Ungulate turnover in the Koobi Fora Formation: Spatial and temporal variation in the Early Pleistocene

Journal Pre-proof Ungulate turnover in the Koobi Fora Formation: Spatial and temporal variation in the Early Pleistocene K. O'Brien, D.B. Patterson, M.D. Biernat, D.R. Braun, T.E. Cerling, A. McGrosky, J.T. Faith PII:

S1464-343X(19)30313-9

DOI:

https://doi.org/10.1016/j.jafrearsci.2019.103658

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AES 103658

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Journal of African Earth Sciences

Received Date: 3 July 2019 Revised Date:

21 September 2019

Accepted Date: 30 September 2019

Please cite this article as: O'Brien, K., Patterson, D.B., Biernat, M.D., Braun, D.R., Cerling, T.E., McGrosky, A., Faith, J.T., Ungulate turnover in the Koobi Fora Formation: Spatial and temporal variation in the Early Pleistocene, Journal of African Earth Sciences (2019), doi: https://doi.org/10.1016/ j.jafrearsci.2019.103658. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Ungulate turnover in the Koobi Fora Formation: Spatial and temporal variation in the

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Early Pleistocene

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K. O’Briena,b*, D. B. Pattersonc,d, M. D. Biernate,f, D. R. Braund,g,h,i, T. E. Cerlingj,k, A.

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McGroskye,f, and J. T. Faitha,b

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a

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UT 84112 USA

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b

Department of Anthropology, University of Utah, 260 S. Central Campus Drive, Salt Lake City,

Natural History Museum of Utah, University of Utah, 301 Wakara Way, Salt Lake City, UT

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84108 USA

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c

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USA

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d

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George Washington University, 800 22nd St NW, Suite 6000, Washington, DC 20052 USA

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e

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Tempe, AZ 85281 USA

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f

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g

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Oxford, OX2 6PN United Kingdom

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h

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South Africa

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i

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Deutscher Platz 6, 04103 Leipzig Germany

Department of Biology, University of North Georgia, 159 Sunset Drive, Dahlonega, GA 30597

Center for the Advanced Study of Human Paleobiology, Department of Anthropology, The

School of Human Evolution and Social Change, Arizona State University, 900 Cady Mall,

Institute of Human Origins, Arizona State University, 951 Cady Mall, Tempe, AZ 85281 USA Institute of Cognitive and Evolutionary Anthropology, Oxford University, 64 Banbury Rd,

Department of Archaeology, University of Cape Town, Private Bag X3, Rondebosch 7701

Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology,

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j

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UT 84112 USA

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k

Department of Geology and Geophysics, University of Utah, 115 S 1460 E #383, Salt Lake City,

Department of Biology, University of Utah, 257 S 1400 E, Salt Lake City, UT 84112 USA

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Correspondence email

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[email protected]

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Declarations of interest: none

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Abstract

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The Koobi Fora Formation, located east of Lake Turkana in northern Kenya, is well-known

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for its well-sampled vertebrate fossil record. Thousands of identifiable remains have been

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collected from three Early Pleistocene geologic members of the Koobi Fora Formation: the

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Upper Burgi (1.98-1.87 Ma), KBS (1.87-1.56 Ma), and Okote (1.56-1.38 Ma). The large sample

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from these members permits analysis of spatial and temporal variation in species composition,

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particularly as it relates to broader changes within the basin. To investigate paleoecological

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trends in the Koobi Fora Formation during this period, we examine spatial and temporal variation

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in the proportional abundance of a large sample (NISP = 3,713) of ecologically diverse ungulate

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taxa, in addition to variation in enamel isotope δ13C values. We find that the Karari subregion

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experienced significant turnover within key large mammalian families (Bovidae, Suidae, and

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Equidae) during this period, while changes in other subregions (Ileret and Koobi Fora Ridge)

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were relatively minimal. Our analyses indicate drier conditions in the Karari during the

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regression of Paleolake Lorenyang as the dominant water-dependent taxa (e.g., the bovid tribe

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Reduncini and the suid genus Kolpochoerus) were largely replaced with taxa adapted to xeric

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habitats including the bovid tribe Alcelaphini and the suid genus Metridiochoerus. Little change

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was detected in proportional abundances of broad dietary groups (grazers : mixed-feeders :

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browsers). These findings provide a more resolved spatiotemporal resolution for the mammalian

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communities occupying the region during this time. They further highlight the importance of

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considering localized habitat differentiation in eastern Africa during the Early Pleistocene,

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acknowledging that fossil records document complex and dynamic systems which varied

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spatially and temporally on many scales, much like modern African ecosystems.

