Environmental comparisons of the Awash Valley, Turkana Basin and lower Omo Valley from upper Miocene to Holocene as assessed from stable carbon and oxygen isotopes of mammalian enamel

Journal Pre-proof Environmental comparisons of the Awash Valley, Turkana Basin and lower Omo Valley from upper Miocene to Holocene as assessed from stable carbon and oxygen isotopes of mammalian enamel

Jesseca Paquette, Michelle S.M. Drapeau PII:

S0031-0182(20)30547-2

DOI:

https://doi.org/10.1016/j.palaeo.2020.110099

Reference:

PALAEO 110099

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date:

30 July 2019

Revised date:

24 October 2020

Accepted date:

26 October 2020

Please cite this article as: J. Paquette and M.S.M. Drapeau, Environmental comparisons of the Awash Valley, Turkana Basin and lower Omo Valley from upper Miocene to Holocene as assessed from stable carbon and oxygen isotopes of mammalian enamel, Palaeogeography, Palaeoclimatology, Palaeoecology (2020), https://doi.org/10.1016/ j.palaeo.2020.110099

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© 2020 Published by Elsevier.

Journal Pre-proof Environmental comparisons of the Awash Valley, Turkana Basin and lower Omo Valley from upper Miocene to Holocene as assessed from stable carbon and oxygen isotopes of mammalian enamel

Jesseca Paquettea,* & Michelle S.M. Drapeaua,** a

Département d‟anthropologie, Université de Montréal, C.P. 6128, Succ. Centre-ville,

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Montréal, QC, Canada, H3C 3J7

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* email: [email protected]

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**Corresponding author: Ph: +1-514-343-6490, email: [email protected]

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Abstract

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The Awash Valley, Turkana Basin and lower Omo Valley of East Africa are three regions that have been particularly important for documenting the environment from the

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late Miocene to the Holocene, but these basins have never been compared throughout that

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large temporal sequence. In this context, we compare changes in the diet of herbivores with mixed diets (hippopotamids, elephantids, suids and bovids), the total largevertebrate diet in the ecosystem, as well as water deficit from these three basins between 7.4 Ma and 10 ka to determine how they were different. Our sample consists of a compilation of more than 3000 published mammalian stable isotopic values. Our results show that the Awash valley becomes more arid through time, corresponding broadly with an increase in C4 plants, but that relationship is not clear in the other two basins. The 1

Journal Pre-proof Awash and Turkana are broadly similar in overall aridity while the lower Omo Valley is clearly more mesic between 4 and 2.5 Ma. However, the Turkana and Omo are similar in ecosystem values, while it is the Awash that presents a landscape with more C4 plants. When comparing the diets, the three basins are similar, with an increase in C4 plants after ~4 Ma (after 3.0-3.4 Ma in the lower OmoValley), with all taxa converging by 1.92.4 Ma on a similar diet with mostly C4 plants. Elephants vary little throughout the

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sequence when compared to the other taxa. Our data suggest that, except for elephantids,

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all taxa studied track the expansion of C4 plants in East Africa and that this expansion

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may be, in part, related to a general increase in aridity.

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1. Introduction

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Keywords: Diet, Aridity index, Ecosystem, Water deficit, Herbivores, Mixed feeders

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The lower Omo Valley, the Turkana Basin and the Awash Valley are three regions that have been particularly important for understanding past habitats and environments in East Africa (Fig. 1). Part of the effort has been to estimate climatic conditions and available habitat types at different periods and in different regions, as well as to better understand how these environmental aspects varied temporally or geographically (e.g., Levin 2015). Proxies to estimate climatic conditions and habitats have included faunal uniformitarianism (e.g., Vrba 1975), faunal proportions (e.g., Bobe

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Journal Pre-proof et al. 2002), ecomorphology (e.g., Kappelman et al. 1997), pollen and phytolith proportions (e.g., Bonnefille et al. 2004; Barboni et al. 1999), and sedimentology (e.g., Butzer 1971). More recently, methods using carbon stable isotopes have been developed. Carbon stable isotopes from paleosol carbonates have been used to infer the proportion of woody cover (Cerling et al., 2011) while stable isotopes from enamel carbonate has been used to determine the diet of fossil mammals (including hominins) (e.g., Lee-Thorpe et

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al. 1994), and infer the proportions of trees and bushes vs. grasses in tropical settings

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(e.g., Cerling et al. 2015), or habitat structure (e.g., Bedaso et al. 2013). Finally, oxygen

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stable isotopes have been used to estimate aridity (e.g., Levin et al. 2006). However, few

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studies have investigated temporal trends that extend throughout the whole sequence, from late Miocene to Holocene, in all these regions.

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This study proposes to analyze stable carbon and oxygen isotopes of herbivores‟

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enamel from 7.4 Ma to 10 ka with the goals of determining 1) whether taxa that have a mixed C3/C4 diet (mixed feeders) follow similar trends within each basin, 2) whether all

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three basins follow similar dietary trends for each mixed feeders, 3) whether the

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ecosystem values (as estimated from the total vegetation consumed by macroherbivores) and the aridity (as estimated from enamel oxygen isotopes) provide synchronous signals within basins, and 4) whether the ecosystem values and aridity indices follow the same trends among basins. The broad aim of the study is to document how each basin varies through time and relative to each other and whether the various signals provide a coherent picture of past environmental conditions. Although numerous studies have compared localities, basins at specific time periods, and types of signals, none has 3

Journal Pre-proof compared all these signals in all three basins through the end of the Miocene to the Holocene for a more global perspective on how these different signals vary within and among basins and possibly differ through time.

[INSERT FIG. 1 ABOUT HERE]

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2. Background

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Stable carbon isotopes from mammalian enamel and paleosol carbonates have been used to infer climatic and vegetational trends in East Africa. C4 grasses become a

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significant presence in the late Miocene and they are detected through the carbon isotopic

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values in the enamel of East African herbivores (~ 7 Ma; e.g. Morgan et al. 1994), a

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presence also inferred from paleosol carbon isotopes (Cerling 1992; Ségalen et al. 2007; Cerling et al. 2011). By the early Pliocene, C4 grasses can sometime dominate the

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landscape. For example, they are estimated to cover up to 80% the landscape at Galili

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(Bedaso 2011) and they dominated the diet in the Ardipithecus-bearing locality at Gona

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(Levin et al. 2008). However, generally fossil herbivore diets clearly show the presence of both C3 and C4 vegetation at variable proportions throughout the Pliocene in both the Turkana and Awash basins (Bedaso et al. 2013; Cerling et al. 2015). The early Pleistocene is generally characterized by an opening of the environment. Indeed, stable carbon isotopes of tooth enamel have signaled a mixed diet in herbivores before 2.4 Ma, but an increase of C4 resources in diets between 2 Ma and 1 Ma in the Turkana Basin (e.g.: Harris et al. 2008; Cerling et al. 2013a, 2013b, 2015), and at 2.7 and 2 Ma in the

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Journal Pre-proof lower Omo Valley (Negash et al. 2020) suggesting an increased proportion of tropical grasses and a decrease of woody plants. Between 0.8 and 0.6 Ma, the environment in the Awash Valley seems particularly xeric (Bedaso et al. 2013) with a near-absolute dominance of C4 vegetation (Bedaso et al. 2010), and similar condition in the Turkana at

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that time period (Wynn, 2004; Cerling et al. 1988, 2011).

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2.1 Within basin comparisons of trends in herbivore’s diet

Although some have warned about making climatic inferences from animals‟ diets

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(e.g., Levin et al. 2015), some taxa are considered good markers of the surrounding

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vegetation since they are opportunistic feeders and will consume both C3 and C4 plants

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depending on availability (Vogel 1978). However, herbivore taxa have often modified their diets through time (e.g., Cerling et al. 1999; Uno et al. 2011; Cerling et al. 2015) in

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part, but not solely, in response to increase in C4 plant availability. For example, in the

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Pliocene of Kenya, suids have increased their C4 intake (Uno et al. 2011; Cerling et al.

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2015), elephants increase their C3 intake sometime after 1 Ma (Cerling et al. 1999), while hippos maintained a mixed diet (Cerling et al. 2015). However, it is unknown whether these past trends are limited to the Turkana or are generalized to other areas of East Africa, as would be expected if it tracks C4-plant expansion. Du and colleagues (2019), comparing carbon stable isotopes from paleosols to those herbivore enamel, underscore that the herbivore‟s enamel provide a signal that is temporally narrow but geographically wider relative to that of paleosols. Similarly,

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Journal Pre-proof different mammalian taxa can be expected to vary in their feeding and migrating ranges and may provide signals that represent various landscape coverage. For example, hippos will generally forage in the vicinity of the body of water in which they reside (e.g., Boisserie et al. 2005), while elephants cover greater distances daily and seasonally, having ranges that can be more than 1000 km2 (Leuthold and Sale 1973; Rodgers and Elder 1977). Despite some taxa variability in C3/C4 diets, mixed feeders are generally

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considered indicators of relative C3/C4 plants availability in the landscape. Thus,

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comparing mixed-feeder diets within basins can provide insight into differences between

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signals that are more local and small scale and those that are of larger scale. Finally, by

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comparing directly mixed feeders, it is possible to determine if they have similar signals through time of whether their diet evolves differently, and if so, whether it is generalized

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throughout East Africa or more basin specific.

