Buluk, Kenya stable isotopes

Stable isotope (ẟ¹³C, ẟ¹⁸O) paleoecology of the late Early Miocene mammalian fauna from Buluk, Kenya

Irisa Arney, Ellis M. Locke, Ellen R. Miller, and Isaiah O. Nengo

Article number: 27.1.a19
https://doi.org/10.26879/1335
Copyright Society of Vertebrate Paleontology, March 2024

Author biographies
Plain-language and multi-lingual abstracts
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Submission: 31 August 2023. Acceptance: 10 January 2024.

 ABSTRACT

Early Miocene deposits at Buluk in northern Kenya have produced an abundant and diverse community of mammalian fossils, including catarrhine primates, and the site is an important resource for characterizing habitat heterogeneity across East Africa during the early-middle Miocene transition. Here we present the results of stable carbon and oxygen isotope analyses of fossil tooth enamel from Buluk’s ruminant artiodactyls, suoids, anthracotheres, rhinocerotids, proboscideans, and hyraxes, to address the nature of the C₃ vegetation (i.e., open-canopy versus closed-canopy and/or degree of water stress) and to determine whether C₄ plants were a dietary component of Buluk herbivores. δ¹³C_enamel and δ¹⁸O_enamel values indicate that most Buluk herbivores foraged in a C₃-dominated mosaic of open canopy woodland habitats with no evidence of closed-canopy habitats, consistent with soil and biomarker analyses. Despite paleosol evidence for the presence of C₄ vegetation at Buluk, δ¹³C_enamel results show no evidence for the consumption of C₄ biomass. This discrepancy may be caused by the presence of C₄ vegetation on the landscape that was not consumed by local herbivores in quantities that can be captured by bulk δ¹³C_enamel, lack of sampling of taxa that did consume C₄ vegetation, or it may reflect differences in scale between these two data sources. Overall, the range of δ¹³C_enamel values from Buluk are like the early Miocene site of Moroto and are significantly more enriched than the middle Miocene localities Maboko Island and Fort Ternan, suggesting more water-stressed environments predominated at these earlier Miocene sites in East Africa.

Irisa D. Arney. Western University of Health Sciences, College of Osteopathic Medicine of the Pacific Northwest, Lebanon, Oregon 97355, USA. (corresponding author) iarney@westernu.edu
Ellis M. Locke. Idaho College of Osteopathic Medicine, Meridian, Idaho 83642, USA. elocke@icom.edu
Ellen R. Miller. Wake Forest University, Department of Anthropology, Winston Salem, North Carolina 27109, USA. millerer@wfu.edu
Isaiah O. Nengo, deceased. Formerly of Turkana Basin Institute, Social and Behavioral Sciences, Stony Brook University, Stony Brook, New York 11794, USA.

Key Words: carbon isotopes; East Africa; Miocene; paleoenvironment; herbivores; C3 diets

Final citation: Arney, Irisa D., Locke, Ellis M., Miller, Ellen R., and Nengo, Isaiah O. 2024. Stable isotope (ẟ¹³C, ẟ¹⁸O) paleoecology of the late Early Miocene mammalian fauna from Buluk, Kenya. Palaeontologia Electronica, 27(1):a19.
https://doi.org/10.26879/1335
palaeo-electronica.org/content/2024/5121-buluk-kenya-stable-isotopes

Copyright: March 2024 Society of Vertebrate Paleontology.
This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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INTRODUCTION

Buluk is the easternmost early Miocene primate locality in Africa, and as such the site is a meaningful point of comparison for documenting both spatial and temporal heterogeneity of East African habitats across the early-middle Miocene transition. A growing body of paleoenvironmental evidence suggests that the early Miocene of East Africa was characterized by substantial spatial and temporal variation in climatic conditions and vegetation (Retallack et al., 2002; Bonnefille, 2010; Michel et al., 2014, 2020; Lukens et al., 2017a, 2021; Butts, 2019; Liutkus-Pierce et al., 2019; Baumgartner and Peppe, 2021; Peppe et al., 2023). While forests are documented at some early Miocene sites in western Kenya (e.g., Michel et al., 2014, 2020; Oginga, 2018; Baumgartner and Peppe, 2021), open woodland and wooded grassland environments are also well documented both in western Kenya (Driese et al., 2016; Lukens et al., 2017a) and northern Kenya (Lukens et al., 2017b, 2021; Butts, 2019; Liutkus-Pierce et al., 2019).

Soil and plant wax geochemical and palaeobotanical evidence for some contribution of C₄ vegetation to open woodland habitats has been recovered at several early Miocene localities in East Africa, including at Buluk (Lukens et al., 2017a, 2021; Peppe et al., 2023). Whether early Miocene herbivorous mammals had begun to incorporate these C₄ resources into their diets is less clear. Dental morphological data suggest that grazing adaptations were not present among ruminants until the late Miocene (Hall and Cote, 2021), and published information about herbivore enamel isotopic evidence prior to 10 Ma is largely lacking (Cerling et al., 1997; Arney et al., 2022; MacLatchy et al., 2023). Buluk’s mammalian assemblage provides an opportunity to add to the regional record of habitat variation in the early Miocene of East Africa and to retrodict the dietary behavior of herbivores that inhabited these environments. To this end, we report the results of enamel stable isotope analyses (δ¹³C_enamel, δ¹⁸O_enamel) from a range of fossil mammalian herbivores at Buluk with two objectives: 1) to establish dietary isotopic ranges for fossil herbivore guilds; and 2) to examine inter-site variation in δ¹³C_enamel values to assess niche partitioning among Early and Middle Miocene herbivores.

Toward the first objective, we establish carbon isotopic profiles for the major mammalian herbivore groups from Buluk to examine dietary niche partitioning at the site and to determine whether C₄ vegetation was a dietary component for any of the sampled groups. Several past δ¹³ C_enamel studies exploring the vegetation history of East Africa during the Neogene found that landscapes in the Early and Middle Miocene were C_3-dominated with little to no C₄ biomass (Cerling et al., 1991, 1997; Uno et al., 2011, 2016; Feakins et al., 2013; Polissar et al., 2019). This paradigm was recently challenged by the recovery of geochemical evidence from plant wax and soil carbonates, which indicated heterogeneous environments including C₄ grasses were present at multiple early Miocene localities (Peppe et al., 2023). δ¹³C_enamel values from herbivorous mammals at Buluk will allow us to assess whether C₄ grasses or water-stressed C₃ vegetation were being ingested by these animals. In addition, δ¹⁸O values are used to characterize the drinking habits utilized by mammalian herbivores, and to assess the implications for local environmental aridity at Buluk.

The second objective is to compare Buluk δ¹³C_enamel values with those from published records of two Middle Miocene localities in Western Kenya: Maboko Island (~15-14 Ma; Feibel and Brown, 1991) and Fort Ternan (13.7 Ma; Pickford et al., 2006), and the Ugandan Early Miocene locality Moroto (21 Ma; MacLatchy et al., 2023) to examine inter-site variation among common taxa. Inferred open habitats at Maboko (Evans et al., 1981; Retallack et al., 2002; Arney et al., 2022) and Fort Ternan (Evans et al., 1981; Shipman et al., 1981; Shipman, 1986; Retallack et al., 1990; Kappelman, 1991; Retallack, 1992; Cerling et al., 1991, 1997; Dugas and Retallack, 1993) have played a key role in framing the early-middle Miocene ecological transition. Additionally, Moroto, Maboko, and Fort Ternan are the only East African localities with δ¹³C_enamel values that provide comparative values for interpreting niche partitioning among presumed C_3-dominated feeders before the spread of C₄ grasslands at 10 Ma. Together with information from Buluk, these four enamel carbon isotope datasets will provide a snapshot of the temporal and spatial habitat variation across three distinct regions in the developing East African Rift Valley System during the early to middle Miocene.

