Fossil elephantoids, Awash paleolake basins, and the Afar triple junction, Ethiopia

ELSEVIER

Palaeogeography,Palaeoclimatology,Palaeoecology114 (1995) 357-368

Fossil elephantoids, Awash paleolake basins, and the Afar triple junction, Ethiopia J o n E. K a l b Vertebrate Paleontology Laboratory, Balcones Research Center, University of Texas, Austin, TX 78712, USA Received 10 May 1994; revised and accepted 15 September 1994

Abstract

The most diverse collection of fossil elephantoids from a single area, are contained in the 1-kin-thick hominidbearing Awash Group. These deposits range from late Miocene to Holocene in age and are found in the Awash Valley of the Afar Depression, Ethiopia. Uniquely, the Afar elephantoids inhabited a series of internal lake basins splayed out across an evolving, subaerial triple junction created by the separation of the African, East African and Arabian plates. The spatial and temporal distribution of elephantoids in these basins demonstrates that these animals were progressively drawn into the central Afar with the divergence of the three plates. As such, the elephantoids moved from the higher margins of the East African and African plates to the depressed lowlands of the triple junction, where subsiding and migrating lake basins served as ideal habitats for large herbivores with high water requirements. This pattern of migration serves as a model for the migration of animals into intercontinental areas and for their dispersal across plate boundaries.

1. Introduction

The Afar Depression is unique as a subaerial triple junction formed by the intersection of the East African, Red Sea, and Gulf of Aden rifts (Pilger and Rrsler, 1975) (Fig. 1). These rifts, formed as a result of the separation of the African, East African and Arabian plates, contain numerous smaller graben structures aligned with the major rift trends. These grabens are host to a number of sedimentary basins whose progressive development has reflected the overall evolution of the triple junction (Kalb, 1978; Kalb et al., 1982d). The basins along the western and central Afar have been subject to high rates of deposition for extended periods largely as a result of runoff from the adjacent uplifted highlands. As such, thick sedimentary deposits contain high concentrations of preserved mammalian fossils, including large 0031-0182/95/$9.50 © 1995Elsevier ScienceB.V. All rights reserved SSDI 0031-0182(94)00088-3

numbers of fossil hominids (e.g. Johanson et al., 1982; Kalb et al., 1984; White et al., 1993). In addition, other mammalian groups are well represented (Kalb et al., 1982c, table 2), most prominently the proboscideans, which are represented by no less than 16 taxa of the superfamily Elephantoidea (Kalb and Mebrate, 1993). Because of the abundance and often excellent preservation of elephantoid remains, particularly dentition, these fossils have for years been used in eastern Africa for biostratigraphic purposes (e.g. Coppens, 1965; Maglio, 1970, 1973; Beden, 1985). In this paper, elephantoids are shown to reflect the age differences and the distribution of a unique series of late Miocene to Holocene lake basins spread along the three arms of the triple junction. The fossils and sedimentary deposits in these basins are all part of the 1-km-thick Awash Group, which crops out in the southwestern and central Afar

J.E. Kalb/'Palaeogeography, Palaeoclimatology, Palaeoecology114 (1995)357-368

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J.E. Kalb/Palaeogeography, Palaeoclimatology, Palaeoecology114 (1995) 357-368

along the drainages of the Awash River (Fig. 1). The stratigraphy and age of the Awash Group is summarized below as a context to discussion of the age and distribution of fossil elephantoids as they are known in Africa and in the 35,000 km 2 Awash Valley. Finally, a model is presented depicting the paleobiogeography of Awash elephantoids over a 10 Ma period. Specifically shown is the temporal distribution of elephantoid fossils in a series of paleolake basins formed during intercontinental breakup and the creation of the triple junction.

