Dietary palaeoecology of an Early Cretaceous armoured dinosaur (Ornithischia; Nodosauridae) based on floral analysis of stomach contents

The exceptionally well-preserved holotype of the armoured dinosaur Borealopelta markmitchelli (Ornithischia; Nodosauridae) from the Early Cretaceous (Clearwater Formation) of northern Alberta preserves a distinct mass within the abdominal cavity. Fourteen independent criteria (including: co-allochthony, anatomical position, gastroliths) support the interpretation of this mass as ingested stomach contents—a cololite. Palynomorphs in the cololite are a subset of the more diverse external sample. Analysis of the cololite documents well-preserved plant material dominated by leaf tissue (88%), including intact sporangia, leaf cross-sections and cuticle, but also including stems, wood and charcoal. The leaf fraction is dominated (85%) by leptosporangiate ferns (subclass Polypodiidae), with low cycad–cycadophyte (3%) and trace conifer foliage. These data represent the most well-supported and detailed direct evidence of diet in an herbivorous dinosaur. Details of the dietary palaeoecology of this nodosaur are revealed, including: selective feeding on ferns; preferential ingestion of leptosporangiate ferns to the exclusion of Osmundaceae and eusporangiate ferns such as Marattiaceae; and incidental consumption of cycad–cycadophyte and conifer leaves. The presence of significant (6%) charcoal may represent the dietary use of recently burned conifer forest undergoing fern succession, early evidence of a fire succession ecology, as is associated with many modern large herbivores.


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

Recommendation?
Accept with minor revision (please list in comments) Comments to the Author(s) I wish to reveal my identity as a reviewer, because I think it is important to discuss science openly. Anyway, I have only good things to say about this paper, with the exception of the misinterpretation and misapplication of the results from our work (Hummel et al. 2008; Gee 2011), which likely stem from a too-quick reading of our papers or a lack of deeper understanding of how we came to our rating of the various plant groups as possible sauropod food plants. However, these issues can be cleared up with a bit of rewriting. I discuss these points and offer suggestions for improvement in the attached pdf (Appendix A). I look forward to continuing the good research into as well as the interesting conversation on food plants in herbivorous dinosaurs.

Dear Dr Brown
On behalf of the Editors, I am pleased to inform you that your Manuscript RSOS-200305 entitled "Dietary palaeoecology of an Early Cretaceous armoured dinosaur (Ornithischia; Nodosauridae) based on floral analysis of stomach contents" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the referees' comments at the end of this email.
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Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) This manuscript is about the stomach content of an exceptionally preserved armored dinosaur, unique to the history of dinosaur research. There is no doubt about that it is a stomach content; 14 of the 16 criteria supporting this are fulfilled for this finding. And the content of the stomach itself preserved the remains of the last meal in such detail that we can have a very detailed picture of what this herbivorous animal have consumed. After many decades of research, there is finally a herbivorous dinosaur finding where much of the food consumed can be identified, in some cases, at the genus level. The discovery contradicts some earlier theories that we can learn a lot about the feeding mode and food preference of ornithischian dinosaurs. The manuscript is well written, well legible and covers every detail. The most important available methods were used to examine the material in question. The illustrations are informative and show all relevant details. Besides a few small comments, I fully support the publication of this manuscript.
Comments: A small but not negligible part of the stomach contents contains charred plant residues. The authors interpret this as saying that this herbivorous dinosaur may have been grazing various ferns in an area where fire had previously destroyed the forest. This allowed the charred pieces of plant to enter the animal's stomach. However, it is well known that in many vertebrate animals, including domestic animals, sometimes consume carbonized plant material, which contributes to their better digestion and, among others, the binding of harmful substances. For pets, Schmidt et al. (2019, PeerJ) compiled all relevant work on the topic, which is an excellent demonstration of the need for vegetable charcoal from time to time for many animals, not just herbivores. (In the case of domestic animals, this is added as an additive to certain foods). I suggest the authors to discuss the possibility that this animal may have consciously consumed these pieces of carbonized plant remains, perhaps in the context of its digestion. Since we knew the last food that this animal consumed, could this not be related to the fact that this animal was possibly ill, may have had e.g. digestive problems and therefore had taken the charred pieces of plant? Comments to the Author(s) I wish to reveal my identity as a reviewer, because I think it is important to discuss science openly. Anyway, I have only good things to say about this paper, with the exception of the misinterpretation and misapplication of the results from our work (Hummel et al. 2008; Gee 2011), which likely stem from a too-quick reading of our papers or a lack of deeper understanding of how we came to our rating of the various plant groups as possible sauropod food plants. However, these issues can be cleared up with a bit of rewriting. I discuss these points and offer suggestions for improvement in the attached pdf (Review_of_Brown_et_al_Royal_Open_Science.pdf). I look forward to continuing the good research into as well as the interesting conversation on food plants in herbivorous dinosaurs.

