Dinosaurs from the Santonian–Campanian Atlantic coastline substantiate phylogenetic signatures of vicariance in Cretaceous North America

During the Cretaceous, diversifications and turnovers affected terrestrial vertebrates experiencing the effects of global geographical change. However, the poor fossil record from the early Late Cretaceous has concealed how dinosaurs and other terrestrial vertebrates responded to these events. I describe two dinosaurs from the Santonian to Early Campanian of the obscure North American paleolandmass Appalachia. A revised look at a large, potentially novel theropod shows that it likely belongs to a new clade of tyrannosauroids solely from Appalachia. Another partial skeleton belongs to an early member of the Hadrosauridae, a highly successful clade of herbivorous dinosaurs. This skeleton is associated with the first small juvenile dinosaur specimens from the Atlantic Coastal Plain. The tyrannosauroid and hadrosaurid substantiate one of the only Late Santonian dinosaur faunas and help pinpoint the timing of important anatomical innovations in two widespread dinosaur lineages. The phylogenetic positions of the tyrannosauroid and hadrosaurid show Santonian Appalachian dinosaur faunas are comparable to coeval Eurasian ones, and the presence of clades formed only by Appalachian dinosaur taxa establishes a degree of endemism in Appalachian dinosaur assemblages attributable to episodes of vicariance.


Recommendation? Accept with minor revision (please list in comments)
Comments to the Author(s) I have now read the revised manuscript from Brownstein, as well as their reviewer responses.
While I remain personally unconvinced about the need to formally name this as a new tyrannosauroid taxon, the data the author has added certainly strengthens their argument, so I won't object to it further. I think some issues remain with the quantitative/morphometric analyses however (detailed below), so when proceeding with naming this taxon I think they should lean primarily on their comparative descriptions and qualitative results to frame their argument, doubly so given the revised descriptions and improved emphasis on hypothesized autapomorphies in the manuscript.
In terms of the specific changes in the article since the last version, I think the Discussion is much improved, particularly with respect to addressing issues raised with Appalachian assemblages being described as refugia, and in their reframing of the biogeography discussion.
Regarding the quantitative analyses, the issues I raised in the last review were largely unaddressed in the revised version. In the review response, the author acknowledges the issues in the analyses (particularly in this case the morphometrics and robusticity index comparisons), but their response of adding a few additional specimens does not actually address the issues that I described. To be clear, I don't think the issues in the quantitative analyses are make-or-break for this study, and I think these analyses could be minimized or removed, with the focus kept on the comparative description and biogeography. If the author wanted to keep the quantitative analyses though, they will need some work, which I will go through below.

1) Robusticity Index
The issues raised about the robusticity analysis relate to the data quality and variability. Data for each species derive from point samples of single individuals, with the exception of Albertosaurus (where N=5). The author argues that the lower robusticity index value associated with their new taxon is reflective of phylogenetic position, with it being most similar to older tyrannosauroids. However, the robusticity index results provided in Figure 3A show that Cryptotyrannus is about as similar in RI to basal tyrannosauroids as it is to later tyrannosaurids like Albertosaurus. As well, the 5 sampled individuals of Albertosaurus demonstrate that a substantial variation in RI can be expected in a single taxon (spanning about 40% of the total range of RI values recorded across all taxa in the analysis). While it is possible that this measure can be used to some effect in contrasting non-tyrannosauroids and tyrannosauroids (as argued in Peecook et al 2014), the large amount of variation and equivocal positioning of Cryptotyrannus in the overall RI distribution would suggest that this analysis adds little to no support to the author's arguments.
To illustrate my point, consider that in the text (lines 312-325) the authors argue that the metatarsus of Cryptotyrannus is most similar in robusticity to taxa like Alectrosaurus and distinct from the more robust condition shown in tyrannosaurids. Looking at the measured RI values, Cryptotyrannus has an RI value that is different from Albertosaurus by 0.027, and different from Alectrosaurus by 0.016. So the statement is technically true that Cryptotyrannus is more similar to Alectrosaurus than to most tyrannosaurids, but it is also misleading for several reasons. First, this difference represents not only a very small difference in the sense of the overall range of RI measured across the various species sampled, but also that within the only species where data for more than one individual exists (Albertosaurus), there is a range of RI of ~0.09 (from 0.32 to 0.41). The range of variation within this taxon alone is more than 3x the magnitude of difference that exists separating Cryptotyrannus from Albertosaurus. As well, the closest actual RI value to Cryptotyrannus is not even Alectrosaurus, but rather a juvenile of Bistahieversor (RI of .295 vs .297) Relatedly, the authors in the manuscript text also argue that Cryptotyrannus is very similar in robusticity to 'older tyrannosauroids' like Moros or Suskityrannus while referring to Figure 3A. As neither of those taxa are included in the RI analysis, I think this statement is at best a misleading reference to Cryptotyrannus being most similar in the RI analysis to 'older tyrannosauroids' like Alectrosaurus, though again as noted above it is actually most similar to a juvenile Bistahieversor (which although is a tyrannosauroid, isn't exactly 'older', occurring in younger rocks than Cryptotyrannus does), and fairly close in RI between Alectrosaurus and Albertosaurus specimens.
With all of that in mind, I think the robusticity analysis should probably be removed. I think at best it is unrelated or unsupportive of your argument that Cryptotyrannus is a distinct taxon, and at worst is actively working against your argument.