2. Stratigraphy and age The elephantoid-bearing units of the Awash Group are, from bottom to top, the: Chorora, Adu-Asa, Sagantole, Hadar, Matabaietu, and Wehaietu formations (Kalb et al., 1982d,e; Kalb, 1993) (Fig. 2). The Chorora Formation crops out along the southern escarpment and the Hadar Formation crops out to the north in the area of Hadar (Fig. 3). The remaining units are exposed between these two areas along drainages on either side of the Awash River. These formations, their sub-units, associated isotopic dates, and the stratigraphic distribution of elephantoid taxa within the Awash Group are illustrated in Fig. 2. [In this report, the Mio-Pliocene and Plio-Pleistocene boundaries are placed at 5.2 and 1.64 Ma, respectively (Harland et al., 1990).] The Chorora Formation is bracketed by the Bacca rhyolites (10.5-10.7 Ma) which are contemporaneous with the Fursa basalts (~9-11 Ma) underlying the Adu-Asa Formation (Zannetin and Justin-Visentin, 1974; Sickenberg and Schrnfeld, 1975; Tiercelin et al., 1979; Zannetin, 1992) (Fig. 2). Overlying the basal Adu Member in this formation, the Asa Member has distinctive tufts, the Asa Tuff (AAT)-I, -2, and -3 below, and the Rodent Tuff (RDT) above. These tufts have not yet been dated, but they are younger than the Fursa basalts and older than the maximum known age (4.4 Ma) of basalts of the Lower Afar Stratoid Series (LASS) at the base of the Sagantole Formation (Varet, 1978; Kalb et al., 1982d). Faunal studies suggest that the Adu-Asa Forma-

359

tion ranges from basal Pliocene to late Miocene in age (~4.5-6.0 Ma; Kalb et al., 1982c). The nearly non-tuffaceous Haradaso Member at the base of the Sagantole Formation is overlain by the tuffaceous Aramis Member and the undifferentiated Beearyada beds, as these units have been described from the west side of the Awash River (Kalb et al., 1982d,e; Kalb, 1993). Subsequent stratigraphic work surrounding hominid-bearing levels on the east side of the Awash River has led to the dating of tufts, which tentatively appear to be equivalent to tufts in the Beearyada beds. The Vitric Tuff (VT)-I (3.89 Ma) occurs near the bottom of the eastern Middle Awash sequence, the Cindery Tuff (CT) (3.85 Ma) and the VT-3 (3.75 Ma) near the middle, and the Sidiha Koma (SHT) (3.40 Ma) at the top (Hall et al., 1984; Walter et al., 1985a,b; Walter and Aronson, 1993; White et al., 1993) (Fig. 2). Haileab and Brown (1993) have tentatively described the ERV-032 ( ~ 3.6 Ma) as also occurring near the top of the eastern Awash sequence. The VT-1 and the ERV-032 have been identified as the Moiti and Lomogol tufts present at Lake Turkana (Haileab and Brown, 1993); the SHT was originally described from Hadar and is equivalent to the Tulu Bor-Beta Tuff from Lake Turkana (Walter and Aronson, 1993). The hominid-bearing Hadar Formation is bracketed below by the Oudaleita Tuff (OT) and above by the Bouroukie Tuff(BKT)-2 (Taieb et al., 1978; Kalb, 1993). The BKT-2 has been dated (2.9 Ma) (Walter and Aronson, 1982), as has the SHT (3.40 Ma) which overlies the OT (Brown, 1982; Cerling and Brown, 1982; White et al., 1993; Walter and Aronson, 1993). Dates from the Kada Damoumou Basalt (KDB) (~3.0 Ma), which is part of the Upper Afar Stratoid Series (UASS) (Kalb et al., 1982d), the Kada Hadar Tuff (KHT), and other Hadar tufts have thus far proven unreliable (Walter and Aronson, 1982; Haileab and Brown, 1992). No isotopic dates have been yet reported from the artifact-rich Matabaietu or Wehaietu formations, but their faunas suggest a late Pliocene age (,~ 2.0-2.5 Ma) for the former and an early to late Pleistocene time range for the latter (Kalb et al., 1982c; Kalb, 1993). Nevertheless, these formations

360

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J.E. Kalb/Palaeogeography,Palaeoclimatology,Palaeoecology114 (1995) 357-368 contain numerous tufts which serve as reliable marker beds for distinguishing fossil levels. These include: the Matabaietu Tufts (MT); the Equus Tuff (EQT), which unconformably overlies the Matabaietu Formation and apparently lies stratigraphically above the Dakanihyalo Member; and the Bodo Tufts (BT), which appear to bracket the hominid-bearing Upper Bodo Sand Unit (UBSU) of the Bodo Member and the Meadura Tufts (MET) of the Meadura Member (Kalb et al., 1980; Kalb, 1993) (Fig. 2). The EQT and the MET-1 may prove to correlate with the BT-1 and the BT-2, respectively. Near the top of the Awash Group are the unconformable Middle Pleistocene Awash (MPA) Gravels, which are overlain by the Andalee Member and the Halibee Tuff (HT), which lies at the base of the undifferented Halibee beds.

