This study is interesting in that the authors compare Miocene-aged Gomphotherium molars to a modern African elephant molar in order to infer paleoenvironmental conditions and resulting impacts on gomphothere diet. The authors’ approach is advanced in that they use high-precision XRF analyses across the molar enamel surfaces to analyze trace element patterns in conjunction with the more “standard” carbon and oxygen isotope analyses typical of traditional paleoenvironmental studies. The authors conclude that Gomphotherium may have been geophagous during arid and/or warm periods, similar to modern African elephants.

A study like this is important in that it helps us understand how some animals may respond to Earth’s modern warming climate conditions. Further, in many ways, Miocene atmospheric patterns may be analogous to our current warming scenario, suggesting we can learn a lot from studying this period in Earth’s history. Despite this, our scientific understanding of the Miocene climate on the terrestrial realm is less understood, so studies like this one are vital to building up that knowledge-base.

The research questions and aims in this study were presented in a logical manner. The introduction included an excellent amount of detailed background information necessary to understanding much of the methods and results. With that said, it would be beneficial to the reader access to more raw data in better-organized tables (or in the appendices depending on how much data is available). I did find myself asking many questions related to data I could not locate and I lay out these and other specific questions below:

Section 1.1: It is safe to presume Gomphotherium filled a similar niche as modern African elephants. What is that important in terms of climate change? I.e., Why should we care how modern elephants might respond to climate change?  (I think there are a lot of cool ways to answer this!)

Section 1.3, line 117: Just looking for some clarification here: Can one modern proboscidean tooth capture 15 years? And would this imply that less time is captured in low-crowned gomphothere molars?

Section 1.3: Is there a reason we don’t look at the isotopic record in tusks? Does it have to do with how they grow?

Section 1.5, lines 169-170: C4 vegetation existed long before 8-5 Mya. Do you mean that it wasn’t established the study area until 8-5 Mya?

Section 1.5, lines 179-181: Are you implying that the present-day climate of the Guinea Gulf is tropical? And were the sample sites still so close to the water during the Miocene (was the paleogeography similar)?

Section 1.5, lines 183: “Subsequently?” This seems right in the middle of the Burdigalian and Langhian stages. I think this entire paragraph is a bit confusing as written. Consider revising.

Section 2.1: Is it possible that the gomphothere molars belong to juveniles (how can you rule this out?). The inclusion of juvenile teeth could have implications for stable isotope interpretations.

Section 3.1: Playing devil’s advocate here, but are three gomphothere molars and one modern elephant molar enough to base your conclusions upon? Might be worth a sentence explaining the significance/rarity of these specimens for any non-paleontologist readers.

Section 4.1, line 328: Which teeth are you referring to here, all three? Or are these abundances from various sample locations along a single tooth? Please clarify.

Section 4.1, TABLE 1: Since this is only three gomphothere molars, it is extremely important to see a more detailed and better organized table that actually lays out the data values for each specimen. If it is a lot of data, this could go into an appendix.

Section 4.3, lines 396-397: Sentence here is confusing as written. Please reword.

Section 5.2.1: This entire section seems a bit out of place here. This information seems like it should be included in section 1.2.

 Section 5.2.1, lines 525-527: Enamel biomineral composition responds to the physiological and taxonomical characteristics of the environment? Please clarify.

OTHER IMPORTANT COMMENTS:

• An appendix is referred to in some of the figure captions, but I cannot locate it (no appendix in supplementary data file).
• The metadata xml file is inaccessible (some sort of error).
• Please provide a data table of stable isotopes (C and O)!
• Did authors perform a seasonal analysis of d13C and d18O within the teeth? Again, a data table that clarifies this would be helpful, but it would also be interesting to see this isotope data laid out similar to figures 11-13.

This is an interesting, thorough, and carefully presented paper. It uses valid geochemical proxies to understand the palaeoclimate signal for Mid-Miocene gomphotheres from Iberia as compared to similar data from a modern zoo elephant. I have reviewed the manuscript in detail, including about three dozen  marginal annotations. Those comments can be gleaned from the attached pdf.

The range of analytical methods used is comprehensive and appropriate for the interpretations that the authors are trying to make.

My primary concerns about this paper include the following: (1) The small sample size (3 fossil teeth, 1 modern tooth) upon which this research study is based, and the interpretations that follow from them. (2) The interpretation of the d18O signatures, which are potentially more complex than the authors indicate. (3)  The conclusions and comparisons of the fossil versus modern data are not as clearly integrated and argued as they should be.