BACKGROUND

Geology and Fauna

Buluk is a late Early Miocene fossil site in northern Kenya, situated between Lake Turkana and Lake Chew Bahir (Figure 1A). Fossiliferous sediments at Buluk are part of the Buluk Member of the Bakate Formation (Figure 1B), which is exposed along the uplift of the Suregai-Asille plateau (Harris and Watkins, 1974; Watkins, 1983; McDougall and Watkins, 1985). The upper section of the Buluk Member is a series of sterile, reworked pantelleritic tuffs, while the lower section includes fossiliferous channel lag deposits underlying a rhyolitic layer capped by a basalt flow (McDougall and Watkins, 1985; Hurford and Watkins, 1987). The published date of this capping basalt is 17.2 ± 0.2 Ma (McDougall and Watkins, 1985). At the time of fossil deposition, the landscape at Buluk was dominated by a mature river system associated with an extensive floodplain (Watkins, 1989).

Buluk preserves a rich mammalian fauna (Table 1) including proboscideans, hyracoids, primates, carnivorans, hyaenodonts, rhinocerotids, suoids (i.e., sanitheriids and suids), tragulids, and pecorans (e.g., giraffoids, bovids). Since the initial publication of Early Miocene fossils from Buluk (Harris and Watkins, 1974), ongoing fieldwork has led to the collection of >1500 mammalian fossils, prompting several lineage-specific revisions of the fauna (Geraads et al., 2013; Locke et al., 2020; Morlo et al., 2021; Nishimura et al., 2022). The taxonomic composition of the mammalian fauna from Buluk largely overlaps with many other Early Miocene East African localities, with a few taxa hinting at affinities with Middle Miocene faunas. This is most apparent in the artiodactyls - a group that shows marked taxonomic and gradistic changes during the Miocene. The ruminant artiodactyl community at Buluk is like many other Early Miocene sites, being comprised primarily of tragulids in the genus Dorcatherium, the giraffid Canthumeryx, and multiple species of small-bodied pecorans of unknown familial status (Cote, 2010; Cote et al., 2018). Buluk lacks the more derived artiodactyl families found at Middle Miocene localities like Fort Ternan and Maboko (e.g., climacoceratid giraffoids, bovids, and hippopotamids), but shares with Maboko two closely related species of suids in the genus Lopholistriodon and Megalochoerus (Retallack et al., 2002; Pickford, 2006).

The diverse community of fossil primates from Buluk includes the large-bodied hominoid Afropithecus turkanensis (Leakey and Walker, 1985; Leakey and Walker, 1997), a small-bodied non-cercopithecoid catarrhine comparable in size to Simiolus enjiessi (Rose et al., 1992; Leakey et al., 2011; Nishimura et al., 2022) and at least two other as-yet-undescribed non-cercopithecoid catarrhine taxa (E.M.L. and E.R.M, personal observations). Buluk has also yielded numerous fossils of the stem cercopithecoid Noropithecus bulukensis (Leakey, 1985; Miller et al., 2009; Locke et al., 2020). In contrast to many Early Miocene sites, where cercopithecoids are scarce or absent, N. bulukensis comprises roughly half of all primate fossils recovered from Buluk (Locke et al., 2020).

Stable Carbon Isotopes

The ratio of stable carbon isotopes ¹³C/¹²C (often described as δ¹³C, see ‘Materials and Methods’ for equations) in plants consumed by herbivores is recorded in organismal tissues during development (Lee-Thorp et al., 1989, 1991; Cerling and Harris, 1999; Sponheimer et al., 2003). The high hydroxyapatite content of tooth enamel makes it resistant to diagenetic alteration during the fossilization process, making stable carbon isotope values from fossil herbivore enamel (δ¹³C_enamel) a valuable tool for dietary inference of extinct taxa and paleoenvironmental reconstruction (e.g., Bocherens et al., 1996; Cerling et al., 1997; 2003a, b; Schoeninger et al., 2003; Kingston and Harrison, 2007; Levin et al., 2008; Uno et al., 2011, 2018; Cerling et al., 2015; Du et al., 2019; Paquette and Drapeau, 2021; Robinson, 2021).

The utility of δ¹³C_enamel values derives from the fact that the δ¹³C composition of vegetation consumed by herbivores reliably discriminates between the C₃ and C₄ photosynthetic pathways used by plant groups for producing chemical energy. In modern eastern African ecosystems, woody vegetation and high-elevation grasses utilize the C₃ photosynthetic pathway, which discriminates against the heavier ¹³C and results in lower δ¹³C values (-36‰ to -23‰) (Tieszen et al., 1979; Young and Young, 1983; Cerling and Harris, 1999). C₄ vegetation includes low-elevation grasses, sedges, and some shrubs, which have relatively higher δ¹³C values (-14‰ to -10‰) (Tieszen et al., 1979; Young and Young, 1983; Cerling and Harris, 1999). The C4 pathway also discriminates against ¹³C but at a lower degree than the C₃ photosynthetic pathway.

Variation in environmental and physiological factors that influence δ¹³C values in plants results in a greater range of δ¹³ C values for C₃ plants than for C₄ plants, reflecting a greater influence of environmental conditions on carbon isotope values using the C₃ photosynthetic pathway. For example, the lowest δ¹³C values among C₃ plants occur in closed canopy forests due to the recycling of ¹³C-depleted CO₂ from soil respiration in the forest understory (Vogel, 1978; Ehleringer et al., 1986; van der Merwe and Medina, 1989, 1991). C₃ plant parts found higher in the canopy and in more open canopy environments, where there is more exposure to solar radiation or more xeric conditions, have more positive δ¹³C values (e.g., Ehleringer et al., 1986; Heaton, 1999; Kohn, 2010). The δ¹³C ratios of C₃ plants are also negatively correlated with mean annual precipitation (MAP), meaning plants from lower precipitation environments exhibit higher δ¹³C values. (Kohn, 2010).

Stable Oxygen Isotopes

In conjunction with δ¹³C_enamel values, the stable oxygen isotope values of herbivore tooth enamel (δ¹⁸O_enamel) can provide additional insights into aspects of an animal’s diet and interactions with the environment. Environmental factors such as variation in temperature, humidity, and precipitation influence δ¹⁸O values in drinking water and water within plant material, resulting in a downstream effect on δ¹⁸O _enamel values of mammalian herbivores (Dansgaard, 1964; Rozanski et al., 1993; Higgins and McFadden, 2004). In addition to environmental variation and the δ¹⁸O composition of atmospheric oxygen, the drinking habits, physiology, and metabolic mechanisms of an animal also influence its δ¹⁸O_enamel values (Bryant and Froelich, 1995; Kohn et al., 1996, 1998; Kohn and Law, 2006; Sponheimer and Lee-Thorp, 1999; Levin et al., 2006). This confluence of biotic and abiotic factors complicates the interpretation of δ¹⁸O_enamel values.

Despite the intricacies involved in the oxygen flux within herbivore tissues, some general patterns have emerged from the literature. Plant tissue δ¹⁸O values are especially sensitive to changes in local temperature, humidity, and evapotranspiration (Sternberg et al., 1989). The δ¹⁸O_enamel values of herbivores that rely on plant resources to obtain their water (i.e., non-obligate drinkers) track local variation in humidity and evapotranspiration due to canopy height (Kohn et al., 1996; Levin et al., 2006; Krigbaum et al., 2013; Nelson, 2013; Carter and Bradbury, 2016; Blumenthal et al., 2017; Green et al., 2022). Even the extent of folivory might influence δ¹⁸O_enamelvalues (Carter and Bradbury, 2016; see also Fannin and McGraw, 2020). Herbivores that rely on drinking from local water sources for their water intake (i.e., obligate drinkers) track the δ¹⁸O of meteoric water more closely, and their δ¹⁸O_enamel values are depleted in ¹⁸O relative to those of non-obligate drinkers (Levin et al., 2006). Occupying an aquatic or semi-aquatic niche also affects δ¹⁸O_enamel values, with aquatic and semi-aquatic taxa having lower δ¹⁸O_enamel values relative to terrestrial herbivores (Cerling et al., 2003b; Levin et al., 2006; Kingston and Harrison, 2007; Clementz et al., 2008).