2) Linear Morphometrics & PCA
The issues raised with the linear morphometrics & PCA related primarily to the results not accounting for size-related effects. The authors may have added in some new measurements and specimens, but it hasn't fundamentally altered that problem. Looking at the PC data in the supplementary information, PC1 in the revised analysis now explains 87% of the variance (compared to 85% in the original analysis), with PC2 explaining about 7% of the variance (down from 12% in the original version). So even with the measurements from the three ornithomimosaurs & the juvenile tyrannosauroid, you're still getting results which are almost entirely reflecting the absolute size differences of the specimens (primarily in length, to a lesser extent circumference). This is also reflected in the loadings, where you can see that length and circumference are the most strongly related measured to PC1 and PC2. Consequently, the resulting plots don't provide a result that supports the argument that this is a distinct species based on various morphological characteristics, it just shows that it is most similar in size to a juvenile specimen of Bistahieversor. Moros remains an outlier on PC2 (though consistent with the overall size-driven pattern on PC1), but as about half of the measurements for Moros' are missing data (particularly for values like circumference and proximal transverse width which are strongly related to the PC2 loadings), it is difficult to fully characterize what is going on there. As an aside, the author doesn't explain in the Methods section what they did to account for missing measurements in their PCA. There are several ways to deal with missing data, but it's unclear here which of those they did. In any event though, the morphometric results still have the issue I noted last time, namely that as they are almost entirely related to the size of the specimen being measured, they aren't providing anything informative to support your argument of Cryptotyrannus being a distinct taxon. I suppose you could use these results to argue that it's something very similar to Bistahieversor, and/or the juvenile of a larger more robust tyrannosauroid, but I suspect you would prefer not to do so, given that it would work against the argument from the qualitative data / comparative description that this material represents a new taxon. To that end, I think you may want to consider removing the morphometric analyses. If you are going to keep them in, then I really need to emphasize the need to broaden them not by adding more taxa, but by plotting the results against a known size measurement you can account for in order to standardize things and hopefully tease out some more informative results about the shape differences between these sampled specimens.
Some more minor corrections: Line 776: South America, not South American Lines 824-828: I think there may be some words missing in this sentenc Lines 883-903: the font changes somewhat abruptly here, not sure if that is just an issue in my copy of the PDF, but perhaps best to confirm Lines 893-903: perhaps could be interesting to compare and contrast what you're seeing between Laramidian tyrannosaurines (large size, big heads, small forelimbs) vs Appalachian dryptosaurids with the independent evolution of some of the features of the former group (large size, big heads, small forelimbs) in South American carcharodontosaurs while these animals similarly co-existed with distantly-related allosauroids that didn't go down that path. Not to imply that these groups were totally convergent, given that they did differ in some other features, such as growth rate (e.g. Cullen et al 2020), but there may be some similarity in the patterns underlying the evolution of those suites of characteristics, and how in both times/places you are seeing some large predator clades with those features and some related clades without them.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? Yes