2.2 Within basins comparisons of aridity and ecosystem proxies

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Aridity, at a local scale, has been estimated from the faunal proportion of specific

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taxa (e.g., Bobe 2006), paleosol carbon and oxygen isotopes and depth of the pedogenic carbonate to the calcic horizon (e.g., Cerling et al. 1988; Wynn 2004), or the presence of certain pollens or phytoliths (e.g., Bonnefille 2010; Barboni et al. 1999). However, these methods have either methodological limits or track the abundance of C4 plants that may not be necessarily related to aridity (Levin 2015). The method developed by Levin and colleagues (2006) specifically estimates aridity by comparing the stable oxygen isotope values of taxa that acquire much of their water from their diet (evaporation sensitive) to 6

Journal Pre-proof taxa that depend on surface water (evaporation insensitive), the difference between them being proportional to the water deficit (or aridity). Although mixed-feeder diets are used as a proxy of the proportion of C3 and C4 plants in the landscape, each taxon only provides partial information on the available habitat at a locality and is determined, in part, by taxon-specific preferences and evolution through time. In order to assess more globally the general structure of the

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landscape, Bedaso and colleagues (2010) developed an δ13C average value of all

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macroherbivores that is weighted to their daily food intake and faunal proportions

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(δ13Cecosystem). This provides a more global view of how habitat structure (vegetation

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types and proportions) varied through time.

Environmental trends, habitat reconstructions and aridity, all inferred from a

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variety of proxies, do not always appear to concur when directly compared. For example,

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at Aramis, the 4.4 Ma locality where Ardipithecus was also found, Cerling et al. (2011) analysis of carbon isotopes of paleosols suggested settings such as a grassland or woody

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grassland (but see WoldeGabriel et al. 2009 for an interpretation of more closed settings)

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and water deficit, estimated from oxygen isotopes, suggests dry conditions (White et al. 2009; Blumenthal et al. 2017; Faith 2018a). On the other hand, faunal abundance suggests a habitat that was wooded and relatively closed, principally because of the preponderance of kudus and colobines (WoldeGabriel et al. 1994; White et al. 2006, 2009) and paleobotanical analyses suggested a vegetation thriving in wooded habitat with groundwater, resulting in an interpretation of a rather humid habitat (WoldeGabriel et al. 2001). In the Turkana, carbon isotopes from paleosol carbonates indicate that C4 grasses 7

Journal Pre-proof expanded after ~4.3 Ma even if C3 plants remained dominant, thus creating a mosaic environment (Cerling 1992; Kingston et al. 1994; Levin et al. 2004, 2011; Wynn 2004), while aridity does not appear to increase during that period (Faith 2018a, Quinn et al. 2020). Isotopic analyses of paleosol carbonates show that open ecosystems became more prevalent in the Pleistocene (Ségalen et al. 2007; Cerling et al. 2011), with an increase of

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C4 grasses after 1.7 Ma in the Turkana Basin (Cerling 1992) and at ~1.5 Ma in the Awash

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Valley (Levin et al. 2004). After 2 Ma, the Awash Valley and the Omo-Turkana Basin is

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dominated by grazers (Gibernau and Montuire 1996; Reed 1997; Reed and Russak 2009),

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agreeing with enamel and pedogenic isotopic data that suggest predominance of C4 plants (Quinn et al. 2020). However, aridity indices based on oxygen isotopes indicate both

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xeric and humid settings between 2.5 and 1.5 Ma depending on localities studied (Braun

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et al. 2010; Blumenthal et al. 2017; Faith 2018a), suggesting that the aridification of the environment may not be uniform among all localities and basins in East Africa in the

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early Pleistocene. For the more recent periods, published aridity indices give conflicting

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results, signaling either mixed/arid conditions at 0.2 Ma and more arid conditions at 0.01 Ma (Blumenthal et al. 2017) or more xeric conditions at 0.2 Ma than at 0.01 Ma (Robinson et al. 2016).

It has been noted that increase in C4 plants is not necessarily coupled with signs of increased aridity in the Pliocene-Pleistocene, strongly suggesting that aridity is not the sole factor that drives C4 expansion in East Africa (Levin 2015; Blumenthal et al. 2017;

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Journal Pre-proof Faith et al. 2018; Pollisar et al. 2019). Given that various signals provide variable interpretations, it appears important to directly compare estimates of aridity to habitat structures as estimated from the total vegetation in an ecosystem that was consumed by macroherbivores to determine how these two signals vary throughout a large timescale.

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2.3 Among basins comparisons of herbivore’s diets, aridity and ecosystem proxies

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Isotopic studies on paleosols and enamel as well as analysis on faunal abundance suggest that the Awash Valley and the Turkana Basin were largely similar through time,

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but that environmental change towards more wooded conditions in the Middle Pliocene

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(Cerling et al. 2011) and towards more open and possibly more arid conditions in the

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Late Pliocene were not synchronous (Bobe et al. 2007; Robinson et al. 2017). Indeed, according to isotopic analysis of paleosol carbonates, there was a change of vegetation

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toward more closed habitats in the Middle Pliocene in both regions, but the peak of

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wooded vegetation in the Awash Valley would have occurred later than in the Turkana

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Basin (Cerling et al. 2011). In the Late Pliocene, paleosol isotopes and faunal proportions show the opening of the environments in the lower Awash Valley while the Turkana Basin maintained more mixed C3/C4 vegetation (Robinson et al. 2017). In contrast, Levin et al. (2011), based on soil carbonate oxygen isotopes, noted that the Turkana and possibly even the lower Omo valley show signs of increased aridity in the Pliocene through the mid-Pleistocene, but notes that there is no such trend in the Awash, underscoring, again, that aridity may not be the sole determinant of shift in C3/C4

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Journal Pre-proof vegetation and animal diets. Furthermore, the lower Omo Valley would have been distinct from the Turkana basin between 4 and 2 Ma with more wooded and humid conditions, based on faunal abundance, carbon isotopes of paleosol carbonates and mammalian diets (Bobe et al. 2007; 2011; Levin et al. 2011; Drapeau et al. 2014). Despite recorded differences among these basins, the diet, habitat structure within an ecosystem, and water deficit have not been systematically compared throughout the

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Plio-Pleistocene sequence. Comparison of these various signals will help establish

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whether the among-basin observed differences, sometime inferred from different

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methods, are supported when comparing the same proxies and will help underscore the

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specific nature (vegetation vs. water deficit changes) of the among-basin variation.

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In summary, these studies have mostly focused on specific lithostratigraphic

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formations or taxa, and those that compare the Awash Valley with the Omo and Turkana basins while looking at enamel, aridity and ecosystems trends simultaneously focused

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only on shorter time spans, such as the late Pliocene for example (Robinson et al. 2017).

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It is still unclear how these three basins compare at a longer temporal scale and for different environmental signals. The purpose of this study is to investigate whether the diet of different mixed feeders, the habitat structure within an ecosystem and the water deficit provide similar signals within each basin, and how these three signals differed in the Awash Valley, the Turkana basin and the lower Omo Valley through the upper Miocene to the Holocene.