3. Temporal distribution of elephantoids As summarized in Figs. 2 and 4, the presence of a Tetralophodon longirostris-like elephantoid in the Chorora Formation (10.5-10.7 Ma) conforms with the Tortonian age (~9.0-11.5 Ma) for this species in Europe and North Africa (Kalb et al., in press a). The Anancus in the Adu-Asa and lower Sagantole formations (>4.1---6.0 Ma), similar to A. kenyensis from Member A of the Lukeino Formation (Lukeino-A) (~5.6-6.2 Ma), Kenya, indicates that this conservative form preceded the progressive tetralophodont Anancus from Langebaanweg, South Africa (~4.0--4.5 Ma) and a pentalophodont Anancus from the Sagantole Formation (>3.6~<4.1 Ma) (Kalb and Mebrate, 1993). Stegotetrabelodon orbus, Stegodibelodon, cf. S. schneiderL Stegodon cf. kaisensis, and Primelephas gomphotheroides are all confined to the Adu-Asa Formation. As mentioned, the total fauna from this formation is consistent with an early Pliocene to late Miocene age (~4.5-6.0 Ma). The elephantoids and associated taxa compare with those from such units in Kenya as the lower Kanam beds (~4.5-5.2 Ma) (Pickford, 1986), Unit 1 at Lothagam (~5.5-6.0 Ma) (Smart, 1976), Lukeino-A (~5.6-6.2 Ma) (Tassy, 1986), and

361

Kolinga/MenaUa, Chad (~4-5 Ma) (Coppens, 1965, 1972). Stegodon in the lower Adu-Asa Formation is one of the earliest records of the genus in Africa, while its presence in southern Ethiopia in the lower Shungura Formation (~ 3.0 m.y) is the latest confirmed record (Beden, 1987). The association of Primelephas and the early loxodont, "cf. Loxodonta" (of the Loxodonta Group; sensu Kalb et al., in press b) from Lukeino-A, is the earliest record of these genera occurring contemporaneously in Africa (Tassy, 1986). Loxodonta adaurora is present in the eastern Awash in the upper Sagantole Formation and in the Hadar Formation (>2.9-3.9 Ma), while L. exoptata (also of the Loxodonta Group) is present in the middle Hadar and Matabaietu formations (~2.0-3.0 Ma) (Kalb and Mebrate, 1993; White et al., 1993). The early Elephas, E. recki brumpti, is also present in the eastern Awash in the upper Sagantole Formation (3.4-3.9 Ma), and reportedly in the middle Hadar Formation (> 2.9-~ 3.0 Ma), while E. ekorensis is reportedly present in the lower and middle Hadar levels (Beden, 1985). More derived forms of E. recki (E, r. shungurensis) come from the upper Hadar and Matabaietu formations (~2.0-3.0 Ma) and the Wehaietu Formation (E. r. recki) (Kalb and Mebrate, 1993). Finally, an early elephant, similar to Mammuthus subplanifrons from South Africa, is present in the eastern Awash in the upper Sagantole Formation (and possibly in the Kuseralee Member) while a more derived mammoth is reported in the Hadar Formation and possibly the uppermost Sagantole Formation (~3.0-3.9 Ma) (Maglio and Hendey, 1970; Mebrate and Kalb, 1981; Beden, 1985; Kalb and Mebrate, 1993).

