# Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context

Abigail K. Parker, Jean-Renaud Boisserie, Johannes Müller, Christopher A.

Brochu, Jason J. Head

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Abigail K. Parker, Jean-Renaud Boisserie, Johannes Müller, Christopher A. Brochu, Jason J. Head. Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context. Peer Community In Paleontology, 2025, 10.31233/osf.io/2p6e3_v3. hal-05461140

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RESEARCH ARTICLE

Body size histories of Shungura Formation
reptiles in biotic and abiotic environmental
context

Abigail K Parker<fnref n="1" />,<fnref n="2" /> ID, Jean-Renaud Boisserie<fnref n="3" />,<fnref n="4" /> ID, Johannes Müller<fnref n="5" /> ID, Christopher A Brochu<fnref n="6" /> ID & Jason J Head<fnref n="7" /> ID

**Cite as:** Parker A, Boisserie JR, Müller J, Brochu CA, and Head JJ (2025). Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context. *PaleoXiv* 2p6e3, ver. 3 peer-reviewed by PCI Paleo. DOI: 10.31233/osf.io/2p6e3_v3

Published: 4th September 2025

Recommender:

Andrew Farke

Reviewers:

John Rowan and Pedro Godoy

Correspondence:

abigail.parker@helsinki.fi

THIS ARTICLE WAS PEER-REVIEWED BY PCI PALEO

Read the Editor's recommendation and Referees' reports at DOI:10.24072/pci.paleo.100402

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Abstract

To investigate reptile body size as an ecological indicator and the relationships between size and environmental variables through time, we compared patterns of maximum size from the Plio-Pleistocene Shungura Formation of Ethiopia. For this previously-undescribed reptile fossil record, we provided estimates of body mass for pythonid snakes, aquatic pelomedusid and trionychid turtles, terrestrial testudinid turtles, whose carapace lengths reach over 1m, and crocodylians including *Crocodylus*, cf. *Mecistops*, and the tubulirostrine *Euthecodon*, which is the largest known crocodylian from the Early Pleistocene. Body size maxima in aquatic taxa correspond to lake level, with large body size observed in aquatic turtles, crocodylians, and hippopotamids during lake high stands on the north side of the Turkana Depression. However, these semi-aquatic groups display heterogenous trends over time and relationships to hydrologic proxies, indicating that their differential niches in these aquatic habitats were linked to different conditions and food resources. Terrestrial tortoises (Testudinidae) exceed 100 kg in mass in three members of the Shungura Formation, but are absent at large sizes between 2.3 Ma and 2.1 Ma, during the main period of hominin stone tool production. We tested for correlation between reptile maximum sizes, mammal maximum sizes and faunal metrics, δ¹⁸O and δ¹³C records from paleosols and mammal tooth enamel, and paleotemperature estimates across members of the Shungura Formation. After correction for multiple comparisons, no correlation tests between reptile size and paleoenvironment or mammal metrics are significant. However, high correlation coefficients between size maxima and paleosol δ¹⁸O values suggest temporal coincidence between size change and shifts in hydrological regimes and evaporation

<footnote n="1">¹ University of Cambridge, Department of Zoology and University Museum of Zoology – Cambridge, United Kingdom</footnote>

<footnote n="2">² University of Helsinki, Department of Computer Science - Helsinki, Finland</footnote>

<footnote n="3">³ Centre Français des Études Éthioppiennes, CNRS & Ministère de l'Europe et des affaires étrangères - Addis Ababa, Ethiopia</footnote>

<footnote n="4">⁴ Laboratoire Paléontologie Évolution Paléoécosystèmes Paléoprimatologie, UMR 7262 PALEVOPRIM, CNRS & Université de Poitiers – Poitiers, France</footnote>

<footnote n="5">⁵ Museum für Naturkunde – Berlin, Germany</footnote>

<footnote n="6">⁶ School of Earth, Environment, and Sustainability, University of Iowa – Iowa City, Iowa, USA</footnote>

<footnote n="7">⁷ University of Cambridge, Department of Zoology and University Museum of Zoology – Cambridge, United Kingdom</footnote>
levels. These results identify links between maximum size in reptiles and local environments that, if confirmed by trends in other settings, could be utilized in the future as paleoenvironmental proxies for terrestrial vegetation and aquatic habitats.

**Keywords:** reptile ; crocodylian ; turtle ; body size ; correlation
RÉSUMÉ

Afin d'envisager la taille corporelle des reptiles en tant que marqueur écologique et d'étudier
les relations entre ce proxy et différentes variable environnementales au cours du temps, nous
avons comparé les variations de taille maximale parmi des taxons non-squamates de la
Formation plio-pléistocène de Shungura en Éthiopie. A partir de ce registre fossile
herpétologique inédit, nous fournissons des estimations de la masse corporelle des serpents
pythonids, des tortues aquatiques pélomedusidés et trionychidés, des testudinidés terrestres
dont la longueur de la carapace atteint plus de 1m, et des crocodiliens, notamment *Crocodylus*,
des formes apparentées à *Mecistops*, et le genre tubulirostre *Euthecodon*, le plus grand
crocodilien connu du Pléistocène Ancien. Les tailles corporelles maximales chez les taxons
aquatiques sont corrélées aux niveaux lacustres : la taille corporelle des tortues aquatiques,
crocodiliens, et hippopotamidés augmente parallèlement à l'augmentation du niveau du lac au
nord de la Dépression Turkana. Cependant, ces taxons semi-aquatiques présentent des tendance
hétérogènes au cours du temps et des relations avec des indicateurs hydrologiques, indiquant
que leur niches différentielles dans ces habitats aquatiques étaient liées à des conditions et des
ressources alimentaires différentes. Les grandes tortues terrestres (Testudinidae) dépassent les
100 kg dans trois membres de Shungura, mais sont absentes entre 2,3 Ma et 2,1 Ma, de manière
concomitante à la production importante d'outils lithiques. Nous avons testé la corrélation entre
les tailles maximales des reptiles, les tailles maximales et les mensurations des mammifères,
les données de δ¹⁸O et δ¹³C des paléosols et de l'émail dentaire des mammifères, et les
estimations de paléotempérature pour les membres de la Formation de Shungura. Après
correction pour plusieurs comparaisons, aucun test de corrélation entre les tailles des reptiles et
les paramètres environnementaux ou mammifères n'est significatif. Néanmoins, des
coefficients de corrélation élevés entre les tailles maximales et les valeurs de δ¹⁸O des paléosols
suggèrent une congruence temporelle entre les changements de taille corporelle et les
changements de régime hydrologique et des niveaux d'évaporation. Nos résultats identifient
des liens entre la taille maximale des reptiles et les milieux localement disponibles qui, s'ils
sont confirmés par des tendances dans d'autres contextes, pourraient être utilisés à l'avenir
comme marqueurs paléoenvironnementaux de la végétation terrestre et des habitats aquatiques.

Mots clefs : Reptile ; Crocodilien ; Tortue ; Taille corporelle ; Corrélation
INTRODUCTION

Body size is a key ecological trait reflecting diet, locomotion, and life history strategies (Peters
1983; Schmidt-Nielsen 1984). For poikilotherms including reptiles, body size plays an
additional role in thermoregulation, specifically through its influence on the ratio of surface
area to volume which governs heat exchange, and influences metabolic rates (Ashton and
Feldman 2003; Makarieva et al. 2005; Rodrigues et al. 2018). Therefore, body size trends
within poikilothermic groups such as reptiles provide insight into ecological niche availability
over time and can potentially be used as paleoenvironmental proxies (Head et al. 2009, 2013;
Godoy et al. 2019: Stockdale and Benton 2021; Parker et al. 2023;). Body size distributions
within reptile communities may be useful indicators of paleohabitats, but they have rarely been
measured in the fossil record because environment reconstructions based on the terrestrial
vertebrate fossil record have relied mainly on mammalian faunal proxies (e.g. Hernández
Fernández & Vrba 2006; Cerling et al. 2011; Liu et al. 2012; Bibi & Kiessling 2015; Plummer
et al. 2015; Zliobaite et al. 2016). The Shungura Formation of the Omo Group (Lower Omo
River Valley, southwest Ethiopia) presents an excellent natural laboratory for using the reptile
fossil record to characterize the abiotic and biotic conditions underpinning body mass changes
in terrestrial and aquatic poikilotherms over time.

The Shungura Formation is a chronostratigraphically well-constrained and nearly
continuous section of fluviolacustrine sedimentary sequences that preserves a rich, high-
resolution record of faunal and environmental change during the Plio-Pleistocene (Arambourg
1948, Howell & Coppens 1974, Heinzelin 1983, Boisserie et al. 2008). The formation is divided
into 12 members identified as Basal, then A to L (excluding I) from oldest to youngest. The
depositional lithologies of these members represent changes in fluviolacustrine architecture on
the paleolandscape, most notably expansion and reduction in lake extent (Heinzelin 1983, Nutz
et al. in prep). While mammalian evolution and diversity in the Shungura Formation have been
described in detail, the only previous descriptions of the non-mammalian vertebrate faunas have
been in taxonomic works on fishes, turtles, and crocodylians (Arambourg 1948, de Broin 1979,
Tchernov 1986, Stewart and Murray 2008). Despite this limitation, the sedimentological
interpretations and densely-sampled isotopic and mammalian faunal records of the Shungura
Formation provide a unique opportunity to place changes in local reptile body size maxima in
the context of environmental change, as informed by geochemical and mammalian fossil
records. Here, we estimate the masses of the best-preserved reptile taxa from the Shungura
Formation— crocodylians, turtles, and snakes— and investigate their potential relationships to
biotic and abiotic factors in the environment of the Shungura Formation. We use the detailed time series of environmental and mammalian faunal changes in the Shungura sequence (Alemseged 2003; Bobe & Behrensmeyer 2004; Hernández Fernández & Vrba 2006; Cooke 2007; Boisserie et al. 2010; Passey et al. 2010; Levin 2015; Plummer et al. 2015; Blondel et al. 2018; Negash et al. 2020; Bibi 2023) to place body size histories of Shungura Formation reptiles within the context of local climate and community composition. We compare the maximum size time series for reptiles with mass estimates for various large mammal groups from Bibi and Cantalapiedra (2023) to test whether co-occurring reptiles and mammals exhibited common size trends, indicating shared environmental pressures, or inverse relationships, which could indicate competition or opposing habitat preferences.

Shungura Formation turtle body size histories at community scales were previously examined to estimate paleotemperature and paleoprecipitation using models relating modern turtle community body size to climate (Parker et al. 2023). These results indicated that, although climate variables are only loosely linked to size in modern communities, these models can provide information about relative environmental change through time based on the Shungura record. The tests for correlation with geochemical proxies and mammal faunas we present here expand on that study to identify factors influencing reptile size maxima, which can be evaluated in other contexts in the fossil record to better understand what conditions are necessary for the evolution of large body sizes.

## MATERIAL AND METHODS

### THE SHUNGURA FORMATION REPTILE FOSSIL RECORD

We examined reptile fossils derived from field expeditions, primarily the International Omo Research Expedition (IORE, Howell & Coppens 1974) and Omo Group Research Expedition (OGRE, Boisserie et al. 2008, 2010) housed in the National Museum of Ethiopia (NME)/ Ethiopian Heritage Authority (EHA, ex-Authority for Research and Conservation of the Cultural Heritage) in Addis Ababa. Some of the largest turtle specimens discovered in the field were not collected due to resource constraints and were measured *in situ* during OGRE field campaigns.

Although mostly undescribed, the reptile fossil record of the Shungura Formation is composed of similar taxa to the more extensively studied records from more southern Neogene and Pleistocene localities in the Turkana Depression (Harris et al. 2003; Storrs 2003; Wood
2003; Brochu 2020; Head & Müller 2020). The fossil record of turtles consists of semi-aquatic trionychid taxa, including specimens referrable to *Cycloderma* (Fig. 1A) and *Trionyx* (Parker et al. 2023), pelomedusids, including specimens comparable to *Pelusios* (Fig. 1B), and fully terrestrial testudinids (Fig. 1C), including giant specimens comparable to *Centrochelys* (Parker et al. 2023). Among examined squamates, *Python* (Fig. 1D) is represented by precloacal vertebrae. Other squamates, including *Varanus*, and small-bodied snakes, are not included here due to their sparse record with prohibitively small sample sizes. Crocodylians from the Shungura include cranial remains referable to the extremely tubulirostrine genus *Euthecodon* (Fig. 1F) and members or close relatives of the extant longirostrine genus *Mecistops* (Fig. 1E). We refer to the latter as cf. *Mecistops* here, as some specimens were previously *Crocodylus cataphractus*, a synonym of *Mecistops cataphractus*, the extant African sharp-nosed or slender-snouted crocodile (McAliley et al. 2006), but note that they are morphologically distinct from modern *Mecistops* and their taxonomic assignment is tentative pending phylogenetic analysis (Brochu *et al. in prep*). Additionally, numerous brevirostrine specimens (Fig. 1D) are similar to species of *Crocodylus* from other eastern African sequences (Brochu 2020). Species diagnoses within the genus *Crocodylus* of the Plio-Pleistocene are subject to ongoing revision (Storrs 2003; Brochu 2001, 2020; Brochu *et al.* 2010; Brochu & Storrs 2012). Cranial specimens that we identify as *Crocodylus* here include specimens comparable to the robust species *C. thorbjarnarsoni*, but further taxonomic study is required to determine whether other species from the genus are also present in the Shungura Formation.
<figure><img src="image_9.png" /><figcaption>FIG. 1. — Representative specimens of examined reptile taxa from the Shungura Formation. **A**, *Cycloderma* sp. (OMO 229) carapace in dorsal view; **B**, *Pelusios* sp. (L 3-10, F-3, ca. 2.31 Ma) carapace in dorsal view; **C**, Testudinidae indet. (OMO 18/inf-10074, C-4-8, ca. 2.57-2.76 Ma) right humerus in dorsal/capitular view; **D**, *Python* sp. (OMO 340-10193, L-2, ca. 1.34 Ma) precloacal vertebra in anterior view; **E**, cf. *Mecistops* (OMO 372-10016, upper G, ca 2.057-1.911 Ma) partial rostrum in palatal view.</figcaption></figure>
F, *Euthecodon brumpti* (IORE collections unnumbered) skull in dorsal view. *Crocodylus* sp. (L 875-1, F-3, ca. 2.32 Ma) partial skull in dorsal view. Scale bars equal 5 cm for A-C, E-G, 1 cm for D.

Occurrences and abundances of reptile taxa are not uniformly distributed throughout the members of the Shungura Formation. Member G, spanning 2.27 Ma to 1.91 Ma, is divided into “lower G” and “upper G”, which have yielded distinct fossil assemblages, likely due to the lacustrine system present during the deposition of upper G. Therefore, we have considered these two levels separately alongside the other members. Including all specimens cataloged from 1967-1976 (IORE) and 2006-2018 (OGRE), the highest sampling is from Member E (2.39 Ma-2.33 Ma), with 699 reptile specimens in total (nearly 12 % of the total fossil record for this member). Over 100 specimens have also been collected from members C (2.94 Ma-2.54 Ma), F (2.32 Ma-2.29 Ma), and lower G (2.27 Ma-2.06 Ma). Sampling of turtles is also sufficient for the oldest members A (3.60 Ma-3.44 Ma) and B (3.44-2.94 Ma), with pelomedusid specimens recovered from every member from B through upper G (2.06 Ma-1.91 Ma). However, trionychid fossils complete enough for body size estimation have only been collected from four of those members, B, E, F and upper G. Only 1-2 turtle specimens are known from each member from H (1.91 Ma-1.78 Ma) through L (1.38 Ma-1.09 Ma). Abundances of crocodylians collected vary greatly across members, with 572 specimens from Member E, but zero from A and under 10 for B, D (2.53 Ma-2.39 Ma), H, and J (1.76 Ma-1.56 Ma). No occurrences of *Euthecodon* are recorded from members A and D, while *Crocodylus* is absent from members A, B, and H. Cf. *Mecistops* specimens are known from members C, F through J, and L.

## BODY SIZE ESTIMATION

We estimated body length and body mass for each specimen and determined the maximum reconstructed body mass for turtle and crocodylian groups in each member of the Shungura Formation. For turtles, we used carapace length as the metric for body length. For incomplete specimens, we estimated carapace length using linear regressions from other element measurements based on complete specimens (Supplementary Table 1). We used clade-specific linear regressions to estimate body mass from these carapace lengths. These linear regressions (Table 1) are derived from the data in Regis & Meik (2017).

For *Python*, we used the regression for total body length from the width between the prezygapophyses from McCartney et al (2018); this regression is based on a sample of extant snakes, and the equation is ln(Total Body Length) = 1.095*ln(x) + 4.528, where x is the trans-
prezygapophyseal width in mm. Then, we used the regression for body mass from total length for Pythonidae from Feldman & Meiri (2013).

<table><caption>TABLE 1. – Regression equations used to estimate body mass (in kg, unless noted otherwise) from body lengths for turtles and crocodylians.</caption><thead><tr><th>Taxon</th><th>Measurement, x (cm)</th><th>Regression to Body Mass</th><th>Regression Source</th></tr></thead><tbody><tr><td>Testudinidae</td><td>Carapace length</td><td>BM = 2.751*log10(x) -3.424</td><td>Regression derived from data in Regis and Meik (2017)</td></tr><tr><td>Trionychidae</td><td>Carapace length</td><td>log10(BM) = 1.344*log10(x) -1.049</td><td>Regression derived from data in Regis and Meik (2017)</td></tr><tr><td>Pelomedusidae</td><td>Carapace length</td><td>log10(BM) = -3.814 + 2.861*log10(x)</td><td>Regression derived from data in Regis and Meik (2017)</td></tr><tr><td>Crocodylia</td><td>Total length</td><td>log10(BM) = -4.67 + 2.79*log10(x)</td><td>Slavenko et al. (2016)</td></tr><tr><td>Python</td><td>Total length</td><td>log10(BM) = -5.131 + 2.611*log10(x) [mass unit: g]</td><td>Feldman and Meiri (2013)</td></tr></tbody></table>

For crocodylians, we used regressions relating skull length to total body length or body mass for extant taxa (Hutton 1987; Webb & Messel 1978; Thorbjarnarson 1988; Verdade 2000; Sereno *et al.* 2001; Iijima *et al.* 2016). For incomplete specimens, we estimated skull length using regressions from postcranial or mandibular measurements (Supplementary Table 2), or, in the case of vertebral specimens, estimated total length using the regressions from vertebral centrum length of Iijima and Kubo (2020, Supplementary Table 2).

For cf. *Mecistops*, identifiable cranial fragments most frequently comprise the anterior premaxilla and maxilla or the mandibular symphysis of the dentary. We relate the measurement of the narrowest width of the premaxilla, across the diastema between the premaxillary and maxillary alveoli, to the total skull length, based on the ratio observed in the complete specimen NME L 398 2508A (Supplementary Table 2). Similarly for mandible specimens, we estimate skull length using the ratio between the length of the mandibular symphysis and skull length observed in NME L 398 2508A and 2508B (Supplementary Table 2). Because Shungura specimens identified here as cf. *Mecistops* have broader snouts relative to skull length than either of the two extant species of *Mecistops* (*M. cataphractus* and *M. leptorhynchus*; Brochu *et al.* *in prep.*), this method for total skull length measurement is better practice than estimating from a regression on modern specimens (for reference, the equation $SL = 10.719*x + 10.225$
relates the narrowest premaxilla width of five specimens of *Mecistops* housed at the Natural History Museum, London to total skull length).

