I find the subject of this study worth exploring and the results interesting and promising. However, it seems to me that some key concepts have been slightly misunderstood by the authors and that's why I must recommend a major revision to the text. My main concern is that the authors use the apparent lack of correlation between dietary signal (mesowear) and the patterns of change in the dental traits (hypsodonty and enamel thickness) as their main argument for stating that there is a lag between ecological change and the evolutionary patterns that would call for alternative explanations. However, as has been recently shown (Saarinen and Lister 2023), dental evolutionary patterns in proboscideans are in fact associated with major climatic changes (aridification pulses), although not being driven by a shift to grazing diet (at least not at this late evolutionary change when elephants were already largely grazers and had a rather derived tooth morphology to begin with). The authors consider this mysterious and point at need for alternative explanations, but in fact all that is needed is a careful re-evaluation of results that already exist, and correction of some misconceptions. For example, more grazing mesowear signal does not have to mean more arid and open environment (grazing resources are most prominent in intermediate precipitation range). Mesowear is above all a signal of grazing (grass consumption), not aridity, openness or grit consumption (see Saarinen and Lister 2023, and references therein). Thus, a pattern emerges where the steps in dental trait evolution (similar in the Shungura formation based on the new data presented in this manuscript and for elsewehere in East Africa (Saarinen and Lister 2023)) are associated above all with peaks of aridification (that may involve factors such as shift to more drought-adapted plant diets and exogenous grit (dust)), but not shift to grazing (because at this point the elephants were already largely grazers). Another major issue in this manuscript (apart from the need to revise the interpretation of results) is the treatment of "Elephas recki" as a coherent taxonomic entity, despite recent evidence suggesting that it consists of more than one species (and likely more than one genus). The authors are clearly aware of this, but decide not to address this other than saying that the purpose of this study is not to revise taxonomy. However, the taxonomic uncertainty is a critical issue for interpreting whether the observed patterns of dental trait evolution represent changes within an evolutionary lineage or differences between taxa. For this reason, at minimum I think it should be more clearly acknowledged throughout the text that “Elephas recki compex” is an informal and uncertain taxonomic group. Thus, I suggest at the very minimum to add quotation marks to "Elephas recki" throughout the text. In summary, the results and their presentation seem fine, but the interpretation of the results needs revision. See more detailed comments in the annotated manuscript file.

Title and abstract

• Does the title clearly reflect the content of the article? – Yes
• Does the abstract present the main findings of the study? – Yes (but see my comments on how the interpretation of the results needs to be revised)

Introduction

• Are the research questions/hypotheses/predictions clearly presented? – Mostly yes, but see my comments, especially on the taxonomic position of the "Elephas recki complex"
• Does the introduction build on relevant research in the field? – Yes

Materials and methods

• Are the methods and analyses sufficiently detailed to allow replication by other researchers? – Yes, as far as I can tell
• Are the methods and statistical analyses appropriate and well described? – Yes, as far as I can tell

Results

• In the case of negative results, is there a statistical power analysis (or an adequate Bayesian analysis or equivalence testing)? – Maybe not applicable in this case
• Are the results described and interpreted correctly – Mostly yes (but see detailed comments)

Discussion

• Have the authors appropriately emphasized the strengths and limitations of their study/theory/methods/argument? – Mostly yes.
• Are the conclusions adequately supported by the results (without overstating the implications of the findings)? – No (More discussion is needed and concepts need to be clarified. Now there is too much confusion and misunderstandings which make the results look more confusing in relation to other studies. See detailed comments in the text on my suggestions of what should be done to improve this)