18-May-2020
Dear Dr Brown, It is a pleasure to accept your manuscript entitled "Dietary palaeoecology of an Early Cretaceous armoured dinosaur (Ornithischia; Nodosauridae) based on floral analysis of stomach contents" in its current form for publication in Royal Society Open Science. The comments of the reviewer(s) who reviewed your manuscript are included at the foot of this letter.
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Thank you for your fine contribution. On behalf of the Editors of Royal Society Open Science, we look forward to your continued contributions to the Journal. This manuscript is a well--written and careful study of ancient digestive remainsthe cololite, or stomach contents-found in a well--preserved Early Cretaceous armored dinosaur. The fossil material is spectacular, the approach of the study is good, and the identification of plant remains from the cololite is solid. Together with the uniqueness of this fossil occurrence, this study represents a meaningful contribution to the literature. Thus, this well--documented study of a single meal of an armored Borealopelta dinosaur-its last supper, so to speak-is novel and worthy of being published.
Thank you for giving me the opportunity to review this interesting manuscript. I have been working on the dietary preferences of sauropods for over a decade now and am pleased that Brown et al. have found empirical evidence for food plants in an herbivorous dinosaur. However, there are several differences between their small-stature Borealopelta ankylosaur and the sauropods that we have studied. The major difference concerns size, and the ecological feeding strategies and digestive physiology associated with it. Borealopelta was a small--stature herbivorous dinosaur, with short legs and a short neck, and thus, its feeding height, dietary preferences, and even digestive strategies cannot be compared to those of sauropods. Furthermore, because of the relatively small body mass of Borealopelta, it is doubtful that it had as urgent a need to optimize its intake of highly digestible plant matter as the much larger sauropods did.
In fact, even without knowing the contents of the cololite described in Brown et al.'s study, I would have predicted Borealopelta fed on ferns and other low--growing plants based on its low height, downward--oriented head, short neck, and relatively small body mass. And if the feeding habitat of Borealopelta, which is interpreted by Brown et al. in this manuscript, was indeed one that was affected by wildfire, I would have not expected the ingestion of Equisetum, for this is not an environment in which horsetails are commonly found. For these reasons, I generally concur with the interpretations put forth in this manuscript.
What I disagree with is the misinterpretation and misapplication on the part of Cheirolepidaceae.
• In the second paragraph of the Results section, the spelling of "diametre" and "diametres" would be more consistent than "diameter" and "diameters" given the type of English used in the rest of this manuscript (see "centimetre" in the same paragraph).
• In the second paragraph of section 4.3, shouldn't be call--out for figures 4 and 5 have an s at the end of "figure"?
• p. 8, line 6, it would be more elegantly phrased this way: The plant fragments counted. . .
• p. 8, line 16, I don't understand the call--out here to figure 3b.
• p. 8, line 30, again, I don't understand the call--out here to figure 3.
• p. 8, line 41: Cladophlebis is considered to represent the Mesozoic foliage of the fern family Osmundaceae. This is supported by the association of the fertile fossil leaves of Todites that are morphologically the same as those of the living Todea of the Osmundaceae, with Cladophlebis leaves, known from the Jurassic onwards.
• p. 10, line 11, add "for sauropod dinosaurs" so that it reads: . . .would have been the most likely food sources for the sauropod dinosaurs.
• p. 10, lines 15 to 16, add "for fully grown sauropods" so that it reads: . Borealopelta could not reach the foliage on the taller conifer trees and was probably simply limited to lower growing leaves. Alternatively, ankylosaurs may have sought out fern--dominated patches in closed forests to avoid space competition with fully grown sauropods. Nutrition, feeding strategies, and the occupation of various feeding niches were likely as complex in the Mesozoic as they are today. At any rate, I suggest deleting the entire last sentence of this paragraph because of its inaccuracy.
• p. 10, line 26, add the Osmundaceae to this list of fern families, for Cladophlebis is a member of this famiy.
• P. 10, line 42 to 46: The last two sentences in this paragraph are incorrect. The first of the two sentences is wrong due to a lack of understanding of the results and interpretations in Gee (2011). As explained above, the low likelihood of sauropods targeting Angiopteris and Osmunda was based on the habitat preference of these ferns for closed forests today, and the unlikeliness that giant sauropods could maneuver and readily forage in such a habitat; ankylosaurs would not have had this size--related problem. The second sentence excludes the Osmundaceae, which is represented in the local flora as Cladophlebis pinnae.
• p. 10, line 57, add "non--avian" so that it reads: . . . best available evidence of diet in a nonavian herbivorous dinosaur. [Note that the fossil seeds found in the abdomen of • p. 11, line 54: replace "are typically tree--like" with "form medium to tall trees" Of the two, I find that the wording and approach taken in paragraph on p. 12 are excellent, because this second paragraph very elegantly remarks on the differences between sauropod and ankylosaur feeding strategies linked to height limitations, as well as touches on the complexity of discerning dietary preferences in extinct animals. Once again, thanks for asking me to review this very interesting study. I'm sure that after the numerous small changes that I have suggested have been made, this will be an amazing paper.