For *Euthecodon*, most specimens from the Shungura Formation are rostral fragments. We developed an equation relating the spacing between maxillary or mandibular tooth alveoli to total skull length (Fig 2). We excluded measurements between the anteriormost two alveoli because those teeth are spaced further apart than those along the rest of the snout, which may be due to ontogenetic exclusion of the second premaxillary tooth by the large third tooth, which occurs early in ontogeny, leaving four premaxillary teeth (Brochu 2021). We then estimated total body length for *Euthecodon* based on the relationship between dorsal cranial length (DCL) and total body length (TL) for the extant tubulirostrine crocodylian *Gavialis gangeticus*, $TL = -69.369 + 7.4*(DCL)$ (Sereno *et al.* 2001).

*Crocodylus* cranial specimens from the Shungura Formation have intermediate snout lengths, with similar snout to skull length ratios to the extant saltwater crocodile *C. porosus*, so we used the equation trained on that extant species from Sereno *et al.* (2001): $TL = -20.224 + 7.717*(DCL)$. Using these two different equations to estimate total body length takes into account the difference in the proportion of body length made up by the skull in crocodylians with varying snout morphologies. We used a single regression equation (Table 1) to estimate body mass from body length (Slaveno *et al.* 2016). No species-specific body mass regressions have been published for extant tubulirostrine crocodylians, but the mass regression for general Crocodylia performed consistently and conservatively in estimating crocodylian mass in comparison to species-specific regressions (Webb & Messel 1978; Chabreck & Joanen 1979; Hutton 1987; Thorbjarnarson 1996).
<figure><img src="image_10.png" /><figcaption>FIG. 2. – Estimation of skull length for fragmentary specimens of *Euthecodon*. **A**, partial mandible of *E. brumpti* (L 32-203, C 5-7, c. 2.6-2.7 Ma) in dorsal view, scale equals 5 cm; **B**, tooth spacing along the rostra of four complete skulls used to train the regression, plotted as points, with smoothed curves showing how alveolar spacing changes along each snout, with the anteriormost alveoli the greatest distances apart; **C**, the linear regression equation used to relate that average tooth spacing value (excluding the anteriormost two alveoli) to dorsal skull length, based on the best-fit line through points representing the average tooth spacing of each dentary and maxilla in the four complete skulls. Abbreviations: **Ant**, anterior; **ER**, East Rudolf; **KNM**, National Museums of Kenya; **KP**, Kanapoi; **LT**, Lothagam; **Pos**, posterior.</figcaption></figure>

## MAMMAL AND ENVIRONMENT DATA SOURCES

We compiled mammal body size and environmental proxy datasets from the literature to compare with the trends of Shungura Formation maximum reptile size, including maximum size for mammalian taxa, reconstructed lake level in the Turkana Depression, paleosol carbon and oxygen isotopes, faunal composition of mammalian herbivores, and herbivore carbon and oxygen isotopes. We used member-level stratigraphic resolution for all data.

We took estimates of maximum mammalian body masses for each Shungura member using the methods and data in Bibi & Cantalapiedra (2023), which use loglinear regressions between tooth length and body mass trained on extant species with masses in the Pantheria database (Jones *et al.* 2009). We applied regressions specific to each order of mammals and each tooth position (Bibi & Cantalapiedra 2023). We applied these regressions to obtain mass estimates
for all of the Shungura specimens in the Mammal Dental Metrics Database (Bibi 2023). Because this database includes no proboscidean specimens from the Shungura Formation, we avoided the issues with order-specific regressions for Proboscidea noted by Bibi & Cantalapiedra (2023). We also followed their method for removing duplicates by averaging mass estimates for individual specimens for which multiple teeth were measured. From the dataset, we removed specimens that were not attributable to a specific Shungura member, and subsequently took the maximum mass from each group in each member. For Equidae, most specimens were described prior to the differentiation of Member G into upper/lower units (Hooijer 1975); we assigned these specimens to lower G, to which the majority of the terrestrial mammal specimens in Member G date (Bibi 2023). The bovid tribe Tragelaphini was well-sampled across members, unlike other tribes, so we also tested for correlation with its maximum sizes. We excluded Rhinocerotidae due to low sampling in this dataset. For Hippopotamidae, we instead used unpublished mass estimates based on astragali measurements (522 specimens measured by JRB) using Martinez and Sudre (1995)'s equation for terrestrial cetartiodactyls. They belong to three distinct lineages: aff. *Hippopotamus protamphibius* and its likely successor aff. *Hippopotamus karumensis* (see Harris 1991), the dominant hippopotamus lineage in the Shungura Formation, which is endemic to the Turkana Depression; aff. *Hippopotamus aethiopicus*, a pygmy hippopotamid (Coryndon & Coppens 1975) possibly related to aff. *Hip. protamphibius/karumensis* but of much smaller size, known only in the upper part of the sequence (from Member G to Member L); and *Hippopotamus* sp., less frequent that the other hippopotamids in the Omo Valley during the Plio-Pleistocene and distinguished by particular massive and wide astragali differing from the more slender build of the postcranials belonging to the other lineages. Today *Hippopotamus* shares semi-aquatic habitats with crocodylians and turtles. The largest Shungura specimens belong to *Hippopotamus* sp. and to aff. *Hippopotamus karumensis*, which also displays morphological features (high orbits) and a geochemical signature (δ¹⁸O; Harris *et al.* 2008) indicating semiaquatic behavior.

We used the reconstruction of lake levels in the Turkana Depression by Nutz *et al.* (2020), which is based on study of the sedimentary formations of the Nachukui Formation, south of the Shungura Formation on the western bank of Lake Turkana. This lake level record is suited for comparison to ecological and environmental changes in the region, although lake level change is at least partially diachronous between the Shungura and Nachukui Formations (Lepre 2014). We used member average lake level values on a relative scale based on the curve presented in fig. 15 of Nutz *et al.* (2020). Differential patterns of lake extent between the west and north shores of paleo-lakes in Turkana are minimal relative to the temporal durations included in
these averaged member ages. For an additional metric of Shungura Formation-specific lake extent, we quantified the depositional environments of each member using the coding provided by Heinzelin (1983), which scored each unit's facies in the following scheme: 0 for ephemeral streams, 1-4 for fluviatile facies (from channel to levee, floodplains, and swamps), 5 for mudflats, 6 for deltaic, and 7-8 for lacustrine (nearshore to deeper water). For each member, we used the maximum score in this scheme across units as a metric for the presence of lake conditions in the member.

Paleosol isotope data for both carbon and oxygen comes from the measurements of pedogenic carbonates compiled by Levin *et al.* (2011). This dataset includes 49 samples of Shungura paleosols dated between 3.2 Ma-1.18 Ma. We calculated the average δ¹⁸O and δ¹³C values for each member. We additionally drew on published data for mammalian tooth enamel δ¹⁸O and δ¹³C, based on isotopic ratios measured for over 1,000 dental specimens from nine herbivore families by Negash *et al.* (2020). We use member-average values for all specimens as a metric for the dietary composition of all mammalian herbivores, derived from a subsample of this dataset only including stable carbon isotopic values of specimens for which element identification is documented in the Omo Database, and attributed to a M2 or M3 tooth.

We used two Shungura Formation mammalian faunal composition datasets. Bobe & Behrensmeyer (2004) defined a subset of mammals that they took to be grassland indicators, including bovids in the families Alcelaphini and Antilopini, the suid *Metridiochoerus, Equus, and Theropithecus oswaldi*. We used the proportion of mammal fossils from each member falling in this group as a metric for grassland habitat, although these raw fossil abundances may not accurately represent the relative abundances of taxa in the true paleocommunity due to filtering in taphonomy and collection biases (Bobe & Behrensmeyer 2004). However, with relatively constant taphonomic conditions and collections procedures, changes in abundance values across time bins represent shifts in the relative proportion of taxa, regardless of their absolute abundances. The second dataset (Bobe *et al.* 2007) provided an alternative faunal proxy for the presence of grasslands, measuring separately the percentage of mammal fossils from each member belonging to reduncin bovids and bovids in the clades Antilopini, Alcelaphini, and Hippotragini ("AAH bovids"). Reduncin bovids indicate open but moist habitats with fresh grass, while the AAH bovids today live only in at least seasonally dry grasslands or bushlands (Levin 2015).

Finally, we used two numeric estimates of paleoenvironmental variables sampled across Shungura members. Passey *et al.* (2010) provided estimates of soil temperatures based on measurements of clumped isotopes in pedogenic carbonates. Hernández Fernández & Vrba
(2006) reconstructed mean annual precipitation at sites including individual Shungura members based on models of mammal community structures trained on modern sites. Fig. 3 summarizes all data series used as paleoenvironmental proxies to which we compared the maximum body size time series.

<figure><img src="image_11.png" /><figcaption>FIG. 3. – Schematic summarizing all paleoenvironmental variables used in analyses of correlation with reptile body sizes (colored ovals), excluding paleo-lake level (Nutz et al. 2020) and depositional lake scores (Heinzelin 1983), which were estimated independently through sedimentological synthesis. The data series are color-coded by their sources. Faded lines run from each variable (colored ovals) to the concept for which it acts as a proxy. Components influencing the paleosol isotopic ratios are linked to them by solid lines, as are the processes which filter those isotopic ratios in fossil material relative to paleosol values.</figcaption></figure>

## CORRELATION ANALYSES

To test for coordinated patterns of maximum size change over time between groups and with environmental variables, we used correlation tests. For all body mass variables, we used log₁₀ transformation of mass estimates in kilograms. We employed standard Pearson product-moment correlation tests (rcorr function from the R package Hmisc). We also ran Spearman's
rank correlation tests, appropriate for non-normal data, for comparisons including data series whose values were not normally distributed. We also tested for correlation between maximum size in each reptile group and fossil abundance by member for all reptiles and for only turtles or crocodylians. Abundance counts came from the catalog of specimens identified to these groups collected in the Shungura Formation between 1967 and 2023. These correlation tests reveal whether maximum size is higher in members where reptiles are more frequently preserved or more intensely sampled.

Because we ran many tests for pairwise correlation between the same set of data series, it was necessary to correct for multiple comparisons (Wright 1992). For each set of correlation tests whose results are presented in Tables 2-6, we used sequential Bonferroni correction to adjust the significance cutoff for the resulting p-values. This correction takes into account that some of a large set of pairwise statistical tests will have p-values under 0.05 by chance, rather than due to true significance. It adjusts the significance level for the pairwise test with the lowest p-value by dividing it by the number of tests, then adjusts the significance level for the next lowest p-value to (0.05 / n(comparisons) - 1), and so on, until the next lowest p-value does not fall under the adjusted significance level (Holm 1979).

It is important to note that correlation tests do not take into account the order of the data points and, therefore, cannot test hypotheses of causal effects of one variable on another over time. Tests differentiating correlative and causal relationships between time series require more data points than are available from sampling at the member level within the Shungura Formation (Reitan and Liow 2019; Sugihara et al. 2012). However, we were able to test whether two time series change together between members using correlation tests between the first differences of the series' values. These tests use the value of a variable in one member subtracted from its value in the next member, calculated iteratively for each data series. Correlation between first differences shows that the magnitude and direction of change between bins for two series is coordinated across the time series. The results for first difference correlation are more informative than the correlation tests on raw values because first differencing detrends the data series; if independent directional trends exist in two series over time, significant correlation in raw variable values is likely even without any causal relationship between the variables.

ABBREVIATIONS

NME/EHA National Museum of Ethiopia/ Ethiopian Heritage Authority (ex-Authority for Research and Conservation of Cultural Heritage).
# RESULTS

## SIZE ESTIMATES

While some body size estimates reported here fall within the range of local extant faunas, there are extraordinary size maxima present in the Shungura Formation. For crocodylians (*Euthecodon*) and tortoises, the Turkana Depression is home to the largest representatives of those groups anywhere in the world during the Early Pleistocene. Notably, the estimated mass of *Euthecodon* sp. from Shungura Member L of 2,300 kg makes it the largest Early Pleistocene reptile globally. The large body size of *Euthecodon* in the Turkana Depression contributes to a global trend of increased mean crocodylian body size during the Plio-Pleistocene (Godoy *et al.* 2019; Godoy and Turner 2020). The high diversity of crocodylians occurring in Turkana during this period indicates the availability of multiple distinct niches, including macro-predatory ones, for these semi-aquatic carnivores within a large lake system (Drumheller & Wilberg 2020; Scheyer *et al.* 2013). Of the specimens we attribute to *Crocodylus*, the largest has an estimated mass just over 1,000 kg (Member F), while the largest cf. *Mecistops* measured is slightly smaller, at an estimated 860 kg (Member L).

The *in situ* tortoises from Shungura Members E and H, with preserved carapace lengths of 110 and 100 cm, respectively, are the largest known testudinids from the Early Pleistocene of Africa. Tortoises from 1 m-1.7 m in length are present continuously in eastern Africa from the Early Miocene through the Middle Pleistocene (de Lapparent de Broin 2000). Thereafter, these massive tortoises survived only in Madagascar into the Holocene (Bour 1984). Freshwater turtles in Africa have maintained a maximum body size of 40 cm-60 cm since the Oligocene (Meylan *et al.* 1990; Pérez-García 2019). Most soft-shelled turtle specimens (Trionychidae) from the Shungura Formation fall within this range, but the largest partial specimen, from Member B, is larger, with an estimated carapace length of 71 cm. Modern species of *Pelusios* range in size from 12 cm-55 cm carapace length (Itescu *et al.* 2014), so the Shungura Formation pelomedusids, whose maximum sizes center around 30 cm, fall around the middle of this range. Similar to *Euthecodon* and cf. *Mecistops*, the largest pelomedusid specimen is from Member L, with a carapace length of 42 cm.

Size estimates for *Python* are consistent with maxima of extant African *P. sebae* and *P. natalensis* (e.g. Pitman 1974, Alexander 2018). A single vertebra from Member F (OMO 33-3613), with trans-prezygapophyseal width 52.1 mm, represents an estimated body length of 7
m, larger than reliable estimates for modern African pythons and equivalent to the largest verifiable lengths of any extant snake species (Murphy and Henderson 1997).

## SIZE PATTERNS THROUGH TIME

The three crocodylian groups measured have relatively similar body length estimates in the early Members of the Shungura Formation (452 cm for cf. *Mecistops*, Member C, 300-500 cm for *Crocodylus* from members A-D, and ~600 cm for *Euthecodon* from Members B-F). All three groups increase in size in Member F, where the largest *Crocodylus* specimen, a robust partial skull (OMO 221-1973-2716) was found. From 2 Ma-1 Ma, the maximum size pattern for *Crocodylus* diverges from that of *Euthecodon*, as *Euthecodon* increases in size into Member L, while *Crocodylus* body length decreases to under 5 m in members K and L. In the final member, cf. *Mecistops* (OMO 341-10040) exceeds the *Crocodylus* in size, which is not observed outside of Member L. Differences in postcranial morphologies, which could indicate how mass per body length varies between groups, have not been studied in these fossil taxa; therefore, we applied the same length to mass regressions for all three groups. Such unknown variation in mass per length between crocodylians makes trends within each of these groups over time more reliable than absolute differences between groups (Fig. 4).
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
FIG. 4. — Maximum body size of crocodylians (A), turtles (B), and *Python* (C) by member in the Shungura Formation in comparison with sampling based on specimen counts by member from Omo collections.

Testudinids have estimated size maxima over 50 kg in all members where they are recorded: B, C, D, E, G, and H. Tortoises are figured as absent in Member F because no specimens complete enough for size estimation have been collected; the only tortoise material known from F is small fragmentary material from an excavation in unit F-0 (2.34-2.321 Ma). In lower G, the only known tortoise specimen comes from unit G-13 (2.062-2.072 Ma), so there is an apparent ~200,000 year gap in the presence of large terrestrial turtles during that interval, despite high sampling of reptiles in members F and G (Fig. 4).

Aquatic turtle maximum size is greatest in the members where trionychids occur— B, E, F, and upper G—with the overall largest aquatic turtle estimated at 28 kg in Member B.

Pelomedusid maximum size remains approximately constant across the Formation, with its minimum occurring at around 2 kg in upper G and its maximum of 6.8 kg in Member L.

The largest estimated body masses for *Python* come from the middle of the Shungura sequence, in Member F and upper G. Python maximum size is lowest, under 3 m in length (or 10 kg in mass), in Members B, E, and J. In the remaining members, typical *Python* maximum length is between 4 m-5.5 m.
## CORRELATION WITH SAMPLING

Maximum body size across the Shungura Formation is not correlated to reptile sampling across members (Fig. 4). Table 2 lists correlation coefficients testing for relationships between maximum size in each group and the number of occurrences per member of all reptiles, crocodylians, and turtle specimens, as well as correlation between maximum size and member age. No significant correlation is observed with time; maximum size in all groups both increases and decrease across the sequence. *Crocodylus* has near-significant positive correlation with crocodylian sampling. However, there is a highly negative correlation coefficient for the relationship between trionychid maximum size and turtle sampling.

<table><caption>TABLE 2. – Correlation tests between the maximum sizes of Shungura Formation reptiles and sampling (overall reptile or order-specific) or member age. Each cell contains the Pearson correlation coefficient and the p-value for the test. For the sampling comparisons (data series with non-normal distributions), each cell also shows the Spearman rank correlation coefficient and corresponding p-value. Significant results after sequential Bonferroni correction are marked with **. P-values under 0.05 which are not significant after Bonferroni correction are marked with *, and correlation coefficients over 0.75 are marked in bold.</caption><thead><tr><th>Taxon</th><th>Reptile sampling</th><th>Turtle sampling</th><th>Crocodylian sampling</th><th>Squamate sampling</th><th>Time</th></tr></thead><tbody><tr><td>Aquatic turtles</td><td>0.217, p= 0.605<br>0.071, p= 0.867</td><td>0.24, p= 0.567<br>0.171, p= 0.686</td><td></td><td></td><td>-0.229, p= 0.586</td></tr><tr><td>Trionychidae</td><td><b>-0.933</b>, p= 0.067<br>-0.8, p= 0.2</td><td><b>-0.907</b>, p= 0.093<br>-0.738, p= 0.262</td><td></td><td></td><td>-0.578, p= 0.422</td></tr><tr><td>Pelomedusidae</td><td>0.132, p= 0.755<br>0.548, p= 0.16</td><td>0.109, p= 0.798<br>0.366, p= 0.373</td><td></td><td></td><td>0.284, p= 0.495</td></tr><tr><td>Testudinidae</td><td>0.595, p= 0.159<br>0.45, p= 0.31</td><td>0.697, p= 0.082<br>0.414, p= 0.355</td><td></td><td></td><td>0.024, p= 0.96</td></tr><tr><td>Euthecodon</td><td>-0.406, p= 0.215<br>-0.073, p= 0.831</td><td></td><td>-0.448, p= 0.167<br>-0.087, p= 0.8</td><td></td><td>0.336, p= 0.313</td></tr><tr><td>Crocodylus</td><td>0.574, p= 0.065<br>0.45, p= 0.165</td><td></td><td>0.552, p= 0.078<br>0.626, p= 0.04*</td><td></td><td>0.238, p= 0.48</td></tr><tr><td>Cf. Mecistops</td><td>0.4, p= 0.433<br>0.543, p= 0.266</td><td></td><td>0.426, p= 0.399<br>0.429, p= 0.397</td><td></td><td>0.189, p= 0.719</td></tr><tr><td>Python</td><td>0.003, p= 0.994<br>0.25, p= 0.516</td><td></td><td></td><td>0.227, p= 0.558</td><td>0.141, p= 0.718</td></tr></tbody></table>
## CORRELATION BETWEEN GROUPS AND WITH PALEOENVIRONMENTAL VARIABLES

No tests for pairwise correlation between reptile groups' sizes show result in significant correlation after correction for multiple comparisons (Table 3). The observed correlation coefficient between the maximum sizes of terrestrial tortoises and *Python* (0.997) is the highest. High negative correlation coefficients are observed between trionychid turtle size and the size of both *Crocodylus* and terrestrial tortoises.