Recommendation? Reject
Comments to the Author(s) While I find that the biogeographic issues have been solved by their removal, I still have issues with the taxonomy and the characters used to justify the identification of the theropod material. I remain unconvinced that the metatarsus can be referred to Tyrannosauroidea, or that it is sufficiently diagnostic to erect a new taxon. The other reviewer raised these issues in the first submission and although my review of that version focussed on biogeography, I agree with their points.
My issues fall along two main lines, focusing on the theropod section. First, I believe broader comparisons are necessary to establish that the metatarsus is, in fact, tyrannosauroid. Again, I realize that this conclusion stems from earlier work, but seeing as this re-description overlaps significantly with the 2017 paper, I think re-examining these points is justified. Second, I am unconvinced that the material is complete enough or the characters distinct enough from other tyrannosauroids to erect a new taxon. The author themselves commented on this issue in their 2017 paper, stating that "Based on the incomplete nature of YPM VPPU.021795, the author does not think it wise to name a new taxon based on this specimen." (Brownstein 2017, pg. 14). I do not see how the situation has changed. The only new specimen is the partial caudal vertebra, which does not provide any characters to distinguish the material from other tyrannosaurs.
As a separate point, the proximal views of the metatarsals reveal labels "II" and "IV" that contradict the identification presented in the manuscript. Why were these identifications switched? I don't see a comment on this in either this manuscript or the 2017 paper.
The author hinges the identification as a tyrannosaur on four main characters: 1) the presence of a deep, V-shaped notch on the proximal end of Metatarsal IV [also see previous comment]; 2) large, teardrop-shaped facets for Metatarsal III on the outer metatarsals; 3) an arctometatarsalian metatarsus where the proximal end of Metatarsal III is hidden in anterior view; and 4) visible muscle insertions on the posterior side of the metatarsals.
The author states that the four characters ally the specimen with eutyrannosaur tyrannosauroids, and exclude it from other theropod groups. Each of these characters is problematic, however. With regards to the first character, a deep notch on metatarsal IV, there are several issues. This character seems to stem from Peecook et al. (2014), wherein they describe this notch in tyrannosaurs as 'pointed', not 'deep'. First, this region of the specimen is heavily reconstructed and the depth of this notch may be overexaggerated. Second, if this is actually Metatarsal II, as the labelling in the figure shows, then the distribution of this character will have to be reassessed. Third, the author states that only two ornithomimid genera approach tyrannosaurs in robustness, but that they do not have a deep notch on Metatarsal IV. However, the author also shows that their specimen is less robust than other tyrannosaurs, so it is unclear why only the robust ornithomimids formed the comparison. Indeed, other ornithomimids, such as the species of Ornithomimus, do possess a deep notch on metatarsal IV (Claessens and Loewen 2015). Furthermore, nearly all arctometatarsalian theropods (caenagnathids, ornithomimids, troodontids, and tyrannosaurs) have a notch of some kind in this position, as noted by Peecook et al. (2014) in their initial description of the character. Without a quantitative analysis, it is unclear where the cutoff between 'deep' and 'shallow' might be, and it is also unclear from reading Peecook et al. (2014) whether the depth of the notch or its shape is the most critical factor. Considering the extensive reconstruction of this region, I do not think the shape can be confidently determined. With regards to the second character, teardrop-shaped facets for Metatarsal III, I am not convinced that these can actually be identified in the specimen because of its incompleteness. Metatarsal II is broken distally, so the distal extent and shape of this facet cannot be determined. The same is partly true for Metatarsal IV, as well, and this region is extensively damaged, if not completely missing. Without being able to observe the entire extent of the buttressing surface, I am not sure how its shape can be confidently determined. I will agree that the shape does expand distally, but this is true of all arctometatarsalian theropods, to my knowledge. My interpretation of this character is that the key feature is the rounded distal terminus of the facet, as other theropods sometimes have pointed or oblique termini to the facet. However, I do not see how a rounded distal end of the facet could be established when these regions are so damaged. I even have trouble making out the proximal parts of the facet from the figures, and these could be better shown with an illustration or 3D model. Furthermore, the author specifically notes that caenagnathids differ from tyrannosaurs in having a "poorly developed" facet for Metatarsal III, but I disagree with this assessment, as those of caenagnathids are actually fairly prominent. The author claims that these facets 'abut the distal condyles', citing one of my figures as support for this assertion. However, that figure is not sufficiently detailed to allow this to be evaluated, and, indeed, this interpretation is false. The facets for Metatarsal III are separated from the distal condyles by a region of smooth bone as in all arctometatarsalian theropods. In that figure, this region exists where the shaft of the metatarsal is constricted in anterior view. I have also attached photographs of ROM 781, a caenagnathid metatarsus, that clearly show this (see Appendices A & B). The third character, focused on the structure of the arctometatarsus, is perhaps the most problematic. The author states that tyrannosaurs are distinguished by an arctometatarsalian structure in which Metatarsal III is hidden from the anterior surface at its proximal end. This has been considered a diagnostic feature of caenagnathids (Currie and Russell 1988, Currie 1989, Funston et al. 2016, Funston 2020, but in re-examining its distribution, I find it is also present in some ornithomimids (e.g. Ornithomimus velox Claessens et al. 2015), although less welldeveloped because metatarsal III is narrower at this point. Thus, this feature cannot be used to distinguish tyrannosaurs from caenagnathids or ornithomimids, and it may not even be useful for distinguishing any theropod family in its current phrasing. Finally, M. gastrocnemius lateralis should be present in the same position in almost every theropod, although I am not sure whether its insertion varies in morphology between groups. However, it is difficult to assess the source for this character as all of the references for diagnostic characters are listed at the end of the sentence, rather than alongside the information they provided. Nonetheless, I am suspicious that differences in prominence of a scar of a muscle all theropods share might be more attributable to ontogeny or body mass, rather than taxonomy.
The author also provides some other features to distinguish the specimen from caenagnathids, but they have confused Metatarsals II and IV. The image they figure and the comparisons they draw are actually metatarsal IV, not metatarsal II. Metatarsal II can have a posterior process (e.g., Funston et al. 2015 Cret Res), although this is variable even within the group (and thus suggestive that it is too variable to clearly define differences between families). It is also worth noting that in their figure, the caenagnathid metatarsal (which is IV, not II) is covered by distal tarsal IV, which obscures the proximal shape.
As for the comparisons to other tyrannosaurs, some of the characters in the diagnosis are problematic in light of the contradictory labelling on the metatarsals. For example, the absence of a deep facet on the proximal surface of metatarsal II would be untrue if the labels shown in the figures are correct, as could the difference in proximal shape of metatarsal IV from Dryptosaurus. Finally, it is worth noting that many of the autapomorphies are simply the absence of the autapomorphies of Dryptosaurus, and these character states are widely distributed within Tyrannosauroidea (but not the two-taxon Dryptosauridae). Thus, most of these characters cannot be used to distinguish the taxon from other tyrannosaurs (e.g. the apex of the distal condyles, the mediolateral constriction, and the sulcus between the distal hemicondyles). In my opinion, these diagnostic characters are weak and may be oversplitting, by taking variation at face value with no consideration of allometric, ontogenetic or individual variation. This is especially problematic because of the fragmentary and reconstructed nature of the material. I agree with the other reviewer that the material is not sufficient to confidently state that the variation described is significant, without a broader analysis of individual and taxonomic variation within tyrannosaurs.
The PCA is insufficient, and needs to be size-scaled in some way. As mentioned by the other reviewer and even by the author in the description of the results, it is mostly size-driven. Ornithomimids clearly fall into the range of tyrannosaurs on the y-axis, so if size were eliminated, they may overlap considerably. I also do not think that including three ornithomimosaur points (but no other kinds of theropods) is sufficient, especially when there is significant variation within each group (e.g. Gallimimus, Deinocheirus, Anserimimus, which are all considerably different in metatarsal structure but aren't included). Furthermore, ornithomimosaurs and other theropods are not included in the robusticity analysis, and the author has not responded to the other reviewer's criticisms of the use of robusticity index, which are quite valid. If anything, the quantitative analyses show to me that the author has not included enough comparisons in the study to be confident that the variation they show is meaningful.
To summarize my criticisms on the theropod part of the paper, I am not satisfied that the author has compared the material broadly enough to demonstrate that it pertains to any particular group of theropod dinosaurs. The characters they use are present in multiple groups but they restrict the comparisons unnecessarily-particularly with regards to focusing on ornithomimids, and then only a subset of ornithomimids. Based on the description and figures, I am unconvinced that the material can be referred to a more specific clade than Coelurosauria indet., and I think erecting a new taxon for the material is unsupported.
Considering that I do not think the "tyrannosaur" taxon should be named, I do not see a logical connection between it and the rest of the manuscript. To me, it is mostly a rehash of the 2017 paper, and provides little beyond the inclusion into new phylogenies. Now that the biogeographic section has been removed, there is little to link the two sections of the manuscript. I think the tyrannosaur section should be dropped, and the focus should be placed solely on the hadrosaur material. To my eye, the hadrosaur section of the manuscript is much stronger because there is more material and more comparisons are drawn with other taxa.

Review form: Reviewer 3
Is the manuscript scientifically sound in its present form? Yes

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
This is a welcomed and much needed contribution to our understanding of dinosaurian diversity, anatomy, relationships and biogeography in Appalachia, particularly given that the dinosaurian evolutionary history in Appalachia has been obscured by a patchy fossil record. The paper is generally well written and full of interesting and useful data. I cannot comment on the tyrannosauroid, as I am not an expert on this clade and I lack sufficient knowledge on tyrannosauroid anatomy to evaluate that part of the MS. I can, however, comment on the hadrosaurid. Also, I must say that I already reviewed an earlier version of this MS, previously submitted to another journal. Therefore, I do not have a lot to say this time, particularly because I see that my issues with that version have been addressed in the present one. I only have some minor remarks, listed below. Otherwise, I recommend publication of this paper pending those minor revisions.

Systematic Paleontology
Description Line 530: "the presence of dental battery". Actually, non-hadrosaurid hadrosauroids also posses a dental battery. The difference is that this battery become more complex in hadrosaurids, having more than 30 tooth positions and with an occlusal plane reaching a maximum of three in width in the dentary dental battery.
Line 540-541: "The holotype and referred dentary likely come from individuals nearing or at osteological maturity….". You are not erecting a new genus or species, thus what holotype are you referring to? "Potentially" new hadrosaurid does not mean it is a new hadrosaurid.
Line 607: "The inside of the dentary is perforated by a hole.." Not hole, "foramen" is the word. Line 776: should be "South America", not "South American".
Line 810: not "evinces" but "evidences". Figures 2, 4, 5, and 6 are missing. Maybe you forgot to upload them? Or a glitch of the manuscript site of the journal? Anyway, I cannot fully evaluate many of the characters of the osteology without seeing those images. Figure S4B: The cladogram shows Atlantohadros, a genus name that does not exist as you are not erecting it (probably a remnant from an earlier version of the manuscript?).