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Journal Pre-proof 3. Materials and methods

Carbon stable isotopic values differentiate the two principal photosynthetic pathways used by tropical plants. In the tropics, the C3 pathway is used by most trees, shrubs, forbs and non-grassy herbs, while the C4 pathway is used by tropical grasses, sedges, and rare dicots that are often found in hot and arid conditions (Ambrose and

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DeNiro 1986; Ehleringer and Monson 1993; Koch 1998; Schoeninger 1995; Wynn 2000;

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Ehleringer and Cerling 2002; Hatté and Schwartz 2003; Kohn 2010; Cerling et al. 2013a; Klein 2013). In the African tropics, a third photosynthesis pathway, Crassulacean acid

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metabolism (CAM), is used mostly by succulent plants and they have stable carbon

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isotopic values similar to C4 plants. However, they are generally relatively rare and often

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toxic (Peters and Vogel 2005). Because the isotopic composition in tooth enamel is positively correlated with that of the plants consumed by animals (Schoeninger 1995;

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Cerling and Harris 1999; Passey and al. 2005; Peters and Vogel 2005; Lee-Thorp and

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Sponheimer 2006; Kingston 2007), stable carbon isotopes of enamel have been used

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extensively to estimate the proportion of woody plants and tropical grasses in the diet (e.g.: Ericson et al. 1981; Cerling 1992; Levin 2008; Bedaso 2011). In East Africa, mammals that consume leaves from trees or bushes (browsers) will have enamel with a C3 signal defined as δ13C values under -8.00, animals that predominantly consume grasses or sedges will have a C4 signal, defined as δ13C values more than -1.00, and animals that consume both types of plants have a mixed signal with δ13C values generally between -1.00 and -8.00. Animals in the latter group are called mixed feeders and will

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Journal Pre-proof consume both C3 and C4 plants depending on ambient availability, which can be used to assess relative presence of these different types of plants in the past (Ambrose and DeNiro 1986; Koch 1998; Kingston 2007; Sponheimer et al. 2013). In addition, stable carbon isotopes of enamel can also be used to calculate the average carbon value in an ecosystem (ecosystem carbon value as a proxy of habitat structure) by calibrating data with faunal abundance and the daily food intake of each large herbivore (Bedaso et al.

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2010, 2013).

Stable oxygen isotopes of enamel are in variable proportions in herbivorous taxa

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depending on the source of their water intake. Evaporation-sensitive (ES) taxa track

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aridity because their water intake comes from their leafy diet, in which the oxygen value

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changes according to leaf transpiration (Gonfiantini et al. 1965; Dongmann et al. 1974; Epstein et al. 1977; Ayliffe and Chivas 1990; Luz et al. 1990; Gat 1996). Because water

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molecules with lighter isotopes evaporate more readily from the site of evaporation

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within the leaf (Farquhar et al. 2007), leaf water is more enriched in heavier isotopes in

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more xeric environment and herbivores that acquire water predominantly from their diet will be themselves enriched in heavier isotopes. Evaporation-insensitive (EI) taxa are obligated drinkers and, therefore, track meteoric rainfall as their water intake comes from surface water (Longinelli 1984; Luz et al. 1984, 1990; Kohn et al. 1996; Koch 1998). The enrichment of heavy stable oxygen isotopes (18O) of enamel between ES and EI taxa within one locality reflects the enrichment between leaf water and source water, which increases with aridity (Kohn et al. 1996; Koch 1998). Thus, the value of the ES-EI

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Journal Pre-proof enrichment creates an aridity index as developed by Levin et al. (2006), can be translated to water deficit (WD), distinguishing mesic (WD< 0) and xeric (WD>0) environments. Though aridity affects the abundance and distribution of vegetation (Blumenthal et al. 2017), C4 plants are not exclusive to arid environments and can be found in mesic settings, creating both an open and relatively humid environment (Sankaran et al. 2005), while C3 plants can be found in relatively arid environments (Kohn 2010). Consequently,

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the C3/C4 signal and the aridity signal can provide different information on past habitats.

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Isotopic data were collected from 35 scientific articles published between 1981 to

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2017 (Supplementary material A). Most samples were chemically treated (see Supplementary materials C and D for details), but samples too small to be treated had

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isotopic values corrected by Cerling et al. (2015) using the formula 13Ccorrected = 1.13

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13Cuntreated + 0.64. Faunal abundance data were collected from 14 publications

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(Supplementary material B), the Public Turkana Database, the merged catalogue of the American and French teams of the International Omo Research Expedition, and personal

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communications (Supplementary material B). Our database consists of 2845 stable

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carbon isotopic values and 577 stable oxygen isotopic values of herbivores from the Awash Valley, the Turkana Basin and the lower Omo Valley (Tables 1, 2; Supplementary material C and D), gathered from publications. The time span is between 7.4 Ma to 10 ka. Data were grouped per lithostratigraphic units from different localities and age of these units were gathered from the literature (Supplementary materials C and D). Finally, lithostratigraphic units of overlapping age were grouped within wider age categories that encompassed the age-range of the included units. The fairly wide 4.6-3.9 Ma period in 13

Journal Pre-proof the Awash Valley was separated in two overlapping categories, 4.6–4.2 and 4.4-3.9 Ma, in order to gain a better temporal resolution. As a result of the groupings of lithostratigraphic units, age categories are not uniformly distributed through time, vary in scale, are not always coincident between basins and there are gaps in the sequences (Table 3). We acknowledge that there are interpretative limitations because the variable age categories (Behrensmeyer et al. 2007), but this broad approach allows us to

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appreciate and compare long-term dietary and environmental trends through the last 7 Ma

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in different basins of East Africa.

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Isotopic data collected from previous publications were reported according to the

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Vienna Pee Dee Belemnite (VPDB) isotopic standard (0.01118 ratio), and presented as δ-

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values per mil (‰), following:

)

(1)

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δ13C or δ18O = (

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Where Rsample and Rstandard correspond to the 13C/12C ratios in carbon and to the 18O/16O ratios in oxygen.

3.1 Diet For the diet comparisons, we included stable carbon isotopic data from tooth enamel of taxa that are known to have a mixed diet through time: elephantids, hippopotamids, suids and bovids. Extant elephants are primarily browsers, but in the past

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Journal Pre-proof their diet included much more C4 plants and fell in the mixed-feeder category (Cerling et al. 1999). Suids‟ diet appears to have changed significantly throughout the PlioPleistocene, shifting from a mixed diet in the late Miocene and early Pliocene, to more C4 in the mid-Pliocene, and with some taxa shifting back to a browsing diet (Uno et al. 2011; Cerling et al. 2015). As a group, however, they present a dietary flexibility and are indeed mixed feeders for most of the period studied. Hippopotamids are generally observed to

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graze, but studies of extant animals have shown that they are opportunistic feeders and

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that their carbon isotopic values reflect the surrounding vegetation cover (Boisserie et al.

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2005; Cerling et al. 2008, 2015). Bovids are characterized by a variety of diets, some

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being mixed-feeders, but other specializing in either C3 or C4 diets, but when the family as a whole is considered, the diet is mixed. However, given the large variation of diet

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among the bovid tribes, it is necessary to weight the isotopic data according to the

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abundance of the different tribes within the lithostratigraphic units in order for stable carbon isotopic values to reflect accurately the overall diet of bovids. Similarly, because

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enamel isotopic sampling of museum collections is not necessarily proportional to the

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faunal abundance of the lithostratigraphic unit, we also weighted isotopic samples to the faunal proportions. To do so we multiplied each isotopic value by a factor calculated as:

(2)

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Journal Pre-proof Where Wt corresponds to the weight we give each tribe into each lithostratigraphic units, At is the tribe faunal abundance (NISP) relative to all other tribes in the lithostratigraphic unit, St is the number of isotopic samples for the tribe, and Tst is the total number of isotopic samples for all tribes in the lithostratigraphic unit. Unfortunately, faunal proportions are not available for all lithostratigraphic units, so there are fewer age categories for bovids than other taxa. For suids, we weighted the samples by genus using

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the same formula as above and substituting suid genera for bovid tribes. However, since

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the diet of the different genera of suids does not vary as widely as that of different tribes

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of bovids and in order to avoid reducing the age categories further for that taxa, we used the raw values if genus abundance was not available. Unweighted values of bovids and

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suids are presented in the supplementary materials (Fig. S1). Hippos and elephants‟

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values were not weighed since either there were few specimens securely identified to the

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genus or there was only one genus identified within lithostratigraphic units. Each lithostratigraphic unit, regardless their respective isotopic sample sizes, were

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given the same weight within each age category. However, to avoid giving too much

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weight to very small isotopic samples, we identified lithostratigraphic units with sample sizes that were less than 1/5 of the largest sample in the same age category per region. If such small samples were found, we lumped them with the next smallest sample within the same age category to have more balanced sample sizes within each age category.

[INSERT TABLES 1, 2 AND 3 ABOUT HERE]

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Journal Pre-proof 3.2. Aridity Aridity is calculated by comparing δ18O of evaporation-sensitive (ES) and evaporation-insensitive (EI) taxa that are surface-water dependent. Following the recommendations of Blumenthal et al. (2017), we used oxygen isotopic data from hippopotamuses, elephants, and rhinoceros as evaporation-insensitive (EI) taxa and data from giraffes and tragelaphins as evaporation-sensitive (ES) taxa (Table 2). However, we

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did not use hippotragini has an ES taxon because of concerns that some extant species of

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that taxon have very high-water dependence (Faith, 2018a) and it is likely that extinct

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species did too. In addition, there is evidence that Sivatherium became a grazer after

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3 Ma (Cerling et al. 2015), so they were not used as ES taxa after that period (effectively resulting in the removing six specimens from our database from the Awash Basin only).