Respectfully submitted, Carole Gee University of Bonn, Germany
Dear editors, Thank for you for the efficient handing of our recent submission to Royal Society Open Science entitled "Dietary palaeoecology of an Early Cretaceous armoured dinosaur (Ornithischia; Nodosauridae) based on floral analysis of stomach contents" (Manuscript ID: RSOS-200305), and for the opportunity to submit a revised version. We have gone through the reviewers' comments and have revised the manuscript to address the concerns of Drs Gee and Ősi. We have itemized our response to each reviewer comment below. We feel that these revisions, based on the reviewers' suggestions, have significantly improved the quality of the manuscript, and have added this recognition to our Acknowledgements.
In addition to the changes suggested by Drs Gee and Ősi, we have also made a small number of changes to the manuscript to correct minor typographical errors, clarify some language and added one previously overlooked reference. These changes are minor and do not alter our interpretation or reporting of the results. All changes to the manuscript can been seen in the Track Changes version of both the main document as well as the ESM. One additional change was an alteration of author order. This was done at the request of Donald Henderson who felt the contributions of two co-authors (Jessica Kalyniuk and Dennis Braman) exceeded his own, and should be listed before his name.
All of the authors have contributed to the revisions or at least reviewed them, and have approved the revisions and this response to reviewers. AUTHORS: This is a valid point. This discussion in this section has been modified to reflect that we do not know whether the consumption was intentional or incidental, and this reference has been added. Regardless, much of the remaining discussion is unchanged, as the animal would still need to have encountered a recently burnt area. The text now reads: "Although charcoal is known to be intentionally consumed by multiple wild animals [see review in 106], it may also be incidentally consumed during feeding in an area recently subject to wildfire. In this case, it is unclear if the consumption was intentional or incidental, but regardless indicates that the environment occupied by the animal prior to death was recently subject to wildfire." AUTHORS: Yes, this was incorrect and should refer to figure S3 (rather than figure 3). It has now been corrected throughout the document.
-In the first paragraph of the Discussion,  Ősi et al. (2016, Hist. Biol.), since this work gives more update and diverse picture on ankylosaur jaw mechanism and feeding.
AUTHORS: This reference has now been added.
-On figure 5, the explanation for label "D" is missing.
AUTHORS: The explanation for labels B_C were misplaced in the caption, and D omitted. This error has been corrected.
We thank Dr Ősi for his thoughtful and careful review of our submission. Several useful corrections were incorporated in the revised version.

Reviewer 2: Carole Gee
This manuscript is a well-written and careful study of ancient digestive remains-the cololite, or stomach contents-found in a well-preserved Early Cretaceous armored dinosaur. The fossil material is spectacular, the approach of the study is good, and the identification of plant remains from the cololite is solid. Together with the uniqueness of this fossil occurrence, this study represents a meaningful contribution to the literature. Thus, this well-documented study of a single meal of an armored Borealopelta dinosaur-its last supper, so to speak-is novel and worthy of being published.
Thank you for giving me the opportunity to review this interesting manuscript. I have been working on the dietary preferences of sauropods for over a decade now and am pleased that Brown et al. have found empirical evidence for food plants in an herbivorous dinosaur. However, there are several differences between their small-stature Borealopelta ankylosaur and the sauropods that we have studied. The major difference concerns size, and the ecological feeding strategies and digestive physiology associated with it. Borealopelta was a small-stature herbivorous dinosaur, with short legs and a short neck, and thus, its feeding height, dietary preferences, and even digestive strategies cannot be compared to those of sauropods. Furthermore, because of the relatively small body mass of Borealopelta, it is doubtful that it had as urgent a need to optimize its intake of highly digestible plant matter as the much larger sauropods did. In fact, even without knowing the contents of the cololite described in Brown et al.'s study, I would have predicted Borealopelta fed on ferns and other low-growing plants based on its low height, downward-oriented head, short neck, and relatively small body mass. And if the feeding habitat of Borealopelta, which is interpreted by Brown et al. in this manuscript, was indeed one that was affected by wildfire, I would have not expected the ingestion of Equisetum, for this is not an environment in which horsetails are commonly found. For these reasons, I generally concur with the interpretations put forth in this manuscript.
What I disagree with is the misinterpretation and misapplication on the part of Brown et al. of our work (Hummel et al. 2008;Gee 2011). I am of the belief that their results do not conflict with ours, as they state repeatedly on pages 10 to 12, because one cannot compare the feeding height, foliage attainability, or feeding strategy of ankylosaurs to that of sauropods. Please note that all the intepretations and hypotheses made by Hummel et al. (2008)  AUTHORS: This has now been corrected.
-In the second paragraph of the Results section, the spelling of "diametre" and "diametres" would be more consistent than "diameter" and "diameters" given the type of English used in the rest of this manuscript (see "centimetre" in the same paragraph).
-p. 8, line 11, please check the call-out for figure 4b; shouldn't it be 4h?
AUTHORS: This has now been corrected.
-p. 8, line 16, I don't understand the call-out here to figure 3b.
AUTHORS: 3b was included here in error. This has now been removed.