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Keywords

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Paleoecology; Faunal Abundance; East Turkana; Paleolake Lorenyang

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1. Introduction

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Basin-scale fossil and sedimentary evidence from eastern Africa during the Early Pleistocene

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(2.588-0.781 Ma: Cohen et al., 2013) indicates that pulses of aridification left the region with a

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relatively xeric array of ecosystems during the time in which the genus Homo spread and

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diversified (Leroy and Dupont, 1994; Bobe and Behrensmeyer, 2004; Wynn, 2004; Bobe and

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Leakey, 2009; Potts, 2012; Bobe and Carvalho, 2019). Though attributable to a complex suite of

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factors, paleoclimatic studies suggest that continental environmental shifts had particularly

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variable effects at regional scales (deMenocal, 1995; 2004; Blome et al., 2012; Campisano,

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2012; Lepre, 2014). These continental shifts resulted in heterogeneous local environments across

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eastern Africa’s sedimentary basins, including those relevant to interpreting hominin ecology

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(Levin, 2015). Existing research has largely been regional in scale; however, it is well-known

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that analyses conducted at large geographic scales can mute paleoecological trends due

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paleogeographic variation in the responses of ecosystems to climatic changes (e.g., Barnosky,

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2001). It follows that understanding organism-environment interactions at finer scales will

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potentially shed light on the mechanisms by which broader paleoclimatic changes impacted

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mammalian evolution in eastern Africa (see Lee-Thorp et al., 1994; Cerling et al., 2011a,b; 2013;

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Quinn et al., 2013; Sponheimer et al., 2013; Patterson et al., 2017a, 2019; Bobe and Carvalho,

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2019). Here we focus on the Koobi Fora Formation in the Omo-Turkana Basin due to its tight

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geochronological control, highly fossiliferous deposits, and coexistence of multiple Early

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Pleistocene hominin taxa including Paranthropus boisei, Homo habilis, H. rudolfensis, and H.

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erectus (Wood and Leakey, 2011) (Figs. 1,2).

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Fig. 1: (Left) Example of a composite stratigraphic section from the Koobi Fora Formation, redrawn from Brown and McDougall (2011) and (Right) the greater Horn of Africa with the OmoTurkana Basin’s Pleistocene deposits enlarged, highlighting the three subregions used in this study. Redrawn from McDougall and Brown (2006)

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Through the past several million years, a series of paleolakes with periodic outflows to both

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the Indian Ocean and the Nile River dominated the landscape in the region of the modern Omo-

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Turkana Basin (Feibel, 2011; Joordens et al., 2011; Levin et al., 2011; Boës et al., 2018). One

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closely-studied hydrologic transition is the transgression and regression of an ancient lacustrine

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sequence often described as Paleolake Lorenyang. This lake phase began with tectonic uplift and

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volcanic activity in the southeastern Omo-Turkana Basin resulting in high lake levels from 2.2 -

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1.7 Ma (see Boës et al., 2018 for details) (Fig. 2). These paleolake dynamics occurred against the

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backdrop of gradual environmental change increasing the abundance of tropical grasses in

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eastern African ecosystems between 4 and 1 Ma (Levin et al., 2011; Cerling et al., 2011b;

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Cerling et al., 2015; Levin, 2015), resulting in increased abundance of grassland-indicative taxa

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(Bobe and Behrensmeyer, 2004; Bobe et al., 2007; Bibi and Kiessling, 2015; Fortelius et al.,

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2016; Bobe and Carvalho, 2019).

Fig. 2: Chronology of the Koobi Fora Formation through the time period analyzed in this study

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Our aim here is to examine how patterns of faunal turnover vary in relation to changing

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paleogeography during the regression of Paleolake Lorenyang. We do so through analysis of

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three fossiliferous subregions dating to the Early Pleistocene of the Koobi Fora Formation:

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Koobi Fora Ridge, Karari, and Ileret (Fig. 1). The shoreline of Paleolake Lorenyang was

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dynamic, and shoreline deposits are found in all three subregions, but the Koobi Fora Ridge and

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Ileret subregions are positioned closer to the axis of the Basin than the Karari subregion

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(Behrensmeyer, 1978; Levin et al., 2011). The distance of the Karari subregion from the axis of

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the Basin may have allowed for a unique environmental setting in this area relative to the axis

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(Fig. 1). Specifically, we expect faunal assemblages from 1.98-1.38 Ma of the Koobi Fora Ridge

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and Ileret subregions to reflect stable edaphic grasslands due to their lower-energy fluvial

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systems and proximity to the axis of the basin (see Behrensmeyer, 1985). Meanwhile, we predict

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localized replacement of edaphic grassland-indicative ungulates with dry-grassland taxa in the

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Karari. Further reflecting this variation, we expect the enamel δ13C value depletion in grazing

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ungulates to be heterogeneous through time and across space.