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Although there is evidence that the diet of some Tragelaphini may have been more C4 in

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the past (Negash et al. 2015; Levin et al. 2015), tooth meso- and micro-wear analyses indicate that the C4 component of their diet was of dicot or humid grasses (Curran and

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Haile-Salassie 2016; Blondel et al. 2018) and, therefore, that there is no reason to assume

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that this taxon changed its drinking behavior in the past. Enrichment (εES-EI) was calculated using only four types of pairs demonstrated by Blumenthal and colleagues (2017) to have a significant relationship with water deficit (εGiraffidae-Hippopotamidae, εTragelaphini-Hippopotamidae, εTragelaphini-Elephantidae and εTragelaphiniRhinocerotidae; as stated above we excluded εHippotrageni-Hippotamidae). The enrichment factor is:

A-B = (A-B – 1) x 1000 17

(3)

Journal Pre-proof where the fractionation factor, αA-B, corresponds to:

A-B = RA/RB =

(4)

and where δA is the ES taxon δ18O value and δB that of an EI taxon.

Following Blumenthal et al. (2017), for each type of pairs, we calculated the

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average oxygen values for ES and for EI taxa that we used to calculate a water deficit

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(WD) specific to that type of pair (Supplementary material D). For each lithostratigraphic unit, we averaged the WD of the different types of pairs. Finally, we average the WD

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values of all the lithostratigraphic units within an age category. It is suggested to use at

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least four, and in the best circumstances ten specimens each of ES and EI taxa (Levin et

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al. 2006; Blumenthal et al. 2017). Most age categories usually contain at least four ES and EI specimens, but considering the relative scarcity of fossils for which oxygen

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isotope chemical compositions has been measured, the smaller samples were also

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included and age categories with less than 16 ES-EI pairs were labelled in the graph with

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their respective number of pairs. We acknowledge the fact that the classification of taxa as ES or EI is based on the principle of taxonomic uniformitarianism and assumes that taxa have not shifted their diet through time, which would change their predominant water source intake (Faith 2018a, 2018b). However, we were careful to remove taxa for which there were evidence of changes in how they consumed water (see above) and agree with Blumenthal and colleagues (2018) that it remains a useful method to depict a part of paleoclimatic conditions in East Africa through time.

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3.3. Ecosystem measure and habitat structure Following Bedaso‟s (2011) method, we calculated the ecosystem carbon isotope composition (δ13Cecosystem), where gross carbon values are calibrated by faunal abundance and the estimated daily biomass consumption, following:

ro

of

(5)

-p

Where Xtaxa is the faunal proportion in fraction, Q is the daily food intake in kg, enamel-diet

is equal to - 14,

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δ13Cenamel,taxa is the mean carbon value of each taxon in ‰, and

lP

which represents the difference in fractionation between plants (diet) and δ13Cenamel. Then, δ13Cecosystem values can be used to infer habitat structure (Bedaso et al. 2013).

na

We used all taxa available in each lithostratigraphic unit for which we had faunal

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proportions to calculate δ13Cecosystem, for a total of 2761 stable carbon isotopic samples. To

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calculate daily food intake, we used allometric regression equation correlating body mass and absolute dry matter intake in herbivorous mammals (Clauss et al. 2007). In order to take into account changes in body mass through time, we used, when available, estimated masses of fossil taxa. All information on body mass were collected from 14 articles (Supplementary material E), but were mainly found from Bedaso (2011), Bibi and Kiessling (2015) and Clauss et al. (2003, 2007). From the mass range reported, we calculated the mean average food intake of 30 taxa according to their type of

19

Journal Pre-proof morphophysiological design of digestive tract. When the relative faunal proportions were not available for the species or genus, we grouped the data into family categories for elephantids, hippopotamids, rhinocerotids, giraffids, cercopithecids and equids. However, for bovids, we used the tribe proportions since body mass can differ largely between tribes, and a general category for bovids would not give the expected definition. Thus, if the relative proportions of bovid tribes were missing for a lithostratigraphic unit, that unit

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was excluded from the calculation of δ13Cecosystem.

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4.1. Within-Basin diet comparisons

-p

4. Results

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The diets of the four groups tend to follow similar broad trends within each basin

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with a few exceptions for some taxa at specific time periods (Fig. 2, Table 4). Subtle variations in trends are observable, however. Prior to approximately 5.2 Ma, suids tend to

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have a more C3 diet than elephantids and hippopotamids. Sometime after 4 Ma, the diet

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of bovids becomes more C4 relatively to the other mixed feeders while, in contrast, the diet of hippos tends to become relatively more C3. Elephants tend to have the most C4 diet throughout the sequence until around 1.5 Ma when the other taxa‟s diet becomes similar to that of elephantids. Indeed, in all three basins, there is a tendency for the diet of the four groups to converge through time with less inter-group variation in diets in the most recent periods. In the Awash and Turkana basins, the two basins where it is possible to compare elephants and hippos through time, the diet of the latter has greater overall 20

Journal Pre-proof amplitude of variation and also tends to vary more from one time period to the other in the Awash. Finally, the diet of all the mixed feeders from both the Awash and Turkana basin show a greater proportion in C3-plant consumption around 4 Ma relative to the previous and following periods (Fig. 2AB), and in all basins, all groups show an overall increase of C4 diet afterward.

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[INSERT FIG. 2 AND TABLE 4 ABOUT HERE]

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4.2. Within basin water deficit and δ13Cecosystem comparisons

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There is not a clear trend between calculated WD and δ13Cecosystem values (Fig. 3,

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Tables 5 and 6). Variations in WD through time can be wide within each basin. There is no clear temporal trend of aridification in the Turkana basin for the whole sequence, but

na

there is an increase in aridity starting at 4.0 Ma coupled with an increase C4-plant

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proportions (with one outlier WD value at 2.5-2.4 Ma, based on only one ES-EI pair). In the Awash basin, there is a long term aridification from the late Miocene to the

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Pleistocene as well as a gradual increase of δ13Cecosystem values. However, the WD indicates large mesic-xeric fluctuations with no obvious close correspondence of the δ13Cecosystem values. The WD index directly concurs with the δ13Cecosystem values between 3.9-4.4 and 3.6-3.2 Ma, the aridity and ecosystem values both show, first a swing for more arid condition and more a C4-plant-based ecosystem, and then a reversal of these conditions (Fig. 3A). There are few data points in the lower Omo Valley, but the

21

Journal Pre-proof Pliocene was more mesic than the recent Pleistocene (0.2 to 0.1 Ma; Fig. 3C). Values of the δ13Cecosystem suggest a gradual opening of landscape starting at 4 Ma with the mostC3ecosystem at 2.4-2.3 Ma.

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[INSERT FIG. 3 AND TABLE 5 AND 6 ABOUT HERE]

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4.3. Among basin comparisons of diet

The diet of the different taxa generally shows the same pattern in the Turkana and

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Awash basins (Fig. 4). The lower Omo Valley herbivores tend to have a more C3-rich

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diet than in the other two regions from 4 to 3 Ma, while there is no difference around 1.5-

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1.3 Ma (Fig. 4). All taxa have a peak in C3-diet at around 4 Ma (the only exception is for the hippopotamids in the Omo that are more C3 at 3.4-3.0 Ma than at 4 Ma). This is

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followed by a gradual increase in C4 plants afterwards in hippopotamids, suids and

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bovids, while elephantids do not show much of an increase after 3.6 Ma. The only period

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for which there are data for elephants in the three basins, at 4 Ma, their diet is very similar, while bovids in the lower Omo Valley and the hippopotamids from the Awash tend to have a much more C3 diet at that period. The diet of hippos tends to be slightly more C4 in the Awash than in the Turkana, a difference most notable around 3.0-2.5, as the Awash hippos appear to shift to a C4-dominated diet more quickly than what is observed in the Turkana (Harris et al. 2008).

22

Journal Pre-proof [INSERT FIG. 4 ABOUT HERE]

4.4. Among basin comparisons of water deficit and δ13Cecosystem None of the three basins show a similar pattern of WD through time (Fig. 5A). The Awash Basin appears to become slightly more arid over time, but with high

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fluctuations in successive periods, particularly between ~4 and 2.7 Ma (Fig. 5A). The

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Turkana Basin does not show that trend, but it is similarly characterized by high fluctuations through time (Fig. 5A). The lower Omo Valley has very low values of water

-p

deficit when compared to the other two basins (Fig. 5A). However, more aridity

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measurements for the 3 Ma to 0.2 Ma interval are needed in the lower Omo Valley to

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better understand its environmental conditions and transformations and to compare it to the other basins.