MATERIALS AND METHODS

Enamel Sampling

Samples of fossil tooth enamel (n=67) were collected from the postcanine teeth of 12 genera of non-primate mammalian herbivores from Buluk (Table 1). These samples represent a phylogenetically and ecologically diverse set of mammals including Hyracoidea, Proboscidea (Deinotheriidae, Gomphotheriidae, Mammutidae), Perissodactyla (Rhinocerotidae), and Artiodactyla (Anthracotheriidae, Giraffidae, Suidae, Sanitheriidae). Proboscidean specimens were identified by William Sanders, while all other previously unpublished specimens were identified by E.M.L and E.R.M. All specimens sampled for this study are housed in the collections of the National Museums of Kenya (NMK) in Nairobi, Kenya, and the Turkana Basin Institute (TBI) in Ileret, Kenya. All sampling was conducted with prior authorization from the NMK and by the Kenyan National Commission for Science, Technology, and Innovation (NACOSTI).

Sampling protocols were restricted to fragmentary or damaged teeth, which limits the identification of some specimens to higher taxonomic levels. Non-deinotheriid proboscidean teeth that could not be identified as gomphotheriid or mammutid were classified as “Elephantimorph indet.”. Rhinocerotid teeth that could not confidently be assigned to Brachypotherium minor were designated as “Rhinocerotidae indet.”, given evidence for the presence of multiple rhinocerotid taxa at Buluk (Geraads and Miller, 2013). These higher taxonomic level groups potentially include specimens representing multiple genera. For the entire sample, 75% were identified to the species level, 9% were identified to genus, and 16% were identified to superfamily (Elephantoidea) and family (Rhinocerotidae). When possible, enamel was collected from molars, specifically second and third molars, to ensure adult diet was captured.

Prior to sampling, fossil teeth were inspected for a suitable sampling surface along a broken face that would not interfere with occlusal surface morphology. The enamel surface was cleaned using a tungsten-carbide bur bit to remove matrix, cementum, and/or dentin before samples of pure enamel were removed. The mass of enamel collected reflects a compromise between obtaining a sufficient volume of enamel powder for analysis while preserving informative tooth morphology. Approximately 5 or more milligrams of enamel were collected for most taxa, except for those with small or thinly enameled dentitions (i.e., Tragulidae, Hyracoidea, and Sanitheriidae), for which samples were limited to a maximum of 3 mg.

Pretreatment and Isotopic Analysis

Following Uno and colleagues (2011), a subset of enamel samples (n=15) was analyzed with and without pretreatment to determine the effect of treatment on δ¹³C_enamel values. Enamel powder samples with a mass >6 mg were selected for pretreatment/treatment comparisons. Pretreatment was performed at the Paleoecology Lab at the University of Michigan. Samples undergoing pretreatment were soaked in 3% sodium hypochlorite (NaOCl) for 12 hours. Samples were centrifuged and NaOCl was decanted using a pipette without disturbing the enamel pellet. Enamel powder was rinsed with deionized water to neutrality. Samples were then treated with 0.1M solution of acetic acid (CH₃ COOH) for 12 hours and rinsed with deionized water to neutrality. The enamel powder pellet was transferred to a glass vial and excess moisture was removed by freeze drying overnight. Results of treated versus untreated Buluk δ¹³C_enamel and δ¹⁸O_enamel values are shown in Appendix 1. The difference in δ¹³C_enamel and δ¹⁸O_enamel values between treated and untreated samples was less than ±0.6‰ for all 15 samples, indicating a minimal effect of treatment (Appendix 2).

Since a considerable amount of sample powder can be lost during pretreatment, standard protocols are to generally leave samples with ~1-4 mg of enamel powder untreated and to convert their values to match pretreated samples using regression equations (e.g., Levin et al., 2008; Cerling et al., 2015). Here, based on the results shown in Appendix 1, all other untreated samples were converted to match pretreated samples using the respective carbon and oxygen regression equations:

δ¹³C _final = 0.96 * δ¹³C _untreated - 0.67, R² = 0.97 (1)

δ¹⁸O _final = 0.99 * δ¹⁸O _untreated - 0.26, R² = 0.96 (2)

Treated and untreated samples were sent to the University of Florida, Department of Geosciences Light Stable Isotope Mass Spec Lab for analysis and determination of ¹³C/¹²C ratios. In glass vials, 3-5 mg of enamel powder sample and NBS-19 standards were reacted with 99% phosphoric acid (H₃ PO₄) at 70°C. The CO₂ produced from this reaction was analyzed using mass spectrometry with a Kiel III carbonate preparation device coupled with a Finnigan-Mat 252 isotope ratio mass spectrometer. The standard deviation for NBS-19 standard analyses was ~0.03‰ for δ¹³C and ~0.07‰ for δ¹⁸O. Results are reported in standard parts per million notation (‰):

δ13C = (18O) = (R_sample/R_standard - 1) * 1,000 (3)

R = ¹³C/¹²C or ¹⁸O/¹⁶O (4)

Interpretation of Enamel Stable Isotope Ratios

The δ¹³C value of atmospheric CO₂ (δ¹³C_atm) that is incorporated into plant tissues fluctuates through time and thus impacts the δ¹³C values of plant tissues. This flux in δ¹³C_atm values must be standardized before comparing δ¹³C_enamel values from different time periods. Since the Industrial Revolution (estimated to be approximately A.D. 1750), the generation of ¹³C-depleted CO² from anthropogenic processes has decreased global δ¹³C_atm values (Francey et al., 1999). During the Early and Middle Miocene, estimated average δ¹³C_atm values from North Atlantic benthic foraminifera varied between -4.8‰ to -6.6‰ (Tipple et al., 2010). A pre-Industrial Revolution δ¹³C_atm value of −6.3‰ (δ¹³C₁₇₅₀) was used to standardize fossil enamel carbon isotope values and any modern enamel carbon isotope values mentioned in the text.

For the Early Miocene age (17.2±0.2 Ma, McDougall and Watkins, 1985), the δ¹³C_atm values for 17.0 - 17.4 Ma from the high-resolution benthic dataset from Tipple et al. (2010) were averaged (mean = -5.66‰). Therefore, -0.64‰ was added to δ¹³C_enamel values for standardization to δ¹³C₁₇₅₀ values. Recently published paleoecological work has placed the age of Buluk closer to 16 Ma (Peppe et al., 2023). However, this relies on radiometric K-Ar (alkali feldspar) and fission-track (zircon) ages of 16.1± 0.2 Ma from the volcaniclastic tuffs overlying the Bakate Formation (Figure 1B; Watkins, 1989; Hurford and Watkins, 1987). We conservatively retain the use of the 17.2 Ma date to standardize enamel samples from Buluk with modern and other fossil carbon isotope data. For completeness, a comparison of the differences in δ¹³C_enamel value correction using a date of 16 Ma versus 17.2 Ma is provided in Appendix 3. All fossil and modern δ¹³C _enamel results are reported as δ¹³C₁₇₅₀ values to accommodate fluctuations in atmospheric ẟ¹³C so modern and fossil enamel isotopic values can be compared. Published δ¹³C_enamel values from Moroto, Maboko, and Fort Ternan were also corrected using the high-resolution benthic δ¹³C_atm estimates from Tipple et al. (2010) as follows: raw Maboko δ¹³C_enamel values were corrected following Arney et al., (2022); for the interval of 13.40-14.0 Ma (mean δ¹³C_atm = -5.48‰), -0.82‰ was added to published Fort Ternan δ¹³C_enamel values; for the interval of 20.71-21.38 Ma (mean δ¹³C_atm = -6.04‰), -0.26‰ was added to published Moroto δ¹³C_enamel values.

Isotopic dietary ranges that correspond with general biomes were used to assess habitat structure among presumed C₃-dominated ecosystems (closed canopy vs. open canopy; mesic vs. xeric conditions). The following dietary categories are modified from the diet classifications described in Cerling and colleagues (2015): C₃ closed canopy diets are defined as those with δ¹³C_enamel values < -14‰; C₃ open canopy diets are defined as those with δ¹³C_enamel values between -14‰ and -8‰; and C₃/C₄ mixed diets are characterized as those with δ¹³C_enamel values > -8‰.