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2. Materials and methods

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In this study, specimens are grouped by both taxonomy and broad dietary categories. We

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analyzed the paleontological dataset from Patterson et al. (2017a), with the addition of new field

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collections from 2016, 2017, and 2018, and additional unpublished specimens from the Nairobi

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National Museum (Supplementary Dataset S1). Specimens derive from the three most

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fossiliferous geologic members of the Koobi Fora Formation spanning 600 kyr of the Early

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Pleistocene. These members encompass the transgression and regression of Paleolake

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Lorenyang: the Upper Burgi (1.98-1.87 Ma), the KBS (1.87-1.56 Ma), and the Okote (1.56-1.38

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Ma) (McDougall and Brown, 2006; Brown and McDougall, 2011) (Fig. 1). Field collections

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from 2016, 2017, and 2018 were obtained following the Koobi Fora Research and Training

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Program standardized collection procedure (Patterson et al., 2017a). We targeted areas with few

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published specimens to increase sample sizes for all subregions in each geologic member. We

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collected all terrestrial ungulate specimens identifiable to tribe for the family Bovidae, and genus

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for other families. Spatially (i.e., within five meters) and stratigraphically (i.e., within the same

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geologic lens) associated fossils from the same taxon are consolidated as a single individual

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during field collection in the absence of adequate information to justify them as independent

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elements. This procedure permits combining field collections with museum collections in which

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a , b , c , d specimens associated with a single accession number have been combined into a

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single individual (see Patterson et al., 2017a for additional details). Specimens excavated from

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archaeological sites were not included in this study.

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Ungulate (Artiodactyla and Perissodactyla) specimens were aggregated into analytic taxa at

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the tribe level. Taxa were incorporated into the analysis if greater than 50 specimens were

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recovered in each tribe, with the exception of Hippopotamidae (Table 1). Hippopotamids were

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omitted due to well-documented taphonomic biases (see Behrensmeyer, 1985) and differential

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collection protocol skewing spatial abundance. The bovid tribes Cephalophini and Hippotragini

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were omitted due to their low sample sizes (n = 3 and 19, respectively), along with the

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rhinocerotid tribe Dicerotini (n = 48). In total, this left 3,713 specimens for analysis. We

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combined the specimens from Notochoerus and Phacochoerini (genus Metridiochoerus) for this

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analysis. This analytical approach is supported by several factors. Importantly, Notochoerus goes

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extinct by the end of the time period in question, with its niche likely filled by Metridiochoerus.

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Table 1: Total counts for all analytic taxa, in each member and subregion

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Notochoerus and Metridiochoerus have shared ecomorphological adaptations for xeric

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woodland, bushland, or mixed habitats and isotopic evidence of xeric habitat preferences in

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contrast to the possibly more water-dependent Kolpochoerus (see Bishop, 1999; Harris and

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Cerling, 2002; Patterson et al., 2017a; Rannikko et al., 2017).

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Taxa were further grouped into three dietary classes (grazers, mixed-feeders, and browsers)

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based on categorization by Cerling et al. (2015) and Faith et al. (2018). The only true browsers in

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our final dataset belong to the genus Giraffa, while mixed-feeders include Aepyceros, Antilopini,

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Tragelaphus, and Sivatherium of the lower members (Upper Burgi and KBS). Sivatherium from

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the Okote Member are considered grazers due to the dietary shift in the Early Pleistocene noted

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by Cerling et al. (2015) based on East Turkana δ13C enamel isotope data. Other grazers include

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Alcelaphini, Bovini, Reduncini, Kolpochoerus, Metridiochoerus, Notochoerus, Equus, and

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Eurygnathohippus.

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It is important to note the taphonomic and collection biases present in a dataset of this size. For example, suid fossils have historically been preferentially collected in the region due to their

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extensive use in biochronology (e.g., Harris, 1983; Brown et al., 1985). Postcrania are unlikely to

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be collected due to their abundance, a dearth of storage space, and a lack of taxonomically

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identifiable markers (Behrensmeyer, 1985). However, these biases are likely consistent through

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time and across space within this well-sampled geologic member sequence, meaning that they

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should not impact our results. Furthermore, the exclusion of archaeological data to limit biases in

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accumulation of remains and limiting our study to terrestrial ungulate taxa lessens the impact of

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taphonomy on our results.