<table><caption>TABLE 3. – Correlation between maximum size of reptile groups sampled in the Shungura Formation across the members of the formation. Each cells contains the Pearson correlation coefficient and the p-value for the test. **A** shows results for pairwise correlation tests between log₁₀ mass values and **B** shows results for pairwise correlation tests for the first differences in those values between members, not including results for Trionychidae because the sampled specimens come from too few consecutive members to calculate correlation in their first differences. NA indicates comparison with too few observations of change between bins in common between the two series to run correlation test. P-values under 0.05 which are not significant after Bonferroni correction are marked with *, and correlation coefficients over 0.75 are marked in bold.</caption><thead><tr><th></th><th>Crocodylus</th><th>Cf. Mecistops</th><th>Pelomedusidae</th><th>Testudinidae</th><th>Trionychidae</th><th>Python</th></tr></thead><tbody><tr><td>Euthecodon</td><td>-0.276, p= 0.44</td><td>0.513, p= 0.298</td><td>0.25, p= 0.551</td><td>-0.464, p= 0.295</td><td>0.555, p= 0.445</td><td>0.38, p= 0.313</td></tr><tr><td>Crocodylus</td><td></td><td>-0.209, p= 0.691</td><td>-0.257, p= 0.538</td><td>-0.139, p= 0.793</td><td><strong>-0.85</strong>, p= 0.15</td><td>-0.22, p= 0.569</td></tr><tr><td><strong>A</strong> Cf. Mecistops</td><td></td><td></td><td>0.41, p= 0.492</td><td>0.283, p= 0.817</td><td>NA</td><td>0.547, p= 0.262</td></tr><tr><td>Pelomedusidae</td><td></td><td></td><td></td><td>0.674, p= 0.142</td><td>-0.401, p= 0.599</td><td>-0.172, p= 0.684</td></tr><tr><td>Testudinidae</td><td></td><td></td><td></td><td></td><td><strong>-0.88</strong>, p= 0.315</td><td>-0.12, p= 0.82</td></tr><tr><td>Trionychidae</td><td></td><td></td><td></td><td></td><td></td><td>-0.286, p= 0.714</td></tr><tr><td>Euthecodon</td><td>-0.239, p= 0.568</td><td>NA</td><td>0.238, p= 0.65</td><td>0.102, p= 0.87</td><td>NA</td><td>0.086, p= 0.914</td></tr><tr><td>Crocodylus</td><td></td><td>NA</td><td>-0.079, p= 0.881</td><td>0.141, p= 0.859</td><td>NA</td><td>-0.216, p= 0.784</td></tr><tr><td><strong>B</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pelomedusidae</td><td></td><td></td><td></td><td><strong>0.826</strong>, p= 0.174</td><td>NA</td><td>0.165, p= 0.835</td></tr><tr><td>Testudinidae</td><td></td><td></td><td></td><td></td><td>NA</td><td><strong>0.997</strong>, p= 0.051</td></tr></tbody></table>

After correction for multiple comparisons, none of the relationships between reptile size and paleoenvironmental or mammalian faunal variables are significant. The sequential Bonferroni correction reduces the significance cutoff for correlation tests within Table 4 (comparing reptile and mammal sizes) to 0.0009; none of the observed p-values are below this threshold which
applies to the pairwise test with the highest significance. Therefore, the correlations denoted with * in Tables 4-6 do not exceed the number that are expected to exhibit that level of correlation by chance alone. However, they do identify which reptile and mammal taxa show the relatively strongest covariance in maximum size over time. The highest correlation coefficient in Table 4A is for a negative relationship between maximum size of *Crocodylus* and aff. *Hippopotamus aethiopicus* (the pygmy hippos), while both cf. *Mecistops* and *Python* maximum size are positively related to aff. *Hip. aethiopicus*. The next strongest relationship is positive, between size in pelomedusid turtles and *Hippopotamus sp*.

No significant correlation is observed between change between members in maximum size of reptiles and mammals (Table 4B). The highest positive correlation coefficient there is between aquatic turtles and aff. *Hippopotamus protamphibius/karumensis*.

<table><caption>TABLE 4. – Correlation statistics for reptile masses vs. mammal masses by taxon (Trag, tragelaphins; Cerco, cercopithecids; *aHpk*, aff. *Hip. protamphibius/karumensis*; *aHa*, aff. *Hip. aethiopicus*; *Hsp.*, *Hippopotamus* sp.. **A** shows results for pairwise correlation tests between log₁₀ mass values and **B** shows results for pairwise correlation tests for the first differences in those values between members. The columns marked (S) has non-normally-distributed data, so the correlation test results shown in (A) are the Spearman rank correlation coefficient and corresponding p-value. The cercopithecid, aff. *Hip. aethiopicus*, *Hippopotamus* sp., and trionychid data series are excluded from (B) because they include too few consecutive members sampled. NA indicates other comparisons with too few observations of change between bins in common between the two series to run correlation test. P-values under 0.05 which are not significant after Bonferroni correction are marked with *, and correlation coefficients over 0.75 are marked in bold. Mammal data from Bibi (2023), Bibi and Cantalapiedra (2023), except for *Hippopotamidae* (this study).</caption><thead><tr><th></th><th>Equidae</th><th>Bovidae</th><th>Trag (S)</th><th>Cerco</th><th>aHpk (S)</th><th>aHa</th><th>Hsp.</th></tr></thead><tbody><tr><td>Euthecodon</td><td>-0.42<br>p= 0.349</td><td>-0.559<br>p= 0.074</td><td>-0.37<br>p= 0.293</td><td>0.215<br>p= 0.683</td><td>0.027<br>p= 0.937</td><td>-0.143<br>p= 0.819</td><td>-0.227<br>p= 0.714</td></tr><tr><td>Crocodylus</td><td>-0.575<br>p= 0.177</td><td>0.407<br>p= 0.214</td><td>0.541<br>p= 0.106</td><td>0.473<br>p= 0.421</td><td>0.36<br>p= 0.277</td><td><strong>-0.965</strong><br>p= 0.035*</td><td>-0.158<br>p= 0.8</td></tr><tr><td>cf. Mecistops</td><td>0.427<br>p= 0.573</td><td>-0.557<br>p= 0.251</td><td>0.1<br>p= 0.873</td><td>0.587<br>p= 0.413</td><td>0.143<br>p= 0.787</td><td><strong>0.809</strong><br>p= 0.4</td><td>-0.074<br>p= 0.906</td></tr><tr><td><strong>A</strong> Aquatic turtles</td><td>-0.292<br>p= 0.526</td><td>0.238<br>p= 0.57</td><td>-0.536<br>p= 0.215</td><td>0.487<br>p= 0.406</td><td>0.119<br>p= 0.779</td><td>NA</td><td>-0.489<br>p= 0.511</td></tr><tr><td>Trionychidae</td><td>0.16<br>p= 0.84</td><td>0.208<br>p= 0.792</td><td>-0.6<br>p= 0.4</td><td>NA</td><td>-0.2<br>p= 0.8</td><td>NA</td><td>NA</td></tr><tr><td>Pelomedusidae</td><td>0.522<br>p= 0.229</td><td>-0.262<br>p= 0.532</td><td>0.357<br>p= 0.432</td><td>0.326<br>p= 0.593</td><td>0.119<br>p= 0.779</td><td>NA</td><td><strong>0.949</strong><br>p= 0.051</td></tr><tr><td>Testudinidae</td><td>0.392<br>p= 0.443</td><td>-0.31<br>p= 0.498</td><td>0.179<br>p= 0.702</td><td>-0.255<br>p= 0.745</td><td>-0.071<br>p= 0.879</td><td>NA</td><td>NA</td></tr><tr><td>Python</td><td>-0.222<br>p= 0.632</td><td>-0.114<br>p= 0.771</td><td>0.095<br>p= 0.823</td><td>-0.151<br>p= 0.809</td><td>0.083<br>p= 0.831</td><td><strong>0.914</strong><br>p= 0.267</td><td>-0.722<br>p= 0.168</td></tr><tr><td><strong>B</strong> Euthecodon</td><td>0.127<br>p= 0.81</td><td>-0.468<br>p= 0.172</td><td>-0.073<br>p= 0.852</td><td></td><td>0.048<br>p= 0.895</td><td></td><td></td></tr></tbody></table>
<table><tr><td><i>Crocodylus</i></td><td>-0.558<br>p= 0.25</td><td>0.326<br>p= 0.392</td><td>0.42<br>p= 0.301</td><td>0.274<br>p= 0.476</td></tr><tr><td>Aquatic turtle</td><td>-0.373<br>p= 0.466</td><td>0.061<br>p= 0.908</td><td>0.112<br>p= 0.833</td><td>0.719<br>p= 0.108</td></tr><tr><td>Pelomedusidae</td><td>-0.132<br>p= 0.803</td><td>0.296<br>p= 0.569</td><td>0.122<br>p= 0.819</td><td>-0.629<br>p= 0.181</td></tr><tr><td>Testudinidae</td><td>-0.323<br>p= 0.677</td><td>-0.006<br>p= 0.993</td><td>0.035<br>p= 0.955</td><td>-0.498<br>p= 0.393</td></tr><tr><td>Python</td><td>0.642<br>p= 0.358</td><td>-0.644<br>p= 0.356</td><td>0.696<br>p= 0.304</td><td>0.472<br>p= 0.528</td></tr></table>

Among the comparisons with paleoenvironmental variables (Table 5), there is one correlation test that is significant after sequential Bonferroni correction: a positive relationship between the maximum size of aff. *Hippopotamus protamphibius/karumensis* and paleosol δ¹⁸O. Among the other correlation tests with high correlation coefficients, paleosol δ¹⁸O displays a positive relationship to size in both *Pelomedusidae* and *Hippopotamus sp.* Pelomedusid size is negatively related to paleosol δ¹³C. Three groups display high negative correlation coefficients with lake level in Table 5A: pelomedusids, *Hippopotamus sp.*, and Equidae. Equid size is also negatively related to the depositional environment lake scores of Heinzelin (1983). Finally in Table 5A, soil temperature estimates (Passey *et al.* 2010) are negatively related to the size of *Crocodylus* and aquatic turtles and positively related to tragelaphin bovid size.

Table 5B shows correlation in change between consecutive members in size series and environmental proxies. There, the highest observed positive correlation coefficients are between size in *Python* and paleosol δ¹⁸O, and between aquatic turtle size and lake level (Fig. 8). The greatest negative correlation coefficient is between pelomedusid size and δ¹³C.
<table><caption>TABLE 5. – Correlation statistics of reptile and mammal maximum sizes with a) environmental variables across members of the Shungura Formation. **A** shows results for pairwise correlation tests between log₁₀ mass values and **B** shows results for pairwise correlation tests for the first differences in those values between members, not including results for Trionychidae, Cercopithecidae, or soil temperature because those series have too few consecutive members sampled to calculate correlation in their first differences. Each cell contains the Pearson correlation coefficient and p-value for the correlation test. The row marked (S) has non-normally-distributed data, so the correlation test results shown are the Spearman rank correlation coefficient and corresponding p-value. The cercopithecid, *Hippopotamus* sp., and soil temperature data series are excluded from (B) because they include too few consecutive members sampled. NA indicates other comparisons with too few observations of change between bins in common between the two series to run correlation test. P-values under 0.05 which are not significant after Bonferroni correction are marked with *, p-values which are significant after Bonferroni correction are marked with **, and correlation coefficients over 0.75 are marked in bold. Data from Levin (2015), Nutz *et al.* (2020), Passey *et al.* (2010), and Heinzelin (1983).</caption><thead><tr><th>A</th><th>Paleosol δ<sup>13</sup>C</th><th>Paleosol δ<sup>18</sup>O</th><th>Lake level</th><th>Soil temperature</th><th>Depositional Lake Score</th></tr></thead><tbody><tr><td>Euthecodon</td><td>-0.082<br>p= 0.822</td><td>0.373<br>p= 0.289</td><td>0.224<br>p= 0.533</td><td>0.202<br>p= 0.745</td><td>0.685<br>p= 0.02*</td></tr><tr><td>Crocodylus</td><td>-0.129<br>p= 0.742</td><td>-0.05<br>p= 0.898</td><td>0.192<br>p= 0.594</td><td><strong>-0.824</strong><br>p= 0.176</td><td>0.378<br>p= 0.252</td></tr><tr><td>cf. Mecistops</td><td>-0.302<br>p= 0.621</td><td>0.307<br>p= 0.615</td><td>0.028<br>p= 0.964</td><td>NA</td><td>0.053<br>p= 0.921</td></tr><tr><td>Testudinidae</td><td>0.019<br>p= 0.968</td><td>0.252<br>p= 0.63</td><td>-0.721<br>p= 0.067</td><td>-0.142<br>p= 0.82</td><td>-0.542<br>p= 0.208</td></tr><tr><td>Aquatic Turtles</td><td>0.028<br>p= 0.952</td><td>-0.108<br>p= 0.818</td><td>0.654<br>p= 0.111</td><td><strong>-0.834</strong><br>p= 0.166</td><td>0.098<br>p= 0.818</td></tr><tr><td>Trionychidae</td><td>-0.232<br>p= 0.768</td><td>-0.667<br>p= 0.535</td><td>0.476<br>p= 0.524</td><td>NA</td><td>0.587<br>p= 0.413</td></tr><tr><td>Pelomedusidae</td><td><strong>-0.751</strong><br>p= 0.052</td><td><strong>0.832</strong><br>p= 0.02*</td><td><strong>-0.762</strong><br>p= 0.047</td><td>-0.207<br>p= 0.793</td><td>0.063<br>p= 0.883</td></tr><tr><td>Python</td><td>0.106<br>p= 0.803</td><td>0.005<br>p= 0.99</td><td>0.052<br>p= 0.903</td><td>0.76<br>p= 0.24</td><td>0.003<br>p= 0.995</td></tr><tr><td>aff. Hip. protamphibius-<br>karumensis (S)</td><td>0.588<br>p= 0.08</td><td><strong>0.952</strong><br>p&lt;0.0001**</td><td>0.172<br>p= 0.614</td><td>0.2<br>p= 0.783</td><td>0.33<br>p= 0.294</td></tr><tr><td>aff. Hip. aethiopicus</td><td>0.292<br>p= 0.708</td><td>-0.229<br>p= 0.711</td><td>0.133<br>p= 0.867</td><td>NA</td><td>-0.224<br>p= 0.718</td></tr><tr><td>Hippopotamus sp.</td><td>0.323<br>p= 0.677</td><td><strong>0.815</strong><br>p= 0.185</td><td><strong>-0.901</strong><br>p= 0.099</td><td>NA</td><td>-0.002<br>p= 0.997</td></tr><tr><td>Equidae</td><td>-0.297<br>p= 0.518</td><td>-0.522<br>p= 0.288</td><td><strong>-0.825</strong><br>p= 0.022*</td><td>0.186<br>p= 0.814</td><td><strong>-0.805</strong><br>p= 0.029*</td></tr><tr><td>Cercopithecidæ</td><td>-0.455<br>p= 0.441</td><td>0.629<br>p= 0.181</td><td>0.637<br>p= 0.248</td><td>0.537<br>p= 0.639</td><td>0.232<br>p= 0.658</td></tr><tr><td>Bovidae</td><td>-0.6<br>p= 0.067</td><td>-0.471<br>p= 0.17</td><td>-0.108<br>p= 0.752</td><td>0.168<br>p= 0.787</td><td>-0.411<br>p= 0.185</td></tr><tr><td>Tragelaphini (S)</td><td>-0.273<br>p= 0.448</td><td>0.167<br>p= 0.678</td><td>-0.418<br>p= 0.203</td><td><strong>0.9</strong><br>p= 0.083</td><td>0.005<br>p= 0.989</td></tr></tbody></table>
<table><caption>TABLE 5. – Following.</caption><thead><tr><th>B</th><th>Paleosol δ<sup>13</sup>C</th><th>Paleosol δ<sup>18</sup>O</th><th>Lake level</th><th>Depositional Lake Score</th></tr></thead><tbody><tr><td>Euthecodon</td><td>-0.096<br>p= 0.807</td><td>0.032<br>p= 0.941</td><td>0.017<br>p= 0.965</td><td>0.671<br>p= 0.034*</td></tr><tr><td>Crocodylus</td><td>-0.092<br>p= 0.845</td><td>0.272<br>p= 0.555</td><td>0.127<br>p= 0.765</td><td>0.387<br>p= 0.303</td></tr><tr><td>Aquatic turtles</td><td>0.255<br>p= 0.625</td><td>0.323<br>p= 0.596</td><td><strong>0.813</strong><br>p= 0.049*</td><td>0.024<br>p= 0.964</td></tr><tr><td>Pelomedusidae</td><td><strong>-0.79</strong><br>p= 0.061</td><td>-0.201<br>p= 0.746</td><td>-0.704<br>p= 0.119</td><td>-0.049<br>p= 0.926</td></tr><tr><td>Python</td><td>0.488<br>p= 0.512</td><td><strong>0.803</strong><br>p= 0.406</td><td>-0.65<br>p= 0.35</td><td>-0.707<br>p= 0.293</td></tr><tr><td>Testudinidae</td><td>-0.582<br>p= 0.304</td><td>0.322<br>p= 0.791</td><td>-0.737<br>p= 0.155</td><td>-0.31<br>p= 0.611</td></tr><tr><td>aff. Hip. protamphibius-<br>karumensis</td><td>0.237<br>p= 0.54</td><td><strong>0.895</strong><br>p= 0.003*</td><td>0.495<br>p= 0.145</td><td>-0.034<br>p= 0.92</td></tr><tr><td>aff. Hip. aethiopicus</td><td>NA</td><td>NA</td><td>NA</td><td>-0.436<br>p= 0.713</td></tr><tr><td>Equidae</td><td>0.575<br>p= 0.233</td><td>0.633<br>p= 0.252</td><td>-0.365<br>p= 0.477</td><td>-0.518<br>p= 0.292</td></tr><tr><td>Bovidae</td><td>-0.455<br>p= 0.219</td><td>0.319<br>p= 0.442</td><td>0.003<br>p= 0.993</td><td>-0.325<br>p= 0.33</td></tr><tr><td>Tragelaphini</td><td>0.054<br>p= 0.89</td><td>0.927<br>p= 0.003</td><td>-0.107<br>p= 0.768</td><td>0.182<br>p= 0.615</td></tr></tbody></table>

There are several groups whose maximum size is nearly correlated with mammalian faunal environmental proxies (Table 5A), though not after Bonferroni correction. Mammalian herbivore δ¹⁸O is positively related to body size in *Cercopithecidae*, *Pelomedusidae*, and *Hippopotamus* sp. Aff. *Hippopotamus protamphibius/karumensis* size is higher when the AAH % indicating open-habitat bovids is also high. Also, aff. *Hip. protamphibius/karumensis* size is strongly positively related to the grassland indicator proportion of Bobe & Behrensmeyer (2004). Mean annual precipitation from Hernández-Fernández & Vrba (2006) is negatively related to size in *Cercopithecidae*.