All statistical analyses and plots were made using R (version 4.2.1, R Core Team, 2021). δ¹³C_enamel data from Buluk was compared with published enamel stable isotope values from fossil mammals from Moroto (MacLatchy et al., 2023), Maboko (Arney et al., 2002) and Fort Ternan (Cerling et al., 1997) (Table 2). Shapiro-Wilk tests indicate a normal distribution for δ¹³C_enamel data from Buluk, Maboko, and Fort Ternan, but not from δ¹³C_enamel data Moroto. Intersite δ¹³C_enamel comparisons were conducted using pairwise nonparametric Mann-Whitney U tests. Bulk δ¹⁸O_enamel values between localities were not statistically compared, since each locality has a unique meteoric water source(s) with unknown δ¹⁸O values, which can further complicate interpretations. Results of the intersite carbon analysis and oxygen comparisons are reported in the main text of the Discussion.

RESULTS

General Patterns

Buluk herbivore δ¹³C_enamel values range from -12.7‰ to -9.3‰ with a mean value of -11.2‰ (Figure 2; Table 2). These values are consistent with herbivores foraging in a C₃ -dominated open canopy woodland ecosystem. Carbon isotopic profiles from Buluk herbivores do not fall within the range of values for closed canopy understory foraging, suggesting closed canopy habitats were either absent from the landscape or were not utilized consistently by herbivores sampled here. There is no evidence for the consumption of C₄ resources, even in low proportions.

The δ¹⁸O_enamel values from Buluk range from -6.4‰ to +3.5‰ (Figure 3). Differences in δ¹⁸O_enamel values among taxa potentially reflect variation in water intake strategies, physiological water flux, changes in meteoric water δ¹⁸O values due to environmental factors, and/or contributions from multiple water sources. While it is difficult to make specific inferences about drinking habits or environmental conditions given the lack of extant analogs for many of the fossil taxa sampled here, documenting intra and inter-specific variation in δ¹⁸O_enamel values is valuable for future comparative work. Taxon-specific summary statistics for δ¹³C_enamel and δ¹⁸O_enamel are reported in Table 3, and these values are plotted together for completeness in Appendix 4.

Hyracoidea

Two Afrohyrax championi teeth were sampled in this study, with δ¹³C_enamel values of -11.0‰. These values overlap with the δ¹³C_enamel values of several Buluk taxa, including deinotheres, the giraffid Canthumeryx sirtensis, the upper range of rhinocerotid values, and the low end of values for the anthracotheriid cf. Sivameryx africanus (Figure 2). δ¹⁸O_enamel values of the two A. championi specimens are -0.1‰ and +1.6‰. These oxygen isotopic values are relatively high, overlapping with cf. S. africanus, C. sirtensis, and the large-bodied suid Megalochoerus marymuunguae (Figure 3).

Proboscidea

The δ¹³C values for all sampled Buluk proboscideans range from -12.1‰ to -9.3‰ (Figure 2; Table 3). The range of δ¹³C values for Prodeinotherium hobleyi is -12.1‰ to -9.9‰, consistent with prior isotopic analyses from Buluk showing a pure C₃ browsing signal for this taxon (Cerling et al., 1999). The range for the elephantimorph taxa (i.e., gomphtheriids and mammutids) is -10.7‰ to -9.5‰, suggesting that they may have fed on more water-stressed browse than the relatively δ¹³C depleted deinotheriid. These differences could also be the result of body size variation (Tejada-Lara et al., 2018), plant parts consumed (Carlson and Kingston, 2014; Blumenthal et al., 2016), and interspecific physiological and metabolic variation (Cerling et al., 2021).

The δ¹⁸O_enamel values of Buluk proboscideans range from -6.4‰ to -0.2‰, with a mean value of -3.1‰ (Figure 3). Oxygen signatures for the five proboscidean taxa overlap extensively, with the range of δ¹⁸O_enamel values of P. hobleyi encompassing all other proboscidean values. Overall, the lowest δ¹⁸O_enamel values of all taxa are from elephantimorph proboscideans and P. hobleyi specimens, suggesting proboscideans were dependent on drinking water from a variety of sources (e.g., rivers, springs, ponds, or lakes) for water intake. Proboscideans are the most δ¹⁸O depleted group of mammals sampled from Buluk.

Rhinocerotidae

Ten rhinocerotid teeth were sampled: seven belonging to Brachypotherium minor and three assigned to Rhinocerotidae gen. indet., which may represent a distinct species (Geraads and Miller, 2013). δ¹³C_enamel values for the total Rhinocerotidae sample range from -12.7‰ to -11.0‰. The range in B. minor δ¹³C_enamel values is from -12.4‰ to -11.0‰, which is bracketed by the Rhinocerotidae gen. indet. values (Figure 2). The rhinocerotid carbon isotope values overlap with those from the suid M. marymuunguae, the giraffid C. sirtensis, and the deinothere P. hobleyi, and are lower than δ¹³C values of the sanitheriid Diamantohyus africanus, the tragulid Dorcatherium pigotti, the anthracothere cf. S. africanus, and all elephantimorph proboscideans.

Rhinocerotid δ¹⁸O_enamel values range from -3.6‰ to +0.2‰ and have a mean value of -1.3‰ (Figure 3). These oxygen isotope values are higher than most δ¹⁸O_enamel samples from elephantimorph proboscideans and lower than most samples from C. sirtensis.

Giraffidae

Six Canthumeryx sirtensis teeth were sampled from Buluk. δ¹³C_enamel values for C. sirtensis range from -12.4‰ to -9.4‰ and have a mean value of -10.9‰ (Figure 2). C. sirtensis has the highest range of δ¹³C_enamel values among all sampled mammals from Buluk, which suggests that this taxon has a relatively broad dietary range.

The δ¹⁸O_enamel values for C. sirtensis range from -0.6‰ to +3.5‰, which is relatively enriched in ¹⁸O compared to all other taxa (Figure 3). Extant giraffes in many modern assemblages are similarly enriched relative to other herbivores on the landscape (Kingston and Harrison, 2007; Kohn et al., 1996; Sponheimer and Lee-Thorp, 1999). The ¹⁸O enriched C. sirtensis values are consistent with this taxon preferentially relying on diet for water intake, like extant Giraffa (Levin et al., 2006). These results also indicate C. sirtensis browsed on dietary items enriched in ¹⁸O, such as leaves and/or fruit from high in the woody canopy (Krigbaum et al., 2013; Carter and Bradbury, 2016; Roberts et al., 2017).

Tragulidae

The δ¹³C_enamel values for the five Dorcatherium pigotti teeth sampled range from -10.6‰ to -9.9‰ and have a mean value of -10.2‰ (Figure 2). This range of δ¹³C_enamel values overlaps with many other artiodactyls from Buluk, including Di. africanus, cf. S. africanus, and C. sirtensis. The D. pigotti carbon isotopic results are ¹³C enriched relative to the limited δ¹³C_enamel data available for the forest-dwelling, extant African tragulid, Hyemoschus aquaticus (-13.9‰; Cerling et al., 2004), indicating D. pigotti browsed in more open wooded habitats. There is a second, larger-bodied tragulid species present at Buluk, Dorcatherium chappuisi (Table 1), which could not be sampled. At the Middle Miocene site of Maboko, where these two taxa also co-occur, D. chappuisi is more enriched in ¹³ C than D. pigotti, but whether these taxa exhibit similar niche partitioning at Buluk cannot be determined at this time.

D. pigotti δ¹⁸O_enamel values range from -4.5‰ to -0.3‰ and have a mean value of -2.8‰. The distribution of D. pigotti overlaps with the distribution of δ¹⁸O_enamel values for deinotheres, the higher values of elephantimorphs, the lower values of rhinocerotids, and all other artiodactyls from Buluk (Figure 3).

Suoidea

Suoid species sampled here include three specimens of the sanithere Diamantohyus africanus and five specimens of the large-bodied suid Megalochoerus marymuunguae. Two smaller-bodied suids, Lopholistriodon pickfordi and cf. Kenyasus rusingensis, are also present at Buluk (Table 1) but could not be sampled. D. africanus δ¹³C_enamel values range from -10.8‰ to -9.5‰ and have a mean value of -10.0‰ (Table 3). These carbon isotopic values are ¹³C enriched compared to δ¹³C_enamel values from M. marymuunguae (Figure 2), which range from -11.4‰ to -10.5‰ and have a mean value of -11.2‰. The lower δ¹³C_enamel values of M. marymuunguae indicate it may have had a different foraging strategy than D. africanus, selecting plants or plant parts enriched in ¹³C or foraging in more open canopy.