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The δ13Cecosystem values indicate, for all basins, that the landscape became more

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open with the gradual increase occurring between 4 and 1.5 Ma (Fig. 5B). The Awash

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basin tends to have a more C4-plant signal than the other two basins. The lower Omo Valley presents a landscape that is generally similar to that of the Turkana (Fig. 5B). For the Omo and the Awash, the highest C3 signal is at or around 4 Ma, while in the Turkana it is the second-highest (the most is at 4.0-3.4 Ma).

[INSERT FIG. 5 ABOUT HERE]

23

Journal Pre-proof 5. Discussion 5.1. Comparisons of diets within basin The comparisons of different mixed-feeder diets within each basin illustrates general differences among taxa, with hippopotamids generally showing a slightly more C3 diet than other taxa and elephantids having generally a more C4 diet (Fig. 2). Our data

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show that the shift to a predominantly C3 diet in elephants, observed on modern taxa

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(Cerling et al. 1999), is not observed by 0.8 Ma in the Turkana, nor by 0.6 Ma in the Awash, indicating that it likely occurred in the last 500 000 years in Africa. For suids, our

-p

data supports a general increase in C4 plants in their diet from the Miocene to the

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Pleistocene as previously observed in the Turkana (Uno et al. 2011; Cerling et al. 2015),

lP

although we see a brief C3 shift at 4 Ma. For bovids, a similar gradual increase in C4 plants is observed; before 4 Ma, they tend to plot with hippos with more C3 values, but

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after 4 Ma, they are more C4, with a diet that is more similar to that of elephants, as

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reflected by taxonomic changes through time (e.g., Bobe and Eck, 2001).

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These within-basin comparisons also show that either of these taxa can provide the general temporal trend of a region, although elephants may provide a less sensitive signal. A striking similarity observed in the two basins at time periods with sufficient samplings is the increased in C3 diet around 4 Ma in the Turkana and Awash basins (Fig. 2AB). This increase in C3 diet for all taxa may indicate a contraction of open habitats. However, given that this 4 Ma signal reflects values from only two lithostratigraphic units (Dhidinley in Galili, Awash, and Lonyumun member, Turkana; Table 3), the lower

24

Journal Pre-proof isotopic values may reflect particularities of these localities, and it would only be coincidental that they occur around 4 Ma in both basins. However, given that bovids also present extreme C3 values in the Mursi of the lower Omo Valley, also dated at 4 Ma, lends further reasons to inquire into the possibility of a “C3-event” at that time. Obviously, the 3.9 to 4.2 Ma period would gain to be better documented in these three basins and elsewhere.

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The general increase in C4 diet for all taxa after ~4 Ma is concordant with the

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paleosol carbonate isotopic signal of C4 vegetation expansion after ~4.3 Ma (Cerling

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1992; Kingston et al. 1994; Levin et al. 2004, 2011; Wynn 2004). However, although

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groups show general similarities, some taxa at a certain time period can diverge significantly from the trends observed in the other taxa. It may be due to small sample

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sizes (e.g., n=1 for elephants at 2.5-2.4 Ma in the Turkana Basin), to idiosyncratic

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adaptations of one taxonomic group in a specific context (e.g., the Nyanzachoerus specimen from the Adu-Asa Formation of the Middle Awash, at 6.5-5.2 Ma that present a

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much more C3-signal than the other taxa from the same lithostratigraphic units or than

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any other suids in our samples), or to unusual habitat structures. These deviations, if not due to field collecting bias, could be interesting to investigate further in order to better capture the plasticity of taxa and how it might have evolved through time in different regions. In the two basins where the diet of elephants and hippos can be compared throughout the sequence, hippos show greater variation in diet, which might reflect more heterogeneity in the local water‟s edge habitat through time and space, while elephants‟ values possibly represent more average values for the wider landscape, an overall 25

Journal Pre-proof structure that does not appear to have varied as much. However, it is important to note that since enamel sampling for isotopic analysis is generally localized on the tooth, it does not reflect the diet throughout tooth formation but, instead, a shorter period within the life of the animal. Thus, the interpretation about the wider spatial scale represented by their stable isotopic values remains limited with this type of sampling. It would be interesting to investigate single tooth serial sampling as well as multiple tooth sampling

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in elephants as well as that of less mobile taxa, such as hippopotamids to improve

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definition on, not only the smaller and larger scale signals, but also on seasonality (e.g.,

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-p

Souron et al. 2012).

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5.2. Within basin WD and δ13Cecosystem

The general lack of close correspondence between water deficit values and

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δ13Cecosystem values in the better-documented basins (Awash and Turkana; Fig. 3AB)

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suggests that the vegetation cover is not directly modulated by water availability (Levin

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et al. 2011, Blumenthal et al. 2017; Faith et al. 2018). Although an in-depth comparison with paleosol carbon isotopes values is beyond the scope of this paper, the observed changes through time follow more closely δ13Cecosystem trends than aridity indices (see Cerling et al. 2011), a similarity that should not be surprising since they both are measures of the extent of C3-C4 vegetation, although estimated from different samples (paleosol carbonates and herbivore enamel). There is no clear trend of aridification in the Turkana Basin through the Pliocene (see also Blumenthal and colleagues 2017), despite

26

Journal Pre-proof an increase of C4 plants in the diet of herbivores (Fig. 3B). It is only after 2 Ma that both signal change in the same direction for two successive periods, perhaps signaling an increase in C4 plants that could be driven by increased aridity (see also Wynn 2004, Levin et al. 2011). In the Awash Basin, there appears to be a modest aridification throughout the sequence that is mirrored only broadly by the δ13Cecosystem values, but, as stated above, the

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two signals cannot be matched closely (Fig. 3A). Around 2.8-2.6 Ma, the WD indicates

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relatively mesic conditions but an opening of the landscape (see also Robinson et al.

-p

2017) perhaps suggesting the expansion of mesic grasses. That period is followed by a

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clear increase in aridity that does not appear to be followed by a landscape with more C4 plants.

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In the lower Omo Valley, WD values suggest the most mesic settings of all three

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basins between 3.4-2.5 Ma while the δ13Cecosystem values suggest a more open habitat (Fig. 3C). In the most recent period (Kibish I) at 0.2-0.1 Ma, the WD values indicate xeric

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conditions that correspond to the more open habitat.

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It is important to note that WD calculations are based on a small sample in some age categories and thus might not be a completely accurate measure of the actual conditions. Also, we did not plot propagated errors of the WD in order to see more clearly trends, but the calculation of WD values can sometimes accumulate large errors (Table 4), thus the wide fluctuations of values through time could have been smaller (or possibly larger) in some instances. Finally, seasonal variability in δ18O values have been documented in hippos (Souron et al. 2012), one of the EI taxa used for the calculation, 27

Journal Pre-proof and is likely to be present in other taxa as well. The recorded within-tooth variation of δ18O in extant hippos can reach 2‰ (Souron et al. 2012) which can result in large variation in estimates of water deficits, underscoring that aridity estimates should be interpreted with caution. Although we did not find a perfect correspondence between aridity and δ13Cecosystem, long-term trends of both signals suggest an increase aridity and opening of the habitat in the late Miocene to the late Pleistocene. Although other factors

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than increased aridity might have contributed to the opening of the landscape such as the

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decrease of CO2 atmospheric concentrations (Faith et al. 2018), our data suggest that

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aridity certainly increased overall and must also have been a factor of C4-grass expansion

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in East Africa.

5.3. Diet among basins

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The herbivores of the Turkana and Awash basins seem to generally follow the

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same trend in diet through time (Fig. 4), as shown by previous inter-basin diachronic

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studies of environmental trends in East Africa based on relative faunal abundance and isotopic analysis of enamel and paleosol carbonates (Bobe et al. 2007; Robinson et al. 2017). Since the age categories are different for each basin, it is difficult to determine if changes toward a more open or arid environment occurred synchronously, but our data does not suggest obvious differences in the timing of dietary shifts. This observation contrasts with Cerling and colleagues‟ (2011) paleo-shade proxy that suggested a peak of C3 woody cover at 3.9-3.6 Ma in the Turkana Basin but at 3.4-3.2 Ma in the Awash

28

Journal Pre-proof Valley. In the late Pliocene (2.8-2.7 Ma), hippopotamids, bovids and, to a lesser degree, elephantids diets demonstrate a greater C4 intake in the Awash Valley than in the Turkana Basin (Fig. 4). This agrees with Robinson and colleagues‟ (2017) results from paleosols isotopes, faunal proportions, and enamel isotopic data of a more abrupt shift toward open environments in the Awash Valley than in other regions. When looking at how each taxon compares, the diet of elephants is remarkably

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less variable within basins as well as among basins. Although their diet remains in the

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range of mixed feeders, it consists predominantly of C4 plants. Of the four mixed feeders

-p

examined, elephants are less useful to identify localized environmental shifts. The

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hippos‟ diet is broadly similar among basins, although hippos from the Awash tend to have a diet that is modestly shifted towards a C4 component throughout most of the

lP

sequence. Sampling in the lower Omo Valley is poor making comparisons with other

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basins difficult and 4 Ma is the only period with an adequate sample size comparable to the other basins. At that period, the diet of hippos includes as much C4 plants as those

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from the Turkana Basin and much more than those from the Awash Valley. This runs

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counter to descriptions of a more C3-plant dominated landscape in the lower Omo Valley (Bobe et al. 2007; 2011; Levin et al. 2011; Drapeau et al. 2014). This unexpected result could be due to a sampling issue in the Turkana and Awash that are each represented by only one lithostratigraphic unit for that age period (so is the Omo), or could suggest that the water‟s edge at the in the lower Omo Valley was not particularly different from the water edge in the Turkana at that time period.