As for correlation in change between members (Table 6B), we observed high negative correlation coefficients between mammalian herbivore δ¹³C and size in aquatic turtles and aff. *Hip. protamphibius/karumensis*. The relationship to herbivore δ¹³C was positive for size in aff.
*Hip. aethiopicus*. Pelomedusid size increased when herbivore δ¹⁸O increased (Fig. 8). Changes in percentage of AAH bovids were negatively related to size change in *Pelomedusidae*, positively related to size change in aff. *Hip. protamphibius/karumensis*, and negatively related to size change in aff. *Hip. aethiopicus*. The latter two relationships were reversed with respect to the Reducine bovid proportion; the smaller aff. *Hip. aethiopicus* increased in size as the moist/open-habitat-specialist reduncins became more numerous, while the larger endemic hippopotamid lineage increased in size as the more dry-adapted AAH bovids came to predominate. Aff. *Hip. aethiopicus* size change had a highly negative correlation coefficient vs. the proportion of grassland indicator mammals, as did size change in *Pelomedusidae*. Size change in testudinid tortoises had a correlation coefficient of 0.999 with change in the mean annual precipitation estimates. Pelomedusid size change was also positively related to those precipitation changes. Although none of these relationships are significant, each instance of high correlation indicates where two series change over time in a coordinated manner, possibly due to similar environmental factors affecting both.
<table><caption>TABLE 6. – Correlation statistics of reptile and mammal maximum sizes with faunal variables across members of the Shungura Formation. A shows results for pairwise correlation tests between log₁₀ mass values and B shows results for pairwise correlation tests for the first differences in those values between members, not including results for Trionychidae and Cercopithecidae because the sampled specimens come from too few consecutive members to calculate correlation in their first differences. Each cell contains the Pearson correlation coefficient and p-value for the correlation test. The row and column marked (S) have non-normally-distributed data, so the correlation test results shown are the Spearman rank correlation coefficient and corresponding p-value. P-values under 0.05 which are not significant after Bonferroni correction are marked with *, p-values which are significant after Bonferroni correction are marked with **, and correlation coefficients over 0.75 are marked in bold. Data from Bobe and Behrensmeyer (2004), Hernández Fernández and Vrba (2006), Levin (2011), and Negash *et al.* (2020).</caption><thead><tr><th>A</th><th>Herbivore δ<sup>13</sup>C (S)</th><th>Herbivore δ<sup>18</sup>O</th><th>AAH % (S)</th><th>Reduncin %</th><th>Grassland indicator proportion</th><th>Faunal mean annual precipitation</th></tr></thead><tbody><tr><td>Euthecodon</td><td>0.591<br>p=0.061</td><td>-0.054<br>p=0.875</td><td>0.1<br>p=0.769</td><td>0.238<br>p=0.482</td><td>0.157<br>p=0.645</td><td>0.258<br>p=0.622</td></tr><tr><td>Crocodylus</td><td>-0.077<br>p=0.821</td><td>0.048<br>p=0.889</td><td>0.061<br>p=0.867</td><td>0.036<br>p=0.921</td><td>-0.038<br>p=0.916</td><td>-0.179<br>p=0.734</td></tr><tr><td>cf. Mecistops</td><td>0.029<br>p=1</td><td>0.191<br>p=0.717</td><td>0.086<br>p=0.919</td><td>-0.186<br>p=0.724</td><td>-0.137<br>p=0.796</td><td>0.375<br>p=0.755</td></tr><tr><td>Aquatic Turtles</td><td>-0.619<br>p=0.115</td><td>-0.392<br>p=0.337</td><td>0.539<br>p=0.168</td><td>0.043<br>p=0.919</td><td>0.123<br>p=0.772</td><td>0.324<br>p=0.531</td></tr><tr><td>Trionychidae</td><td>-0.4<br>p=0.75</td><td>-0.645<br>p=0.355</td><td>0.4<br>p=0.75</td><td><strong>0.915</strong><br>p=0.085</td><td>-0.021<br>p=0.979</td><td><strong>1</strong><br>p=0.006*</td></tr><tr><td>Pelomedusidae</td><td>0.31<br>p=0.462</td><td><strong>0.857</strong><br>p=0.007*</td><td>-0.395<br>p=0.332</td><td>0.37<br>p=0.367</td><td>-0.027<br>p=0.949</td><td>0.206<br>p=0.695</td></tr><tr><td>Python</td><td>0.05<br>p=0.912</td><td>-0.194<br>p=0.617</td><td>0.05<br>p=0.898</td><td>-0.38<br>p=0.313</td><td>-0.044<br>p=0.911</td><td>-0.23<br>p=0.66</td></tr><tr><td>Testudinidae</td><td>NA</td><td>0.55<br>p=0.201</td><td>-0.649<br>p=0.115</td><td>-0.104<br>p=0.824</td><td>-0.433<br>p=0.332</td><td>0.085<br>p=0.891</td></tr><tr><td>aff. Hip. protamphibius-karumensis (S)</td><td>0.462<br>p=0.134</td><td>0.35<br>p=0.266</td><td>0.743<br>p=0.0089*</td><td>0.533<br>p=0.091</td><td><strong>0.882</strong><br>p=0.0006*</td><td>-0.543<br>p=0.297</td></tr><tr><td>aff. Hip. aethiopicus</td><td>0.7<br>p=0.233</td><td>-0.492<br>p=0.4</td><td>0.2<br>p=0.783</td><td>0.31<br>p=0.611</td><td>-0.297<br>p=0.627</td><td>NA</td></tr><tr><td>Hippopotamus sp.</td><td>-0.1<br>p=0.95</td><td><strong>0.769</strong><br>p=0.128</td><td>-0.1<br>p=0.95</td><td>0.696<br>p=0.192</td><td>0.561<br>p=0.326</td><td>NA</td></tr><tr><td>Equidae</td><td>-0.286<br>p=0.556</td><td>0.305<br>p=0.507</td><td>-0.342<br>p=0.452</td><td>-0.656<br>p=0.109</td><td><strong>-0.78</strong><br>p=0.038*</td><td>0.575<br>p=0.232</td></tr><tr><td>Cercopithecidae</td><td>-0.029<br>p=1</td><td><strong>0.971</strong><br>p=0.001*</td><td>0.628<br>p=0.173</td><td>-0.045<br>p=0.933</td><td>0.553<br>p=0.255</td><td><strong>-0.864</strong><br>p=0.136</td></tr><tr><td>Bovidae</td><td>-0.431<br>p=0.162</td><td>-0.092<br>p=0.777</td><td>-0.237<br>p=0.482</td><td>-0.456<br>p=0.158</td><td>-0.247<br>p=0.465</td><td>0.138<br>p=0.794</td></tr><tr><td>Tragelaphini (S)</td><td>-0.064<br>p=0.86</td><td>0.6<br>p=0.056</td><td>-0.334<br>p=0.345</td><td>-0.286<br>p=0.424</td><td>-0.103<br>p=0.785</td><td>-0.543<br>p=0.297</td></tr></tbody></table>
<table><caption>TABLE 6. – Following.</caption><thead><tr><th>B</th><th>Herbivore δ<sup>13</sup>C</th><th>Herbivore δ<sup>18</sup>O</th><th>AAH %</th><th>Reduncin %</th><th>Grassland Indicator Proportion</th><th>Faunal Mean Annual Precipitation</th></tr></thead><tbody><tr><td>Euthecodon</td><td>0.098<br>p= 0.787</td><td>-0.056<br>p= 0.878</td><td>0.356<br>p= 0.312</td><td>-0.303<br>p= 0.394</td><td>0.371<br>p= 0.291</td><td>0.465<br>p= 0.43</td></tr><tr><td>Crocodylus</td><td>-0.258<br>p= 0.502</td><td>0.109<br>p= 0.781</td><td>0.342<br>p= 0.406</td><td>-0.47<br>p= 0.24</td><td>0.372<br>p= 0.364</td><td>0.247<br>p= 0.689</td></tr><tr><td>Testudinidae</td><td>0.638<br>p= 0.247</td><td>0.506<br>p= 0.385</td><td>-0.532<br>p= 0.356</td><td>0.628<br>p= 0.257</td><td>-0.611<br>p= 0.273</td><td><strong>0.999</strong><br>p= 0.035*</td></tr><tr><td>Aquatic turtles</td><td><strong>-0.825</strong><br>p= 0.043*</td><td>-0.432<br>p= 0.393</td><td>0.648<br>p= 0.164</td><td>-0.517<br>p= 0.293</td><td>0.624<br>p= 0.185</td><td>0.079<br>p= 0.9</td></tr><tr><td>Pelomedusidae</td><td>0.505<br>p= 0.306</td><td><strong>0.796</strong><br>p= 0.058</td><td><strong>-0.845</strong><br>p= 0.034</td><td>0.693<br>p= 0.127</td><td><strong>-0.856</strong><br>p= 0.029*</td><td><strong>0.766</strong><br>p= 0.131</td></tr><tr><td>Python</td><td>0.553<br>p= 0.447</td><td>-0.084<br>p= 0.916</td><td>0.227<br>p= 0.773</td><td>-0.373<br>p= 0.627</td><td>0.211<br>p= 0.789</td><td>0.559<br>p= 0.622</td></tr><tr><td>aff. Hip. protamphibius-karumensis</td><td><strong>-0.791</strong><br>p= 0.004*</td><td>-0.478<br>p= 0.137</td><td><strong>0.86</strong><br>p= 0.001*</td><td><strong>-0.79</strong><br>p= 0.007**</td><td>0.742<br>p= 0.014*</td><td>0.363<br>p= 0.548</td></tr><tr><td>aff. Hip. aethiopicus</td><td><strong>0.755</strong><br>p= 0.455</td><td>-0.555<br>p= 0.626</td><td><strong>-0.988</strong><br>p= 0.097</td><td><strong>0.941</strong><br>p= 0.219</td><td><strong>-0.954</strong><br>p= 0.193</td><td>NA</td></tr><tr><td>Equidae</td><td>0.175<br>p= 0.741</td><td>0.011<br>p= 0.983</td><td>0.045<br>p= 0.933</td><td>-0.591<br>p= 0.217</td><td>0.231<br>p= 0.66</td><td>-0.158<br>p= 0.799</td></tr><tr><td>Bovidae</td><td>-0.382<br>p= 0.247</td><td>-0.116<br>p= 0.734</td><td>-0.012<br>p= 0.973</td><td>-0.072<br>p= 0.843</td><td>0.012<br>p= 0.975</td><td>0.18<br>p= 0.772</td></tr><tr><td>Tragelaphini</td><td>-0.514<br>p= 0.129</td><td>0.194<br>p= 0.591</td><td>0.482<br>p= 0.189</td><td>-0.643<br>p= 0.062</td><td>0.654<br>p= 0.056</td><td>0.333<br>p= 0.584</td></tr></tbody></table>

DISCUSSION

Body size maxima for these groups are not artefacts of uneven sampling across members because there are no cases of significant positive correlation between size and member-specific occurrence counts of reptile specimens. The negative correlation between trionychid size and sampling increases our confidence that the absence of this taxon from some members is not due to insufficient sampling of turtles in those intervals. However, positive correlation between *Crocodylus* size and crocodylian sampling indicates that maximum size in this group, which includes size estimates based on postcrania that may have been inconsistently collected, may be partially related to sampling effort. Apart from this case, low correlation with sampling
metrics suggests that sampling throughout the formation, while very uneven for reptiles, is not
driving the size trends observed across members. Collection bias against reptiles does not
appear to be size-selective. If anything, larger reptiles are generally more likely to be collected
due to ease of identification relative to, for example, small squamate specimens (although we
note that the very largest specimens, such as intact tortoise carapaces and one cranium of
*Euthecodon*, are left *in situ* due to resource constraints). However, the very low sample counts,
particularly in the most recent members, mean that the maximum sizes reconstructed there may
not accurately reflect the actual maximum sizes in the paleocommunities of every member.

Maximum size trends in the crocodylians and turtles of the Shungura Formation point to
local-scale environmental influences on body size in these reptiles. The two dominantly
terrestrial reptile taxa, Testudinidae and *Python*, have high (though non-significant) correlation
in change in size maxima between members (Table 3B). Fig. 5 plots these size changes against
paleosol carbon isotopes and mammalian herbivore faunal metrics, both providing information
about the openness of vegetation over time. With respect to habitat preferences, extant tortoises
are frequently found in open areas within tropical forest or savannas (Brattstrom 1961). Modern
giant tortoises, which survive only on oceanic islands, have a grazing diet and help to maintain
open habitats known as “tortoise turf” (Falcón & Hansen 2018; Hansen & Galetti 2009).
However, it is unknown whether their ecologies shifted after colonization of those islands from
continental mainlands (Hansen *et al.* 2010). The presence of large tortoises in Member C of the
Shungura Formation, which is reconstructed as having a relatively higher tree cover compared
to later members, suggests that these testudinids were not restricted to open habitats in the
Pleistocene. However, their maximum size increased into Member H coincident with increasing
paleosol δ¹³C values and the proportion of grazing mammals, which suggests that newly open
environments were also then suitable for giant terrestrial tortoises, as they are today. Maximum
size change in tortoises is positively correlated with change in the faunally-derived estimates of
paleoprecipitation by member (Hernández-Fernández & Vrba, 2006, Table 6B). Across the
sequence, tortoise size increased as mammalian herbivore faunas shifted away from being arid-
adapted, a result which undermines the putative open-habitat association of tortoises, although
increased evaporative water loss indicated by δ¹⁸O records may have maintained open habitats
during intervals with higher estimated paleoprecipitation. Modern African testudinids require
at least somewhat open habitats because they use basking behavior for thermoregulation, so we
interpret this pattern as indicating that, in the Shungura sequence, tortoise size most likely
increased during intervals with higher rainfall but also some open environments available.
*Python* is a habitat generalist within African ecosystems, and the relationship between size and environment is not well understood for its largest species. Here, the strongest environmental correlate for size change in *Python* is a positive relationship with pedogenic δ¹⁸O (Table 5B). Pedogenic δ¹⁸O is influenced by the isotopic composition of rainwater and increases to less negative values with more evaporative water loss from soils. δ¹⁸O changes over time with different sources of water input to the Omo River, including direct precipitation (low δ¹⁸O), and high levels of evapotranspiration increase δ¹⁸O. Therefore, this result represents a contrast between precipitation histories reconstructed by faunal proxies, which we observed to change with the tortoise and snake body size series, and isotopic proxies, which indicate *Python* size increase concurrent with shifts to more evaporative water loss from the landscape. Tests of association between tortoise and snake size and isotopic values at other eastern African sites could confirm whether the largest representatives of these groups preferentially inhabited wetter or drier environments.

Tortoises over one meter in length were present in the Turkana Depression from the Late Miocene (at Lothagam, Wood 2003) until the Middle Pleistocene (personal observations in West Turkana, Turkana Basin Institute Turkwel collections). However, no tortoises of this size are documented between 2.3 Ma and 2.1 Ma; this age range is well-sampled for turtles in the Shungura Formation, yet no large tortoises at all have been discovered in Member F or lower G below unit G-13 (though lower G has nearly 16,000 vertebrate remains collected, making it the richest sequence of the formation). The NME/EHA collections include very fragmentary material likely attributable to Testudinidae collected during a 1971-1973 excavation in unit F-0; while too fragmentary for use in any body size regression, these specimens appear to be of tortoises much less than 1 m in length. This temporal gap in the record of giant tortoises aligns with the timing of the Shungura Formation's archaeological record, which is concentrated in Member F and lower G (noted in Fig. 5, Delagnes et al. 2011; Maurin et al. 2014). These stone tools indicate hominin occupations at several locations in the vicinity of the Omo River. The earliest presence of archeological lithic remains is in F-1, after which no tortoises have been recovered from Member F. The next tortoise in age appears in unit G-12. In situ lithics are known from unit G-13, but not thereafter, when tortoise size rebounds to over 1 m/100 kg (Member H, Fig. 5). In the Early Pleistocene and thereafter, there is evidence of consumption of tortoise meat by hominins (Blasco et al. 2011; Klein & Cruz-Uribe 2016; Stiner et al. 2000), with reduction in tortoise body sizes observed globally coincident with the spread of Pleistocene humans (Joos et al. 2022). Large tortoises are easy hunting for humans, and individually contain plentiful meat, so may have been preferentially butchered by humans, who display preferences
toward catching larger prey (Ben-Dor and Barkai 2021). However, without any direct evidence of tortoise butchery in eastern Africa from this time period, we cannot currently determine whether the gap in the record of large tortoises in F and (most of) lower G relates to hominin occupation. Testudinid fossils from diverse localities await further study, from which a high-resolution regional time series could then be assembled to reveal what environmental conditions and interactions restrict tortoise size.

<figure><img src="image_15.png" /><figcaption>FIG. 5. — Maximum testudinid and *Python* body mass by member in the Shungura Formation vs. paleosol $\delta^{13}$C sample values (Levin 2011) and the percentage of Reduncini and AAH (Antilopini, Alcelaphini, and Hippotragini) bovids in each member's mammal fauna (Bobe *et al.* 2007).</figcaption></figure>

Among aquatic turtles, trionychids are consistently larger than pelomedusids (Fig. 6). Both trionychid and pelomedusid turtles today inhabit permanent bodies of water and derive their diets primarily from aquatic animals and plants (Akani *et al.* 2001), though the pelomedusid species *Pelusios niger*, with a maximum carapace length of around 35 cm, comparable to many of the Shungura Formation pelomedusid specimens, incorporates a higher proportion of terrestrial vertebrate prey items than smaller-bodied species of *Pelusios* (Luiselli *et al.* 2021).
Extant *Trionyx triunguis* has also been observed scavenging carcasses of herbivorous mammals, though fish and frog meat are much larger components of their omnivorous diets (Akani *et al.* 2001). With functional links to terrestrial and freshwater habitats, trait structure observed in terrestrial and aquatic turtles can inform environmental conditions where fossil communities lived (Conley & Samuels 2022; Parker *et al.* 2023).

