the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jun 2025
Detection of dietary stress and geophagic behaviour forced by dry seasons in Miocene Gomphotherium
Abstract. To access the impact of anthropogenic emissions and land use change on Earth’s climate and biodiversity, studies into the environment and ecology of natural ecosystems during past warm periods are required. The Miocene Climatic Optimum is a key reference period for future global warming scenarios. However, studies uncovering Miocene climate have heavily favoured marine environments, leaving the impact of warming on terrestrial ecosystems understudied. Here, we present a multi-disciplinary study into the chemical composition of fossil Gomphotherium angustidens (Proboscidea, Mammalia) teeth from the Middle Miocene Vb division (~15.9–16.1 Ma) of western Portugal (Chelas Valley, Lisbon, Lusitanian basin) and their sedimentological context. Trace element and stable isotope compositions in these fossil teeth are compared with similar measurements in molars of a taxonomically related modern African elephant (captive Loxodonta africana). Results reveal seasonal-scale variability in trace elements in both fossil and modern proboscidean tooth enamel, which are interpreted as evidence for seasonal changes in diet. Periodic increases in Na, Fe and Si in G. angustidens demonstrate intake of sediment in the diet during fixed times of the year, a behaviour type previously described in modern elephants during dry seasons. In combination with the heavier carbon and oxygen isotopic composition in G. angustidens compared to L. africana, the terrestrial climate in Miocene Portugal appears characterized by seasonally dry periods, which forced geophagy behaviour of these large mammals and likely had significant consequences for the composition of Miocene ecosystems (e.g., food/water availability and potential seasonal range shifts) in southwestern Europe.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Biogeosciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-1770', Anonymous Referee #1, 28 Jul 2025
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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.
Citation: https://doi.org/10.5194/egusphere-2025-1770-RC1
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