4. Paleobiogeography of African elephantoids As indicated in Fig. 4, the "gomphotheres" (as this term is used in the informal sense), specifically T. longirostris, Paratetralophodon hasnotensis (from the Siwaliks) and Anancus, appear to have had a Eurasian origin in the middle Neogene (~8-16 Ma), along with the early elephantid Stegolophodon (Tassy, 1983, 1990; Kalb et al., in press a). Reports of Stegotetrabelodon of Turolian

362

ZE. Kalb/Palaeogeography, Palaeoclimatology, Palaeoecology114 (1995) 357 368

age from China and Abu Dhabi argue for a Eurasian origin for this genus as well (Madden et al., 1982; Tobien et al., 1988; A. Hill, pers. commun., 1993), as opposed to an African origin (Maglio, 1973). Successive onsets of reduced global sea levels and temperatures during the early and middle Tortonian (at ~10.6 Ma and ~8.3 Ma) (Haq et al., 1988) likely triggered the southern migration into Africa of T. longirostris, early Ananeus (like A. perimensis; Tassy, 1986), and the putative ancestor(s) of African elephantids (Fig. 4). "Mastodon" grandineisivus (sensu Tobien, 1978) and Stegotetrabelodon may also have migrated to Africa at this time. By the late Miocene ( ~ 6.0 Ma), A. kenyensis inhabited eastern Africa and by the middle Pliocene (4.0 Ma) more derived Anancus species covered the length of the continent, from South Africa (Langebaanweg) to North Africa (A. petroechii of Libya) and northeastern Africa (the Middle Awash) (Kalb and Mebrate, 1993). The African elephantids, Stegotetrabelodon, Stegodibelodon and Stegodon, inhabited Africa north of the Equator during the Messinian (Kalb et al., in press a). Stegotetrabelodon syrticus (= lybicus) is known only from north of the Sahara (Libya) while Stegotetrabelodon orbus is subSaharan (Maglio, 1973; Gaziry, 1987). Stegodibelodon schneideri inhabited the Chad basin (Coppens, 1972) while a similar form occupied the Middle Awash area (Kalb and Mebrate, 1993). Although both Stegolophodon and Stegodon have long been regarded as endemic to Asia (e.g. Osborn, 1942; Saegusa, in press), the cladistic analysis of Kalb et al. (in press b) suggests that both the immediate ancestors of Stegodon and its sister group, the Elephantinae, are of African origin, suggesting an African origin for Stegodon as well (Fig. 4). In this regard, Stegodon is found commonly in Africa in association with early

elephantids, the earliest elephants, and Anancus (Kalb and Mebrate, 1993). Kalb et al. (in press a) also suggest that the earliest migration of African elephantids into southern Asia apparently began with Stegodon. This may have occurred in the early or later Messinian (Fig. 4) accompanying well documented low sea levels in the Mediterranean, the Red Sea and elsewhere (Haq et al., 1988). Even more plausible, however, Stegodon may have crossed into Eurasia from Africa over land bridges that existed in the middle Messinian (Courtillot et al., 1980; Girdler, 1984) when lower sea levels and warmer temperatures during this period would have favored such a migration (Haq et al., 1988). Despite the rapid expansion and diversification of later elephants in Africa, Stegodon continued to survive on the continent until the middle Pliocene ( ~ 3.0 Ma) (Beden, 1987). In Asia, however, the genus flourished and lasted well into the Pleistocene in widespread association with elephants resulting from later migrations out of Africa (Maglio, 1973; Saegusa, in press).

5. Paleobiogeography of Awash elephantoids: A model In terms of the paleobiogeographic distribution of elephantoids in the Afar Depression, a model is given in Fig. 3 showing the location of sedimentary depositional centers throughout the region from the late Miocene to the Holocene, which served as a locus of habitats for elephantoids and other animals. In particular, this model focuses on the location of paleolake basins within the unique tectonic setting of the Afar based on the known stratigraphy and sedimentology of the Awash Group. As a subsiding and expanding subaerial triple

Fig. 3. A schematic model of paleolake basin chronology and biostratigraphy showing elephantoid assemblages in the Awash Valley of the Afar triple junction. Attenuated rift zones lie between present day plate boundaries (simplified in heavy dashed lines) and uplifted plateaus (black), Arrows depict directions of rift openings. Positions of reconstructed Mio-Pleistocene lake basins are shown at their location at the time of deposition. Miocene depositional basins along the western escarpment are approximated based on Zannetin and Justin-visentin (1975), Merla et al. (1973) and Sickenberg and Sch6nfeld (1975). Legend schematically depicts ages of individual paleolakes (as specifically discussed in text) and downfaulted basins at successively lower elevations. Present day elevations (in meters above sea level ) reflect relative differences in elevations of paleolake deposits.