Across the Turkana Depression, trionychid specimens of around half a meter in length have been collected at many Plio-Pleistocene sites. Extant *Trionyx triunguis* which inhabits the Omo and Lake Turkana can reach over 100 cm in carapace length (Taskavak & Akcinar 2009), but body sizes in the 40 cm-60 cm range reported here from the Shungura Formation would also be common in extant populations. To understand whether maximum body size in this species, or size distributions within populations, have changed over time, more comprehensive collection of trionychid fossil material is necessary. Past field collections have had a strong bias against collection of fragmentary turtle material, although it is relatively abundant. If the gaps in the trionychid record from all members except for Basal, B, E, F, and upper G are true absences, and not due to collection bias, then the lack of large trionychids could provide information about the depositional environments preserved from those ages.

The positive correlation coefficients between overall aquatic turtle maximum size and lake level (Table 5B, Figs. 6, 8) suggest that large size in these turtles is dependent on habitat availability. Aquatic turtle size maxima are driven by the presence of trionychids in members B, E, F, and lower G, all of which sample lake high stands. Where lacustrine environments in the Lower Omo Valley had smaller volumes, large-bodied trionychids may have avoided those areas in favor of other areas of the Turkana Depression where lake volume or food resources were higher. In contrast, pelomedusid body size displays negative correlation with lake level, potentially suggesting that the smaller-bodied aquatic turtle group was more successful in smaller water bodies (Table 5A). Trionychid body size is also negatively related to the size of terrestrial tortoises and *Crocodylus*, the latter of which is a possible predator of turtles. Pelomedusid size change, while not dramatic across the sequence, also shows links to terrestrial environmental proxies. It is negatively correlated with paleosol $\delta^{13}$C (Table 5B) and shows positive correlation with herbivore $\delta^{18}$O and the faunal precipitation estimates (Table 6B). Again, these non-significant correlation results present mixed signals as to whether reptile size maxima relate to more open/arid environments and mammal faunas, or the opposite. However, the relationship between $\delta^{18}$O and pelomedusid size (Figs. 6, 8) indicates that this trait is responding to changes in regional hydrology. The changes in sources of water input reflected in soil $\delta^{18}$O could correspond to differences in nutrient availability or water temperature
relevant to these aquatic turtles' niches. Overall aquatic turtle size is higher in members where Passey et al. (2010)'s soil temperature estimates are lower.

<figure><img src="image_16.png" /><figcaption>FIG. 6. – Maximum body size of aquatic turtles (trionychid or pelomedusid specimens indicated by color) by member in the Shungura Formation vs. reconstructed relative lake level in the Turkana Depression (Nutz et al. 2020), δ¹⁸O values from paleosol samples (Levin et al. 2011), and δ¹⁸O values from mammal herbivore dental samples (Negash et al. 2020).</figcaption></figure>

The hippopotamid lineage aff. *Hip. protamphibius-karumensis* has size maxima significantly correlated to paleosol δ¹⁸O (Table 5), providing evidence that these semi-aquatic mammals were responding to regional hydrologic changes. Additionally, this hippopotamid lineage' size maxima occurred concurrently with higher AAH percentage in mammal communities, which reflects the abundance of dry grass-adapted bovids (Table 6). This indicates that these hippopotamids increased in size during intervals with higher evaporative water loss and coincident with the spread of grassier habitats. This echoes the proposed relation
between C₄ grassland developments and the success of hippopotamines (Boisserie *et al.* 2011), and the dental enamel δ¹³C results from Harris *et al.* (2008) indicating a nearly pure C₄ diet for this lineage. However, with respect to δ¹³C, the lineage aff. *Hip. protamphibius-karumensis* has a negative correlation coefficient in our results, while the smaller aff. *Hip. aethiopicus* has high size maxima in more C₄-enriched members (Table 6B). Such opposite relationships for these two hippopotamid groups are also seen in correlation with overall mammal herbivore δ¹⁸O (Table 6B), suggestive of niche partitioning between the hippopotamids. *Hippopotamus* sp. is not consistently present in the Shungura sequence, but where it is, size maxima are negatively related to lake level and positively related to paleosol δ¹⁸O (Table 5A). This result suggests that the third group of hippopotamids, who are generally present in the region during lake high stands, seem to decrease in size when lake area expands and evaporation decreases.

Another semi-aquatic group whose evolution to large size is of interest in the Shungura Formation are the otters, a very large (~200 kg) species of which was present in members B and C and which may have competed with crocodylians for aquatic or terrestrial prey (Grohé *et al.* 2022). The gigantism observed in both *Euthecodon* and this otter suggests the presences of high resource and habitat availability for aquatic vertebrate carnivores in the Omo Valley during several intervals of the Shungura Formation’s deposition. In particular for *Euthecodon*, large aquatic range sizes would be necessary to support a population of these extremely large carnivores (Scheyer *et al.* 2013). We propose that these body size increases are due to indeterminate growth exhibited by these reptiles; in permissive habitats with abundant food and/or limited mortality, individuals reached larger sizes.

The three crocodylian morphotypes considered, the extremely tubulirostine *Euthecodon*, *Crocodylus*, and cf. *Mecistops*, have both similarities and differences in their maximum size changes over time (Fig. 7). Both *Euthecodon* and cf. *Mecistops* attain their overall size maxima in the most recent Shungura member. Both *Crocodylus* and *Euthecodon* increase in size from members E-F. From Member F onwards, there is an apparent decrease in *Crocodylus* size. Specimens from members B, D, and F (OMO 28-1968-3213, L 40-29, and OMO 221-1973-2716) are all extremely robust cranial fragments, likely comparable to *C. thorbjarnarsoni* (Brochu & Storrs 2012), while specimens from later members have unresolved taxonomy and attain smaller size maxima. Looking at high-resolution temporal sequences like this with multiple crocodylians present can help untangle dynamics of their niche partitioning and faunal turnover in the context of paleoenvironments and prey species presence (Gardin *et al.* 2024). In our tests, there were few instances where high correlation coefficients were observed between crocodylian size maxima and environmental proxies. *Crocodylus* size showed negative
correlation with the size of terrestrial tortoises and aff. *Hip. aethiopicus* (Fig. 8), as well as
negative correlation with estimated soil temperatures. The latter is of interest in light of the
theoretical expectation that ectothermic reptiles can attain the largest body sizes only at high
temperatures (Head *et al.* 2009; Parker *et al.* 2023).

<figure><img src="image_17.png" /><figcaption>FIG. 7. – Maximum body size of crocodylians (*Euthecodon*, *Crocodylus*, and cf. *Mecistops*) and hippopotamids (brown line series for the local *protamphibius-karumensis* lineage) by member in the Shungura Formation vs. reconstructed relative lake level in the Turkana Depression (Nutz *et al.* 2020), and the maximum lacustrine facies score by member from Heinzelin (1983).</figcaption></figure>

The assembly of high-resolution time series datasets for vertebrate traits through particular
stratigraphic sections like this one enables the testing of hypotheses of differential responses to
environmental change. The use of regressions like those deployed here (see S.I.) based on
isolated skeletal elements makes estimating body mass tractable for large sample sizes of reptile
specimens. In thoroughly-studied depositional contexts like the Turkana Depression,
comparison of time series data from vertebrate, invertebrate, plant, and geochemical metrics
can identify processes influencing evolution in both community composition and individual taxa's traits. Beyond correlation tests, future studies with only a small increase in the number of time steps used for sampling can test for causal relationships between such variables (Sugihara et al. 2012; Reitan & Liow 2019). Such analyses have previously been carried out mainly in marine settings, but their application to temporal sequences in the terrestrial fossil record has high potential to reveal how co-evolution and trait-based responses to environmental change have occurred in the past (Liow et al. 2015; Hannisdal & Liow 2018; Reitan & Liow 2019; Lidgard et al. 2021). Identifying such trait changes within faunas can, in this case, provide information about habitat shifts that occurred where early humans lived, and more broadly, allow us to build predictive models of faunal responses to future climate changes (Polly et al. 2011; Parker et al. 2023).

<figure><img src="image_18.png" /><figcaption>FIG. 8. – Alternative visualizations of correlation tests, with the two data series compared on the two axes. The color of the points indicates the age of the member for each observation. All mass data are shown with units log₁₀(kg).</figcaption></figure>

CONCLUSION

We situated reptile body size histories in their environmental context through time in the Shungura Formation in order to characterize the abiotic and biotic conditions underpinning
body mass changes in these groups. The observed patterns of maximum body size through time in relation to environmental proxy data demonstrate how local environmental shifts can drive body size evolution. While correlation tests were non-significant, we identify several reptile size-environment relationships, both matching and challenging previous expectations, that can be developed as proxies through testing in other contexts.

Several of the reptile groups studied display relationships between their body size and estimates of paleo-temperature, paleo-precipitation, and vegetation openness, all of which could be clarified through comparison to other sequences in the Turkana Depression. The presence and size of aquatic trionychid turtles is positively related to lacustrine conditions. Both crocodylians and hippopotamids also attain size maxima during lake high stands. The associations between body size in these reptile groups and habitat that we identify here can serve in future to assist in reconstructing climate and vegetation patterns in the eastern African record, if they prove generalizable to other sedimentary sequences. Reptile size is simple to measure and the previously-understudied fossil record of turtles, snakes, and crocodylians can be tapped into as a proxy to inform habitat availability in both terrestrial and aquatic habitats.

## Acknowledgements

We are grateful to the National Museum of Ethiopia/Ethiopian Heritage Authority (ex-ARCCH, Ministry of Tourism) for allowing to access collections and perform research. We deeply thank the NME staff (T. Getachew, S. Melaku, and G. Tekle Yemanebirihan) for guidance and support with collection study and Blade Engda Redae for assistance with specimen numbers and databasing. We thank the many members of the Omo Group Research Expedition for helpful discussion that contributed to the content of this paper. We thank F.K. Manthi and E. Ndiema (National Museums of Kenya) for access to comparative specimens. This work was funded by a Herchel Smith Fellowship, Emmanuel College Panton Trust grant, and a Cambridge University Worts Travelling Scholars grant to A.K.P., NERC NE/W007576/1, NSF 2124836, and a Cambridge Africa ALBORADA grant to J.J.H. and J.M., with additional support from the Integrated Climate Change Biology and Conservation Paleobiology in Africa programmes of the IUBS to J.J.H. We are deeply indebted to the hundreds of people who participated to the fieldwork missions of the IORE, of the OGRE and of other research programs, who managed collections and databases, who prepared specimens, who contributed to their study, who provided financial support, who helped with administrative processes, and who provided advice and moral support. The OGRE is a joint program of PALEVOPRIM, the CFEE and the EHA principally funded by the Ministry of Europe and Foreign Affairs, the
National Research Agency, the Région Nouvelle-Aquitaine, CNRS INEE, PALEVOPRIM, and the Fyssen Foundation. The OGRE is extremely grateful to the EHA, the SNNPR, the South Omo Zone, the Nyangatom and Dassanetch Weredas and their people for their help and reception.

## Data Availability

All data and code used are available at https://doi.org/10.5281/zenodo.15213585.

## REFERENCES

AKANI G.C., CAPIZZI D. & LUISELLI L. 2001. — Diet of the softshell turtle, Trionyx triunguis, in an Afrotropical forested region. *Chelonian Conservation and Biology* 4 (1): 200–201.

ALEMSEGED Z. 2003. — An integrated approach to taphnomy and faunal change in the Shungura Formation (Ethiopia) and its implication for hominid evolution. *Journal of Human Evolution* 44: 451–478.

ARAMBOURG C. 1948. — Contribution a l'étude géologique et paléontologique du bassin du Lac Rodolphe et de la basse vallée de L'Omo: Deuxième partie: Paléontologie, *Mission Scientifique de l'Omo, 1932-1933* Vol. 1: géologie-anthropologie. Paris, Editions du Muséum. p. 231–592.

ASHTON K.G. & FELDMAN C.R. 2003. — Bergmann's Rule in Nonavian Reptiles: Turtles Follow It, Lizards and Snakes Reverse It. *Evolution* 57 (5): 1151–1163.

AUFFENBERG W. 1981. — The Fossil Turtles of Olduvai Gorge, Tanzania, Africa. *Copeia* 1981 (3): 509–522. https://doi.org/10.2307/1444555.

BEN-DOR M. & BARKAI R. 2021. — Prey Size Decline as a Unifying Ecological Selecting Agent in Pleistocene Human Evolution. *Quaternary* 4 (1): 7. https://doi.org/10.3390/quat4010007.

BIBI F. 2023. — The Mammal Dental Metrics Database: a compilation of fossil and extant mammal tooth measurements. https://doi.org/10.5061/dryad.69p8cz968.

BIBI F. & CANTALAPIEDRA J.L. 2023. — Plio-Pleistocene African megaherbivore losses associated with community biomass restructuring. *Science* 380 (6649): 1076–1080. https://doi.org/10.1126/science.add8366.

BIBI F. & KIESSLING W. 2015. — Continuous evolutionary change in Plio-Pleistocene mammals of eastern Africa. *Proceedings of the National Academy of Sciences* 112 (34): 10623–10628. https://doi.org/10.1073/pnas.1504538112.
BLASCO R., BLAIN H.-A., ROSELL J., DÍEZ FERNÁNDEZ-LOMANA J.C., HUGUET PÀMIES R., RODRÍGUEZ J., ARSUAGA J.L., BERMÚDEZ DE CASTRO J.M. & CARBONELL E. 2011. — Earliest evidence for human consumption of tortoises in the European Early Pleistocene from Sima del Elefante, Sierra de Atapuerca, Spain. *Journal of Human Evolution* 61: 503–509.

BLONDEL C., ROWAN J., MERCERON G., BIBI F., NEGASH E., BARR W.A. & BOISSERIE J.-R. 2018. — Feeding ecology of Tragelaphini (Bovidae) from the Shungura Formation, Omo Valley, Ethiopia: Contribution of dental wear analyses. *Palaeogeography, Palaeoclimatology, Palaeoecology* 496: 103–120. https://doi.org/10.1016/j.palaeo.2018.01.027.

BOBE R. & BEHRENSMEYER A.K. 2004. — The expansion of grassland ecosystems in Africa in relation to mammalian evolution and the origin of the genus Homo, Evolution of grass-dominated ecosystems during the late Cenozoic Session at the North American Paleontological Convention, 2001. *Palaeogeography, Palaeoclimatology, Palaeoecology* 207 (3): 399–420. https://doi.org/10.1016/j.palaeo.2003.09.033.

BOBE R., BEHRENSMEYER A.K., ECK G.G. & HARRIS J.M. 2007. — Patterns of abundance and diversity in late Cenozoic bovids from the Turkana and Hadar Basins, Kenya and Ethiopia, *Hominin environments in the East African Pliocene: an assessment of the faunal evidence*. Springer. p. 129–157.

BOISSERIE J.-R., DELAGNES A., BEYENE Y. & SCHUSTER M. 2010. — Reconstructing the African background to human expansions in Eurasia: New research in the Shungura Formation, Ethiopia, Oldest Human Expansions in Eurasia: Favouring and Limiting Factors. *Quaternary International* 223–224: 426–428. https://doi.org/10.1016/j.quaint.2009.07.013.

BOISSERIE J.-R., GUY F., DELAGNES A., HLUKSO L.J., BIBI F., BEYENE Y. & GUILLEMOT C. 2008. — New palaeoanthropological research in the Plio-Pleistocene Omo Group, Lower Omo Valley, SNNPR (Southern Nations, Nationalities and People Regions), Ethiopia. *Comptes Rendus Palevol* 7 (7): 429–439. https://doi.org/10.1016/j.crpv.2008.07.010.

BRATTSTROM B.H. 1961. — Some New Fossil Tortoises from Western North America with Remarks on the Zoogeography and Paleoecology of Tortoises. *Journal of Paleontology* 35 (3): 543–560.

BROCHU C.A. 2021. — If Dr. Seuss made an animal with a woodwind instrument growing out of its head: The systematics and paleoecological significance of the hyper-tube-snouted osterolaimine crocodylid Eutheocodon from the Late Cenozoic of Africa. *Journal of Vertebrate Paleontology* (Program and Abstracts): 67.
BROCHU C.A. 2020. — Pliocene crocodiles from Kanapoi, Turkana Basin, Kenya, Kanapoi: The Paleobiology of a Pliocene Site in the Turkana Basin, Kenya. *Journal of Human Evolution* 140: 102410. https://doi.org/10.1016/j.jhevol.2017.10.003.

BROCHU C.A. 2001. — Crocodylian Snouts in Space and Time: Phylogenetic Approaches Toward Adaptive Radiation1. *American Zoologist* 41 (3): 564–585. https://doi.org/10.1093/icb/41.3.564.

BROCHU C.A., NJAU J., BLUMENSCHINE R.J. & DENSMORE L.D. 2010. — A New Horned Crocodile from the Plio-Pleistocene Hominid Sites at Olduvai Gorge, Tanzania. *PLOS ONE* 5 (2): e9333. https://doi.org/10.1371/journal.pone.0009333.

BROCHU C.A. & STORRS G.W. 2012. — A Giant Crocodile from the Plio-Pleistocene of Kenya, the Phylogenetic Relationships of Neogene African Crocodylines, and the Antiquity of Crocodylus in Africa. *Journal of Vertebrate Paleontology* 32 (3): 587–602. https://doi.org/10.1080/02724634.2012.652324.

CERLING T.E., WYNN J.G., ANDANJE S.A., BIRD M.I., KORIR D.K., LEVIN N.E., MACE W., MACHARIA A.N., QUADE J. & REMIEN C.H. 2011. — Woody cover and hominin environments in the past 6 million years. *Nature* 476 (7358): 51–56. https://doi.org/10.1038/nature10306.

CHABRECK R.H. & JOANEN T. 1979. — Growth Rates of American Alligators in Louisiana. *Herpetologica* 35 (1): 51–57.

CONLEY J.A. & SAMUELS J.X. 2022. — Community Structure Analysis of Turtles with Application to the Early Pliocene Gray Fossil Site of Northeastern Tennessee. *Journal of Vertebrate Paleontology* (Program and Abstracts): 112–113.

COOKE H.B.S. 2007. — Stratigraphic variation in Suidae from the Shungura Formation and some coeval deposits, *Hominin environments in the East African Pliocene: an assessment of the faunal evidence*. Springer. p. 107–127.

CORYNDON S.C. & COPPENS Y. 1975. — Une espèce nouvelle d'Hippopotame nain du Plio-Pléistocène du bassin du lac Rodolphe (Ethiopie, Kenya). *Comptes Rendus de l'Académie des Sciences* 280: 1777–1780.

DE BROIN F. 1979. — Chéloniens du Miocène et du Plio-Plistocène d'Afrique orientale. *Bulletin de la Société Géologique de France* XXI (3): 323–327.