Decision letter (RSOS-210127.R0)
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Royal Society Open Science openscience@royalsociety.org on behalf of Dr Julia Brenda Desojo (Associate Editor) and Kevin Padian (Subject Editor) openscience@royalsociety.org Subject Editor Comments to Author (Professor Kevin Padian): Comments to the Author: Thanks for your resubmission. We have three somewhat different reviews here with some common threads and some strong concerns. First, the reviewers appreciate the improvement from the previous submission. And there are several remaining issues.
--the two reviewers who feel confident to assess the theropod taxonomy don't agree that this has unique features that warrant naming a new species, and one reviewer is not convinced that it is even a tyrannosauroid.
--one reviewer believes that it is better concentrate on the hadrosaurid material, and another offers some improvements in the interpretation of that material.
--one reviewer is very concerned about the statistics and morphometrics and does not believe that they are appropriately applied and so they should be removed. Our AE, without comment, has recommended "major revision." I will support that, and hope that you will reconsider some of the focus of your paper and whether it is really justified to name a new tyrannosaur. Whatever your decision, please notify our editorial office if you need more time to revise. Best wishes.
Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) I have now read the revised manuscript from Brownstein, as well as their reviewer responses.
While I remain personally unconvinced about the need to formally name this as a new tyrannosauroid taxon, the data the author has added certainly strengthens their argument, so I won't object to it further. I think some issues remain with the quantitative/morphometric analyses however (detailed below), so when proceeding with naming this taxon I think they should lean primarily on their comparative descriptions and qualitative results to frame their argument, doubly so given the revised descriptions and improved emphasis on hypothesized autapomorphies in the manuscript.
In terms of the specific changes in the article since the last version, I think the Discussion is much improved, particularly with respect to addressing issues raised with Appalachian assemblages being described as refugia, and in their reframing of the biogeography discussion.
Regarding the quantitative analyses, the issues I raised in the last review were largely unaddressed in the revised version. In the review response, the author acknowledges the issues in the analyses (particularly in this case the morphometrics and robusticity index comparisons), but their response of adding a few additional specimens does not actually address the issues that I described. To be clear, I don't think the issues in the quantitative analyses are make-or-break for this study, and I think these analyses could be minimized or removed, with the focus kept on the comparative description and biogeography. If the author wanted to keep the quantitative analyses though, they will need some work, which I will go through below.

1) Robusticity Index
The issues raised about the robusticity analysis relate to the data quality and variability. Data for each species derive from point samples of single individuals, with the exception of Albertosaurus (where N=5). The author argues that the lower robusticity index value associated with their new taxon is reflective of phylogenetic position, with it being most similar to older tyrannosauroids. However, the robusticity index results provided in Figure 3A show that Cryptotyrannus is about as similar in RI to basal tyrannosauroids as it is to later tyrannosaurids like Albertosaurus. As well, the 5 sampled individuals of Albertosaurus demonstrate that a substantial variation in RI can be expected in a single taxon (spanning about 40% of the total range of RI values recorded across all taxa in the analysis). While it is possible that this measure can be used to some effect in contrasting non-tyrannosauroids and tyrannosauroids (as argued in Peecook et al 2014), the large amount of variation and equivocal positioning of Cryptotyrannus in the overall RI distribution would suggest that this analysis adds little to no support to the author's arguments.
To illustrate my point, consider that in the text (lines 312-325) the authors argue that the metatarsus of Cryptotyrannus is most similar in robusticity to taxa like Alectrosaurus and distinct from the more robust condition shown in tyrannosaurids. Looking at the measured RI values, Cryptotyrannus has an RI value that is different from Albertosaurus by 0.027, and different from Alectrosaurus by 0.016. So the statement is technically true that Cryptotyrannus is more similar to Alectrosaurus than to most tyrannosaurids, but it is also misleading for several reasons. First, this difference represents not only a very small difference in the sense of the overall range of RI measured across the various species sampled, but also that within the only species where data for more than one individual exists (Albertosaurus), there is a range of RI of ~0.09 (from 0.32 to 0.41). The range of variation within this taxon alone is more than 3x the magnitude of difference that exists separating Cryptotyrannus from Albertosaurus. As well, the closest actual RI value to Cryptotyrannus is not even Alectrosaurus, but rather a juvenile of Bistahieversor (RI of .295 vs .297) Relatedly, the authors in the manuscript text also argue that Cryptotyrannus is very similar in robusticity to 'older tyrannosauroids' like Moros or Suskityrannus while referring to Figure 3A. As neither of those taxa are included in the RI analysis, I think this statement is at best a misleading reference to Cryptotyrannus being most similar in the RI analysis to 'older tyrannosauroids' like Alectrosaurus, though again as noted above it is actually most similar to a juvenile Bistahieversor (which although is a tyrannosauroid, isn't exactly 'older', occurring in younger rocks than Cryptotyrannus does), and fairly close in RI between Alectrosaurus and Albertosaurus specimens.
With all of that in mind, I think the robusticity analysis should probably be removed. I think at best it is unrelated or unsupportive of your argument that Cryptotyrannus is a distinct taxon, and at worst is actively working against your argument.