29

Journal Pre-proof The suids‟ diet shows an increase in C4 plants in all three basins, which corresponds to an increasing dental adaptation for grazing through time (e.g., Cooke 2007). However, in the lower Omo, the diet is more C3 at the 3.4-3.0 Ma than in the other two basins. It is followed by an “abrupt” C4-shift at 3.0-2.5 Ma (Bibi et al. 2013). Vrba (1988) and Bobe et al. (2007) suggested that the lower Omo Valley was a wooded refugium in the late Pliocene until 2 Ma while the other regions became more open.

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However, our data suggest an earlier date for the C4 shift, possibly at the beginning of the

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Pleistocene. Except for values from the Miocene, the suid‟s diet has less amplitude of

-p

variation in the Awash than in the Turkana or Omo perhaps indicating, in the former, less variability in the available habitats through time. However, unlike what is observed with

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the hippos‟ diet, the suids‟ diet is not more C4 in the Awash than in the Turkana. After

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2.5 Ma, the suids‟ diet is strikingly similar among all three regions sampled and it

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contains the greatest proportion of C4 plants, suggesting a landscape with less woody cover throughout East Africa as generally inferred from a number of proxies.

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The diets of bovids all show an increase in C4 plants through the Plio-Pleistocene,

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increases that occur at generally similar rates in all basins. The Awash and Turkana have broadly similar values, but present small differences: bovids from the Awash have a more C4 signal between 4 and 3.2 Ma and again between 2.8 and 2.6 Ma suggesting more C4 plants. The difference at the later period was also observed by Robinson and colleagues (2017) based on faunal proportions and isotopic analyses of paleosol carbonates (see also Cerling et al 2011) and by our results on hippos (see above), lending strong support for more open habitats in the Awash at that period. In the lower Omo Valley, the diet of 30

Journal Pre-proof bovids is more C3 when compared to that of the Turkana and Awash until the 1.5-1.3 Ma period. This concurs with interpretations of a more wooded refugium in the lower Omo Valley in the Pliocene inferred from bovid abundance, isotopes of paleosol carbonates, suids‟ diets and community analysis (Vrba 1975; Bobe et al. 2007, 2011; Levin et al. 2011; Souron 2012; Robinson et al. 2017). Unlike what we observed with the suids (or hippos), the diet of bovids suggests that the Omo ceased to be different from the other

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two basins after 2 Ma, concurring with a similar conclusion drawn from the proportions

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of bovid tribes (Vrba 1988, Bobe et al. 2007). Although the diet of hippos, suids, and

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bovids from all basins suggest an increase in C4 plants through time, and that the lower

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Omo Valley may have acted as a wooded refugium, only the bovids suggest that it lasted

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well into the Pleistocene.

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5.4. Among basin comparisons of WD and δ13Cecosystem

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In terms of aridity trends, our results show that the Turkana Basin and Awash

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Valley were not very different on average, while the few values for the lower Omo Valley signal that it was clearly more mesic (Fig. 5A) supporting similar interpretations from numerous other proxies, including Bobe and colleagues‟ observation that the lower Omo Valley is characterized by fewer arid-adapted taxa (2007) than the contiguous Turkana Basin. In the Turkana, there is only a weak increase in aridity through time, more marked for the later part of the sequence sometime after 2.4 Ma. This trend is similar to that of soil carbonate oxygen isotopes, that showed that there is no overall

31

Journal Pre-proof increase in aridity in the Turkana basin until approximately 2 Ma (Levin et al. 2011). The Awash basin, on the other hand, tends to become slightly more arid through the PlioPleistocene. The trend, however, is not observed with soil carbonate oxygen isotopes, suggesting instead that the Awash showed no change in aridity (Levin et al. 2011). However, paleosol carbonates often measure conditions at the proximal floodplain zone, hence may not provide a representative picture of the landscape (Du et al. 2019), while

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herbivores are more likely to sample a larger radius of the available habitats. If so, aridity

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measured from herbivores‟ enamel may provide a broader picture of the environment.

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Better sampling of mammalian enamel for oxygen isotopes would help clarify whether

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the increase observed in our data is robust, but it suggests that aridity did increase through time and may have contributed to C4-grass expansion.

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In contrast to the aridity values, the δ13Cecosystem values of the lower Omo Valley

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are not significantly different from that of the Turkana, while the Awash is reconstructed to have more open habitats with more C4 plants. These results are not unexpected

ur

considering that the enamel isotopic values tend to be more C4 in that region. All three

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basins have habitats that are gradually opening up after 4 Ma (Fig. 5B). Overall, the signals among basins are often broadly congruent and present a picture of a landscape that includes an increasing quantity of C4 plants in the diets of herbivores. There is some evidence of aridification in the Turkana and possibly in the Omo after 2.4 Ma, but the clearest evidence for aridification was observed in the Awash throughout the sequence, a trend that was not observed with other aridity proxies (Levin

32

Journal Pre-proof et al. 2011). This underscores how more data is required to further our understanding of how East Africa changed through time.

6. Conclusions The goal of this research was to compare dietary signals of mixed-feeder

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herbivores, water deficits and δ13Cecosystem values in the Awash Valley, Turkana Basin and

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lower Omo Valley between 7.4 and 0.01 Ma. Our data show a similar pattern in dietary trends among taxa in the Awash Valley and Turkana Basin, where there is a modest

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increase of C3 diets at around 4 Ma followed by a general increase for all taxa except

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elephants in the consumption of C4 plants afterwards. Water deficit and δ13Cecosystem

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values are broadly similar in the Turkana Basin and Awash valley. In contrast, the lower Omo Valley is clearly more mesic and herbivores have a more C3 diet than those in the

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Awash Valley and the Turkana Basin before 2.5 Ma, while it is much less different after

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that period when comparisons are possible.

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Our results suggest that the δ13Cecosystem values do not strictly follow aridity signals, with peaks of aridity not closely coinciding with an increased intake of C4 plants and vice versa. In the Awash Valley, δ13Cecosystem values and the diet of herbivores show, to various degrees, an increased presence of C4 vegetation starting at around 4 Ma and aridity gradually increases during that period. The lower Omo Valley has dramatically more mesic conditions when compared to the Awash and Turkana basins but shows no clear differences in δ13Cecosystem values when compared to the Turkana. Thus, our results

33

Journal Pre-proof suggest a complex relationship between aridity and vegetation that is not uniform among basins. However, our observation of a long-term trend of increasing aridity in the Awash suggests that aridity is, in part, responsible to the expansion of C4 plants in that region. This study shows that hippopotamids, suids and bovids all track the expansion of C4 plants in East Africa, but that elephantids do not. There appear to be an increase in C3 plants consumption at around 4 Ma, but better documentation of that period will be

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needed to confirm this trend, which is based on a limited number of localities. When

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comparable, the lower Omo Valley is more mesic than the other two basins in the

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Pliocene and there is some evidence that the herbivore diets were more C3 (although not

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uniformly so across taxa), but the overall landscape appears to have been relatively similar to that of the Turkana basin. This underscores that profiles provided by various

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proxies are not equivalent and, considered together, they yield a more complete picture of

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Acknowledgements

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the past.

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We would like to particularly thank Drs. R. Bobe, Y. Haile-Selassie, R. Potts, D. Geraads and B. Viola for kindly providing unpublished relative faunal abundance. We also thank Drs. M. Pickford, T. Luedecke, J. Fleagle, J. Harris, L. Bishop, T. Harrison, E. Fara, R. Gallotti, C. Egeland, C. Gilbert and M. Shoeninger for their assistance. Finally, we thank Laurence Dumouchel and Enquye Negash for their comments on an earlier version of this manuscript.

34

Journal Pre-proof Funding: This work was supported in part by the Hominin Dispersals Research Group (HDRG) with a grant from the Fonds de la recherche du Québec – Société et culture (#179537).