J.E. KalblPalaeogeography, Palaeoclimatology, Palaeoecology 114 (1995) 357-368

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junction between the African, East African and Arabian plates (Fig. l), the Afar area has "absorbed" sedimentary basins into the three hoststructures of the East African, Red Sea and Gulf of Aden rifts (Burke, 1977; Kalb, 1978; Girdler, 1984; Tiercelin, 1990) (Fig. 3). Basin development is best expressed by paleolakes that have not only migrated to lower and lower elevations onto a single subsiding rift floor, but also that have been drawn into a central zone of subsidence created by three separating plates (Pilger and Rrsler, 1975). Local basin geometry has been greatly influenced by the counterclockwise rotation of the Danakil horst, or "microplate", which presently straddles the southern end of the Red Sea and

forms much of the uplifted eastern area of the Afar Depression (Tazieff et al., 1972; Souriot and Brun, 1992). The southeast movement of this horst with the opening of the Red Sea resulted in intense grabenization of the central Afar, which allowed paleolakes in the western Afar to migrate to the north and east. Throughout their development from the late Miocene to the present these lakes have been fed by rivers flowing down, between, and along the base of plate margins, particularly by the tributaries that fed the proto-Awash River and that now flow into the present day Awash Valley. Tetralophodon cf. T. longirostris from the Chorora Formation comes from the oldest known

J.E. Kalb/Palaeogeography, Palaeoclimatology, Palaeoecology114 (1995) 357-368

(10.5-10.7 Ma) and formerly highest lake basin in the Awash region and is presently exposed along the uplifted southern Afar margin (Figs. 2 and 3). This early lake is indicated by diatomite-rich deposits presently lying between 1200 and 1400 m (above sea level) whose extent indicate a former lake the size of present-day Lake Turkana (Sickenberg and Sch6nfeld, 1975). To the north, along the foothills of the present day western escarpment (725-815 m), A. kenyensis and the early elephantids of the lower Adu-Asa Formation inhabited the region of a much younger ( ~ 5.5-6.0 Ma) and lower lake basin. This basin is indicated by thick diatomite deposits of the Adu Member, which thin out northwardly and are abruptly overlain by well-stratified and laminated tufts of the overlying Asa Member (Kalb et al., 1982c). A later (>4.4~5.0 Ma) but still early elephantoid fauna has been recovered from the Kuseralee Member, which crops out to the northeast between 640 and 665 m. Fluviatile or fluvio-deltaic deposits there appear equivalent to lacustrine sediments in the eastern Middle Awash that are overlain by thick LASS(?) basalts (Kalb et al., 1982d). The later elephantoids from the Sagantole Formation (>3.4~<3.9 Ma) are associated with a succession of fluvial, swamp, near shore, and northwardly thickening lacustrine sediments (575-665 m) that reflect both progressive tectonic subsidence as well as climatic fluctuations (Kalb et al., 1982d; Williams et al., 1986; Adamson and Williams, 1987; Kalb, 1993, figs. 11-13). The diverse elephants from the lower and middle Hadar Formation (Beden, 1985) (~3.0-3.40 Ma and 450-525 m) (Fig. 2) are associated with delta plain and lacustrine sediments that indicate a northeastwardly migrating lake subject to both large scale fluctuations and tectonism (Aronson and Taieb, 1981; Tiercelin, 1986). This lake would have been fed principally by rivers flowing southsoutheast (the Red Sea trend) and north-northeast (the East African Rift trend) analogous to the present day Mil6 and Awash rivers (Fig. 3). Thick, tuff-filled channels at Amado and Halsaiya (adjacent to the Mil6 River) northwest of Hadar (Fig. 3) and channels to the south in the Sidiha Koma Tuff (SHT) and the overlying Kada Hadar Tuff (KHT) (Fig. 2) at Hadar indicate successive rift-river sys-