The δ¹⁸O_enamel values for D. africanus range from -2.8‰ to -2.2‰ and have a mean value of -2.6‰. The δ¹⁸O_enamel values for M. marymuunguae range from -1.0‰ to +1.4‰ and have a mean value of +0.2‰, which are higher than the δ¹⁸O_enamel values of D. africanus (Figure 3). The ¹⁸O depleted δ¹⁸O_enamel values in D. africanus compared to M. marymuunguae oxygen profiles is consistent with a dependence on a more ¹⁸O depleted diet and water sources. Differences in body size and physiological metabolic processes could also have contributed to the divergent carbon and oxygen profiles between these two suoids (Bryant and Froelich, 1995; Kohn et al., 1996; Passey et al., 2005; Tejada-Lara et al., 2018; Cerling et al., 2021), as D. africanus is markedly smaller than M. marymuunguae (van der Made, 1996; Grossman, 2008). The higher δ¹⁸O_enamel values for M. marymuunguae, however, do not follow the expectations outlined in Bryant and Froelich (1995) for a much larger species, where larger body sizes take in more liquid water and rely less on food metabolism for oxygen intake.

Anthracotheriidae

Four anthracothere teeth were sampled, which are identified as cf. Sivameryx africanus. Anthracothere δ¹³C_enamel values range from -11.0‰ to -9.5‰ with a mean value of -10.4‰. The distribution of anthracothere δ¹³C_enamel values overlaps with those of tragulids, sanitheriids, and elephantimorph proboscideans (Figure 2).

The δ¹⁸O_enamel values for anthracotheres range from -5.2‰ to +1.9‰ and have a mean value of -2.7‰. Most anthracothere δ¹⁸O_enamel values are depleted in ¹⁸O and overlap with δ¹⁸O_enamel values of all proboscideans and D. pigotti. One δ¹⁸O_enamel value of cf. S. africanus is relatively ¹⁸O enriched, overlapping with δ¹⁸O_enamel values of C. sirtensis (Figure 3). While this sample came from a maxillary P⁴ of cf. S. africanus, the relatively enriched δ¹⁸O enamel value likely reflects a shift in either surface water and/or dietary δ¹⁸O values rather than a weaning signature, as the P4s of most extant ruminant and non-ruminant herbivores form and erupt after the permanent upper molars (Spinage, 1976; Davis, 1980; Levine, 1982; Hillman-Smith et al., 1986; Fricke and O’Niel, 1996; Kohn, 1996; Kohn et al., 1998; Fricke et al., 1998; Green et al., 2018, 2022).

DISCUSSION

Herbivore Enamel Isotopes, Diets, and Paleoecology

The range of Buluk herbivore δ¹³C_enamel values (-13.1‰ to -9.8‰) indicates these mammals had C₃-dominated diets. Elephantimorph proboscideans exhibit the highest δ¹³C_enamel values in the sample and rhinocerotids exhibit the lowest values (Figure 2). Stable carbon isotope data from pedogenic carbonates at Buluk indicates the presence of a C₄ component on the landscape (Peppe et al., 2023), but evidence from δ¹³C_enamel samples suggests the herbivorous mammals at Buluk were likely not consuming this resource. Using methods published by MacLatchy et al. (2023), estimated fractions of C₄ biomass at Buluk from pedogenic carbonate δ¹³C values indicate a wide range of possible C₄ biomass, with the lowest reconstructed C4 percentage of 19% (95% confidence interval = 0% to 46%; δ¹³C_carbonate = -8.7‰) and the largest reconstructed C₄ percentage of 72% (95% confidence interval = 46% to 97%; δ¹³C_carbonate = -0.8‰).

Due to the opportunistic nature of the sampling strategy, it remains possible that some Buluk herbivores that were unable to be sampled for this analysis did indeed utilized C₄ resources. It is also possible that the abundance of C₄ vegetation was too low to be consistently incorporated into herbivore diets, or that C₄ plants were not preferred dietary items. The tightly constrained range of δ¹³C_enamel values from Buluk suggests similar foraging patterns among mammalian herbivores, or niche partitioning along axes that are not reflected in isotopic analyses (Arney et al., 2022). The overall range of herbivore δ¹³C_enamel values reflects feeding in open environments without closed canopy components, water-stressed C₃ plants, or abundantly consumed C₄ plants.

These enamel carbon isotope results are consistent with many, but not all previous paleoecological findings from Buluk. Preliminary analyses of paleosol geochemistry and morphology suggest a relatively open woodland environment within a seasonal, subhumid precipitation regime (Lukens et al., 2017b, 2021). In this way, paleoenvironmental reconstructions at Buluk are consistent with the interpretation that heterogeneous open habitats were present across East Africa during the Early Miocene (Peppe et al., 2021). The fact that the Buluk herbivores exhibit C₃-dominated diets within this open canopy habitat is also consistent with previous local and regional stable isotopic work showing that East African herbivore diets were C_3-dominated prior to 10 Ma (Cerling et al., 1991, 1997; Uno et al., 2011; Feakins et al., 2013; Polissar et al., 2019).

Enamel oxygen profiles from Buluk mammals indicate variation in water intake strategies between herbivore guilds (Figure 3). The giraffid C. sirtensis displays the highest δ¹⁸O_enamel values, consistent with an interpretation of this taxon as a non-obligate drinker receiving most of its water intake from dietary vegetation, as seen in most extant giraffes (e.g., Sponheimer and Lee-Thorpe, 1999; Cerling et al., 2003a, b; Schoeninger et al., 2003; Levin et al., 2008) and δ¹⁸O_enamel results from Maboko and Fort Ternan (Appendix 4). The lowest δ¹⁸O_enamel values among the Buluk fauna are found in the elephantimorph proboscideans, which suggests that these taxa had a higher reliance on standing water (Levin et al., 2008). Although Buluk’s depositional environment reflects a mature river system (Watkins, 1989), intra- and inter-specific variation in δ¹⁸O_enamel values may reflect multiple sources of drinking water with varying δ¹⁸O values across the landscape. It is important to note that, while δ¹⁸O_enamel values have been used to draw inferences about obligate vs. non-obligate drinking herbivores from Plio-Pleistocene fossil localities (e.g., Blumenthal et al., 2017), the lack of modern analogs for some Early and Middle Miocene taxa makes inferences about their drinking behaviors (i.e., obligate vs. non-obligate) more tentative.

Precipitation seasonality has been previously proposed to be a driver in catarrhine evolution, specifically a shift from low to high precipitation seasonality in the Late Miocene (e.g., Temerin and Cant, 1983; Andrews and Martin, 1991; Pickford, 1995). Evidence for Miocene climatic seasonality, however, is limited both for the Turkana Basin and for eastern Africa more broadly (Green et al., 2022). The range of herbivore δ¹⁸O_enamel values from Buluk (9.9‰, n=67) is relatively high when compared to the bulk δ¹⁸O_enamel values of herbivores from Kalodirr (range = 7.8‰, n=66), an Early Miocene (~17 Ma) site in West Turkana where primate molar serial oxygen profiles have yielded clear evidence of seasonal precipitation variation (Green et al., 2022). While differences in δ¹⁸O_enamel values between Kalodirr and Buluk may reflect differences in local hydrological dynamics (i.e., different sources of rainwater with different δ¹⁸O_enamel values) and/or taxonomic sampling, the greater range of oxygen values from Buluk this provisionally could indicate that Buluk, like Kalodirr, was characterized by strong wet/dry seasonality. The frequency and intensity of wet/dry seasons at Buluk is currently unknown but may be better understood by future serial sampling of enamel from primates and/or associated fauna.