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The three subregions were defined as follows for this study, acknowledging that the divisions

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are based on spatial associations due to modern topography: Koobi Fora Ridge sediments lie

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between the ephemeral Il Derati-Il Alia and Il Iriyamang’werr, and are dominated by the

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Aberagaiya Ridge and the sediments laterally exposed adjacent to the shore of Lake Turkana.

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Karari sediments average 30 km inland, from Il Iriyanmang’werr to Il Warata. It is a series of

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westward-facing outcrops along the Karari Ridge approximately 30 km from the shores of Lake

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Turkana that exhibit substantial exposure due to capping Late Pleistocene sediments resistant to

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headward erosion. Topography in this region relates directly to the tectonic uplift of the

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northeastern Turkana Basin (Levin et al., 2011; Quinn et al., 2013). Ileret comprises sediments

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north of Il Warata (Fig. 1). It exhibits sediments that are exposed by the erosion associated with

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the large drainage systems to the northeast of the basin—most notably the Il Eriet and Il Dura

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seasonal fluvial systems.

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The fossil records of the three subregions (Karari, Koobi Fora Ridge and Ileret) were

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compared to one another within each geologic member using Pearson’s chi-squared (χ2) test.

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Chord distance was additionally calculated to indicate dissimilarity of the subregions when

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compared within members (Faith and Lyman, 2019). All analytic taxa and dietary classes were

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then evaluated for directional temporal variation using χ2 tests for linear trend (χ2trend) (Lyman,

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2008), with each subregion analyzed independently. Due to the multiple tests being run, we use

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an alpha of 0.01 for determining significance to reduce the likelihood of false positives.

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δ13C enamel isotope data from Harris and Cerling (2002), Braun et al. (2010), Cerling et al.

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(2011a), Cerling et al. (2015), Patterson et al. (2017b; 2019) were combined (Supplementary

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Dataset S2). Taxa with data from three subregions of the Koobi Fora Formation and sample sizes

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of ≥10 individuals were analyzed. Sufficient data were available for Alcelaphini, Reduncini,

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Tragelaphus, and Metridiochoerus in the Karari and Ileret subregions. No taxon met this

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threshold for the Koobi Fora Ridge subregion. Isotope values were compared temporally across

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members using the Kruskal-Wallis nonparametric test.

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3. Results

3.1. Spatial variation

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When comparing taxa from the three subregions within each geologic member, the

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divergence between assemblages increases through time. In the Upper Burgi Member, taxonomic

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abundances were not significantly different (p<0.01) between any pairing of subregions: Karari

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vs. Koobi Fora Ridge (χ2 = 8.30; p = 0.686), Karari vs. Ileret (χ2 = 8.88; p = 0.633), and Koobi

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Fora Ridge vs. Ileret (χ2 = 8.61; p = 0.658). In the KBS Member, the abundances diverge near

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significance in Karari vs. Koobi Fora Ridge (χ2 = 23.88; p = 0.013) and Karari vs. Ileret (χ2 =

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24.19; p = 0.012), but not for Koobi Fora Ridge vs. Ileret (χ2 = 2.49; p = 0.996). In the Okote

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Member, differences in analytic taxa yield significant results for Karari vs. Koobi Fora Ridge (χ2

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= 41.10; p < 0.001) and Karari vs. Ileret (χ2 = 43.13 and p < 0.001). The two axial subregions

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(Koobi Fora Ridge and Ileret) are also near significant difference from one another in the Okote

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Member assemblage (χ2 = 22.33; p = 0.022). This pattern is further reflected by chord distances

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between subregions through time, showing relatively similar conditions throughout the Koobi

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Fora Formation in the Upper Burgi Member followed by the sharp divergence of the Karari from

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other subregions by the KBS Member, and the subsequent separation of the Koobi Fora Ridge

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and Ileret in the Okote Member (Fig. 3).

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Fig. 3: Chord distances between paired subregions indicating divergence through time, for which higher chord distance = more dissimilar

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3.2. Temporal variation

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Of the nine significant temporal abundance shifts (p <0.01) within analytic taxa, seven

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occurred in the Karari subregion (statistical results in Table 2, proportions in Fig. 4). Across the

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three geologic members, directional increases in abundance occurred in the Karari in

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Alcelaphini, Bovini, Metridiochoerus + Notochoerus, and Equus. Decreases were detected in

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Antilopini, Reduncini, and Kolpochoerus. Notably, Eurygnathohippus does not track the related

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and morphologically similar Equus and instead appears to have decreased. Outside the Karari,

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Aepyceros proportions decreased in abundance in Ileret, while the Koobi Fora Ridge had both an

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increase in Eurygnathohippus and a positive trend near significance in Bovini. Across the three

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broad ungulate dietary classes (grazers (C4), mixed-feeders (C3/C4) and browsers (C3)), no trends

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are statistically significant (Fig. 5).