365

terns feeding "Lake Hadar" (Johanson et al., 1978; Aronson and Taieb, 1981; Kalb, 1993). The 30-40 m thick Denen Dora Member comprising the middle Hadar beds reflects a mosaic of environments, and reportedly contains no less than five species of elephants (E. ekorensis, E. recki brumpti, L. adaurora, L. exoptata, Mammuthus sp.) (Beden, 1985), as well as a profusion of hominid fossils (Aronson and Taieb, 1981). The basal Denen Dora levels also indicate a period of maximum lake extension (Aronson and Taieb, 1981) and may correlate with lithologically similar deposits at the site of Geraru 35 km northeast of Hadar. Similarly aged fossils are also found at Gerara (Kalb, 1993), including the baked remains of an elephant found lying next to a basalt flow. The elephants (E. r. shungurensis, L. exoptata) of the upper Hadar and Matabaietu formations (,~ 2.0-3.0 Ma) are associated with distinctly fluviatile sediments (Aronson and Taieb, 1981; Kalb et al., 1982d; Beden, 1985), while the elephants (E. r. recki, L. africana?) of the Pleistocene Wehaietu Formation are associated with a combination of fluvial and lacustrine deposits (Kalb et al., 1982d; Kalb and Mebrate, 1993). Quaternary lake deposits with fossil remains overlying older strata exist over much of the Awash Valley and reflect continued lake development during wetter periods (Taieb, 1974; Kalb et al., 1982d). Deposits of the lower Awash Valley, however, best reflect the continued migration of lake basins into the central Afar Depression. Former Pleistocene lake shorelines (< 100,000? yr B.P., 450-500 m) and subaqueous basalt flows in the rhomb-shaped Kariyu plain east of Hadar, a series of late Quaternary shorelines surrounding the Tendaho-Asaita plain (<27,000 yr B.P., 380-410 m) and recent deposits surrounding Lake Abbe (< 2700 yr B.P., 240-315 m) (Fig. 3) reflect both oscillating climates and regional tectonism (Gasse and Rognon, 1973; Taieb, 1974; Rognon, 1975; Gasse and Street, 1978; Varet, 1978). Lake Abbe itself is the final drainage basin of the Awash River, a river that uniquely courses its way through the three arms of a triple junction. Elephants, certainly L. africana, were reported in the region as recently as the early 1930s (Largen and Yalden, 1987).

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6. Conclusions Uniquely, the elephantoid-bearing formations of the 1-km-thick Awash Group stretch from the northern margin of the East African plate (the Chorora area) to the increasingly attenuated northeastern margin of the African plate (the western Awash), to the fault-controlled interior basins of a splayed-out triple junction (the central Afar). Given the proximity of the Afar area to the Arabian plate and the ready access that animal populations have had to and from Arabia from Chorora times (10.6 Ma) to the end of the Miocene (5.2 Ma), it is apparent how and when Eurasian elephantoids ( Tetralophodon, Anancus, Stegotetrabelodon) may have crossed into Africa and when early Stegodon may have first crossed into Arabia and into southern Asia (also see: Kalb et al., in press a) (Fig. 4). Such faunal interchange is made particularly feasible based on reconstructed Afro-Arabian rift models (e.g. Pilger and ROsier, 1975; Courtillot et al., 1980; Girdler, 1984, 1991; Acton et al., 1991; Souriot and Brun, 1992) and times of major shifts in temperatures and sea levels (Haq et al., 1988). Also, it is apparent how later elephantoid populations were attracted to the Afar lowlands by following rivers down and along plate margins, and then into the central Afar along a progression of subsiding and migrating lake basins that served as ideal habitats for large herbivores with high water requirements. Overall, this model has broader implications on the step-bystep process of animal migrations into and dispersal across inter-continental areas prior to complete plate separation. Finally, another look at the hominid-rich Hadar Formation shows that it is a product of a depositional center that lay astride two intersecting rift systems and proximal to a third (Figs. 1 and 3). As such, the depositional and preservational setting of the area lies at the locus of tectonomagrnatic forces that both split continents apart and helped create ideal animal habitats and burial grounds between continents. Thus, the history of Hadar, with its "Lucy" skeleton and whole skeletons of elephants, is one sandwiched within the much longer and more complex history of an

evolving triple junction that by any standards makes the Afar region so extraordinarily unique.

Acknowledgements For helpful comments, information support I am grateful to my former colleagues, also D. Froehlich, G. Bell, J. P. Tassy, H. Saegusa, A. Azzaroli, C. Saenz, Eve Davis and W.M. Reid.

or other RVRME Shoshani, Jolly, M.

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