Data Availability

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Datasets related to this article are in Supplementary materials C and D.

Declarations of interest

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None

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Journal Pre-proof WoldeGabriel, G., S. H. Ambrose, D. Barboni, R. Bonnefille, L. Bremond, B. Currie, D. DeGusta, et al. 2009. The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus. Science 326 (5949): 65-65e5. https://doi.org/10.1126/science.1175817. WoldeGabriel, G., Y. Haile-Selassie, P.R. Renne, W.K. Hart, S.H. Ambrose, B. Asfaw, G. Heiken, and T. D. White. 2001. Geology and Palaeontology of the Late Miocene

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Middle Awash Valley, Afar Rift, Ethiopia. Nature 412 (6843): 175-8.

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https://doi.org/10.1038/35084058.

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WoldeGabriel, G., T. D. White, G. Suwa, P. Renne, J. de Heinzelin, W. K. Hart, and G.

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Heiken. 1994. Ecological and Temporal Placement of Early Pliocene Hominids at Aramis, Ethiopia. Nature 371 (6495): 330-3. https://doi.org/10.1038/371330a0.

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Wynn, J. G. 2000. Paleosols, Stable Carbon Isotopes, and Paleoenvironmental

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Interpretation of Kanapoi, Northern Kenya. Journal of Human Evolution 39 (4): 411‑32. https://doi.org/10.1006/jhev.2000.0431.

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Wynn, J. G. 2004. Influence of Plio-Pleistocene Aridification on Human Evolution:

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Evidence from Paleosols of the Turkana Basin, Kenya. American Journal of Physical Anthropology 123 (2): 106‑18. https://doi.org/10.1002/ajpa.10317. Wynn, JG, M. Sponheimer, W. H. Kimbel, Z. Alemseged, K. Reed, Z. Bedaso, and J. N. Wilson. 2013. Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia. Proceedings of the National Academy of Sciences 110 (26): 10495-10500. https://doi.org/10.1073/pnas.1222559110.

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Journal Pre-proof Wynn, J. G., K. E. Reed, M. Sponheimer, W. H. Kimbel, Z. Alemseged, Z. K. Bedaso, and C. J. Campisano. 2016. Dietary Flexibility of Australopithecus afarensis in the Face of Paleoecological Change During the Middle Pliocene: Faunal Evidence from Hadar, Ethiopia. Journal of Human Evolution 99: 93–106.

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Figure 1. Map of the Awash Valley, lower Omo Valley and Turkana Basin in East Africa. Modified from http://www.lifeinternational.org.uk/horn1.html and http://biofocuscommunicatie.nl/africa-map-outline-with-scale/.

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Figure 2. Mean values ±1 standard error enamel carbon isotope values of herbivores in the Awash Valley (A), the Turkana Basin (B) and the lower Omo Valley (C). Carbon value were plotted for each age category. Values for the bovids and suids (when possible) are weighted according to faunal abundance. Values of more than -1.00 indicate C4 plant-based diet, values under -8.00 indicate a C3 plant-based diet, and values in between suggest a mixed diet.

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Figure 3. Mean values ±1 standard error of water deficit and δ13Cecosystem values in the Awash Valley (A), the Turkana Basin (B) and the lower Omo Valley (C). Water deficit and δ13Cecosystem values were calculated for each STUs, then averaged and plotted for each age category. Positive values indicate a more arid or open environment, whilst negative values indicate mesic or more closed settings. Small numbers next to error bars indicate sample size when fewer than 16 ES-EI pairs could be used to calculate the water deficit. Figure 4. Mean values ±1 standard error of enamel carbon isotope values of elephantids (A), hippopotamids (B), suids (C) and bovids (D). Values for the bovids andsuids (when possible) are weighted according to faunal abundance. Values of more than 1.00 indicate C4 plant-based diet, values under -8.00 indicate a C3 plant-based diet, and values in between suggest a mixed diet. (Same data as in Figure 2, but organized by taxon.) Figure 5. Mean values ±1 standard error of water deficit (A) and 13Cecosystem values (B) for all three basins. Water deficit and carbon value were calculated for each STUs, then averaged and plotted for each age category. For the water deficit, positive values indicate a more arid environment, whilst negative values indicate mesic settings. Small numbers next to error bars indicate sample size when fewer than 16 ES-EI pairs could be used to calculate the water deficit. (Same data as in Figure 3, but organized by type of signal.) 53

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Table 1. Sample size of carbon values included in this study. Awash Valley

Lower Omo Valley

Total

412

241

1028

Elephantidae

86

103

18

207

Hippopotamidae

90

182

14

286

238

193

112

543

28

80

163

111

72

74

Cercopithecidae

130

34

Deinotheriidae

14

Equidae Giraffidae

Gomphotheriidae

7

Camelidae

0 1203

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Total

1

109

7

281

4

150

0

164

36

13

63

5

1

13

1

0

1

1231

411

2845

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Rhinocerotidae

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Suidae

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375

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Bovidae

Turkana Basin

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Table 2. Sample size of oxygen values included in this study.

Taxa EI Total

Tragelaphini

Turkana Basin

Lower Omo Valley

Total

113

19

21

153

40

19

2

61

Hippopotamidae

86

104

13

203

Elephantidae

63

27

18

108

Rhinocerotidae

33

18

1

52

335

187

577

55

Giraffidae

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Taxa ES

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Awash Valley

Table 3. Lithostratigraphic units included in each age categories for the Awash, Turkana and lower Omo Valley basins. Units labeled with * were used in the δ13Cecosystem calculations and units labeled with # were used for the water deficit calculations. Awash Valley 54

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Lithostratigraphic units (Areas) Busidima Formation (Asbole)* # Busidima Formation (Hadar) # Lee Adoyta Fault Block (Ledi-Geraru)* # Gurumaha Fault Block (Ledi-Geraru)* # Kada Hadar II Subember, Kada Hadar Formation (Hadar)* # Upper Woranso-Mille Time Interval (Woranso-Mille)* Upper Basal Member, Hadar Formation (Dikika)* # Shabeley Laag Member, Mount Galili Formation (Galili)* Sidi Hakoma Tuff, Hadar Formation (Hadar)* # Sidi Hakoma Member, Hadar Formation (Dikika)* # Middle Woranso-Mille Time Interval (Woranso-Mille) Denen Dora Member, Hadar Formation (Dikika) Denen Dora-2 Member, Hadar Formation (Hadar)* # Lower Woranso-Mille Time Interval (Woranso-Mille)* # Base of the Basal Member, Hadar Formation (Dikika)* # Dhidinley Member, Mount Galili Formation (Galili)* # Satkawhini Member, Mount Galili Formation (Galili)* # Lower Aramis Member, Sagantole Formation (Aramis)* # Segala Noumou Member, Sagantole Formation (Gona) # Gona Western Margin South Member, Sagantole Formation (Gona) # As Duma Member, Sagantole Formation (Gona)# Sifi Tuff, Adu-Asa Formation (Gona) Kebo‟o Tuff, Adu-Asa Formation (Gona)#

4.6 – 5.2 5.2 – 6.5

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4.2 – 4.6

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3.9 – 4.4

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3.6 – 3.8

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3.2 – 3.6

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#

Turkana Basin Age category (Ma) 0.8 – 1.3 1.3 – 1.5

Lithostratigraphic units (Areas) Nariokotome Member, Nachukui Formation (West Turkana)* Natoo Member, Nachukui Formation (West Turkana)* Okote Member, Koobi Fora Formation (East Turkana)* # 55

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3.4 – 4.0 4.0 4.2 4.3 – 5.2 5.2 – 6.5

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6.5 – 7.4

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3.0 – 3.4

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2.5 – 3.0

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2.4 – 2.5

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1.9 – 2.4

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1.5 – 1.9

Kaitio Member, Nachukui Formation, (West Turkana)* # KBS Member, Koobi Fora Formation (East Turkana)* # Upper Burgi Member, Koobi Fora Formation (East Turkana)* # Kalochoro Member, Nachukui Formation (West Turkana)* # Lokalalei Member, Nachukui Formation (West Turkana) # Upper Tulu Bor Submember, Koobi Fora Formation (East Turkana) Upper and Middle Lomekwi Submembers, Nachukui Formation (West Turkana)* # Lower Tulu Bor Submember, Koobi Fora Formation (East Turkana)* Lower Lomekwi Submember, Nachukui Formation (West Turkana)* # Kaiyumung Member, Nachukui Formation (Lothagam, West Turkana) Kataboi Member, Nachukui Formation (West Turkana) Lokochot Member, Koobi Fora Formation (East Turkana)* Lonyumun Member, Koobi Fora Formation (Allia Bay, East Turkana)* # Kanapoi Formation (Kanapoi)* # Apak Member, Nachukui Formation (Lothagam, West Turkana)* Upper Nawata Member, Nawata Formation (Lothagam, West Turkana)* # Lower Nawata Member, Nawata Formation (Lothagam, West Turkana)*

Age category (Ma) 0.01 0.1 – 0.2 1.3 – 1.5 1.9 – 2.3 2.3

Lower Omo Valley

Lithostratigraphic units Kibish IV Member, Kibish Formation* Kibish III Member, Kibish Formation* Kibish I Member, Kibish Formation* # L Member, Shungura Formation* E Member, Shungura Formation* F Member, Shungura Formation* 56

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G Member, Shungura Formation D Member, Shungura Formation* C Member, Shungura Formation* # B Member, Shungura Formation * # Mursi Formation (Yellow Sands)* #

Table 4: Mean, standard error of the mean, and sample size of δ13C values per age category.