Analytic Taxon Aepyceros Alcelaphini Antilopini Bovini Reduncini Tragelaphus Giraffa Sivatherium Metridiochoerus + Notochoerus Kolpochoerus Equus Eurygnathohippus

Ileret p -value Trend Trend ↘ 7.05 0.008 1.79 0.181 ↗ 1.81 0.178 ↘ 3.21 0.073 ↗ 0.30 0.582 ↘ 0.18 0.674 0.03 0.856 0.29 0.587 2 χ trend

1.41 1.75 0.25 2.20

0.235 0.185 0.614 0.138

↗ ↘ ↗

Karari Koobi Fora 2 2 χ trend p -value Trend χ trend p -value 2.33 0.127 1.60 0.206 18.68 <0.001 0.28 0.597 8.16 0.004 0.20 0.651 12.44 <0.001 4.67 0.031 18.75 <0.001 3.41 0.065 0.79 0.373 0.02 0.899 2.70 0.100 1.20 0.274 0.72 0.397 0.02 0.880 20.97 40.42 12.95 2.53

<0.001 <0.001 <0.001 0.112

↗

2.01 0.01 2.44 13.82

0.156 0.932 0.118 <0.001

Table 2: χ2trend results, with statistically significant (<0.01) trends indicated with arrows and bold text

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Fig. 4: Spindle diagram of relative faunal abundance as a percent of the total for each analytic taxon, with significant temporal changes marked and taxon sample size listed for each subregion

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Fig. 5: Relative abundance of each dietary class indicating relatively constant proportions of the three groups

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3.3. Isotopic variation

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Metridiochoerus δ13C values (total n = 96) became more depleted in the Karari through time

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(χ2 = 16.13; p < 0.001) but this trend was not detected elsewhere. δ13C value enrichment or

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depletion is not significant in any other taxon, although trends are seen in Alcelaphini (total n =

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97) in the Karari (χ2 = 6.10; p = 0.047) and in Ileret (χ2 = 6.83; p = 0.033). No clear trends were

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detected for Reduncini (total n = 70) nor Tragelaphus (total n = 38) (Fig. 6).

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Fig. 6: δ13C value results for the four taxa analyzed. C4 grazing: δ13C > -1; C3 browsing: δ13C < -8; mixed C3/C4 intermediate, following Cerling et al. (2015)

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4. Discussion 4.1. Spatiotemporal variation

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This study was conducted to investigate the abundance dynamics of the East Turkana

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mammalian community during the regression of Paleolake Lorenyang during the Early

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Pleistocene (Boës et al., 2018). Our spatiotemporal analysis of faunal abundance helps

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contextualize the Koobi Fora Formation at a subregional scale rather than evaluating it as a

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single ecologically-continuous area. We have detected considerable spatial and temporal

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variations in the faunal assemblage, potentially tied to the dynamic water system patterns and

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drainage pathways noted by Quinn et al. (2013) and Boës et al. (2018). Our analyses indicate a

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localized ecological shift in the Karari subregion between 1.98 and 1.38 Ma unmatched in the

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Ileret and Koobi Fora Ridge subregions. The Karari faunal turnover was characterized by

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replacement of water-dependent Reduncini and Kolpochoerus (Harris and Cerling, 2002;

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Kingdon et al., 2013) with Alcelaphini, Bovini, Equus, and Metridiochoerus—taxa adapted to

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xeric habitats. This occurred in conjunction with a significant decrease in the abundance of

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Antilopini, comprised mainly of species characteristic of mixed-C3/C4 environments (Cerling et

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al., 2015). Overall, this suggests that the previously documented abundance shifts in the region

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such as those in Alcelaphini, Antilopini, and Suidae (Patterson et al., 2017a) were driven by

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substantial changes that principally occurred within the Karari as edaphic grasslands were

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replaced by drier, secondary grasslands. Importantly, these changes were not mirrored in the

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Koobi Fora Ridge nor in Ileret. These findings would be undetectable at a larger scale, e.g.

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treating the entirety of East Turkana as a single landscape rather than a variable, dynamic

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collection of ecosystems.

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The lack of significant shifts in enamel δ13C in most taxa are likely due to sample size, but

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the positive results can be informative. The isotopic shift indicating δ13C depletion in the diet of

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Metridiochoerus and a decreasing trend in Alcelaphini can be interpreted in multiple ways. First,

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it may indicate an increase in xeric scrubland in the Karari as was suggested by Patterson et al.

