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Thankyou for your careful consideration in addressing each of the comments from the reviewers. I have assessed the responses myself and am satisfied that their concerns have been resolved. think this manuscript is ready for publication.
One quick note to address in production, in lines 2763 and 2764 "gingiva" is repeated twice. I assume this is a typo that will need to be fixed during typesetting.
[# PeerJ Staff Note - this decision was reviewed and approved by Laura Wilson, a PeerJ Section Editor covering this Section #]
I would like to echo the three reviewers in commending the authors on the quality of their manuscript.
All three reviewers recommend minor revisions to the work.
Please consider the reviews and annotated PDFs provided below and return a revised manuscript.
Of particular importance please:
1) Provide details on how the ontogenetic stage of each specimen was established.
2) As highlighted by Dr Erich Fitzgerald please consider adding the proposed additional figures to support the conclusions of the work.
Kind regards
Matthew McCurry
The English is very good, for both the descriptions and discussions. I spotted some typos and possibly awkward/unclear formulations (see annotated pdf). However, note that I am not a native English speaker, and that alternative wording may work better.
The reference list is complete, demonstrating the in-depth knowledge of the many topics tackled in this long manuscript by the authors. You will see in the annotated pdf some suggestions for additions, but in some cases the requested references appear later in the text. I also apologize in advance for focusing a bit my suggestions on Mystacodon (which I co-described), but in some case the results of the longer description and interpretation of this taxon may be better used (e.g., volume of oral cavity, shape of the elbow joint, anatomy of the innominate...).
The article is finely organized, both for the descriptions and discussion. Figures are adequately referenced in the text.
Figures are all excellent and extremely useful. The inclusion of reconstructions is especially helpful. In a few cases the connecting lines seem to be missing from the figure, sometimes the lines are not easy to see on the darker background, and there are a few typos in labels, but this will be very easily corrected. In some cases, I made suggestions for the addition of more information in the figure caption, for example on the ontogenetic stage of each specimen (even something like juvenile/adult would be useful).
The research questions are adequately stated. It is made very clear how this great sample fills some major gaps in the fossil record of Oligocene toothed mysticetes. I made some suggestions about the potential interest of further discussing the contribution of the new material (and new anatomical interpretations) to the debate on the feeding technique(s) of this highly unusual group, but these are only suggestions!
All the analyses and descriptions seem to have been performed more than adequately. I did not find any major flaw.
All methods are described with enough detail, making it easy to reproduce the analyses (though I did not try to reproduce the phylogenetic analyses due to software issues on my side).
Though it is not requested to assess the impact and novelty of this work, I want to state that the addition of so many new specimens greatly improves our knowledge of Coronodon, and this strong contribution will undoubtedly become a key step in the study of the archaeocete-neocete transition. In this context, the detailed discussion of potential Neoceti synapomorphies is especially welcome.
Conclusions are perfectly stated. Alternative interpretations are adequately discussed and the authors remain very cautious about the results of their phylogenetic analyses. Directions for future work and limitations are also clearly mentioned.
- Because the sample includes several individuals that are interpreted as juveniles (of different ontogenetic stages), and because the anatomy of these juveniles is later used to discuss some very interesting ontogenetic trends, I would strongly recommend starting the description of each species with a separate paragraph explaining how the ontogenetic stage of the different specimens was established. This would certainly help the reader to follow your reasoning and, for example, remove any feeling of circular reasoning. I certainly do not question your conclusions on this matter but gathering all the ontogenetic data would greatly help.
- I was quite surprised not to find a separate section in the discussion elaborating on the very interesting and provocative hypothesis proposed for the feeding strategy of Coronodon in earlier work. One would have expected to see a discussion on what the new observations bring to the interpretation of Coronodon as a tooth filter-feeder. As briefly mentioned in the text, this hypothesis has been challenged in another study (Hocking et al.), and it would be great to evaluate how the new specimens contribute to the debate, with the inclusion of comments on tooth shape, the disposition of alveoli (for example this interesting observation of imbricated upper molars on a juvenile, a feature that is lost in adult), and even tooth wear (see below). Also, the surprising similarities of crown shape with other groups of heterodont cetaceans (for example Inticetus) may be included in such a discussion, as it seems that such a crown pattern evolved independently in distant groups with a quite different rostral morphology.