DE LAPPARENT DE BROIN F. 2000. — African chelonians from the Jurassic to the present: phases of development and preliminary catalogue of the fossil record. *Palaeont. afr.* 36: 43–83.
DRUMHELLER S.K. & WILBERG E.W. 2020. — A synthetic approach for assessing the interplay of form and function in the crocodyliform snout. *Zoological Journal of the Linnean Society* 188 (2): 507–521. https://doi.org/10.1093/zoolinnean/zlz081.

FALCÓN W. & HANSEN D.M. 2018. — Island rewilding with giant tortoises in an era of climate change. *Philosophical Transactions of the Royal Society B: Biological Sciences* 373 (1761): 20170442. https://doi.org/10.1098/rstb.2017.0442.

FELDMAN A. & MEIRI S. 2013. — Length–mass allometry in snakes. *Biological Journal of the Linnean Society* 108 (1): 161–172. https://doi.org/10.1111/j.1095-8312.2012.02001.x.

GARDIN A., PUCÉAT E., GARCIA G., BOISSERIE J.-R., EURIAT A., JOACHIMSKI M.M., NUTZ A., SCHUSTER M. & OTERO O. 2024. — Stable oxygen isotopes of crocodilian tooth enamel allow tracking Plio-Pleistocene evolution of freshwater environments and climate in the Shungura Formation (Turkana Depression, Ethiopia). *Biogeosciences* 21 (2): 437–454. https://doi.org/10.5194/bg-21-437-2024.

GODOY P.L., BENSON R.B.J., BRONZATI M. & BUTLER R.J. 2019. — The multi-peak adaptive landscape of crocodylomorph body size evolution. *BMC Evolutionary Biology* 19 (1): 167. https://doi.org/10.1186/s12862-019-1466-4.

GODOY P.L. & TURNER A.H. 2020. — Body Size Evolution in Crocodylians and Their Extinct Relatives, *eLS.American Cancer Society*. p. 442–452. https://doi.org/10.1002/9780470015902.a0029089.

GROHÉ C., UNO K. & BOISSERIE J.-R. 2022. — Lutrinae Bonaparte, 1838 (Carnivora, Mustelidae) from the Plio-Pleistocene of the Lower Omo Valley, southwestern Ethiopia: systematics and new insights into the paleoecology and paleobiogeography of the Turkana otters. *Comptes Rendus. Palevol* (30): 681–705.

HANNISDAL B. & LIOW L.H. 2018. — Causality from palaeontological time series. *Palaeontology* 61 (4): 495–509. https://doi.org/10.1111/pala.12370.

HANSEN D.M., DONLAN C.J., GRIFFITHS C.J. & CAMPBELL K.J. 2010. — Ecological history and latent conservation potential: large and giant tortoises as a model for taxon substitutions. *Ecography* 33 (2): 272–284. https://doi.org/10.1111/j.1600-0587.2010.06305.x.

HANSEN D.M. & GALETTI M. 2009. — The Forgotten Megafauna. *Science* 324 (5923): 42–43.

HARRIS J.M., LEAKEY M.G., CERLING T.E. & WINKLER A.J. 2003. — Early Pliocene tetrapod remains from Kanapoi, Lake Turkana Basin, Kenya, Natural History Museum of Los Angeles County. *Contributions in Science* 498: 39–114.
HEAD J.J., BLOCH J.I., HASTINGS A.K., BOURQUE J.R., CADENA E.A., HERRERA F.A., POLLY P.D. & JARAMILLO C.A. 2009. — Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures. *Nature* 457 (7230): 715–7.

HEAD J.J., GUNNELL G.F., HOLROYD P.A., HUTCHISON J.H. & CIOCHON R.L. 2013. — Giant lizards occupied herbivorous mammalian ecospace during the Paleogene greenhouse in Southeast Asia. *Proceedings of the Royal Society B: Biological Sciences* 280 (1763). https://doi.org/10.1098/rspb.2013.0665.

HEAD J.J. & MÜLLER J. 2020. — Squamate reptiles from Kanapoi: Faunal evidence for hominin paleoenvironments. *Kanapoi: The Paleobiology of a Pliocene Site in the Turkana Basin*, Kenya. *Journal of Human Evolution* 140: 102451. https://doi.org/10.1016/j.jhevol.2018.01.007.

HEAD J.J. & MÜLLER J. 2018. — Squamate reptiles from Kanapoi: Faunal evidence for hominin paleoenvironments. *Journal of Human Evolution*. https://doi.org/10.1016/j.jhevol.2018.01.007.

HERNÁNDEZ FERNÁNDEZ M. & VRBA E.S. 2006. — Plio-Pleistocene climatic change in the Turkana Basin (East Africa): Evidence from large mammal faunas. *Journal of Human Evolution* 50 (6): 595–626. https://doi.org/10.1016/j.jhevol.2005.11.004.

HOLM S. 1979. — A Simple Sequentially Rejective Multiple Test Procedure. *Scandinavian Journal of Statistics* 6 (2): 65–70.

HOOIJER D.A. 1975. — Miocene to Pleistocene Hipparions of Kenya, Tanzania and Ethiopia. *Zoologische Verhandelingen* 142 (1): 1–80.

HOWELL F.C. & COPPENS Y. 1974. — Inventory of remains of Hominidae from Pliocene/Pleistocene formations of the lower Omo basin, Ethiopia (1967–1972). *American Journal of Physical Anthropology* 40 (1): 1–16. https://doi.org/10.1002/ajpa.1330400102.

HUTTON J.M. 1987. — Growth and Feeding Ecology of the Nile Crocodile Crocodylus niloticus at Ngezi, Zimbabwe. *Journal of Animal Ecology* 56 (1): 25–38. https://doi.org/10.2307/4797.

IIJIMA M. & KUBO T. 2020. — Vertebrae-Based Body Length Estimation in Crocodylians and Its Implication for Sexual Maturity and the Maximum Sizes. *Integrative Organismal Biology* 2 (1): obaa042. https://doi.org/10.1093/iob/obaa042.

IIJIMA M., TAKAHASHI K. & KOBAYASHI Y. 2016. — The oldest record of Alligator sinensis from the Late Pliocene of Western Japan, and its biogeographic implication. *Journal of Asian Earth Sciences* 124: 94–101. https://doi.org/10.1016/j.jseaes.2016.04.017.
ITESCU Y., KARRAKER N.E., RAIA P., PRITCHARD P.C.H. & MEIRI S. 2014. — Is the island rule general? Turtles disagree. *Global Ecology and Biogeography* 23 (6): 689–700. https://doi.org/10.1111/geb.12149.

JONES K.E., BIELBY J., CARDILLO M., FRITZ S.A., O'DELL J., ORME C.D.L., SAFI K., SECHREST W., BOAKES E.H., CARBONE C., CONNOLLY C., CUTTS M.J., FOSTER J.K., GRENYER R., HABIB M., PLASTER C.A., PRICE S.A., RIGBY E.A., RIST J., TEACHER A., BININDA-EMONDS O.R.P., GITTLEMAN J.L., MACE G.M. & PURVIS A. 2009. — PanTHERIA: a species-level database of life history, ecology, and geography of extant and recently extinct mammals. *Ecology* 90 (9): 2648–2648. https://doi.org/10.1890/08-1494.1.

JOOS J., PIMIENTO C., MILES D.B. & MÜLLER J. 2022. — Quaternary megafauna extinctions altered body size distribution in tortoises. *Proceedings of the Royal Society B: Biological Sciences* 289 (1987): 20221947. https://doi.org/10.1098/rspb.2022.1947.

KLEIN R.G. & CRUZ-URIBE KATHRYN 2016. — Large mammal and tortoise bones from Elands Bay Cave (South Africa): implications for Later Stone Age environment and ecology. *Southern African Humanities* 29 (1): 259–282. https://doi.org/10.10520/EJC-55c1a2524.

LEPRE C.J. 2014. — Early Pleistocene lake formation and hominin origins in the Turkana-Omo rift. *Quaternary Science Reviews* 102: 181–191.

LEVIN N.E. 2015. — Environment and Climate of Early Human Evolution. *Annual Review of Earth and Planetary Sciences* 43 (1): 405–429. https://doi.org/10.1146/annurev-earth-060614-105310.

LEVIN N.E., BROWN F.H., BEHRENSMEYER A.K., BOBE R. & CERLING T.E. 2011. — Paleosol carbonates from the Omo Group: Isotopic records of local and regional environmental change in East Africa. *Palaeogeography, Palaeoclimatology, Palaeoecology* 307 (1): 75–89. https://doi.org/10.1016/j.palaeo.2011.04.026.

LIDGARD S., DI MARTINO E., ZÁGORŠEK K. & LIOW L.H. 2021. — When fossil clades ‘compete’: local dominance, global diversification dynamics and causation. *Proceedings of the Royal Society B: Biological Sciences* 288 (1959): 20211632. https://doi.org/10.1098/rspb.2021.1632.

LIOW L.H., REITAN T. & HARNIK P.G. 2015. — Ecological interactions on macroevolutionary time scales: clams and brachiopods are more than ships that pass in the night. *Ecology Letters* 18 (10): 1030–1039. https://doi.org/10.1111/ele.12485.

LIU L., PUOLAMÄKI K., ERONEN J.T., ATAABADI M.M., HERNESNIEMI E. & FORTELIUS M. 2012. — Dental functional traits of mammals resolve productivity in terrestrial ecosystems past
and present. *Proceedings of the Royal Society B: Biological Sciences* 279 (1739): 2793–2799.

LUISELLI L., DEMAYA G.S., BENANSIO J.S., PETROZZI F., AKANI G.C., ENIANG E.A., AJONG S.N., DI VITTORIO M., AMADI N. & DENDI D. 2021. — A Comparative Analysis of the Diets of a Genus of Freshwater Turtles across Africa. *Diversity* 13 (4): 165. https://doi.org/10.3390/d13040165.

MAKARIEVA A.M., GORSHKOV V.G. & LI B.-L. 2005b. — Gigantism, Temperature and Metabolic Rate in Terrestrial Poikilotherms. *Proceedings: Biological Sciences* 272 (1578): 2325–2328.

MARTINEZ J.-N. & SUDRE J. 1995. — The astragalus of Paleogene artiodactyls: comparative morphology, variability and prediction of body mass. *Lethaia* 28 (3): 197–209. https://doi.org/10.1111/j.1502-3931.1995.tb01423.x.

MCALILEY L.R., WILLIS R.E., RAY D.A., WHITE P.S., BROCHU C.A. & DENSMORE L.D. 2006. — Are crocodiles really monophyletic?—Evidence for subdivisions from sequence and morphological data. *Molecular Phylogenetics and Evolution* 39 (1): 16–32. https://doi.org/10.1016/j.ympev.2006.01.012.

MCCARTNEY J.A., ROBERTS E.M., TAPANILA L. & O'LEARY M.A. 2018. — Large palaeophiid and nigerophiid snakes from Paleogene Trans-Saharan Seaway deposits of Mali. *Acta Palaeontologica Polonica* 63 (2): 207–220.

MEYLAN P. & AUFFENBERG W. 1986. — New land tortoises (Testudines: Testudinidae) from the Miocene of Africa. *Zoological Journal of the Linnean Society* 86 (3): 279–307. https://doi.org/10.1111/j.1096-3642.1986.tb01813.x.

MEYLAN P., WEIG B.S. & WOOD R.C. 1990. — Fossil soft-shelled turtles (family Trionychidae) of the Lake Turkana Basin, Africa. *Copeia*: 508–528.

NEGASH E.W., ALEMSEGED Z., BOBE R., GRINE F., SPONHEIMER M. & WYNN J.G. 2020. — Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia. *Proceedings of the National Academy of Sciences* 117 (36): 21921–21927.

NUTZ A., SCHUSTER M., BARBONI D., GASSIER G., VAN BOCXLAER B., ROBIN C., RAGON T., GHIENNE J.-F. & RUBINO J.-L. 2020. — Plio-Pleistocene sedimentation in West Turkana (Turkana Depression, Kenya, East African Rift System): Paleolake fluctuations, paleolandscapes and controlling factors. *Earth-Science Reviews* 211: 103415. https://doi.org/10.1016/j.earscirev.2020.103415.

PARKER A.K., MÜLLER J., BOISSERIE J.-R. & HEAD J.J. 2023. — The utility of body size as a functional trait to link the past and present in a diverse reptile clade. *Proceedings of the*
*National Academy of Sciences* 120 (7): e2201948119.
https://doi.org/10.1073/pnas.2201948119.

PARKER A.K., MÜLLER J., BOISSERIE J.-R. & HEAD J.J. 2023. — The utility of body size as a functional trait to link the past and present in a diverse reptile clade, in press. *Proceedings of the National Academy of Sciences*.

PASSEY B.H., LEVIN N.E., CERLING T.E., BROWN F.H. & EILER J.M. 2010. — High-temperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates. *Proceedings of the National Academy of Sciences*; 201001824.
https://doi.org/10.1073/pnas.1001824107.

PÉREZ-GARCÍA A. 2019. — New information and establishment of a new genus for the Egyptian Paleogene turtle ‘Stereogenys’ libyca (Podocnemididae, Erymnochelyinae). *Historical Biology* 31 (3): 383–392. https://doi.org/10.1080/08912963.2017.1374383.

PETERS R.H. 1983. — *The Ecological Implications of Body Size*. Cambridge University Press.
(Cambridge Studies in Ecology).

PLUMMER T.W., FERRARO J.V., LOUYS J., HERTEL F., ALEMSEGED Z., BOBE R. & BISHOP L.C. 2015. — Bovid ecomorphology and hominin paleoenvironments of the Shungura Formation, lower Omo River Valley, Ethiopia. *Journal of Human Evolution* 88: 108–126.
https://doi.org/10.1016/j.jhevol.2015.06.006.

POLLY P.D., ERONEN J.T., FRED M., DIETL G.P., MOSBRUGGER V., SCHEIDEGGER C., FRANK D.C., DAMUTH J., STENSETH N.C. & FORTELIUS M. 2011. — History matters: ecometrics and integrative climate change biology. *Proceedings of the Royal Society B: Biological Sciences* 278 (1709): 1131–1140. https://doi.org/10.1098/rspb.2010.2233.

REGIS K.W. & MEIK J.M. 2017. — Allometry of sexual size dimorphism in turtles: a comparison of mass and length data. *PeerJ* 5. https://doi.org/10.7717/peerj.2914.

REITAN T. & LIOW L.H. 2019. — layeranalyzer: Inferring correlative and causal connections from time series data in r. *Methods in Ecology and Evolution* 10 (12): 2183–2188.
https://doi.org/10.1111/2041-210X.13299.

RODRIGUES J.F.M., OLALLA-TÁRRAGA M.Á., IVERSON J.B. & DINIZ-FILHO J.A.F. 2018. — Temperature is the main correlate of the global biogeography of turtle body size. *Global Ecology and Biogeography* 27 (4): 429–438. https://doi.org/10.1111/geb.12705.

SCHEYER T.M., AGUILERA O.A., DELFINO M., FORTIER D.C., CARLINI A.A., SÁNCHEZ R., CARRILLO-BRICEÑO J.D., QUIROZ L. & SÁNCHEZ-VILLAGRA M.R. 2013. — Crocodylian diversity peak and extinction in the late Cenozoic of the northern Neotropics. *Nature Communications* 4: 1907. https://doi.org/10.1038/ncomms2940.
SCHMIDT-NIELSEN K. 1984. — *Scaling: Why is Animal Size So Important?* Cambridge University Press. 260 p.

SERENO P.C., LARSSON H.C.E., SIDOR C.A. & GADO B. 2001. — The Giant Crocodyliform Sarcosuchus from the Cretaceous of Africa. *Science* 294 (5546): 1516–1519. https://doi.org/10.1126/science.1066521.

SLAVENKO A., TALLOWIN O.J.S., ITESCU Y., RAIA P. & MEIRI S. 2016. — Late Quaternary reptile extinctions: size matters, insularity dominates. *Global Ecology and Biogeography* 25 (11): 1308–1320. https://doi.org/10.1111/geb.12491.

STEWART K.M. & MURRAY A.M. 2008. — Fish remains from the Plio-Pleistocene Shungura Formation, Omo River basin, Ethiopia. *Geobios* 41 (2): 283–295. https://doi.org/10.1016/j.geobios.2007.06.004.

STINER M.C., MUNRO N.D. & SUROVELL T.A. 2000. — The Tortoise and the Hare: Small-Game Use, the Broad-Spectrum Revolution, and Paleolithic Demography. *Current Anthropology* 41 (1): 39–79. https://doi.org/10.1086/300102.

STOCKDALE M.T. & BENTON M.J. 2021. — Environmental drivers of body size evolution in crocodile-line archosaurs. *Communications Biology* 4 (1): 1–11. https://doi.org/10.1038/s42003-020-01561-5.

STORRS G.W. 2003. — Late Miocene-Early Pliocene Crocodilian Fauna of Lothagam, Southwest Turkana Basin, Kenya, in *LEAKEY M.G. & HARRIS J.M. (eds.), Lothagam: The Dawn of Humanity in Eastern Africa*. Columbia University Press.

SUGIHARA G., MAY R., YE H., HSIEH C., DEYLE E., FOGARTY M. & MUNCH S. 2012. — Detecting Causality in Complex Ecosystems. *Science* 338 (6106): 496–500.

TASKAVAK E. & AKCINAR S.C. 2009. — Marine records of the Nile soft-shelled turtle, Trionyx triunguis from Turkey. *Marine Biodiversity Records* 2.

TCHERNOV E. 1986. — *Evolution of the Crocodiles in East and North Africa*. Paris, Centre National de la Recherche Scientifique. (Cahiers de Paleontologie).

THORBJARNARSON J.B. 1996. — Reproductive Characteristics of the Order Crocodylia. *Herpetologica* 52 (1): 8–24.

THORBJARNARSON J.B. 1988. — The Status and Ecology of the American Crocodile in Haiti. *Bulletin of the Florida State Museum, Biological Sciences* 33 (1).

VERDADE L.M. 2000. — Regression equations between body and head measurements in the broad-snouted caiman (Caiman latirostris). *Revista Brasileira de Biologia* 60 (3): 469–482. https://doi.org/10.1590/S0034-71082000000300012.
WEBB G.J.W. & MESSEL H. 1978. — Morphometric analysis of Crocodylus porosus from the north coast of Arnhem Land, Northern Australia. *Australian Journal of Zoology* 26 (1): 1–27. https://doi.org/10.1071/zo9780001.

WOOD R.C. 2003. — Fossil Turtles from Lothagam, *Lothagam: The Dawn of Humanity in Eastern Africa*. Columbia University Press. p. 115–136. https://doi.org/10.7312/leak11870-007.

WOOD R.C. 1979. — First Record of a Fossil Trionychid Skull from Africa. *Herpetologica* 35 (4): 360–364.

WRIGHT S.P. 1992. — Adjusted P-Values for Simultaneous Inference. *Biometrics* 48 (4): 1005–1013. https://doi.org/10.2307/2532694.

ŽLIOBAITĖ I., RINNE J., TOTH A.B., MECHENICH M., LIU L., BEHRENSMEYER A.K. & FORTELIUS M. 2016. — Herbivore teeth predict climatic limits in Kenyan ecosystems. *PNAS* 113 (45): 12751–12756.