AUTHORS:
We have changed the wording so it better corresponds to our classification scheme (Tables 3, S2 and Fig. 6) where wood tissue (a grouped category that included woody stems as well as fragmented wood tissue) was 3.6%. -p. 8, line 30, again, I don't understand the call-out here to figure 3.
AUTHORS: This was an error, and reference to fig. 3 has been removed.

-p. 8, line 41: Cladophlebis is considered to represent the Mesozoic foliage of the fern family
Osmundaceae. This is supported by the association of the fertile fossil leaves of Todites that are morphologically the same as those of the living Todea of the Osmundaceae, with Cladophlebis leaves, known from the Jurassic onwards.
AUTHORS: In recent accounts of Mesozoic ferns we have consulted, Cladophlebis is attributed to the Osmundales, but not to the family Osmundaceae owing to the lack of sporangial characters diagnostic of the family. We have made this change and have added citation of a recent source for Cladophlebis being Osmundales.
-p. 10, line 11, add "for sauropod dinosaurs" so that it reads: . . .would have been the most likely food sources for the sauropod dinosaurs.
AUTHORS: This edit has been made as suggested. AUTHORS: This edit has been made as suggested.
dietary preferences of sauropods] are likely applicable to Ornithischian taxa, including ankylosaurs, . . ." As I mention earlier in this review, I think that the feeding strategies of ankylosaurs may have been completely different from those of the giant sauropods; because of their much smaller sizes, the ankylosaurs probably did not need to bulk feed on optimally nutritious foliage like the extremely large sauropods did; Borealopelta may have done just fine on ferns, even if these plants offered less nutrition and fewer calories per mouthful. Moreover, it is clear that Borealopelta could not reach the foliage on the taller conifer trees and was probably simply limited to lower growing leaves. Alternatively, ankylosaurs may have sought out fern-dominated patches in closed forests to avoid space competition with fully grown sauropods. Nutrition, feeding strategies, and the occupation of various feeding niches were likely as complex in the Mesozoic as they are today. At any rate, I suggest deleting the entire last sentence of this paragraph because of its inaccuracy.