Getachew et al. conducted dental morphometric and mesowear analysis on the classic sequence of elephantid fossils from Ethiopia’s Shungura Formation, ascribed to the “Elephas recki” complex. As the authors duly noted in their manuscript (MS hereafter) with appropriate citations, there has been recent research progress which convincingly argued on the basis of cranial morphology, that “Elephas recki” from the Shungura Formation is likely to represent a number of time-successive species lineages, as opposed to a single anagenetic lineage whose dental evolutionary pattern resembles that of a ring species rolled out in time; notwithstanding that the authors’ own data reject the simple linear anagenetic scenario. Therefore, I recommend for the authors to acknowledge, in revising the MS, that they made an entirely valid and pertinent study of community-level ecomorphological shift in Shungura elephantids within the timespan of the stratigraphic sequence they investigated (3.75–1.09 Ma), but their data should not be conflated with investigation of morphological trends within a single species or genus. Therefore, to avoid confusion I recommend the authors use lexicons such as “Elephas recki” complex (ERC hereafter), or “recki” instead of Elephas recki to delineate the taxonomic group they are investigating, and I believe this to be consistent with the authors’ intension to state their positional neutrality on the taxonomic and phylogenetic context of the ERC. This does not detract the value of this contribution to the discipline in the slightest. Lister (2013) and Saarinen & Lister (2023) all used community-level dental trait and foraging ecology approach. I recommend the publication of this MS, contingent on suitable revisions being carried out concerning the feedbacks addressed here.

In the 4th paragraph of their Materials and Methods section, the authors acknowledge the problem I alluded to in the previous paragraph. Without intending to dismiss the authors’ understanding of this issue, my impression is that their wording has slightly misrepresented the debate. There can now be little doubt on the basis of craniodental morphology that the comparatively ‘advanced’, nominotypical recki from late Early Pleistocene East Africa represents a progenitor to subsequent species of Palaeoloxodon (Saegusa & Gilbert 2008; Larramendi et al. 2020; Zhang 2020; Sanders 2024). The central problem is whether earlier materials attributed to the ERC all represent progenitor populations that lead up to nominotypical recki, a proposition not supported by Zhang (2020) on the basis of his comparative studies of the referred skulls of “Elephas recki brumpti” and “E. r. atavus” from the Omo-Turkana Basin. By the nature of the East African fossil record, the earlier materials attributed to the ERC is very likely to contain the ancestor of nominotypical Palaeloxodon recki; and Zhang’s (2020) core argument was simply that materials representing other elephantid lineages were lumped within the “recki” complex by earlier authors such as Arambourg (1948) and Beden (1980; 1983; 1987). By measuring first-hand 140 Shungura elephant molars covering much of the entire stratigraphic span of the ERC, the authors have in fact contributed outstandingly valuable data concerning debate. I thereby encourage the authors to add simple descriptive statistics and multivariate clustering analysis (e.g. linear discriminant or principal component analysis) for molar morphological variance across each stratigraphic bin they have analysed, as a basic means of discerning the number of dental morphotypes present in each stratigraphic bin. Given the authors collated good sample sizes for elephantid dental mesowear at different Shungura horizons, this can potentially detect ‘niche partition’ between different morphotype-groups. If discernible dental morphotype-groups are identified, I would recommend the authors refrain from ascribing taxonomic identities to different morphotypes, beyond descriptions of principal morphologies that characterise each morphotype-group (cf. Todd 2005).

Concerning one statement from the opening paragraph, although it is indeed true that elephantids underwent precipitate net decline within the last two million years, it should not be overlooked that major phylogenetic events within the clade that led to the highly successful modern and Late Pleistocene species took place within this interval. The successful occupation of Northern Eurasia by Mammuthus and Palaeoloxodon during their Pleistocene radiation, amid intense glacial-interglacial cycles, also vastly expanded the ecological envelope of this lineage. Admittedly I am falling short of finding a suitable way to integrate some nuance into the statement about their steep recent decline in a pithy fashion, I would suggest this to be a worthwhile exercise during revision.

I was personally intrigued that the authors cited Arambourg (1938) as the authority for the currently conventional procedure of measuring hypsodonty in proboscideans. Osborn, Gregory and Matthew have all noted in their earlier works trends of hypsodonty increase in herbivorous mammal lineages including proboscideans, although in the time of completing this review I had been unable to find examples of a comparable hypsodonty index employed in their works. I encourage the authors to briefly demonstrate their literature factchecking in revision.

