Dear authors,

This is an excellent and thorough piece of anatomical work, and it is great to see this material finally described. I thus have no hesitation in recommending its publication, and think that the work would do service to Royal Society Open Science and would be wholly appropriate given its history of publishing palaeontological anatomical work. I thank the authors for taking the time and effort to finally bring the taxon to light, and am impressed by their dedication.

I do, however, have a few points which I would like to see addressed prior to publication. These are marked up in the PDF, and more major points are also outlined here:

-The most major point I have is to ask whether there any chance to get the mandible CT scanned? This would add significant points to the description such as the exact articulation of the elements in the mandible, how far the foramina extend into the mandible and to where, and would clarify the tooth count. Given that the rest of the material was scanned and that CT scanning is generally very readily available today it seems somewhat strange that this was not carried out – perhaps it was difficult to scan due to its shape? If so, please explain in the text. If it were able to be scanned, there would be no need to segment the elements out (I know this can be very time consuming). If not CT, then at least surface scanning should be possible, as with the manus, although this would not yield additional anatomical information.

-The hyoid and vertebrae could also easily be surface scanned, although less important. I think – if easy to carry out - such scan data would add significantly to the work, given the rather limited nature of the material and limited wider implications as a whole, and make the work of yet greater use to workers globally.

-It would really help the reader if you cited the anatomical abbreviations (alongside the figure citation) where elements and structures are referred to in the text. I would ask for this thorought the manuscript.

-I may have misunderstood something, but there appear to be no support values for the tree nor details of characters supporting the placement of the taxon. I would expect synapomorphy lists for at least the most relevant nodes, and would also at least expect decay indices and ideally bootstrap too. ACCTRAN and DELTRAN optimisation should ideally also be included in the supplement. Running a Bayesian analysis would also be nice, but I think it’s not strictly necessary I suppose as the focus is the description. I know that the matrix is unmodified from previous work, but an additional taxon can change things…

-The end of the discussion which puts the taxon in wider context is rather lacking currently. It would be great to see it fleshed out, and the taxon discussed in the wider context of size increases in archosaur(omorph)s and ecosystem development and recovery. The final statement regarding the taxon being the last of the “Charig taxa” to be described doesn’t really relate to the rest of the paragraph, and could be fleshed out and its importance explained/underlined to those not familiar with Charig.

-The decision to maintain the taxon and not refer to S. nyassicus is probably reasonable, but could do with some further explanation I think.

-Can you make sure it is very clear where the scan data will be archived, what will be made available, and what the usage rights are?

-The way the elements are highlighted in bold where the section describing them starts is nice, but I personally find it’s not so easy to find the “headings” when scrolling through. Maybe this approach could be replaced by a bold and possibly italicized element name at the start of the first line of the paragraph for clarity/ease of finding sections, e.g. "Palatines. Most parts of the palatines..." ..or indeed with a true (possibly numbered) subheading, but that might be overkill considering the relatively short length of some of the descriptions. Possibly this issue is not so problematic with final formatting though, if the authors prefer to keep the original style….

-Other minor points (e.g. spelling in reference list) are marked up in PDF

Very best wishes,

Roland Sookias (r.sookias@gmail.com - please do not hesitate to contact me with any queries, or if you consider I may have misunderstood anything)