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(2017b). Second, it could be due to the inclusion of C3 riparian dicots along the rivers draining

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the eastern basin margin in the Karari. Third, proportions of species within C3 and C4 categories

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may have changed across the subregion, as different plant families produce slightly different

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δ13C values due to independently derived photosynthetic pathways. Finally, species composition

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within the genus Metridiochoerus is known to have changed during this interval as M.

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compactus, M. hopwoodi, and M. modestus replaced the morphologically distinct M. andrewsi

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(see Harris and Cerling, 2002), and variable diets at the species level are known to exist at the

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generic level of both modern and fossil ungulates (Kingdon et al., 2013; Cerling et al., 2015).

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Harris and Cerling (2002) found no significant difference in the diets of these species, but finer

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taxonomic identification would need to be pursued from a larger sample to improve our

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interpretation of the temporal shift in δ13C values. The proximity of subregions to one another

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may also limit spatial variation in δ13C, as large mammals could almost certainly move freely

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throughout the broader region seasonally or simply due to broad foraging ranges, as are common

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throughout eastern Africa today (Kingdon et al., 2013).

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Even with the decrease in the relative abundance of Antilopini and decreasing trend of

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Aepycerotini, the overall proportions of mixed-feeders are consistent through the three

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subregions through time (Fig. 5). This can be attributed to the relatively stable proportions of the

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predominant mixed-feeder in the environment: Tragelaphus. Despite inhabiting a changing

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environment, the generalist dietary strategy of Tragelaphus relative to other bovids likely

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allowed for more ecological flexibility and lower levels of turnover in this taxon. Tragelaphus

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species are notable for their diet breadth both in the paleontological record (Cerling et al., 2015;

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Blondel et al., 2018) and in extant representatives of the genus (Kingdon et al., 2013). Both the

19 277

bongo-like T. nakuae and the greater kudu T. strepsiceros are known from all geologic members

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considered here. These species alone span a range of diet always inclusive of C3 vegetation but

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with wide variation in C4 consumption (see Cerling et al., 2015), possibly suggesting occupation

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of an equally broad range of habitat. In addition, another species is known contemporaneously

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from elsewhere in the Omo Group, T. gaudryi, with ecological and evolutionary affinities to the

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scrubland-inhabiting T. imberbis (Gentry, 1985; Blondel et el., 2018). Specific classification of a

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greater portion of collected specimens will be needed to properly address abundance shifts

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within this genus in the Early Pleistocene, with likely consequences for paleoecological

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reconstructions.

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Finally, we must note the difference in spatiotemporal patterning of the equid genera

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Eurygnathohippus and Equus. While Equus tracks the increases of other dry grassland taxa like

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Alcelaphini and Metridiochoerus in the Karari, Eurygnathohippus abundance remains constant

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or even decreases; the opposite is true for the Koobi Fora Ridge. The coexistence of these

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morphologically similar genera despite probable competition continues to raise paleoecological

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questions. While Eurasia saw the extinction of tridactyl hipparionin species related to

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Eurygnathohippus soon after the arrival of Equus, eastern Africa maintained two or more species

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of Eurygnathohippus through the Early Pleistocene alongside at least three Equus species

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(Eisenmann, 1983). Unlike their often mixed-feeding Eurasian relatives and Neogene ancestors,

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Eu. cornelianus and other Pleistocene Eurygnathohippus had highly derived morphology

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including hypsodont dentition comparable to that of Equus, often large body size, and enriched

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δ13C values indicative of grazing (Cerling et al., 2015). Niche partitioning and functional

298

divergence are common among bovids and other ungulates, but the diversity of Equidae in the

299

Koobi Fora Formation far exceeds anything existing today. Further analysis will be needed to

20 300

resolve the coexistence of these abundant animals and whether the extinction so

301

Eurygnathohippus can be explained by detectable ecological factors.

302 303

4.2. Habitat structure

304 305

The faunal restructuring in the Karari is likely due to a combination of factors, with the axial

306

subregions more likely than the Karari to retain edaphic grassland fauna. Paramount to the

307

Karari’s distinction would have been its relative distance from the paleolake margin during this

308

period of regression, creating a set of ecosystems comparable to what can be found today just

309

north of the study region in the Lower Omo Valley. Despite little spatial variation in rainfall,

310

temperature, and other climatic factors across the Lower Omo Valley, substantial biotic variation

311

is visible across the region. The banks of the Omo River and its delta are fertile and have

312

historically hosted an array of taxa dependent on the river’s water and nutrients (e.g., Kobus,

313

Redunca, and Potamochoerus). This contrasts with a vastly different landscape comprised

314

mostly of dry scrubland and secondary grasslands with corresponding fauna (e.g., Alcelaphus,

315

Madoqua, and Phacochoerus) found further from the modern riverbanks (Assefa et al., 2008).