2) Linear Morphometrics & PCA
The issues raised with the linear morphometrics & PCA related primarily to the results not accounting for size-related effects. The authors may have added in some new measurements and specimens, but it hasn't fundamentally altered that problem. Looking at the PC data in the supplementary information, PC1 in the revised analysis now explains 87% of the variance (compared to 85% in the original analysis), with PC2 explaining about 7% of the variance (down from 12% in the original version). So even with the measurements from the three ornithomimosaurs & the juvenile tyrannosauroid, you're still getting results which are almost entirely reflecting the absolute size differences of the specimens (primarily in length, to a lesser extent circumference). This is also reflected in the loadings, where you can see that length and circumference are the most strongly related measured to PC1 and PC2. Consequently, the resulting plots don't provide a result that supports the argument that this is a distinct species based on various morphological characteristics, it just shows that it is most similar in size to a juvenile specimen of Bistahieversor. Moros remains an outlier on PC2 (though consistent with the overall size-driven pattern on PC1), but as about half of the measurements for Moros' are missing data (particularly for values like circumference and proximal transverse width which are strongly related to the PC2 loadings), it is difficult to fully characterize what is going on there. As an aside, the author doesn't explain in the Methods section what they did to account for missing measurements in their PCA. There are several ways to deal with missing data, but it's unclear here which of those they did. In any event though, the morphometric results still have the issue I noted last time, namely that as they are almost entirely related to the size of the specimen being measured, they aren't providing anything informative to support your argument of Cryptotyrannus being a distinct taxon. I suppose you could use these results to argue that it's something very similar to Bistahieversor, and/or the juvenile of a larger more robust tyrannosauroid, but I suspect you would prefer not to do so, given that it would work against the argument from the qualitative data / comparative description that this material represents a new taxon. To that end, I think you may want to consider removing the morphometric analyses. If you are going to keep them in, then I really need to emphasize the need to broaden them not by adding more taxa, but by plotting the results against a known size measurement you can account for in order to standardize things and hopefully tease out some more informative results about the shape differences between these sampled specimens.
Some more minor corrections: Line 776: South America, not South American Lines 824-828: I think there may be some words missing in this sentenc Lines 883-903: the font changes somewhat abruptly here, not sure if that is just an issue in my copy of the PDF, but perhaps best to confirm Lines 893-903: perhaps could be interesting to compare and contrast what you're seeing between Laramidian tyrannosaurines (large size, big heads, small forelimbs) vs Appalachian dryptosaurids with the independent evolution of some of the features of the former group (large size, big heads, small forelimbs) in South American carcharodontosaurs while these animals similarly co-existed with distantly-related allosauroids that didn't go down that path. Not to imply that these groups were totally convergent, given that they did differ in some other features, such as growth rate (e.g. Cullen et al 2020), but there may be some similarity in the patterns underlying the evolution of those suites of characteristics, and how in both times/places you are seeing some large predator clades with those features and some related clades without them.
Reviewer: 2 Comments to the Author(s) While I find that the biogeographic issues have been solved by their removal, I still have issues with the taxonomy and the characters used to justify the identification of the theropod material. I remain unconvinced that the metatarsus can be referred to Tyrannosauroidea, or that it is sufficiently diagnostic to erect a new taxon. The other reviewer raised these issues in the first submission and although my review of that version focussed on biogeography, I agree with their points.
My issues fall along two main lines, focusing on the theropod section. First, I believe broader comparisons are necessary to establish that the metatarsus is, in fact, tyrannosauroid. Again, I realize that this conclusion stems from earlier work, but seeing as this re-description overlaps significantly with the 2017 paper, I think re-examining these points is justified. Second, I am unconvinced that the material is complete enough or the characters distinct enough from other tyrannosauroids to erect a new taxon. The author themselves commented on this issue in their 2017 paper, stating that "Based on the incomplete nature of YPM VPPU.021795, the author does not think it wise to name a new taxon based on this specimen." (Brownstein 2017, pg. 14). I do not see how the situation has changed. The only new specimen is the partial caudal vertebra, which does not provide any characters to distinguish the material from other tyrannosaurs. As a separate point, the proximal views of the metatarsals reveal labels "II" and "IV" that contradict the identification presented in the manuscript. Why were these identifications switched? I don't see a comment on this in either this manuscript or the 2017 paper.
The author hinges the identification as a tyrannosaur on four main characters: 1) the presence of a deep, V-shaped notch on the proximal end of Metatarsal IV [also see previous comment]; 2) large, teardrop-shaped facets for Metatarsal III on the outer metatarsals; 3) an arctometatarsalian metatarsus where the proximal end of Metatarsal III is hidden in anterior view; and 4) visible muscle insertions on the posterior side of the metatarsals.
The author states that the four characters ally the specimen with eutyrannosaur tyrannosauroids, and exclude it from other theropod groups. Each of these characters is problematic, however. With regards to the first character, a deep notch on metatarsal IV, there are several issues. This character seems to stem from Peecook et al. (2014), wherein they describe this notch in tyrannosaurs as 'pointed', not 'deep'. First, this region of the specimen is heavily reconstructed and the depth of this notch may be overexaggerated. Second, if this is actually Metatarsal II, as the labelling in the figure shows, then the distribution of this character will have to be reassessed. Third, the author states that only two ornithomimid genera approach tyrannosaurs in robustness, but that they do not have a deep notch on Metatarsal IV. However, the author also shows that their specimen is less robust than other tyrannosaurs, so it is unclear why only the robust ornithomimids formed the comparison. Indeed, other ornithomimids, such as the species of Ornithomimus, do possess a deep notch on metatarsal IV (Claessens and Loewen 2015). Furthermore, nearly all arctometatarsalian theropods (caenagnathids, ornithomimids, troodontids, and tyrannosaurs) have a notch of some kind in this position, as noted by Peecook et al. (2014) in their initial description of the character. Without a quantitative analysis, it is unclear where the cutoff between 'deep' and 'shallow' might be, and it is also unclear from reading Peecook et al. (2014) whether the depth of the notch or its shape is the most critical factor. Considering the extensive reconstruction of this region, I do not think the shape can be confidently determined.