AUTHORS:
We have re-worded the sentences as so: "Although largely in the context of sauropod diet, many these results are also applicable to megaherivorous Ornithischian taxa, including ankylosaurs, other than distinctions due to feeding height limitations." -p. 10, line 26, add the Osmundaceae to this list of fern families, for Cladophlebis is a member of this family.
AUTHORS: consistent with our statement above, we have added Osmundales to this list.
-P. 10, line 42 to 46: The last two sentences in this paragraph are incorrect. The first of the two sentences is wrong due to a lack of understanding of the results and interpretations in Gee (2011). As explained above, the low likelihood of sauropods targeting Angiopteris and Osmunda was based on the habitat preference of these ferns for closed forests today, and the unlikeliness that giant sauropods could maneuver and readily forage in such a habitat; ankylosaurs would not have had this size-related problem. The second sentence excludes the Osmundaceae, which is represented in the local flora as Cladophlebis pinnae.
AUTHORS: The caveat '(although based on hypothesized limited accessibility to closed forest by large sauropods)' has been added to this section to clarify the first point. The second sentence has also been adjusted to read "Of these groups, only Cycadales and Bennettitales were present in the Gates Formation flora, although Cladophlebis is often placed in the Osmundales [102] together with Osmunda (Osmundaceae)." to take into account the reviewers other comment.
-p. 10, line 57, add "non-avian" so that it reads: . . . best available evidence of diet in a nonavian herbivorous dinosaur. [Note that the fossil seeds found in the abdomen of Jeholornis prima, a Cretaceous bird, are widely accepted as gut contents; see Sander et al. 2010.] AUTHORS: This edit has been made as suggested -p. 11, line 30: There are two additional reasons that may explain why Equisetum remains were not found in the cololite, although this genus occurs in the nearby megaflora. One reason is that horsetails may have not been locally abundant at the Borealopelta feeding grounds, which are interpreted here as a fire-swept environment; fossil and living Equisetum prefers damp or even shallow freshwater habitats. The second is that all living species of Equisetum that we have tested in follow-up studies since Hummel et al. (2008) and Gee (2011) are extremely digestible (Gee et al., manuscript in progress), and any horsetails consumed by the Borealopelta may have been digested to a point past recognizability in this cololite AUTHORS: We do comment in the manuscript that rarity of Equisetites in the feeding grounds at the time of Borealopelta's last meal may have contributed to its absence in the cololite. Pg 11 lines 28-31. However, we have amended this statement to more clearly express this concept.
Regarding a lack of preservation within the cololite explaining the absence of Equisetites, we find this an ad hoc and implausible suggestion. On a quick review of literature, we found some studies of extant birds and mammals that routinely fed on Equisetum, and while these studies concur with the finding for high digestibility of Equisetum, these studies found that Equisetum fragments were readily identifiable in these animals' feces. Geese, like our nodosaur, have gastroliths. If extant geese, grizzly bear, and caribou scat preserve Equisetum stems with their more advanced chemical and microbial digestion vs. a nodosaurid, Occam's razor leads to an expectation that had Borealopelta eaten Equisetum it would be recognizable in the cololite. We have added a sentence to make this point. AUTHORS: We disagree that it is incorrect to refer to Borealopelta as a "large herbivore". This is because conservative body mass estimates place Borealopelta at 1,300 kg, well above the 1,000 kg threshold used to define the megaherbivore guild (NB: defined in the 1 st sentence of the 2 nd paragraph of the Introduction), and Borealopelta is the largest documented herbivorous taxon within both its host formation and the geographically proximate penecontemporaneous formations. We do concede that Borealopelta is smaller than many of the sauropod dinosaurs that were the subject of the Gee 2011 paper. To address Dr Gee's concerns, we have clarified that while Borealopelta as a large herbivore, it is smaller than those for which these inferences were made. The sentence now reads, with an added follow-up sentence: "The detection of cycad-cycadophyte leaf fragments in the cololite slides, and only a trace of conifer leaves on the slides, is contrary to previous prediction of diet choice for large herbivorous dinosaurs (although based on sauropods, much larger animals than Borealopelta), which argued against cycads and favoured araucarian leaves based on the energetics of their digestion [12,110]. Adult sauropods, however, would have accessed trees, whereas Borealopelta would have accessed low-stature plants." -p. 11, line 54: replace "are typically tree-like" with "form medium to tall trees" AUTHORS: Change made.
-p. 12, line 4, please check if figure 4f is the correct photo AUTHORS: Change made to cite 4i,j.
-p. 12, lines 27 to 55. Much of the first part of this paragraph repeats what has been said about the predictions of sauropod dietary preferences in the lowermost paragraph of p. 11. I realize that this because the paragraph on p. 11 focuses on fossil leaf fragments, while that on p. 12 refers to the palynoflora. Still, it would be better to edit the writing in the first paragraph to reduce so much repetition in the manuscript. Of the two, I find that the wording and approach taken in paragraph on p. 12 are excellent, because this second paragraph very elegantly remarks on the differences between sauropod and ankylosaur feeding strategies linked to height limitations, as well as touches on the complexity of discerning dietary preferences in extinct animals. Once again, thanks for asking me to review this very interesting study. I'm sure that after the numerous small changes that I have suggested have been made, this will be an amazing paper.
AUTHORS: As we have made changes to the text on p. 11 as above to address the concerns raised by Dr Gee, we believe the change proposed here is moot. The detail presented at lines 27 to 55 on p. 12 we feel sets up the discussion that follows and is necessary for the flow of our argument.
We thank Dr Gee for her careful and thoughtful review. Some useful improvements and corrections have resulted, and we look forward to Dr Gee reading the final published version and appreciate her expectation that this will be 'an amazing paper'.
Caleb Brown and David Greenwood, and on behalf of the other authors