Among the Buluk herbivores, the most positive δ¹³C_enamel values are from elephantimorph proboscideans (gomphotheres and mammutids). Elephantimorph δ¹³C_enamel values overlap with some deinothere samples, but deinotheres exhibit a significantly larger variance in δ¹³C_enamel values than elephantimorphs (F[18,12]=4.733, p=0.009), which extends into a more δ¹³C-depleted range than any other proboscidean sampled from Buluk. This greater range of variation is likely due to the larger sample size of deinothere teeth compared to other fauna (Figure 3), which captures more isotopic variation compared with the limited samples of available gomphothere and mammutid teeth. Despite these sample size limitations, it is also possible that elephantimorph proboscideans consumed vegetation or plant parts that were enriched in δ¹³C compared to deinotheres (Carlson and Kingston, 2014; Blumenthal et al., 2016), or that isotopic enrichment differs between these groups as a result of differences in digestive physiology, metabolism, and body size (Passey et al., 2005; Tejada-Lara et al., 2018; Cerling et al., 2021). Enamel microwear features indicate that Buluk P. hobleyi, Archaeobelodon sp. nov., and Zygolophodon sp. nov. taxa likely fed on tough, fibrous browse (Sanders et al., 2020). Mesowear angles for Archaeobelodon sp. nov. molars suggest browsing with some mixed feeding for this species (Saarinen and Lister, 2023). Mixed feeding among elephantimorphs that incorporated C₃ browse or grasses enriched in ¹³C could be driving the higher δ¹³C_enamel values relative to the other proboscidean taxa sampled.

There is no clear pattern of oxygen isotopic partitioning among the Buluk proboscideans based on δ¹⁸O_enamel values. Deinothere and elephantimorph δ¹⁸O_enamel values overlap extensively between deinotheres, gomphotheres, and mammutids (Figure 3). Deinothere δ¹⁸O values are widely variable and overlap with rhinocerotids, tragulids, sanitheres, suids, and anthracotheres. This broad range of proboscidean δ¹⁸O values may be related to methodology (e.g., sampling different tooth positions and areas of the crown; Kohn et al., 1996; Malone et al., 2021) or dietary ecology (e.g., drinking water from across multiple sources or consuming food sources with widely varying δ¹⁸O values; Kohn et al., 1996; Carter and Bradbury, 2016).

Relatively depleted δ¹⁸O_enamel values from early Miocene anthracotheres at Kalodirr appear to corroborate previous inferences of an aquatic or semi-aquatic niche for this clade (Green et al., 2022), which was previously suggested based on the association of anthracothere skeletal remains with lacustrine sediments (Pickford, 1981, 1983). Extant semi-aquatic herbivores like the hippopotamus (Hippopotamus amphibius) have δ¹⁸O_enamel values lower than associated terrestrial fauna (Bocherens et al., 1996; Clementz et al., 2008). This technique has been applied to fossil anthracotheres from the Eocene and Oligocene of Egypt as well as the Oligocene of Germany to infer both semi-aquatic and terrestrial niches for members of this clade (Clementz et al., 2008; Liu et al., 2008; Tütken and Absolon, 2015). In contrast with Kalodirr specimens of Sivameryx africanus, the Buluk specimens of cf. Sivameryx africanus have relatively enriched δ¹⁸O_enamel values (Figure 3) that overlap with oxygen profiles of terrestrial herbivores from the same site (e.g., elephantimorphs, deinotheres, tragulids). One possible interpretation of these results is that cf. Sivameryx from Buluk was occupying a more terrestrial niche rather than the semi-aquatic one proposed for Sivameryx from Kalodirr.

Buluk preserves a diverse suoid community that includes the small-bodied selenodont sanithere Diamantohyus africanus, the small-bodied lophodont suid Lopholistriodon pickfordi and two bunodont kubanochoerine suids, cf. Kenyasus rusingensis and the much larger Megalochoerus marymuunguae (Pickford, 1983a; van der Made, 1996; Bishop, 2010). Only M. marymuunguae and D. africanus could be sampled for this analysis (Table 1), but variation in their carbon and oxygen isotopic values suggests possible niche partitioning among the Buluk suoids. Megalochoerus marymuunguae has lower δ¹³ C_enamel values than D. africanus, suggesting it fed on more ¹³C depleted vegetation (Carlson and Kingston, 2014; Blumenthal et al., 2016). Given the size difference between D. africanus and M. marymuunguae, variation in body mass-related digestive physiologies and metabolic processes could have contributed to these δ¹³C_enamel results (Passey et al., 2005; Tejada-Lara et al., 2018; Cerling et al., 2021).

The δ¹⁸O_enamel values of M. marymuunguae are enriched relative to most other herbivores at Buluk, including D. africanus, suggesting that this species was less water-dependent and received a larger portion of its water from dietary vegetation. These higher δ¹⁸O_enamel values could also be attributed to consuming foods enriched in δ¹⁸O, such as fruits from higher in the woody canopy or a large proportion of leafy browse (Carter and Bradbury, 2016; Roberts et al., 2017). Other species of large-bodied kubanochoerine suids from Early Miocene deposits on Rusinga Island (Garrett, 2016) and the Middle Miocene deposits on Maboko Island (Arney et al., 2022) exhibit a much lower δ¹⁸O_enamel signal relative to other fauna than M. marymuunguae from Buluk. At both Rusinga and Maboko Islands, kubanochoerine suids exhibit δ¹⁸O_enamel values that are more depleted than most other herbivore taxa (Garrett, 2016; Arney et al., 2022), which is ecologically similar to the depleted δ¹⁸O_enamel values of the extant, water-dependent giant forest hog (Hylochoerus meinertzhageni) and the red river hog (Potamochoerus porcus) (Harris and Cerling, 2002; Nelson, 2013; Cerling et al., 2015; Martin et al., 2015; Lazagabaster et al., 2021). The contrasting enriched δ¹⁸O_enamel values of M. marymuunguae suggest feeding and/or drinking behavior that is unlike these extant taxa and other fossil kubanochoerines from localities with closed forest elements in the local habitat.

Only one of the two tragulid species at Buluk, Dorcatherium pigotti, could be sampled in this study. While no specimens of the larger congeneric D. chappuisi from Buluk were available for sampling, these two species exhibit overlapping δ¹³C_enamel and δ¹⁸O_enamel values at the Early Miocene locality of Kalodirr (Butts, 2019; Green et al., 2022), but disparate carbon and oxygen values at the Middle Miocene site of Maboko (Arney et al., 2022). The δ¹³C_enamel values of D. pigotti from Buluk are enriched (mean δ¹³C_enamel value = -10.7‰) relative to the water chevrotain (Hyemoschus aquaticus), the only extant African tragulid species (mean δ¹³C_enamel value = -13.9‰, Cerling et al., 2004). Extant H. aquaticus is a nocturnal, forest-dwelling herbivore that primarily consumes fallen fruits, but also occasionally consumes insects and carrion (Dubost, 1987; Gautier-Hion et al., 1980; Hart, 1986). Dorcatherium pigotti from Buluk, along with the majority of Miocene tragulids from East Africa and Eurasia, appears to have had a wider range of browsing diets outside of forested habitats than the modern H. aquaticus (Nelson, 2007; Aiglstorfer et al., 2014).

Early and Middle Miocene Intersite Comparisons

Stable carbon isotope data from herbivore enamel is available from relatively few East African Miocene sites older than 9 Ma (Cerling et al., 1997; Arney et al., 2022; MacLatchy et a., 2023), yet the few published δ¹³C_enamel datasets have contributed substantially to our understanding of herbivore dietary isotope ecology within C₃ -dominated habitats. The new δ¹³C_enamel data from Buluk can be compared with data from Moroto (21 Ma), Maboko (~16 Ma), and Fort Ternan (13.7 Ma) to help fill a gap in what is known about East African dietary paleoecology across the early to Middle Miocene transition (16-15 Ma) (Table 2).

The bulk δ¹³C_enamel values from Buluk are not significantly different from the Early Miocene site Moroto (Figure 4; Table 4). Moroto’s paleoenvironment has been reconstructed as a woodland setting in which herbivores consumed primarily water-stressed C₃ vegetation and potentially some C₄ vegetation (MacLatchy et al., 2023). The lack of statistically significant differences between bulk δ¹³C_enamel values from Buluk and Moroto suggests that the landscape at Buluk likely also included open woodland elements with water-stressed C₃ vegetation. Herbivores from Buluk, however, exhibit a narrower range of δ¹³C_enamel values than those from Moroto, and there is no isotopic evidence for any closed canopy forest elements nor of minor C₄ (or CAM) dietary components, as indicated by the end members of the Moroto δ¹³C_enamel distribution (MacLatchy et al., 2023). While bulk δ¹³C_enamel distributions are influenced by the taxonomic sampling and overall sample size, the narrower range of the larger Buluk sample (n=67) compared to the broader range of the smaller Moroto samples (n=25) implies that the differences in δ¹³C_enamel range are likely not an artifact of sampling strategy.