- There is very little description of dental wear in the text. This is surprising considering the beautiful preservation and quantity of teeth. One would have expected a few more lines on attrition versus abrasion and possible changes through ontogeny. Other tooth damages (if present), such as broken tips of denticles, could be informative too, again in the context of the different ecological interpretations of this very interesting taxon. This is only a suggestion, but I think that it would further increase the importance of this remarkable sample.
- I noted a more minor issue in the diagnosis, where a genus is compared to specimens. I would suggest rephrasing a bit, for example adding something like 'unnamed new genus and species represented by specimen ...', to remove any potential confusion.
- In the description and discussion, specimens are sometimes only attributed to the genus Coronodon, or not associated to any taxonomic information. Due to the description of three species in this genus, this sometimes may create confusion.
- Other comments and suggestions for minor changes can be found in the annotated pdf (also in figures, tables and captions at the end of the document).
I am looking forwards to seeing this important contribution published. My congratulations to the authors.
O Lambert
The manuscript by Boessenecker, Beatty and Geisler is generally well written, and although being a lengthy paper, the length is justified by the amount of new data presented and thoughtful discussion of the potential implications of results. This was an informative and enjoyable paper to read, so congratulations to the authors on their fine work!
There are a few instances within the main text in which there are typographical and grammatical errors, or sentences that should be rewritten for clarity, but these are relatively minor and do not detract from the ideas communicated (see annotated PDF for details).
Literature cited is appropriate and relevant to the aims and content of the paper, with only a few minor inaccuracies in citation to fix (see annotated PDF for details). Due to limitations of time, I was unable to exhaustively check every reference in the list.
The included figures are generally excellent, with extremely informative photographs of the fossil specimens. If only more papers describing fossil marine mammals included photographs/figures of the high standard herein. Nice work. There are some minor errors to fix in the figure captions and labelling of figures (see annotated PDF for details). All the figures are beneficial to the presentation of results and interpretation, so I regard them all as necessary. Indeed, there is an argument to be made for a few additional figures that would enhance the communication of ideas in the paper (see General Comments below and annotated PDF for details).
Despite publication of many informative fossils and analyses thereof, the phylogenetic relationships of stem mysticetes are now more unstable than perhaps they were ~10 years ago. Related, the early evolution of filter-feeding, tooth reduction, and baleen, including functional interpretation of the craniodental morphology of key stem mysticete taxa, are contentious. Problematically, few toothed archaic mysticete taxa are described from well preserved, minimally distorted, associated skulls with earbones + teeth, as well as postcrania; and only a couple of described species are known from more than one specimen. Thus intraspecific, especially ontogenetic, variation and patterns in the earliest diverging mysticetes are a ghastly blank. These gaps in knowledge are clearly articulated in the Introduction to this paper and nicely set-up the rationale for the research presented here.
The morphological description of fossils and phylogenetic analyses are performed to a generally high standard, although there are some minor inconsistencies and questionable interpretations of homology that need to be addressed (see annotated PDF for details).
A problem with the manuscript is the omission from Material and Methods of any explanation of the approaches used to estimate ontogenetic stage in the fossils. This is surprising considering the arguably most fundamental advance in this paper being the identification of multiple individuals of one species of toothed mysticete representing ontogenetically younger and older animals. I suggest the authors explain in the Methods the ‘proven’ baseline indicators of ontogenetic stage in living Cetacea/Mammalia that were applied to these fossils to support hypothesised immature vs. mature stages of growth in Coronodon.