The authors duly highlighted Maglio’s (1973) hypothesis of the craniomandibular apparatus evolving as a suite to explain the apparent discordance between direct dietary signal represented by mesowear and signals of dental morphological adaptation. Yet in their wording they slightly misrepresented the relevance of Zhang (2020) here. Zhang (2020) examined cranial osteology in fossil elephantids (including those from the Turkana Depression) primarily for the purpose of inferring taxonomy and phylogeny, rather than from the perspectives of function, modularity and developmental constraints. I agree with the authors that constraints brought about by the gross evolutionary morphology of the craniomandibular apparatus could have been a factor that impeded elephantid dental evolution in the studied region from showing trajectory that aligns ‘perfectly’ with mesowear-based dietary signal, and in my knowledge Zhang (2020) said nothing to explicitly contradict such postulation. This would serve a riveting subject for future research!

Literature cited:​

Arambourg C. 1938. — Mammiferes fossiles du Maroc. Verlag nicht ermittelbar 46: 1–74.

Arambourg C. 1948. — Contribution à l'étude géologique et paléontologique du bassin du lac Rodolphe et de la basse vallée de l'Omo. Deuxième partie : paléontologie, in Arambourg C. (ed.), Mission Scientifique de l'Omo, 1932-1933. Tome 1 : géologie - anthropologie. Vol. Fascicule 3 Editions du Muséum, Paris: 231–562.

Beden M. 1980. — Elephas recki Dietrich, 1915 (Proboscidea, Elephantidae). Evolution au cours du Plio- Pléistocène en Afrique orientale. Géobios 13 (6): 891-901.

Beden M. 1983. — Family Elephantidae, in Harris J. M. (ed.), Koobi Fora Research Project. Volume 2. The fossil ungulates: Proboscidea, Perissodactyla, and Suidae. Clarendon Press, Oxford: 40-129.

Beden M. 1987. — Les éléphantidés (Mammalia-Proboscidea), in Coppens Y. & Howell F. C. (eds), Les faunes plio-pléistocènes de la basse vallée de l'Omo (Ethiopie), tome 2. Vol. Cahiers de Paléontologie. Editions du Centre National de la Recherche Scientifique, Paris: 162 p.

Cantalapiedra J. L., Sanisidro Ó., Zhang H., Alberdi M. T., Prado J. L., Blanco F. & Saarinen J. 2021. — The rise and fall of proboscidean ecological diversity. Nature Ecology & Evolution 5 (9): 1266-1272.

Larramendi A., Zhang H., Palombo M.-R. & Ferretti M. P. 2020. — The evolution of Palaeoloxodon skull structure: Disentangling phylogenetic, sexually dimorphic, ontogenetic, and allometric morphological signals. Quaternary Science Reviews 229: 106090. DOI: https://doi.org/10.1016/j.quascirev.2019.106090.

Lister A. M. 2013. — The role of behaviour in adaptive morphological evolution of African proboscideans. Nature 500: 331-334.

Maglio V. J. 1973. — Origin and evolution of the Elephantidae. Transactions of the American Philosophical Society 63 (3): 1-149.

Owen-Smith R. N. 1988. — Megaherbivores. The influence of very large body size on ecology. Cambridge University Press, 369 p.

Saarinen J. & Lister A. M. 2023. — Fluctuating climate and dietary innovation drove ratcheted evolution of proboscidean dental traits. Nature Ecology & Evolution 7(9): 1490–1502.

Saegusa H. & Gilbert W. H. 2008. — Elephantidae, in Gilbert W. H. & Asfaw B. (eds), Homo erectus. Pleistocene Evidence from the Middle Awash, Ethiopia. University of California Press, Berkeley: 193–226.

Sanders W. J. 2024. — Evolution and fossil record of African pProboscidea (First edition). CRC Press, 346 p.

Zhang H. 2020. —Evolution and systematics of the Elephantidae (Mammalia, Proboscidea) from the Late Miocene to Recent [Ph. D. dissertation: University of Bristol, 319 p.

Steven Zhang​​