The δ¹³C_enamel values from Buluk and from Moroto are significantly more enriched than the two Middle Miocene sites, Maboko and Fort Ternan (Figure 4; Table 4). Middle Miocene deposits at Maboko include multiple time-successive beds, which have been reconstructed as primarily open forest/woodland mosaics without the presence of C₄ biomass or water-stressed C₃ vegetation (Arney et al., 2022). Fort Ternan, despite a long history of paleoenvironmental debate (Shipman et al., 1981; Pickford, 1983a, 1983b; Shipman, 1986; Retallack et al., 1990; Cerling et al., 1991; Kappleman, 1991; Retallak, 1992; Dugas and Retallack, 1993; Cerling et al., 1997), has yielded stable carbon isotope data from soil carbonates and herbivore enamel that indicates a relatively open canopy C₃-dominated ecosystem without the presence of C₄ biomass (Cerling et al., 1991; Cerling et al., 1997). Comparisons of the bulk δ¹³C_enamel distributions between these Middle Miocene sites and Buluk suggest that herbivores at Buluk foraged within more open-canopy, relatively drier habitats with greater proportions of water-stressed C₃ vegetation than at Maboko or Fort Ternan. While multiple paleoecological proxies have demonstrated temporal and spatial habitat heterogeneity across East African Early Miocene localities (Peppe et al., 2023), landscape variation across East Africa during the Middle Miocene is less well known, due in large part to the small number of known Middle Miocene fossil localities, and it is difficult to say whether the differences in δ¹³C_enamel values reported here reflect any spatial and/or temporal trends across the Early-Middle Miocene transition.

A comparison of representative taxonomic groups (hyracoids, proboscideans, rhinocerotids, giraffoids, tragulids, suoids, and anthracotheres) reveals substantial differences in herbivore foraging behaviors between Early and Middle Miocene localities in Kenya (Figure 4). The δ¹³C_enamel values of Buluk taxa are higher than their Middle Miocene counterparts.

Specifically, comparisons of species that occur at both Buluk and Maboko (the deinothere P. hobleyi, the tragulid D. pigotti, and the hyracoid A. championi) demonstrate dietary differences that likely reflect environmental differences between these two sites. While the gomphothere P. macinnesi also occurs at both Buluk and Maboko, the single sample from Buluk limits comparisons that can be made for this taxon, although this specimen is more enriched than any P. macinnesi specimen from Maboko. All D. pigotti and A. championi δ¹³C_enamel values from Buluk are relatively enriched and do not overlap with the values of their conspecifics from Maboko (mean D. pigotti value is 2.1‰ higher; mean A. championi value is 2.0‰ higher). There is overlap among the δ¹³C_enamel distributions of P. hobleyi specimens from Buluk and Maboko, but the mean δ¹³C_enamel value of the Buluk specimens is relatively enriched. These results suggest that species co-occurring between Buluk and Maboko likely had flexible diets and consumed various proportions of dietary items (e.g., leaves, fruits, C₃ grasses) depending on the local environment.

Additional comparisons at higher taxonomic levels broadly demonstrate greater δ¹³C_enamel enrichment at Buluk and Moroto compared to Maboko and Fort Ternan (Figure 4). Among elephantimorphs, both Eozygodon morotoensis from Moroto (which has the highest δ¹³C_enamel values of any elephantimorph in this sample) and the two taxa from Buluk demonstrate more enriched values than the Maboko and Fort Ternan specimens. Among suoids, M. marymuunguae and D. africanus from Buluk have higher δ¹³C_enamel values than their congeners (Megalochoerus khinzikebirus and Diamantohyus nadirum) from Maboko, indicating different dietary preferences and/or vegetation availability between these sites. The sole giraffoid from Buluk, C. sirtensis, exhibits slightly enriched δ¹³C_enamel values relative to Climacoceras africanus from Maboko but overlaps with the large Giraffoidea sp. from Maboko and Giraffokeryx primaevus from Fort Ternan. Rhinocerotidae is the only taxonomic group that demonstrates considerable overlap in δ¹³C_enamel values across Early and Middle Miocene sites. The extensive overlap between the rhinocerotid samples from Buluk, Maboko, and Fort Ternan suggests limited dietary variation among these species and possibly less flexible feeding ecology than other mammalian groups.

Of the taxonomic groups that overlap between Buluk and Moroto, only the anthracotheres show differences in δ¹³C_enamel values. The Moroto anthracothere, Brachyodus cf. aequatorialis, has depleted δ¹³C_enamel values relative to cf. Sivameryx africanus from Buluk. One interpretation of these results is that B. cf. aequatorialis at Moroto inhabited wetter, more densely vegetated habitats relative to cf. S. africanus at Buluk. This may be more indicative of differences in the behavior and biology of these taxa, which differ in size and dental morphology, rather than environmental differences between these sites. The low δ¹⁸O_enamel values of B. cf. aequatorialis relative to other Moroto herbivore δ¹⁸O_enamel values (Appendix 5; see also fig. S12 of MacLatchy et al., 2023) support the idea that this taxon might have been more water dependent than cf. S. africanus at Buluk. Brachyodus and Sivameryx have been suggested to be a hydrophilic taxon based on depositional and taphonomic settings at North African and some East African Miocene localities (Pickford, 1983; Holroyd et al., 2010; Miller et al., 2014), although cranial remains of Sivameryx africanus from the Early Miocene locality of Kalodirr have cast doubt on the proposal that this species was semi-aquatic (Rowan et al., 2015).

Prior to the publication of stable carbon isotope data from the Early Miocene in East Africa, variation in ¹³C among plants in C₃-dominated ecosystems was presumed to be too limited for dietary inference or palaeoecological reconstruction (MacLatchy et al., 2023). Here, however, the collective ranges of dietary carbon signals from herbivore guilds at Moroto, Buluk, Maboko, and Fort Ternan document shifting foraging strategies across the Early and Middle Miocene. While caution should be used when using herbivore δ¹³C_enamel values for paleoenvironmental comparisons, especially when inferring differences in woody cover (Robinson et al., 2021), the considerable C₃ -dominated dietary heterogeneity documented in these intersite results is consistent with independent lines of palaeoecological evidence for variable habitat cover and vegetation at these Early and Middle Miocene sites (e.g., Lukens et al., 2021; MacLatchy et al., 2023; Peppe et al., 2023).

East African Miocene Environments and Catarrhine Evolution

Early models of the environmental context for early catarrhine evolution emphasized pervasive complex canopy cover across eastern Africa during the Early Miocene, eventually giving way to more fragmented woodlands in the late Early Miocene and the Middle Miocene (e.g., Andrews and Van Couvering, 1975; Andrews and Kelley, 2007; Couvreur et al., 2008). A more recent body of soil carbonate and plant wax isotopic datasets from multiple Early Miocene sites has indicated the presence of more eclectic habitats across this region, ranging from closed canopy forests to wooded grasslands with arid conditions that could support substantial C₄ plant components (Lukens et al., 2017a; Liutkus-Pierce et al., 2018; MacLatchy et al., 2023; Peppe et al., 2023). Buluk, along with the Karungu, Moroto, and Napak have been identified as some of the sites with C₄ isotopic biomass signatures from pedogenic carbonates, and the presence of PACMAD (Panicoideae, Arundinoideae, Chloridoideae, Micrairoideae, Aristioideae, Danthonoideae) phytoliths at six Early Miocene sites suggests the presence of potentially C₄ grasses on the landscape (Peppe et al., 2023) (Figure 5A, B).