With regards to the second character, teardrop-shaped facets for Metatarsal III, I am not convinced that these can actually be identified in the specimen because of its incompleteness. Metatarsal II is broken distally, so the distal extent and shape of this facet cannot be determined. The same is partly true for Metatarsal IV, as well, and this region is extensively damaged, if not completely missing. Without being able to observe the entire extent of the buttressing surface, I am not sure how its shape can be confidently determined. I will agree that the shape does expand distally, but this is true of all arctometatarsalian theropods, to my knowledge. My interpretation of this character is that the key feature is the rounded distal terminus of the facet, as other theropods sometimes have pointed or oblique termini to the facet. However, I do not see how a rounded distal end of the facet could be established when these regions are so damaged. I even have trouble making out the proximal parts of the facet from the figures, and these could be better shown with an illustration or 3D model. Furthermore, the author specifically notes that caenagnathids differ from tyrannosaurs in having a "poorly developed" facet for Metatarsal III, but I disagree with this assessment, as those of caenagnathids are actually fairly prominent. The author claims that these facets 'abut the distal condyles', citing one of my figures as support for this assertion. However, that figure is not sufficiently detailed to allow this to be evaluated, and, indeed, this interpretation is false. The facets for Metatarsal III are separated from the distal condyles by a region of smooth bone as in all arctometatarsalian theropods. In that figure, this region exists where the shaft of the metatarsal is constricted in anterior view. I have also attached photographs of ROM 781, a caenagnathid metatarsus, that clearly show this. The third character, focused on the structure of the arctometatarsus, is perhaps the most problematic. The author states that tyrannosaurs are distinguished by an arctometatarsalian structure in which Metatarsal III is hidden from the anterior surface at its proximal end. This has been considered a diagnostic feature of caenagnathids (Currie and Russell 1988, Currie 1989, Funston et al. 2016, Funston 2020, but in re-examining its distribution, I find it is also present in some ornithomimids (e.g. Ornithomimus velox Claessens et al. 2015), although less welldeveloped because metatarsal III is narrower at this point. Thus, this feature cannot be used to distinguish tyrannosaurs from caenagnathids or ornithomimids, and it may not even be useful for distinguishing any theropod family in its current phrasing. Finally, M. gastrocnemius lateralis should be present in the same position in almost every theropod, although I am not sure whether its insertion varies in morphology between groups. However, it is difficult to assess the source for this character as all of the references for diagnostic characters are listed at the end of the sentence, rather than alongside the information they provided. Nonetheless, I am suspicious that differences in prominence of a scar of a muscle all theropods share might be more attributable to ontogeny or body mass, rather than taxonomy.
The author also provides some other features to distinguish the specimen from caenagnathids, but they have confused Metatarsals II and IV. The image they figure and the comparisons they draw are actually metatarsal IV, not metatarsal II. Metatarsal II can have a posterior process (e.g., Funston et al. 2015 Cret Res), although this is variable even within the group (and thus suggestive that it is too variable to clearly define differences between families). It is also worth noting that in their figure, the caenagnathid metatarsal (which is IV, not II) is covered by distal tarsal IV, which obscures the proximal shape.
As for the comparisons to other tyrannosaurs, some of the characters in the diagnosis are problematic in light of the contradictory labelling on the metatarsals. For example, the absence of a deep facet on the proximal surface of metatarsal II would be untrue if the labels shown in the figures are correct, as could the difference in proximal shape of metatarsal IV from Dryptosaurus. Finally, it is worth noting that many of the autapomorphies are simply the absence of the autapomorphies of Dryptosaurus, and these character states are widely distributed within Tyrannosauroidea (but not the two-taxon Dryptosauridae). Thus, most of these characters cannot be used to distinguish the taxon from other tyrannosaurs (e.g. the apex of the distal condyles, the mediolateral constriction, and the sulcus between the distal hemicondyles). In my opinion, these diagnostic characters are weak and may be oversplitting, by taking variation at face value with no consideration of allometric, ontogenetic or individual variation. This is especially problematic because of the fragmentary and reconstructed nature of the material. I agree with the other reviewer that the material is not sufficient to confidently state that the variation described is significant, without a broader analysis of individual and taxonomic variation within tyrannosaurs.
The PCA is insufficient, and needs to be size-scaled in some way. As mentioned by the other reviewer and even by the author in the description of the results, it is mostly size-driven. Ornithomimids clearly fall into the range of tyrannosaurs on the y-axis, so if size were eliminated, they may overlap considerably. I also do not think that including three ornithomimosaur points (but no other kinds of theropods) is sufficient, especially when there is significant variation within each group (e.g. Gallimimus, Deinocheirus, Anserimimus, which are all considerably different in metatarsal structure but aren't included). Furthermore, ornithomimosaurs and other theropods are not included in the robusticity analysis, and the author has not responded to the other reviewer's criticisms of the use of robusticity index, which are quite valid. If anything, the quantitative analyses show to me that the author has not included enough comparisons in the study to be confident that the variation they show is meaningful.
To summarize my criticisms on the theropod part of the paper, I am not satisfied that the author has compared the material broadly enough to demonstrate that it pertains to any particular group of theropod dinosaurs. The characters they use are present in multiple groups but they restrict the comparisons unnecessarily-particularly with regards to focusing on ornithomimids, and then only a subset of ornithomimids. Based on the description and figures, I am unconvinced that the material can be referred to a more specific clade than Coelurosauria indet., and I think erecting a new taxon for the material is unsupported.
Considering that I do not think the "tyrannosaur" taxon should be named, I do not see a logical connection between it and the rest of the manuscript. To me, it is mostly a rehash of the 2017 paper, and provides little beyond the inclusion into new phylogenies. Now that the biogeographic section has been removed, there is little to link the two sections of the manuscript. I think the tyrannosaur section should be dropped, and the focus should be placed solely on the hadrosaur material. To my eye, the hadrosaur section of the manuscript is much stronger because there is more material and more comparisons are drawn with other taxa.
Reviewer: 3 Comments to the Author(s) This is a welcomed and much needed contribution to our understanding of dinosaurian diversity, anatomy, relationships and biogeography in Appalachia, particularly given that the dinosaurian evolutionary history in Appalachia has been obscured by a patchy fossil record. The paper is generally well written and full of interesting and useful data. I cannot comment on the tyrannosauroid, as I am not an expert on this clade and I lack sufficient knowledge on tyrannosauroid anatomy to evaluate that part of the MS. I can, however, comment on the hadrosaurid. Also, I must say that I already reviewed an earlier version of this MS, previously submitted to another journal. Therefore, I do not have a lot to say this time, particularly because I see that my issues with that version have been addressed in the present one. I only have some minor remarks, listed below. Otherwise, I recommend publication of this paper pending those minor revisions.