316

Our findings of divergence between subregions are consistent with expectations given the

317

geography and geology of the basin (Brown and Feibel, 1991; Feibel, 2011; Levin et al., 2011;

318

Quinn et al., 2013). This would be especially true for Ileret due to its position adjacent to the

319

Omo fluvial system throughout the period considered here. Analyses of the Shungura Formation

320

(adjacent to the Omo River in southern Ethiopia, see Fig. 1) faunal assemblage suggest a pattern

321

similar to that of Ileret, with retention of edaphic grassland ungulates (Bobe et al., 2007; Reed

322

and Russak, 2009; Biernat et al., 2018). This followed earlier patterns of vegetation replacement

21 323

detected in the broader Omo-Turkana Basin during the Plio-Pleistocene (see Bibi et al., 2013;

324

Uno et al., 2016a,b). Further studies are needed to further draw links between the axial and

325

marginal regions of the Koobi Formation and the rest of the basin.

326

As drier conditions can be implicated as the primary factor behind turnover patterns in the

327

Karari, an increase in Antilopini abundance would be expected alongside Alcelaphini and

328

Metridiochoerus based on xeric grassland modern analogs for these three groups. The majority

329

of antilopins in the Koobi Fora Formation fossil assemblage from the Early Pleistocene belong to

330

the species Antidorcas recki, which is likely ancestral to the modern A. marsupialis, a species

331

that requires open habitats (Kingdon et al., 2013; Reid, 2005). Today, A. marsupialis is limited to

332

the dry plains and semideserts of southern Africa. However, the significant decline of this taxon

333

in the record while most grassland ungulates increase (a trend noted by Patterson et al., 2017a)

334

can instead be attributed to different ancestral habitat preferences in A. recki. This was first

335

suggested by Plummer and Bishop (1994) based on the ecomorphology of Early Pleistocene

336

specimens from Tanzania’s Olduvai Gorge. Unlike their probable descendants, A. recki had

337

metapodials indicating adaptation to mixed or mesic habitats. A. recki ecomorphology exhibits

338

similarity to Aepyceros and Reduncini—both of which also declined in the Karari. Our findings

339

support the ecological affinity of A. recki with other mixed-feeders and/or to mesic grazers as

340

suggested by Plummer and Bishop (1994). This calls into question the conventional grouping of

341

Antilopini with Alcelaphini and Hippotragini as cursorial, open-country “AAH” bovids based on

342

living taxonomic analogs (e.g., Vrba, 1975; Vrba, 1980; Bobe and Eck, 2001; Bobe, 2006).

343

A combination of the taxon abundance, enamel and paleosol δ13C values, and

344

paleogeographic reconstructions provides a robust interpretation of these habitats during the

345

Pleistocene (Levin et al., 2011; Quinn et al., 2013; Patterson et al., 2017b). The three subregions

22 346

of the Koobi Fora Formation described here all hosted similarly mesic habitats during the time of

347

deposition of the Upper Burgi Member. This contrasts with the Karari in later time intervals.

348

This more distal subregion became dominated by xeric wooded grasslands intersected by high-

349

energy fluvial systems leveling into deltaic floodplains as they approached the paleolake margin.

350

Meanwhile, low-energy systems may have continued to dominate the Koobi Fora Ridge and

351

Ileret during low lake levels as reconstructed by Behrensmeyer (1978), Levin et al. (2011), and

352

Quinn et al., (2013). Our analysis has implemented the ungulate fossil record as a valuable proxy

353

for spatiotemporal variation in the Koobi Fora Formation and documents clear intraregional

354

variation comparable to that found at broader scales in the Omo-Turkana Basin (see Bobe and

355

Leakey, 2009; Patterson et al., 2017a).

356 357

5. Conclusion

358 359

This analysis has illuminated a strong pattern of statistically significant turnover in the Karari

360

subregion of the Koobi Fora Formation and relative stasis in the axial subregions (Koobi Fora

361

Ridge and Ileret), a pattern undetectable at larger scales when treating the Koobi Fora Formation

362

as a single ecological landscape. The general trend documents the replacement of taxa adapted to

363

mesic grasslands with those adapted to xeric habitats. Other factors may have been influential,

364

particularly those related to the proportions and types of C3 vegetation (Quinn et al., 2013). This

365

supports our understanding that multiple factors facilitated the Koobi Fora Formation’s

366

ecosystem evolution from 2.0-1.4 Ma, with the regression of Paleolake Lorenyang of potentially

367

substantial contribution to these changes (Lepre et al., 2007; Lepre, 2014).