2.6 – 2.7

3.2 – 3.6 3.6 – 3.8 3.9 – 4.4 4.2 – 4.6 4.6 – 5.2

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3.1 – 2.9

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2.7 – 2.8

0.11 0.56 2 -1.15 0.61 11 -1.68 0.31 30 -0.96 0.31 16 -3.30 1 -1.78 0.68 9 -2.68

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-1.30 0.15 3

Suidae (Weighed) -0.17 0.19 3 -0.75 0.66 7 -2.48 0.53 3 -1.08 0.68 10 -2.15 0.42 24 -2.22 0.23 80 -2.80 0.49 27 -3.76 0.79 5 -2.36 0.37 49 -1.51

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1.3 – 2.4

-0.90 0.50 2 0.52 0.45 9

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0.6 – 0.8

Elephantidae Hippopotamidae

-0.21 1.52 3 -0.82 1.85 2 -2.68 1.69 3 -4.29 0.62 28 -2.77 0.69 14 -8.30 0.73 5 -3.33 0.51 21 -1.66

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Age category (Ma)

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Bovidae (Weighed) -0.78 0.72 22

0.28 0.40 26 0.65 0.34 79 -1.64 0.74 21 -2.48 0.27 128 -2.64 0.51 8 -7.23 0.96 24 -3.57 0.80 33

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5.2 – 6.5

0.36 4 -1.2 1.3 2

0.74 7 -1.98 1.22 4

0.31 19 -9.18 0.49 9

Turkana Basin

2.4 – 2.5 2.5 – 3.0 3.0 – 3.4 3.4 – 4.0

4.0

4.2 4.3 – 5.2

Bovidae (Weighed) 0.63 0.67 12 -0.41 0.39 56 -1.03 0.27 135 -0.28 0.27 86 0.09 0.81 7 -0.65 0.52 27 -1.82 0.45 39 -4.60 1.51 9 -6.04 1.15 10 -5.65 1.23 17

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Suidae (Weighed) -0.03 0.19 3 -0.04 0.20 23 -0.74 0.22 40 -0.13 0.08 57

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1,9 – 2,4

0.03 0.12 3 -1.15 0.39 16 -1.16 0.36 24 -0.92 0.20 23 -2.30 1 -2.88 1.07 5 -3.62 0.77 10 -4.29 1.02 8 -4.43 0.86 22 -2.04 0.65 7 -4.55 0.92 7

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1.5 – 1.9

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1.3 – 1.5

-0.55 0.25 2 -1.45 1.95 2 -0.06 0.29 14 0.05 0.08 13 -2.5 1 -0.56 0.49 8 -0.89 0.34 9 -1.00 1 -4.56 0.62 17 -2.59 0.38 15 -0.67 0.16 5

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0.7 – 1.3

Elephantidae Hippopotamidae

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Age category (Ma)

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-0.67 0.52 10 -1.61 0.40 7 -2.79 1.95 3 -5.38 1.88 6 -2.00 0.48 11 -2.78 0.72 6

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5.2 – 6.5 6.5 – 7.4

-0.89 0.28 9 -3.42 1.41 4

-2.59 0.42 25 -4.02 0.43 22

-4.80 0.79 8 -7.44 0.38 10

-5.36 2.00 5 -2.74 1.56 5

Suidae (Weighed) -2.46 0.65 7 -1.65 0.27 17 -0.35 0.27 8 -0.89 0.29 16 -0.20 0.22 10 -0.88 0.30 10

Bovidae (Weighed) -1.04 0.85 9 -3.02 0.81 31 -0.22 0.47 23 -2.18 0.41 39 -1.70 0.43 38

Lower Omo Valley Elephantidae Hippopotamidae

0.001

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0.1 – 0.2

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1.4

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1.9 – 2.3

2,5 – 3.0

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2.4 – 2.5

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2.3 2.3 – 2.4

3,0 – 3.4

4.0

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-5.76 1 -4.27 1.16 5

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Age category (Ma)

-3.98 0.43 18

-2.30 1 -7.30 1 -5.27 0.65 6

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-1.67 0.25 13 -6.43 0.86 3 -6.42 0.63 22

-2.38 0.49 28 -2.71 0.40 37 -3.72 0.66 31 -10.79 1.57 4

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Table 5: Water deficit calculation, propagated errors and number of ES-EI pairs used for the calculations for each aggregated age categories. Awash Basin Age category WD Propagated n of pairs (Ma) error 0.6 – 0.8 877 2271 24 1.3 – 2.4 337 916 45 2.6 – 2.7 187 559 8 2.7 – 2.8 -419 1332 44 2.9 – 3.1 821 1807 85 3.2 – 3.6 -64 3443 392 3.6 – 3.8 719 2190 314 3.9 – 4.4 -14 1143 36 4.2 – 4.6 32 2895 628 4.6 – 5.2 60 1416 140 5.2 – 6.5 -279 940 20

Age category (Ma) 0.1 – 0.2 3.5 – 3.0 3.0 – 3.4

Lower Omo Valley WD Propagated error 244 1170 -802 428 -1095 455

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Age category (Ma) 1.3 – 1.5 1.5 – 1.9 1.9 – 2.4 2.4 – 2.5 2.5-3.0 3.0 – 3.4 4.0 4.2 5.2 – 6.5

Turkana Basin WD Propagated error 483 782 45 1141 -226 872 1568 423 154 1246 7 901 -170 2162 1119 1004 153 989

n of pairs 12 378 39 1 22 2 82 18 36

n of pairs 25 8 8 60

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862

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Table 6: δ13Cecosystem calculated values, standard error of the mean (number of lithostratigraphic units averaged), and total number of faunal specimens. Awash Basin 13 Age category δ Cecosystem St. error n (Ma) (nlithostrat.) 0.6 – 0.8 -15.17 (1) 60 2.6 – 2.7 -14.05 (1) 43 2.7 – 2.8 -14.72 (1) 133 2.9 – 3.1 -17.86 (1) 90 3.2 – 3.6 -17.51 0.56 (6) 419 3.6 – 3.8 -17.10 0.31 (2) 131 3.9 – 4.4 -20.02 (1) 36 4.2 – 4.6 -18.25 1.94 (2) 208

Age category (Ma) 0.01 1.3 – 1.5 1.5 – 1.9 1.9 – 2.4 2.5 – 3.0 3.0 – 3.4 3.4 – 4.0 4.0 4.2 4.3 – 5.2 5.2 – 6.5 6.5 – 7.4

Turkana Basin δ Cecosystem St. error (nlithostrat.) -13.86 (1) -15.03 0.84 (2) -16.81 0.50 (2) -16.75 0.12 (2) -18.48 (1) -18.92 2.66 (2) -24.07 (1) -22.29 (1) -18.30 (1) -16.99 (1) -17.18 (1) -18.13 (1)

Age category (Ma)

Lower Omo Valley δ Cecosystem St. error (nlithostrat.)

n

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13

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26 42 283 243 60 83 25 101 68 42 71 82

13

n

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-17.27 -16.28 -13.98 -15.76 -22.15 -16.59 -16.24 -19.74 -22.19

17 63 31 55 50 28 51 35 71

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0.01 0.1 – 0.2 1.3 – 1.5 1.9 – 2.3 2.3 2.4 – 2.5 3.5 – 3.0 3.0 – 3.4 4.0

At ~4 Ma, C3-plant increases in the diet of all taxa in the Awash and Turkana

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Highlights  Diets of bovids, suids and hippos, but not elephants, track the C4-plant expansion

Only bovids‟ diet suggests more C3 plants In the Omo Valley throughout the

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

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basins

Pliocene



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mesic

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Landscape structure was similar in the Omo and Turkana, but the Omo was more

Aridity increases in the Awash, suggesting it contributed to C4-plant expansion

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

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5