Systematic Paleontology Description
Line 530: "the presence of dental battery". Actually, non-hadrosaurid hadrosauroids also posses a dental battery. The difference is that this battery become more complex in hadrosaurids, having more than 30 tooth positions and with an occlusal plane reaching a maximum of three in width in the dentary dental battery.
Line 540-541: "The holotype and referred dentary likely come from individuals nearing or at osteological maturity….". You are not erecting a new genus or species, thus what holotype are you referring to? "Potentially" new hadrosaurid does not mean it is a new hadrosaurid.
Line 607: "The inside of the dentary is perforated by a hole.." Not hole, "foramen" is the word. Line 776: should be "South America", not "South American".
Line 810: not "evinces" but "evidences". Figures 2, 4, 5, and 6 are missing. Maybe you forgot to upload them? Or a glitch of the manuscript site of the journal? Anyway, I cannot fully evaluate many of the characters of the osteology without seeing those images. Figure S4B: The cladogram shows Atlantohadros, a genus name that does not exist as you are not erecting it (probably a remnant from an earlier version of the manuscript?).

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Recommendation? Accept with minor revision (please list in comments)
Comments to the Author(s) As I noted in my previous review, I remain unconvinced by the authors arguments for erecting a new taxon here. My view remains largely unchanged upon reading this subsequent revision. This species description is based on highly fragmentary material, and there is little accounting for intra-specific / ontogenetic variability given the paucity of the available data. But, as I also noted in my prior review, the authors have improved their arguments compared to their initial submission, emphasized the hypothesized autapomorphies, noted several other cases of tyrannosauroid taxa being named based on similarly fragmentary materials, and have added at least some note that this description is potentially contingent on finding less fragmentary material to re-assess in the future, so I am not going to insist they refrain from erecting a new taxon. Indeed, I think our disagreement on this point perhaps simply reflects a difference in philosophical approach to science and degree of caution exercised by this author vs others, as evinced by their prior semi-frequent record of naming (or attempting to name) new species under similar conditions (i.e. highly fragmentary materials and a lack of examination of intra-specific variability and ontogeny). In any event, as noted previously, I've made my concerns here noted, and I think that so long as the authors are upfront about the nature of this material, then it's fine to allow them to publish their hypothesis and name this taxon. In the future, as more material is discovered, the issue can be re-visited and critically examined in a more robust way. I see that the authors have also now removed the problematic quantitative sections. That is probably for the best, given that their analytical results were equivocal at best and working against their arguments at worst. I think that collecting more data and doing some kind of more thorough analyses would ideally have been better than removing them outright, but that would have likely required more data to exist than currently does for their new taxon (which as noted above and previously is sort of the crux of the issue in the paper generally). As it stands, and as I noted in my prior review, the analyses weren't really adding much in the first place so it seems fine to remove them and make it clear that the hypothesis is driven on the qualitative assessments and comparisons.
The updated phylogenetic analysis is welcome, as are the additional comparisons discussions, though the very low support values in their trees (noted by the author as likely relating to the incompleteness of the taxon) nicely underscore the concerns noted above and by the other reviewer about the relatively low data quality here and it's potential implications for naming a new taxon vs. describing it as Dryptosauridae of uncertain association / with some features that are potentially distinct from Dryptosaurus Concerning the hadrosaur, since it is no longer being named, I think some minor changes to the MS text might be warranted, since it is still frequently being referred to as a new taxon and discussed in reference to its holotype, etc. It doesn't really have a holotype since you aren't erecting it as a new taxon. I'd just discuss the material as being from this formation, and potentially new due to the following suite of features (a,b,c), with a note that you are not formally naming it due to the fragmentary nature (and other reasons x,y,z). As well, the hadrosaur is still labelled as 'Atlantohadros' in the Supplementary Information (e.g. Fig S5). Clarifying the text this way is mostly for consistency of the taxonomic hypotheses being presented, and shouldn't diminish the author's discussions of the importance of characterizing the biodiversity of this otherwise poorly characterized time and place. minor note: that font change issue is still present on page 38, lines 855-868 of the pdf.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Comments to the Author(s)
The manuscript re-describes a fragmentary theropod metatarsus, erecting a new taxon for the material, and presents new hadrosaurid specimens. Together, these specimens provide information on the evolution of these clades in the Santonian-Campanian, as well as revealing more of the fauna of Appalachia.
My personal taxonomic philosophy, developed through my own work on very fragmentary theropods, is to be overly cautious and only refer specimens to certain clades or taxa when I can confidently eliminate other potential confounds, like ontogeny, individual variation, or taphonomic distortion. However, I understand that not all palaeontologists approach taxonomy in this way, and that arguments can be made for both sides. Thus, while I personally disagree that the material is sufficient to confidently refer to a tyrannosaur or to erect a new taxon, this is based on my philosophy, and I can provide no evidence against the author's hypothesis. I believe they have done as good a job as can be done to support their assertions with our current knowledge of these dinosaurs. Therefore, I believe the manuscript is fit to publish with some very minor revisions, outlined below: Throughout: As this is being submitted to Royal Society Open Science, a UK journal, distances (miles) should be presented in metric units, or at least metric conversions should be presented in parentheses alongside Imperial units. The same is true of words with different spellings, e.g. palaeontology, colour, etc.
Throughout: In-text citations will need to be changed to RSOS's style.
Line 208: the abbreviation C&D Canal should be presented in parentheses after the first use of the full name. However, seeing as this abbreviation is only used twice, maybe it should just be spelled out in full?