23 368

Our multiproxy analyses are just the latest of many approaches to studying the

369

spatiotemporal heterogeneity of the Omo-Turkana Basin. Together, analyses on multiple scales

370

and different proxies provide a more resolved view of these dynamic ecosystems. This is crucial

371

to understanding the ecological context and evolution of the mammals of eastern Africa during

372

this critical period.

373

24 374

Acknowledgments

375 376

This research was supported by an NSF-IRES grant to KO, the NSF Dissertation

377

Improvement Grant 1424203 and Wenner-Gren Fieldwork Grant to DBP, and OISE awards

378

1358178 and 1358200 and NSF Archaeology Program grant 1624398 to DRB. We would like to

379

thank past affiliates of the KFRTP including Kayla Allen, Lauren Anderson, W. Andrew Barr,

380

Alyssa Enny, Frances Forrest, Ashley Hammond, Laura Hunter, Jackson Kimambo, Silindokuhle

381

Mavuso, Emmanuel Ndiema, Kelvin Opondo, Jonathan Reeves, Chris Ssebuyungo, and R.

382

Brendon Zeller, along with Mikael Fortelius, Meave Leakey, and Deming Yang of the Turkana

383

Basin Institute, the National Museums of Kenya, and The George Washington University. We

384

further thank Joan Brenner-Coltrain, Brian Codding, L. Brock James, and Kaye Reed for their

385

advice and assistance, and Kay Behrensmeyer and Frank Brown, whose research contributions in

386

the Omo-Turkana Basin form the foundation of this project.

387 388

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118.

Table 1:

Family

Tribe Aepycerotini Alcelaphini Antilopini Bovidae Bovini Reduncini Tragelaphini Giraffini Giraffidae Sivatheriini Phacochoerini + Notochoerus Suidae Potamochoerini Equini Equidae Hipparionini

Analytic Taxon Aepyceros Alcelaphini Antilopini Bovini Reduncini Tragelaphus Giraffa Sivatherium Metridiochoerus + Notochoerus Kolpochoerus Equus Eurygnathohippus

Ileret Karari Koobi Fora U. U. U. Burgi KBS Okote Burgi KBS Okote Burgi KBS Okote 8 33 8 32 2 1 13 45 3 13 67 59 91 37 9 21 104 12 6 34 13 57 4 0 13 73 7 1 20 19 14 11 2 4 39 7 44 124 125 231 27 1 48 225 10 14 26 36 59 16 3 24 85 14 6 20 16 19 9 1 13 60 2 0 9 5 4 8 0 4 25 1 36 15 13 10

79 46 41 21

103 30 32 15

161 201 66 32

78 7 28 4

5 1 6 0

Table 2: Ileret Analytic Taxon Trend χ2trend Aepyceros ↘ 7.05 Alcelaphini 1.79 Antilopini 1.81 Bovini 3.21 Reduncini 0.30 Tragelaphus 0.18 Giraffa 0.03 Sivatherium 0.29 Metridiochoerus + Notochoerus 1.41 Kolpochoerus 1.75 Equus 0.25 Eurygnathohippus 2.20

Karari

Koobi Fora

ppvalue Trend χ2trend value Trend χ2trend 0.008 2.33 0.127 1.60 0.181 ↗ 18.68 <0.001 0.28 0.178 ↘ 8.16 0.004 0.20 0.073 ↗ 12.44 <0.001 4.67 0.582 ↘ 18.75 <0.001 3.41 0.674 0.79 0.373 0.02 0.856 2.70 0.100 1.20 0.587 0.72 0.397 0.02 0.235 0.185 0.614 0.138

↗ ↘ ↗

20.97 <0.001 40.42 <0.001 12.95 <0.001 2.53 0.112

↗

pvalue 0.206 0.597 0.651 0.031 0.065 0.899 0.274 0.880

2.01 0.156 0.01 0.932 2.44 0.118 13.82 <0.001

41 27 12 3

151 102 71 29

14 15 11 11

Highlights •

East Turkana was spatially and temporally variable from 2.0-1.4 Ma

•

Most ungulate abundance changes in East Turkana occurred in the Karari

•

Faunal changes coincide with the regression of Paleolake Lorenyang

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: