excellent and welcome contribution. The loss of the description of the new taxon is also probably a good thing in terms of focusing just on the phylogeny and taxonomy.

I have made numerous minor comments throughout the MS, many of which are just spotting typos (please see the attached annotated version of the PDF), but the only moderately substantial issue I would raise concerns your decision of where to separate species/genera and relates to specimen completeness. If a specimen is quite incomplete, then it's likely to be difficult to find enough autapomorphies to reach the required species/genus quota, whereas lots of autapomorphies might have been present in other (non-preserved) parts of the skeleton. In the case of Dinheirosaurus (and it's your proposed synonymization of that taxon with Supersaurus that got me thinking about this issue), there's only a couple of cervical vertebrae and a dorsal series, and yet it "almost" passes the genus test. Perhaps you need to also consider weighting the number of differences by the completeness of the specimen when determining your species/genus determinations too? I realise that this adds a whole extra level of complexity because you're making comparisons between increasing numbers of specimens/taxa, but it's definitely something to at least think about and should probably be discussed, if not enacted here.

I also still think that you might be better served moving the character list to the end of the text, as an Appendix, but included in the main paper. However, if you're determined to keep it in the main text, I think it would be better to have a combined Materials and Methods section, with the character list at the end of this.

I am also happy with the responses to comments from my previous review.

I look forward to seeing the final version of this published.

Best wishes,

Phil Mannion