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Kind regards, Royal Society Open Science Editorial Office Royal Society Open Science openscience@royalsociety.org on behalf of Prof Kevin Padian (Subject Editor) openscience@royalsociety.org Editor comments: Thanks for your revisions. The reviewers are still not convinced of the validity of naming a new taxon, and I am not convinced that just because some people in the past have done it based on scrappy material that it is justifiable. However a new genus name will get into the literature and so is more likely to be searched than "Dryptosaurid indet.", and if the new name comes to be regarded as a nomen dubium it may ultimately find a home within another taxon. I'm submitting a "major revision" decision because the reviewers have identified some pervasive issues of format and consistency that need to be addressed fully and accurately, so as to save work for our editorial staff. Please make sure all of this is taken care of, or we will not be able to accept the submission. Good luck with your revisions.
Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) As I noted in my previous review, I remain unconvinced by the authors arguments for erecting a new taxon here. My view remains largely unchanged upon reading this subsequent revision. This species description is based on highly fragmentary material, and there is little accounting for intra-specific / ontogenetic variability given the paucity of the available data. But, as I also noted in my prior review, the authors have improved their arguments compared to their initial submission, emphasized the hypothesized autapomorphies, noted several other cases of tyrannosauroid taxa being named based on similarly fragmentary materials, and have added at least some note that this description is potentially contingent on finding less fragmentary material to re-assess in the future, so I am not going to insist they refrain from erecting a new taxon. Indeed, I think our disagreement on this point perhaps simply reflects a difference in philosophical approach to science and degree of caution exercised by this author vs others, as evinced by their prior semi-frequent record of naming (or attempting to name) new species under similar conditions (i.e. highly fragmentary materials and a lack of examination of intra-specific variability and ontogeny). In any event, as noted previously, I've made my concerns here noted, and I think that so long as the authors are upfront about the nature of this material, then it's fine to allow them to publish their hypothesis and name this taxon. In the future, as more material is discovered, the issue can be re-visited and critically examined in a more robust way. I see that the authors have also now removed the problematic quantitative sections. That is probably for the best, given that their analytical results were equivocal at best and working against their arguments at worst. I think that collecting more data and doing some kind of more thorough analyses would ideally have been better than removing them outright, but that would have likely required more data to exist than currently does for their new taxon (which as noted above and previously is sort of the crux of the issue in the paper generally). As it stands, and as I noted in my prior review, the analyses weren't really adding much in the first place so it seems fine to remove them and make it clear that the hypothesis is driven on the qualitative assessments and comparisons.
The updated phylogenetic analysis is welcome, as are the additional comparisons discussions, though the very low support values in their trees (noted by the author as likely relating to the incompleteness of the taxon) nicely underscore the concerns noted above and by the other reviewer about the relatively low data quality here and it's potential implications for naming a new taxon vs. describing it as Dryptosauridae of uncertain association / with some features that are potentially distinct from Dryptosaurus Concerning the hadrosaur, since it is no longer being named, I think some minor changes to the MS text might be warranted, since it is still frequently being referred to as a new taxon and discussed in reference to its holotype, etc. It doesn't really have a holotype since you aren't erecting it as a new taxon. I'd just discuss the material as being from this formation, and potentially new due to the following suite of features (a,b,c), with a note that you are not formally naming it due to the fragmentary nature (and other reasons x,y,z). As well, the hadrosaur is still labelled as 'Atlantohadros' in the Supplementary Information (e.g. Fig S5). Clarifying the text this way is mostly for consistency of the taxonomic hypotheses being presented, and shouldn't diminish the author's discussions of the importance of characterizing the biodiversity of this otherwise poorly characterized time and place. minor note: that font change issue is still present on page 38, lines 855-868 of the pdf.
Reviewer: 2 Comments to the Author(s) The manuscript re-describes a fragmentary theropod metatarsus, erecting a new taxon for the material, and presents new hadrosaurid specimens. Together, these specimens provide information on the evolution of these clades in the Santonian-Campanian, as well as revealing more of the fauna of Appalachia.
My personal taxonomic philosophy, developed through my own work on very fragmentary theropods, is to be overly cautious and only refer specimens to certain clades or taxa when I can confidently eliminate other potential confounds, like ontogeny, individual variation, or taphonomic distortion. However, I understand that not all palaeontologists approach taxonomy in this way, and that arguments can be made for both sides. Thus, while I personally disagree that the material is sufficient to confidently refer to a tyrannosaur or to erect a new taxon, this is based on my philosophy, and I can provide no evidence against the author's hypothesis. I believe they have done as good a job as can be done to support their assertions with our current knowledge of these dinosaurs. Therefore, I believe the manuscript is fit to publish with some very minor revisions, outlined below: Throughout: As this is being submitted to Royal Society Open Science, a UK journal, distances (miles) should be presented in metric units, or at least metric conversions should be presented in parentheses alongside Imperial units. The same is true of words with different spellings, e.g. palaeontology, colour, etc.
Throughout: In-text citations will need to be changed to RSOS's style.
Line 208: the abbreviation C&D Canal should be presented in parentheses after the first use of the full name. However, seeing as this abbreviation is only used twice, maybe it should just be spelled out in full?

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Recommendation? Accept as is
Comments to the Author(s) I am pleased to see that the author has taken onboard all of the many changes we reviewers have suggested. I appreciate the author's patience and professionalism going through four rounds of revision on the manuscript, which I understand can be quite frustrating. The manuscript is now very sound and will be an important contribution to the field and is sure to be foundational to all future work on Appalachian dinosaurs. I have no further suggestions for the manuscript.

Decision letter (RSOS-210127.R2)
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Dear Mr Brownstein,
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Kind regards, Royal Society Open Science Editorial Office Royal Society Open Science openscience@royalsociety.org on behalf of Professor Kevin Padian (Subject Editor) openscience@royalsociety.org Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) I've reviewed the latest round of changes made by the author, and think the manuscript is suitable for acceptance/publication. Reviewer: 2 Comments to the Author(s) I am pleased to see that the author has taken onboard all of the many changes we reviewers have suggested. I appreciate the author's patience and professionalism going through four rounds of revision on the manuscript, which I understand can be quite frustrating. The manuscript is now very sound and will be an important contribution to the field and is sure to be foundational to all future work on Appalachian dinosaurs. I have no further suggestions for the manuscript.
Follow Royal Society Publishing on Twitter: @RSocPublishing Follow Royal Society Publishing on Facebook: https://www.facebook.com/RoyalSocietyPublishing.FanPage/ Read Royal Society Publishing's blog: https://royalsociety.org/blog/blogsearchpage/?category=Publishing As shown above, the tyrannosauroid condition is clearly much deeper than that present in Ornithomimus, which arguably shows the deepest mt III facet for mt IV among arctometatarsalian ornithomimosaurs.
With regards to the second character, teardrop-shaped facets for Metatarsal III, I am not convinced that these can actually be identified in the specimen because of its incompleteness. Metatarsal II is broken distally, so the distal extent and shape of this facet cannot be determined. The same is partly true for Metatarsal IV, as well, and this region is extensively damaged, if not completely missing. Without being able to observe the entire extent of the buttressing surface, I am not sure how its shape can be confidently determined. I will agree that the shape does expand distally, but this is true of all arctometatarsalian theropods, to my knowledge. My interpretation of this character is that the key feature is the rounded distal terminus of the facet, as other theropods sometimes have pointed or oblique termini to the facet. However, I do not see how a rounded distal end of the facet could be established when these regions are so damaged. I even have trouble making out the proximal parts of the facet from the figures, and these could be better shown with an illustration or 3D model. Furthermore, the author specifically notes that caenagnathids differ from tyrannosaurs in having a "poorly developed" facet for Metatarsal III, but I disagree with this assessment, as those of caenagnathids are actually fairly prominent. The author claims that these facets 'abut the distal condyles', citing one of my figures as support for this assertion. However, that figure is not sufficiently detailed to allow this to be evaluated, and, indeed, this interpretation is false. The facets for Metatarsal III are separated from the distal condyles by a region of smooth bone as in all arctometatarsalian theropods. In that figure The third character, focused on the structure of the arctometatarsus, is perhaps the most problematic. The author states that tyrannosaurs are distinguished by an arctometatarsalian structure in which Metatarsal III is hidden from the anterior surface at its proximal end. This has been considered a