## SUPPLEMENTARY INFORMATION

Data File: mammals and isotope series across members

<table><caption>SUPPLEMENTARY TABLE 1. — Linear regression equations used to estimate straight line carapace length for turtle specimens. Abbreviations: **UMZC**, Cambridge University Museum of Zoology; **EHA**, National Museum of Ethiopia/Ethiopian Heritage Authority; **KNM**, National Museums of Kenya; **NHML**, Natural History Museum (London); **PPHM**, Panhandle Plain Historical Museum.</caption><thead><tr><th>Taxon</th><th>Measurement, x (cm)</th><th>Regression to carapace length (CL, in cm)</th><th>Regression trained on N=</th><th>Sources for regression data</th></tr></thead><tbody><tr><td>Trionychidae</td><td>Skull length</td><td>CL = 3.5595*x - 5.3869</td><td>3</td><td>UMZC</td></tr><tr><td>Trionychidae</td><td>Hyohypoplastron maximum length</td><td>CL = 2.25*x</td><td>2</td><td>KNM-LT 28483, EHA OMO 229</td></tr><tr><td>Trionychidae</td><td>Costal width at midline (average)</td><td>CL = 10.051*x - 2.9827</td><td>10</td><td>NHML, KNM, & EHA collections</td></tr><tr><td>Pelomedusidae</td><td>Plastron length</td><td>CL = 0.985*x</td><td>1</td><td>KNM-WS 14376</td></tr><tr><td>Pelomedusidae</td><td>Midline length of posterior lobe of plastron</td><td>CL = 4.2047*x - 54.557</td><td>5</td><td>OMO 57/5-1972-324, OMO 38-1968-3640, L 3-10, F 164-10079, F 256-10009, L 182-100002</td></tr></tbody></table>
<table><caption>SUPPLEMENTARY TABLE 2. – Linear regression equations used to estimate skull length for crocodylian specimens; total body length for vertebral regressions from Ijima and Kubo (2020). * measurement for final two rows, vertebral centrum length, is in mm. Abbreviations: **KNM**, National Museums of Kenya; **NHML**, Natural History Museum (London); **TBI**, Turkana Basin Insitute.</caption><tr><td>Testudinidae</td><td>Humerus length</td><td>CL = 1.7523*x + 49.703</td><td>3</td><td>NHML & Hay<br/>1908</td></tr><tr><td>Testudinidae</td><td>Maximum width of acetabulum</td><td>CL = 13.32*x</td><td>2</td><td>NHM 3097,<br/>PPHM 1534</td></tr><tr><td>Testudinidae</td><td>Midline length of neural</td><td>CL = 5.234*x</td><td>1</td><td>KNM-FM 21225</td></tr></table>

<table><thead><tr><th>Taxon</th><th>Measurement, x (cm*)</th><th>Regression to Skull Length (SL in cm, TL in mm)</th><th>Regression trained on N=</th><th>Sources for regression data</th></tr></thead><tbody><tr><td>Euthecodon</td><td>Mandible length</td><td>SL = 0.8754*x + 1.2113</td><td>4</td><td>KNM-ER 757, KNM-KP 18330, KNM-ER 8260, KNM-LT 26306</td></tr><tr><td>Euthecodon</td><td>Mandible length to posterior of dentary articulation</td><td>SL = 0.8754*x + 1.2113</td><td>5</td><td>NMK-ER 757, KNM-KP 18330, KNM-ER 8260, KNM-LT 26306, KNM-KP 66227</td></tr><tr><td>Euthecodon</td><td>Average tooth spacing (excluding two anteriormost tooth sockets)</td><td>SL = 19.966*x + 30.446</td><td>4</td><td>KNM-ER 757, KNM-LT 26306, KNM-ER 8260, KNM-ER 18330</td></tr><tr><td>Cf. Mecistops</td><td>Narrowest width of premaxilla</td><td>SL = 9.97*x</td><td>1</td><td>NME L 398 2508A</td></tr><tr><td>Cf. Mecistops</td><td>Length of mandibular symphysis</td><td>SL= 3.8706*x</td><td>1</td><td>NME L 398 2508A</td></tr><tr><td>Crocodylus</td><td>Mandible length</td><td>SL = 0.6829*x +6.4381</td><td>23</td><td>NHML collections</td></tr><tr><td>Crocodylus</td><td>Mandible length to posterior of dentary articulation</td><td>SL = 0.8206*x + 4.5791</td><td>19</td><td>NHML collections</td></tr><tr><td>Crocodylus</td><td>Length of mandibular symphysis</td><td>SL = 3.1846*x + 17.789</td><td>20</td><td>NHML collections</td></tr><tr><td>Crocodylus</td><td>Height of dentary at posterior end of mandibular symphysis</td><td>SL= 13.158*x</td><td>2</td><td>NME AL 126-11, L.449-4 (Hadar, Shungura Crocodylus)</td></tr><tr><td>Crocodylus</td><td>Length between centers of d9-d12 alveoli</td><td>SL= 5.1384*x + 8.318</td><td>18</td><td>NHML collections</td></tr><tr><td>Crocodylus</td><td>Femur circumference</td><td>SL = 4.8793*x + 1.2857</td><td>4</td><td>NHML collections</td></tr></tbody></table>
Body size of Shungura reptiles

Parker A. *et al.*

<table><tr><td><i>Crocodylus</i></td><td>D3-D10 vertebral centrum length</td><td>log(TL) = 1.906 + 0.993*log(x)</td><td>5</td><td>Ijima and Kubo 2020</td></tr><tr><td><i>Crocodylus</i></td><td>C8-D2 vertebral centrum length</td><td>log(TL) = 2.015 + 0.956*log(x)</td><td>2</td><td>Ijima and Kubo 2020</td></tr></table>
<table><caption>SUPPLEMENTARY TABLE 3. – Specimen data of Testudines by taxa for specimens with maximum body mass estimates for each member of the Shungura Formation.</caption><thead><tr><th>Taxon</th><th>Shungura Member</th><th>Midpoint Age (Ma)</th><th>Specimen number</th><th>Measurement</th><th>Dimension (cm)</th><th>Carapace length (cm)</th><th>Mass reconstruction (kg)</th></tr></thead><tbody><tr><td rowspan="5">Trionychidae</td><td>Basal</td><td>3.672</td><td>OMO 80-1974-903</td><td>Costal width</td><td>4.7</td><td>44.3</td><td>14.6</td></tr><tr><td>B</td><td>3.188</td><td>L 729-10005</td><td>Costal width</td><td>7.4</td><td>71.4</td><td>27.7</td></tr><tr><td>E</td><td>2.356</td><td>OMO 57/5-10025</td><td>Costal width</td><td>4.3</td><td>40.2</td><td>12.8</td></tr><tr><td>F</td><td>2.305</td><td>OMO 306-1976-386</td><td>Costal width</td><td>4.8</td><td>45.3</td><td>15.0</td></tr><tr><td>upper G</td><td>1.986</td><td>F 164-NC3</td><td>Carapace length</td><td>58</td><td>58</td><td>20.9</td></tr><tr><td rowspan="5">Pelomedusidae</td><td>B</td><td>3.188</td><td>OMO 28-1967-960</td><td>Carapace length</td><td>33.5</td><td>33.5</td><td>3.54</td></tr><tr><td>C</td><td>2.734</td><td>OMO 3/1-10038</td><td>Length of posterior lobe of plastron</td><td>21.8</td><td>37.1</td><td>4.74</td></tr><tr><td>D</td><td>2.457</td><td>L 824-13</td><td>Carapace length</td><td>33.1</td><td>33.1</td><td>3.42</td></tr><tr><td>E</td><td>2.356</td><td>OMO 38-1968-3640</td><td>Carapace length</td><td>35</td><td>35</td><td>4.01</td></tr><tr><td>F</td><td>2.305</td><td>L 182-10002</td><td>Carapace length</td><td>34.5</td><td>34.5</td><td>3.85</td></tr><tr><td rowspan="3">Testudinidae</td><td>lower G</td><td>2.167</td><td>OMO 257-1973-5315</td><td>Carapace length</td><td>36</td><td>36</td><td>4.35</td></tr><tr><td>upper G</td><td>1.986</td><td>F164-10079</td><td>Carapace length</td><td>27</td><td>27</td><td>1.91</td></tr><tr><td>L</td><td>1.239</td><td>OMO 346-10082</td><td>Length of posterior lobe of plastron</td><td>23</td><td>42.2</td><td>6.83</td></tr><tr><td rowspan="5">Testudinidae</td><td>B</td><td>3.188</td><td>OMO 28-1967-958</td><td>Neural length</td><td>15.4</td><td>80.6</td><td>66.1</td></tr><tr><td>C</td><td>2.734</td><td>L 823-3</td><td>Plastron length</td><td>103</td><td>108.9</td><td>151.3</td></tr><tr><td>D</td><td>2.457</td><td>[measured in field]</td><td>Maximum width of acetabulum</td><td>6.6</td><td>87.9</td><td>83.9</td></tr><tr><td>E</td><td>2.356</td><td>OMO 70-NC1</td><td>Carapace length</td><td>110</td><td>110</td><td>155.6</td></tr><tr><td>lower G</td><td>2.167</td><td>L 626-105</td><td>Neural length</td><td>16.2</td><td>84.8</td><td>76.0</td></tr><tr><td rowspan="2">Testudinidae</td><td>upper G</td><td>1.986</td><td>F 164-10068</td><td>Humerus length</td><td>15.4</td><td>76.7</td><td>57.7</td></tr><tr><td>H</td><td>1.84</td><td>OMO VE 3-NC1</td><td>Carapace length</td><td>100</td><td>100</td><td>119.7</td></tr></tbody></table>
<table><caption>SUPPLEMENTARY TABLE 4. – Specimen data of Crocodylidae by taxa for specimens with maximum body mass estimates for each member of the Shungura Formation. For *Crocodylus*, measurement type “Dentary height” refers to the height of the mandible at the posteriormost level of the mandibular symphysis. For cf. *Mecistops*, “narrowest width” refers to the width of the narrowest part of the premaxilla, posterior to the premaxillary alveoli. For *Euthecodon*, the measurement “Tooth spacing” is the average distance between the center of alveoli (dentary or maxilla) for all alveoli present in each specimen, excluding the anteriormost two.</caption><thead><tr><th>Taxon</th><th>Shungura<br>Formation<br>Member</th><th>Age<br>(Ma)</th><th>Specimen Number</th><th>Measurement</th><th>Dimension<br>(cm)</th><th>Estimated Skull<br>Length</th><th>Length<br>Reconstruction<br>(cm)</th><th>Mass<br>Reconstruction<br>(kg)</th></tr></thead><tbody><tr><td rowspan="15">Crocodylus</td><td>A</td><td>3.517</td><td>OMO 127-1973-4466</td><td>Length between<br>d9-d12 alveoli</td><td>7.2</td><td>45.3</td><td>329.4</td><td>226.1</td></tr><tr><td>B</td><td>3.188</td><td>OMO 28-1968-3213</td><td>Dentary height</td><td>5.1</td><td>67.1</td><td>497.6</td><td>714.9</td></tr><tr><td>C</td><td>2.734</td><td>L 449-4</td><td>Skull length</td><td>52</td><td>--</td><td>381.1</td><td>339.6</td></tr><tr><td>D</td><td>2.457</td><td>OMO 5/2-1967-617</td><td>D3-D10 vertebral<br>centrum length</td><td>4.3</td><td>--</td><td>342.8</td><td>252.8</td></tr><tr><td>E</td><td>2.356</td><td>L 40-29</td><td>Dentary height</td><td>5.6</td><td>73.7</td><td>548.4</td><td>937.7</td></tr><tr><td>F</td><td>2.305</td><td>OMO 221-1973-2716</td><td>Dentary height</td><td>5.9</td><td>77.6</td><td>578.9</td><td>1080.4</td></tr><tr><td>lower G</td><td>2.167</td><td>OMO 310-1976-549</td><td>C8-D2 vertebral<br>centrum length</td><td>5.5</td><td>--</td><td>477.3</td><td>636.5</td></tr><tr><td>upper G</td><td>1.986</td><td>OMO 2-1967-209</td><td>Length between<br>d9-d12 alveoli</td><td>11.7</td><td>68.4</td><td>507.6</td><td>755.9</td></tr><tr><td>J</td><td>1.658</td><td>OMO 358-10043</td><td>D3-D10 vertebral<br>centrum length</td><td>6.6</td><td>--</td><td>516.2</td><td>792</td></tr><tr><td>K</td><td>1.458</td><td>P 995-1a</td><td>Skull length</td><td>53</td><td>--</td><td>381.1</td><td>339.6</td></tr><tr><td>L</td><td>1.239</td><td>OMO 393-10133</td><td>Femur<br>circumference</td><td>11.5</td><td>57.4</td><td>422.7</td><td>453.6</td></tr><tr><td rowspan="5">Cf.<br>Mecistops</td><td>C</td><td>2.734</td><td>OMO 3-1967-910</td><td>Length of<br>mandibular<br>symphysis</td><td>15.8</td><td>61.1</td><td>451.7</td><td>545.8</td></tr><tr><td>F</td><td>2.305</td><td>L 398-2508A</td><td>Skull length</td><td>65.8</td><td>--</td><td>487.6</td><td>675.4</td></tr><tr><td>lower G</td><td>2.167</td><td>OMO 150-1972-1</td><td>Width of<br>narrowest part of<br>snout</td><td>5.5</td><td>54.8</td><td>402.9</td><td>396.8</td></tr><tr><td>upper G</td><td>1.986</td><td>OMO 372-10016</td><td>Narrowest width</td><td>6.1</td><td>60.8</td><td>449.1</td><td>537.1</td></tr><tr><td>J</td><td>1.658</td><td>OMO 358-10041</td><td>Narrowest width</td><td>5.3</td><td>51.8</td><td>379.9</td><td>336.6</td></tr><tr><td></td><td>L</td><td>1.239</td><td>OMO 341-10040</td><td>Length of<br>mandibular<br>symphysis</td><td>18.5</td><td>71.6</td><td>532.3</td><td>863.2</td></tr></tbody></table>
<table><caption>SUPPLEMENTARY TABLE 5. – Specimen data of *Python* for specimens with maximum body mass estimates for each member of the Shungura Formation.</caption><tr><td>B</td><td>3.188</td><td>L1-151</td><td>Tooth spacing</td><td>3.12</td><td>92.7</td><td>616.9</td><td>1302.3</td></tr><tr><td>C</td><td>2.735</td><td>OMO 18-1968-3215</td><td>Tooth spacing</td><td>3.5</td><td>103.1</td><td>673</td><td>1660.6</td></tr><tr><td>D</td><td>2.547</td><td>L 64-34</td><td>Tooth spacing</td><td>3.15</td><td>93.3</td><td>621.3</td><td>1328.6</td></tr><tr><td>E</td><td>2.356</td><td>OMO 38-1973-4629</td><td>Tooth spacing</td><td>2.8</td><td>89.6</td><td>569.6</td><td>1042.6</td></tr><tr><td>F</td><td>2.305</td><td>OMO 129/a-1972-4</td><td>Tooth spacing</td><td>3.05</td><td>94.4</td><td>606.5</td><td>1242.3</td></tr><tr><td rowspan="6">Euthecodon</td><td>lower G</td><td>2.167</td><td>OMO 6-1967-381</td><td>Tooth spacing</td><td>3.54</td><td>103.9</td><td>679</td><td>1701.6</td></tr><tr><td>upper G</td><td>1.986</td><td>F 164-10102</td><td>Tooth spacing</td><td>3.65</td><td>106</td><td>695.2</td><td>1817.6</td></tr><tr><td>H</td><td>1.84</td><td>F 161-22</td><td>Tooth spacing</td><td>3.16</td><td>96.5</td><td>622.8</td><td>1337.4</td></tr><tr><td>J</td><td>1.658</td><td>OMO 394-10046</td><td>Tooth spacing</td><td>3.09</td><td>95.2</td><td>612.5</td><td>1276.4</td></tr><tr><td>K</td><td>1.458</td><td>OMO 339-NC1</td><td>Skull length</td><td>92</td><td>--</td><td>611.4</td><td>1270.3</td></tr><tr><td>L</td><td>1.239</td><td>OMO K 7-1969-4410</td><td>Tooth spacing</td><td>4.07</td><td>111.6</td><td>756.8</td><td>2303.1</td></tr></table>

<table><thead><tr><th>Taxon</th><th>Shungura Member</th><th>Midpoint Age (Ma)</th><th>Specimen number</th><th>Measurement</th><th>Dimension (mm)</th><th>Total length (cm)</th><th>Mass reconstruction (kg)</th></tr></thead><tbody><tr><td rowspan="10">Python</td><td>B</td><td>3.188</td><td>L1-32b</td><td>Prezygapophyseal width</td><td>20.4</td><td>251</td><td>5.59</td></tr><tr><td>C</td><td>2.735</td><td>L 47-67</td><td>Prezygapophyseal width</td><td>41</td><td>540</td><td>41.13</td></tr><tr><td>D</td><td>2.547</td><td>L 824-12</td><td>Prezygapophyseal width</td><td>40.1</td><td>527</td><td>38.6</td></tr><tr><td>E</td><td>2.356</td><td>L 82-30</td><td>Prezygapophyseal width</td><td>24</td><td>300</td><td>8.9</td></tr><tr><td>F</td><td>2.305</td><td>OMO 33-3613</td><td>Prezygapophyseal width</td><td>52.1</td><td>702</td><td>81.6</td></tr><tr><td>lower G</td><td>2.167</td><td>OO 75-1971-2862</td><td>Prezygapophyseal width</td><td>37</td><td>483</td><td>30.67</td></tr><tr><td>upper G</td><td>1.986</td><td>F 163-11</td><td>Prezygapophyseal width</td><td>44.8</td><td>595</td><td>52.99</td></tr><tr><td>J</td><td>1.658</td><td>OMO 379-10008</td><td>Prezygapophyseal width</td><td>22.9</td><td>284</td><td>7.68</td></tr><tr><td>L</td><td>1.239</td><td>OMO 389-10055</td><td>Prezygapophyseal width</td><td>34</td><td>440</td><td>24.08</td></tr></tbody></table>
<figure><img src="image_19.png" /></figure>

# Untangling links between environment and reptilian body size in the Plio-Pleistocene Shungura Formation in Ethiopia

**Andrew Farke** *based on peer reviews by John Rowan and Pedro Godoy*

Abigail K. Parker, Jean-Renaud Boisserie, Johannes Müller, Christopher A. Brochu, Jason J. Head (2025) Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context. *PaleorXiv, ver. 3, peer-reviewed and recommended by Peer Community in Paleontology*. https://doi.org/10.31233/osf.io/2p6e3_v3

Submitted: 15 April 2025, Recommended: 08 September 2025

**Cite this recommendation as:**

Farke, A. (2025) Untangling links between environment and reptilian body size in the Plio-Pleistocene Shungura Formation in Ethiopia. *Peer Community in Paleontology*, 100402. https://doi.org/10.24072/pci.paleo.100402

Published: 08 September 2025

Copyright: This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/

Relationships between environment and morphology—particularly body size—are a common topic of study in vertebrate paleoecology. In the face of contemporary anthropogenic climate change, the fossil record is a key to understanding what the future might hold for today's ecosystems. Similarly, questions around the origin of humans are often framed against the backdrop of local, regional, and global environmental change. This contribution by Parker et al. (2025) explores relationships between body size and local environmental proxies for reptiles in the Shungura Formation of Ethiopia during the Plio-Pleistocene (~3.5 to 1.2 Ma). This formation has particular significance for understanding human evolution, alongside an extensive non-human vertebrate fossil record and rigorously documented geological context.