The new in-depth analysis of the key (probable) stem mysticete Coronodon by Boessenecker, Beatty and Geisler is an important contribution towards addressing gaps in knowledge. There are seven major outcomes in this paper:
(1) The osteology of Coronodon is described and illustrated in useful detail, providing insights on the morphology of early Neoceti.
(2) A new family Coronodonidae is named for the clade including Coronodon and related, undescribed specimens.
(3) Two new species of Coronodon, C. planifrons and C. newtoni, are named and described in detail.
(4) The morphology of juvenile and adult specimens of C. havensteini are compared, providing baseline data on ontogenetic variation within a species of toothed mysticete.
(5) The joint between the rostrum and braincase, plus mandibular symphysis, of Coronodon are interpreted as kinetic––a skull feature of living mysticetes––and as evidence bolstering the hypothesis that Coronodon was a toothed filter-feeder.
(6) Parsimony analysis under different character weighting schemes yielded different hypotheses for the phylogenetic position of Coronodon, variable placements for other toothed mysticetes, and the possibility that Coronodon + some other toothed mysticetes (e.g., Mystacodon and Borealodon) are excluded from Mysticeti and Neoceti.
(7) Some characters formerly considered diagnostic for Neoceti are perhaps more broadly distributed amongst Cetacea.
Conclusions (1)–(4), (6), and (7) are generally well supported by the results and analyses presented herein, with underlying data included. Some of the descriptions and evidence put forward in support of taxonomic/phylogenetic conclusions (1–4) were challenging, if not impossible, to verify during review because the anatomy was not illustrated/labelled in the included figures: to improve the manuscript, additional figures are required (see General Comments below and annotated PDF for details).
It is clear from recent literature, and the authors’ own admission, that conclusion (6) is another iteration in the quest for some consensus/stability, if not solution, to the interrelationships of toothed mysticetes and the base of Neoceti. More questions are uncovered than answers given, which is a fine thing.
Conclusion (5) and the evidence presented in support, needs some reconsideration by the authors, with my main suggestion for improvement being to rethink how closely comparable the morphology (and function) of the mandibular symphysis and temporomandibular joint of Coronodon are with those of extant filter-feeding mysticetes (see annotated PDF for details).
I really like this paper: the authors are to be commended on what will be a useful resource on cetacean osteology, systematics, and provocative discussion of character evolution across the archaeocete-Neoceti transition.
To sum up, my major suggestions for improving what is already a good manuscript are:
(1) Provide a detailed explanation of the Methods used to estimate the ontogenetic stage of each specimen, especially the ontogenetic series of Coronodon havensteini.
(2) Add the following 5 figures to the paper that will augment the existing figures and enhance comprehension and verification of the written descriptions/comparisons and character codings:
(a) photograph + annotated photograph/diagram of the orbital wall and surrounding region in oblique antero-ventrolateral view.
(b) annotated photograph/diagram of the auditory region of the basicranium (without periotic in articulation) of Coronodon havensteini, labelling all of the many anatomical features described in the text; this could be based on the photo in Fig. 13F.
(c) photograph + annotated photograph/diagram of the anatomical details of the internal acoustic meatus and adjacent features of the pars cochlearis of periotic, in endocranial/dorsomedial view, in Coronodon havensteini.
(d) photograph + annotated photograph/diagram of the anatomical details of the internal acoustic meatus and adjacent features of the pars cochlearis of periotic, in endocranial/dorsomedial view, in Coronodon newtoni.
(e) photograph + annotated photograph/diagram of the anatomical details of the internal acoustic meatus and adjacent features of the pars cochlearis of periotic, in endocranial/dorsomedial view, in Coronodon planifrons.
Thank you for doing such great research and putting together such a well-written, scientifically interesting, and superbly illustrated manuscript.
Erich Fitzgerald
Please see the attached pdf file.
Please see the attached pdf file.
Please see the attached pdf file.
Please see the attached pdf file.
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