In contrast, evidence from herbivore δ¹³C_enamel values reveals the persistence of pure C₃ or C₃ -dominated diets during this time period (Figure 5C). The C₃-dominated diets of herbivores at Buluk, as well as Moroto, contrast with the considerable C₄ signal derived from paleosol carbonates, suggesting that these C₄ plants were not consumed in sufficient quantities to be reflected in the δ¹³C_enamel values of the taxa sampled here. The nature of the opportunistic sampling strategy allows for the possibility that one or more of the taxa that were not sampled may have consumed some amount of C₄ vegetation. That is, sampling biases from the availability of dental remains among both the Buluk and Moroto collections could be underestimating the total range of herbivore δ¹³C_enamel values, particularly at the higher end of the range.

Differences in temporal scale between isotopic proxies may also help explain the discrepancy between paleosol and herbivore enamel isotope results. Enamel isotopes reflect vegetation consumed over a shorter duration of time (months to years), while soil isotopes reflect a local vegetation signal that is averaged over thousands of years (Koch, 1998). The mobility of herbivores also suggests that δ¹³C_enamel values reflect vegetation from a wider geographic area than paleosol carbonates, and the discrepancy between the two proxies is possible if herbivores foraged on C₃ resources in more distant areas from where carbonate nodules were collected (Du et al., 2019). As a result, enamel isotopes likely capture a short-term phase of vegetation and a wider area of foraging where C₄ plants were not abundant or utilized, while carbonates capture long-term, seasonally drier intervals in a more localized area that would have favored spikes in C₄ plants and the formation of soil carbonates (Peppe et al., 2023). In a mature river system, like the one interpreted for Buluk, it is also possible that the pedogenic carbonates might not have recorded a C₃ endmember signal since the time of carbonate formation exceeds that of the channel position with its C₃ vegetation (Levin et al., 2004). In other words, pedogenic carbonate forming beneath a riverbank and its riparian woodland would have migrated further distally or eroded by a change in the river channel position. Additionally, it is also possible that the presence of diagenesis in enamel and/or pedogenic carbonate samples could be influencing this incongruity.

Beginning in the late Oligocene and Early Miocene, tectonic activity in eastern Africa (e.g., rift valley formation, volcanism, uplift of the East African Plateau) is believed to have been a force shaping regional climatic variation and local scale habitat heterogeneity across eastern Africa (Sepulchre et al., 2006; Wichura et al., 2015; Linder, 2017; Peppe et al., 2023; Munday et al., 2023). The accumulation of multi-proxy paleoenvironmental data documenting extensive habitat heterogeneity across East Africa during the Early and Middle Miocene (Michel et al., 2014, 2020; Driese et al., 2016; Lukens et al., 2017; Liutkus-Pierce et al., 2019; Baumgartner and Peppe, 2021; Maclatchy et al., 2023; Peppe et al., 2023) has complicated previous adaptive models of catarrhine evolution that posited the emergence of hominoid diet and locomotion within widespread forest environments (e.g., Napier, 1967; Temerin and Cant, 1983; Hunt, 2016). Concomitant with this emerging picture of eastern African habitat heterogeneity is a growing body of evidence for a wide array of locomotor and dietary diversity among Early and Middle Miocene catarrhines (e.g., Wuthrich et al. 2019; Nishimura et al., 2022; MacLatchy et al., 2023). The Buluk herbivore enamel isotope results presented here provide additional evidence documenting the presence of open canopy woodland habitats inhabited by Early Miocene catarrhines. Additionally, the extent of niche separation among Buluk herbivores, as well as the separation between conspecific and congeneric taxa at Buluk and Maboko demonstrates that Early and Middle Miocene herbivores utilized complex and variable dietary behaviors on this heterogenous East African landscape,and the primates may have done the same.

CONCLUSIONS

Buluk is a terminal Early Miocene locality that preserves a rich mammalian fauna including a diversity of fossil catarrhines. Herbivore δ¹³C_enamel values indicate that Buluk mammals lived and foraged in an arid, C₃ -dominated, open canopy woodland environment, as indicated by the tightly constrained range of δ¹³C_enamel values (-12.7‰ to -9.4‰). Herbivores at Buluk had C₃ -dominated diets with no evidence for consumption of C₄ resources, despite their presence on the landscape (Peppe et al., 2023). Taxa with the lowest δ¹³C_enamel values include rhinocerotids and deinotheres, while the highest values are among elephantimorph proboscideans and sanitheriid suoids. The relatively high δ¹⁸O_enamel range among Buluk herbivores may reflect extensive fluctuation in δ¹⁸O values, which may be a consequence of precipitation seasonality, multiple local water sources, or sampling several water sources during seasonal migration. Results of serial oxygen isotopic analyses of herbivore teeth would test for seasonality (e.g., Green et al., 2022), while comparisons of Buluk herbivore strontium isotopic ratios with bioavailable ⁸⁷Sr/⁸⁶Sr could potentially test for herbivore mobility (Janzen et al., 2020).

Comparisons with δ¹³C_enamel data from fauna at the Early Miocene site Moroto and the Middle Miocene sites Maboko and Fort Ternan indicate a shift from more enriched signals in the Early Miocene to more depleted ones in the Middle Miocene. The similarity between the Moroto and Buluk δ¹³C_enamel value distributions suggests the dominance of water-stressed C₃ vegetation at these sites, although the broader range of values from Moroto implies greater habitat variability, including the presence of some closed-canopy elements and the consumption of limited C₄ grasses, which are not documented at Buluk. Stable carbon isotope data have previously affirmed the similarity in paleoenvironments between the Middle Miocene sites of Maboko and Fort Ternan (Arney et al., 2022). Compared to the herbivores from these Middle Miocene sites, herbivores from Buluk foraged in a more open canopy, water-stressed ecosystem. Shared taxa between Buluk and Maboko (P. hobleyi, A. championi, and D. pigotti) have δ¹³C_enamel values that reflect consistently ¹³C enriched diets at Buluk. These findings are consistent with recent reconstructions of a subhumid woodland environment at Buluk based on geochemical data (Lukens et al., 2017b, 2021) and contribute to the growing body of evidence for environmental and dietary heterogeneity across space and time in the East African Miocene (Peppe et al., 2023).

Despite the growing body of geochemical and palaeobotanical evidence for C₄ biomass in eastern Africa during the Early and Middle Miocene (Kingston et al., 1994; Morgan et al., 1994; Lukens et al., 2017a; MacLatchy et al., 2023; Peppe et al., 2023), δ¹³C_enamel data from Buluk provides no evidence of a C₄ dietary signal among mammalian herbivores. The absence of a C₄ component in herbivore diets could reflect the differences in temporal scale captured by stable isotopes from tooth enamel and pedogenic carbonates. It may be that the C₄ biomass present at Buluk during the lifespan of herbivores sampled was in low enough proportion that it was not a preferred dietary item for herbivorous mammals, or they consumed this material in quantities too low to impact bulk enamel δ¹³C values.

Future research could expand the understanding of individual dietary variability and dietary variation across herbivore guilds in the C₃ -dominated ecosystem at Buluk by applying additional dietary proxies (e.g., mesowear, microwear) and serial isotopic sampling of herbivore molars. To further assess shifting diets and environments across the Early-Middle Miocene transition, more targeted sampling of additional Buluk taxa that persisted into the Middle Miocene (e.g., Dorcatherium chappuisi, Lopholistriodon pickfordi, cf. Kenyasus rusingensis, and cf. Propaleoryx nyanzae; see Table 1) should be prioritized if additional dental specimens are recovered.

ACKNOWLEDGEMENTS

Funding for this work was provided by the Leakey Foundation, the Turkana Basin Institute, National Geographic, and the Foothill De-Anza Foundation. We would like to thank the Kenyan National Commission for Science, Technology, and Innovation (NACOSTI) for permission to conduct this research. We would also like to thank the Department of Earth Sciences at the National Museums of Kenya and the Turkana Basin Institute for access to fossil specimens and permission for sampling. Thanks to B. Sanders for identifying the proboscideans fossils from Buluk and to J. Kingston, N. Levin, and L. MacLatchy for guidance and feedback during the project development phase. We are grateful for comments from K. Uno and one anonymous reviewer. Finally, many thanks to the Buluk field crew without whom this work would not have been possible.

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