In this study, the research team tested for correlations between the maximum size of a variety of reptiles, including snakes, turtles, and crocodylians, with faunal variables and environmental proxies across multiple levels of the Shungura Formation. Due to the necessity of correcting for multiple comparisons as well as relatively small sample sizes, no tested relationship between body length and other variables was statistically significant. Nonetheless, some correlation values are quite strong, suggestive still of a tight relationship in some cases, such as a negative correlation between pelomedusid turtle size and inferred lake level. Unfortunately, but not unexpectedly, the incomplete nature of individual fossils complicated size estimation. As one example, crocodylian body length was estimated from a regression incorporating skull length. However, skull length
often had to be estimated, such as from alveolar spacing in snout fragments. Although this undoubtedly
introduces error, it is a practical necessity for maximizing sample size in this case.

The current stage of work does not have much for statistically definitive answers, but it points the way to great potential for future studies of reptile size evolution in the Shungura Formation. Certainly, additional collecting will be a critical part of this. Although ungulates and hominids often get much of the glory in studies of environmental change in eastern Africa, this new contribution hints at how crocs, turtles, and snakes are another important piece of the puzzle.

**References:**

Parker, A., Boisserie, J.-R., Müller, J., Brochu, C. A., and Head, J. J. (2025). Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context. PaleorXiv, 2p6e3, ver. 3 peer-reviewed by PCI Paleo. https://doi.org/10.31233/osf.io/2p6e3_v3

**Reviews**

**Evaluation round #1**

DOI or URL of the preprint: https://doi.org/10.31233/osf.io/2p6e3_v1

Version of the preprint: 1

**Authors' reply, 13 August 2025**

Download author's reply

Download tracked changes file

**Decision by Andrew Farke, posted 16 June 2025, validated 17 June 2025**

We have received two reviews for your preprint; they are in general very positive, but suggest some minor items to address. Please address these either via incorporation into the updated manuscript or commentary in your response document, as appropriate.

**Reviewed by Pedro Godoy ID, 16 June 2025**

This manuscript by Parker et al. explores different aspects of body size evolution in reptiles, using as a study system the Plio-Pleistocene fossil record of the Shungura Formation in Ethiopia. Specifically, the authors are interested in answering which biotic and abiotic factors are associated with through-time variation of body size in reptiles. The relatively well-known fossil assemblage represented by the Shungura Formation offers an excellent opportunity to investigate these questions, given that the different members of the Formation provide a good temporal record of the faunal and environmental changes, with well-constrained ages. The authors estimated the body sizes (length and mass) of many reptile specimens, including crocodylians, turtles and pythonid snakes. Then, the time series of body size patterns (maximum size) for different groups were tested for correlation with abiotic (e.g., paleotemperature) and biotic (e.g., body size of other groups) factors.

My only comment regarding the methodological approach is that I would use regressions instead of correlation tests between first differences. Specifically, I would use generalised least squares regression (GLS) with a first order autoregressive model to correct for the amount of serial correlation (or "autocorrelation" in time series). Nevertheless, I do believe the results presented by the authors are robust and can be discussed.
I would only consider using GLS regressions in future studies, since it is more appropriate for this kind of
analysis (see Benson et al. 2013 DOI: 10.1016/j.palaeo.2012.10.028, for example).

The authors did a good job exploring the data in many different ways and also discussing the
possible implications of the results. Even though correlation were not significant, it does not mean that the
data does not show association between body size and the investigated factors, especially environmental
factors.

Sincerely,
Pedro Godoy
University of São Paulo

Questions from PCI Paleo review guide:

Title and abstract

Does the title clearly reflect the content of the article? [X] Yes, [ ] No (please explain), [ ] I don't know

Does the abstract present the main findings of the study? [X] Yes, [ ] No (please explain), [ ] I don't know

Introduction

Are the research questions/hypotheses/predictions clearly presented? [X] Yes, [ ] No (please explain), [ ] I don't know

Does the introduction build on relevant research in the field? [X] Yes, [ ] No (please explain), [ ] I don't know

Materials and methods

Are the methods and analyses sufficiently detailed to allow replication by other researchers? [X] Yes, [ ] No
(please explain), [ ] I don't know

Are the methods and statistical analyses appropriate and well described? [X] Yes, [ ] No (please explain), [ ] I don't know

Results

In the case of negative results, is there a statistical power analysis (or an adequate Bayesian analysis or equiva-
lence testing)? [X] Yes, [ ] No (please explain), [ ] I don't know

Are the results described and interpreted correctly? [X] Yes, [ ] No (please explain), [ ] I don't know

Discussion

Have the authors appropriately emphasized the strengths and limitations of their study/theory/methods/argu-
ment? [X] Yes, [ ] No (please explain), [ ] I don't know

Are the conclusions adequately supported by the results (without overstating the implications of the findings)?
[X] Yes, [ ] No (please explain), [ ] I don't know

Reviewed by **John Rowan**, 09 June 2025

Body size histories of Shungura Formation reptiles in biotic and abiotic environmental context

This article presents analyses of body size change in the Plio-Pleistocene Omo reptile faunas and corre-
lates them with different proxy records (d13C and d18O isotopes from paleosols, mammal abundance, lake
level changes, etc) to understand how body size change may have been shaped by environmental factors.
Overall, this is an excellent study on a currently poorly understood (but very interesting!) element of eastern
Africa's late Cenozoic faunas, with their insights into potential reptile-environment relationships encouraging
further analyses of reptiles from other sites. I provide some comments below for the authors to consider.

Major Comments
Throughout the MS and main text tables there are correlations of so many different records presented, often discussing whether they are significant/nonsignificant. A few points on this:

1) It would be useful to have a table of the proxies you look at with basic info about the inferences that can be made, e.g., relative abundance of reduncin bovids = availability of fresh grass grazing lawns adjacent to river/lake systems. While the choice of proxy records makes sense to me (as someone working in eastern Africa), for readers not so familiar with the region and its records this would be helpful (vs having to wander through the text for an explanation of each proxy).

2) I'm not sure how much importance I would give to p-values (most are going to be non-significant because sample sizes are small, power is low, multiple comparisons so you have to Bonferroni correct anyway, etc). For me the most compelling thing are the correlations themselves and you've recovered some strong patterns through the formation, keeping in mind the issues of autocorrelation (and detrending). I would suggest giving less weight to statistical significance and focus more on the overall patterns. While it is nice to see the changes through time (x-axis = time, y-axis = variable of interest) you might also plot the change between adjacent points (e.g., x-axis = Δ proxy variable of interest, y-axis = Δ reptile variable of interest) to show the correlation patterns because they can look quite different (https://janhove.github.io/posts/2016-11-21-what-correlations-look-like/)

I am very, very skeptical of the suggestion that the tortoise gap in 'members F and (most of) lower G' might be driven by hominin impacts (Lines 694-711). So far as we understand, Oldowan hominins are nowhere near Late Pleistocene-Holocene modern humans, so the Klein and Cruz-Uribe and Joos et al links are of very limited relevance. For mammals, there are plenty of taxa in Turkana that blip in and out of the record irrespective of sampling effort (based on overall NISP) but we do not have to posit hominin impacts (and doing it for tortoises without a more concrete analyses might seem like special pleading). The focus on reptile body size in relation to environment is plenty interesting enough, no need to stick your neck out on this suggestion, which would likely become another 'just so' story of human evolution—and that's not your fault, but many would happily and uncritically cite this paper as evidence that Oldowan hominins caused Early Pleistocene tortoise extinctions in Africa, whereas robust evidence for hominin-driven extinctions at this time is non-existent at present.

The y-axes on the multipaneled figures are not always clear, e.g., Fig 5: Is the d18O scale for both herbi-vore and paleosol values? What's the max value (it's hard to tell, +2.5 per mil?); why no scale for relative lake level, and what are the units, change in m?

Minor Comments (line numbers)

55: 'hominid' -> 'hominin'

139: Check '...body size histories of Shungura Formation'

166: Undocumented or undescribed?

175-176: Redundant with close of last paragraph, perhaps combine?

185: 'sections' -> 'sequences'

185: Species diagnoses (not definitions)

310: Here and throughout, check for consistency with capitalization (e.g., Member G), also on 309 lower G is a submember not a member

362: Reduncini is a tribe so this should be 'reduncin' (follow this for bovid tribes throughout)

456: Is this specimen number complete?

663-666: Not necessarily, precipitation is only one aspect shaping habitat openness (and does not consider temperature and PET, what do the d18O from paleosols suggest?)

711: 'anthropic' -> 'anthropogenic'
I do not need to be anonymous

Signed, John Rowan (jr2006@cam.ac.uk)
Dear PCI Paleontology/ Recommender Farke,

We thank both reviewers for their careful consideration of our manuscript and helpful feedback. We are here re-submitting our manuscript with responses to all comments (below). The largest changes we made were two additions to the figures and an update to remove statements claiming hominin hunting temporarily extirpated the largest tortoises from the Shungura sequence, in response to the second reviewer. The latter change retains discussion of the coincidence in the gap in the tortoise record with the timing of stone tool deposits in Shungura, but clarifies that when in geologic history humans first affected tortoise population and body size is a matter for further research.

We appreciated the reviewer's positive views of the contribution of this manuscript to raising the profile of this reptile fossil record and contextualizing it using varied available paleoenvironmental proxies. With the minor revisions we submit now, this paper provides both primary and integrative results establishing the utility of time series studies of body size maxima.

Thanks for your consideration of this manuscript, and we look forward to hearing your response to our revisions.

Sincerely,

Abigail Parker, on behalf of all authors

**Reviews below, responses in blue**

We have received two reviews for your preprint; they are in general very positive, but suggest some minor items to address. Please address these either via incorporation into the updated manuscript or commentary in your response document, as appropriate.

by Andrew Farke, 16 Jun 2025 20:29

This manuscript by Parker et al. explores different aspects of body size evolution in reptiles, using as a study system the Plio-Pleistocene fossil record of the Shungura Formation in Ethiopia. Specifically, the authors are interested in answering which biotic and abiotic factors are associated with through-time variation of body size in reptiles. The relatively well-known fossil assemblage represented by the Shungura Formation offers an excellent opportunity to investigate these questions, given that the different members of the Formation provide a good temporal record of the faunal and environmental changes, with well-constrained ages. The authors estimated the body sizes (length and mass) of many reptile specimens, including crocodylians, turtles and pythonid snakes. Then, the time series of body size
patterns (maximum size) for different groups were tested for correlation with abiotic (e.g., paleotemperature) and biotic (e.g., body size of other groups) factors.

My only comment regarding the methodological approach is that I would use regressions instead of correlation tests between first differences. Specifically, I would use generalised least squares regression (GLS) with a first order autoregressive model to correct for the amount of serial correlation (or "autocorrelation" in time series). Nevertheless, I do believe the results presented by the authors are robust and can be discussed. I would only consider using GLS regressions in future studies, since it is more appropriate for this kind of analysis (see Benson et al. 2013 DOI: 10.1016/j.palaeo.2012.10.028, for example).

The authors did a good job exploring the data in many different ways and also discussing the possible implications of the results. Even though correlation were not significant, it does not mean that the data does not show association between body size and the investigated factors, especially environmental factors.

Sincerely,

Pedro Godoy
University of São Paulo

* We thank Dr. Godoy for this positive review and feedback. On the question of time series statistics, we did explore autoregressive models at the beginning of this project, but deemed them to be inappropriate for data series of the length and nature of the ones we consider here. In particular, serial correlation tests rely on identification of directional trends across time, which may be inaccurate due to uncertainty and missing member data in these series. We also are particularly interested in directional trends over time and where they align, so disagree with the approach of removing such trends, instead opting for tests of correlation between first differences, which explicitly test for similarities in member-to-member stepwise change between series. We also ruled out using multivariate regressions, again because of the short and incomplete time series here. We will in future work look into using GLS as an alternative to correlation tests, though as the paper you link shows, their results usually agree.

Review by John Rowan, 09 Jun 2025 20:36

This article presents analyses of body size change in the Plio-Pleistocene Omo reptile faunas and correlates them with different proxy records (d13C and d18O isotopes from paleosols, mammal abundance, lake level changes, etc) to understand how body size change may have been shaped by environmental factors. Overall, this is an excellent study on a currently poorly understood (but very interesting!) element of eastern Africa's late Cenozoic faunas, with their insights into potential reptile-environment relationships encouraging further analyses of reptiles from other sites. I provide some comments below for the authors to consider.
Major Comments

Throughout the MS and main text tables there are correlations of so many different records presented, often discussing whether they are significant/nonsignificant. A few points on this:

1) It would be useful to have a table of the proxies you look at with basic info about the inferences that can be made, e.g., relative abundance of reduncin bovids = availability of fresh grass grazing lawns adjacent to river/lake systems. While the choice of proxy records makes sense to me (as someone working in eastern Africa), for readers not so familiar with the region and its records this would be helpful (vs having to wander through the text for an explanation of each proxy).

* Thanks for this feedback; in response, we have added a figure (new Fig. 3) summarizing relationships between the proxy series we use, the factors contributing to them, and the aspects of the paleoenvironment and paleocommunities that they represent.

2) I'm not sure how much importance I would give to p-values (most are going to be non-significant because sample sizes are small, power is low, multiple comparisons so you have to Bonferroni correct anyway, etc). For me the most compelling thing are the correlations themselves and you've recovered some strong patterns through the formation, keeping in mind the issues of autocorrelation (and detrending). I would suggest giving less weight to statistical significance and focus more on the overall patterns. While it is nice to see the changes through time (x-axis = time, y-axis = variable of interest) you might also plot the change between adjacent points (e.g., x-axis = Δ proxy variable of interest, y-axis = Δ reptile variable of interest) to show the correlation patterns because they can look quite different (https://janhove.github.io/posts/2016-11-21-what-correlations-look-like/)

* We agree with this assessment; we have kept the formatting of our statistical results including p-values, and discussion of how Bonferroni correction, but in several places changed phrasing to clarify how high correlation coefficients, regardless of lack of significance, can indicate meaningful relationships between pairs of series (lines 566, 574, 622).

* We carried out extensive plotting of proxy vs. reptile size, as suggested here, in preliminary phases of this research. We have added several examples of these plots for the strongest correlation coefficients recovered as an additional figure in the discussion (Fig. 8), and welcome the reviewers' feedback on whether this figure adds to the interpretation of our results.

I am very, very skeptical of the suggestion that the tortoise gap in 'members F and (most of) lower G' might be driven by hominin impacts (Lines 694-711). So far as we understand, Oldowan hominins are nowhere near Late Pleistocene-Holocene modern humans, so the Klein and Cruz-Uribe and Joos et al links are of very limited relevance. For mammals, there are plenty of taxa in Turkana that blip in and out of the record irrespective of sampling effort (based on overall NISP) but we do not have to posit hominin impacts (and doing it for
tortoises without a more concrete analyses might seem like special pleading). The focus on reptile body size in relation to environment is plenty interesting enough, no need to stick your neck out on this suggestion, which would likely become another 'just so' story of human evolution—and that's not your fault, but many would happily and uncritically cite this paper as evidence that Oldowan hominins caused Early Pleistocene tortoise extinctions in Africa, whereas robust evidence for hominin-driven extinctions at this time is non-existent at present.

* We have reconsidered our statement on the temporal coincidence between the Shungura archaeological record and the absence of large tortoises following your comment. We discuss the environmental associations of large tortoise size prior (lines 648-684), but the many habitat metrics considered here do not point to any exceptional conditions in the members lacking large tortoises. With regards to the time period of human impacts on tortoises, we note that tortoise butchery is documented in the Early Pleistocene (Blasco et al. 2011, citation added), and the Joos et al. results also show size reduction beginning in the Early Pleistocene. Ben-Dor and Barkai (2021), among other sources, also discuss size-selective hunting beginning in the Early Pleistocene. Our revised version includes a brief mention of these relevant sources, and retains our discussion of the specific stratigraphic units from which tortoises are lacking. Otherwise, we agree that, as you note, this could be just a "blip;" therefore, we have removed suggestions of a causal or explanatory relationship with hominin occupation, instead leaving this question open for future research.

The y-axes on the multipaneled figures are not always clear, e.g., Fig 5: Is the d18O scale for both herbivore and paleosol values? What's the max value (it's hard to tell, +2.5 per mil?); why no scale for relative lake level, and what are the units, change in m?

* In Fig. 5, we added the +2.5 per mil unit to the scale for d18O, and hope this scale clearly applies to both the herbivore and paleosol series. We also added clarification to one of the scales in Fig. 4, and otherwise rely on the legend of color/shape to link each figured series to the relevant y-axes scales.

* For lake level, this data is (unfortunately) only available on a relative scale, meaning the highest value is the highest lake level reconstructed for the Nachukui Fm and the lowest value is the lowest reconstructed lake level. This series is derived from the Fig. xxx of Nutz et al. (2020), at the recommendation of the authors of that reconstruction of lake extent. The consistency between these relative lake levels and the numeric scores assigned by Heinzelin (1983) characterizing facies (see Fig. 7) gives us confidence that this series reasonably approximates how lacustrine conditions were at the time each member of the formation was deposited. Ideally, we would test for correlation between semi-aquatic animals' body size trends and actual lake extent (m2), volume (m3), or depth, but estimates of those metrics are not available based on current sedimentological and paleohydrological studies. We look forward to future work considering reptile size in the context of forthcoming estimates of local paleo-lake and river extent in the Omo Valley and across the Omo-Turkana Depression.
Minor Comments (line numbers)

55: 'hominid' -> 'hominin'

* Resolved

139: Check '...body size histories of Shungura Formation'

* Resolved, word missing

166: Undocumented or undescribed?

* In many cases, both, but agreed lack of description is the relevant point here

175-176: Redundant with close of last paragraph, perhaps combine?

* Resolved

185: 'sections' -> 'sequences'

* Resolved

185: Species diagnoses (not definitions)

* Resolved

310: Here and throughout, check for consistency with capitalization (e.g., Member G), also on 309 lower G is a submember not a member

* Checked, now consistent (only capitalized when referring to one specific member)

362: Reduncini is a tribe so this should be 'reduncin' (follow this for bovid tribes throughout)

* Resolved

456: Is this specimen number complete?

* Yes, it is OMO 341-10040)

663-666: Not necessarily, precipitation is only one aspect shaping habitat openness (and does not consider temperature and PET, what do the d18O from paleosols suggest?)

* Looking at the comparisons between tortoise size and d18O, all correlation coefficients are positive but low, consistent with larger sizes in members with more evaporative water loss. We add a mention of this in (new version) lines 681-683.

711: 'anthropic' -> 'anthropogenic'

* Line removed