Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 134–139

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Fishes and palaeogeography of the African drainage basins: Relationships betweenChad and neighbouring basins throughout the Mio-Pliocene

Olga Otero a,⁎, Aurélie Pinton a, Hassan Taisso Mackaye b, Andossa Likius b,Patrick Vignaud a, Michel Brunet a,c

a Institut international de Paléoprimatologie, Paléontologie humaine: Evolution et Paléoenvironnements (IPHEP), UMR CRNS 6046, Université Poitiers, SFA, avenue du recteur Pineau,F-86 022 Poitiers, Franceb Département de Paléontologie, Université de N'Djaména, BP 1117, N'Djaména, Tchadc Collège de France, Chaire de Paléontologie humaine, 3 Rue d'Ulm, F-75 231, Cedex 05, Paris, France

⁎ Corresponding author.E-mail address: olga.otero@univ-poitiers.fr (O. Otero

0031-0182/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.palaeo.2009.01.005

a b s t r a c t

a r t i c l e i n f o

Article history:

The Neogene African fossi Received 10 July 2008Received in revised form 9 December 2008Accepted 19 January 2009

Keywords:MiocenePliocenePalaeobiogeographyAfrica

l record of two fishes, i.e. Semlikiichthys (Perciformes indet.) and Calarius(Siluriformes, Ariidae) is reviewed including new material from Chadian outcrops. Their distributionconfirms the existence of a Nilo-Sudan ichthyological province. During an early part of the Miocene an east–west waterway is demonstrated to have crossed Africa. Allopatric speciation in the Chadian and East Africanareas suggests that a split occurred in this system prior to 7 Ma. Moreover, our data support the existence ofhydrographical connections between the Chadian basin and the Syrte basin, after 7 Ma, at about the Mio-Pliocene boundary. Finally, our study also demonstrates that connections between the Niger and Chad basinsexisted at the very end of the Miocene.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

During the Neogene, global climatic changes and geodynamicprocesses strongly influenced faunal distribution in Africa by reshufflingecological barriers. In particular, the development both of a north–southrifting-related uplift in eastern Africa (e.g. Baker et al., 1972), and of aridenvironments in the Saharan zone since the LateMiocene (Schuster et al.,2006), dramatically modified the geographical context of vertebratedispersal between the Nilo-Sudan sub-basins (Roberts, 1975). Because ofthe scarcity of palaeontological data, the rhythm and tempo of theenvironmental modifications and of the related faunal exchanges arepoorly documented. In that context, the Central African faunas from Chadare crucial data sets. Indeed, the four Chadian fossiliferous areasdiscovered by the Mission Paléoanthropologique Franco–Tchadienne(MPFT) have opened four successive Neogenewindows on the vertebratefauna history in Central Africa (Fig. 1; Brunet et al., 1997, 1998, 2000;Vignaudet al., 2002): TMdatedat 7.04±0.18Ma;KBat 5.26±0.23Ma;KLat 3.96±0.48 Ma, and KT at 3.58±0.27 Ma (Lebatard et al., 2008). Theirinterest is enhanced by the first discoveries of fossil hominids far west ofthe Rift Valley, i.e. the Pliocene Australopithecus bahrelghazali (Koro Toro,Brunet et al., 1995) and the Late Miocene Sahelanthropus tchadensis(Toros-Menalla, Brunet et al., 2002, 2005).

One of the peculiar features of the Chadian Mio-Pliocene assemblagesis the richnessof fossilfish,whichhavebeen systematicallycollected in the

).

ll rights reserved.

four areas since 1997. The distribution through time of freshwaterdependant taxa provides some evidence of the palaeogeography of theChad basin. Our current study of the Chadian fish faunas leads us to focuson two fish taxa, whose presence in the Chadian areas and in other Nilo-Sudan basins (Fig. 1) has implications for palaeobiogeography. Theperciform fish Semlikiichthyswas first identified (as Lates rhachirhynchus)in eastern DR Congo deposits (Greenwood and Howes, 1975), and subse-quently in western Turkana basin deposits (Stewart, 2003a,b). A palatinefrom Wadi Natrun in Egypt (Greenwood, 1972), which was identical tothose of S. rhachirhynchus, was also thought to be of that species(Greenwood and Howes, 1975). Recently, a new species was described inChadianLateMiocenedeposits (Oteroet al., 2008). Freshwaterariid catfishare today only known in the Niger basin but they are present in PlioceneChadian (Brunet et al., 2000) and Libyan (Gaudant, 1987) deposits.

The fossil record of these two fishes is established using unpublishedChadian data. The systematic status and habitat are discussed below sothat thedistributionof the taxa caneither be attributed to endemism, or toexchanges between neighbouring basins during the Mio-Pliocene. Thepalaeobiogeographical information from the fossil fish is discussed in thelight of the available geological and environmental data, and comparedwith biogeographical information based on mammal taxa.

2. Material and geological context

The Chadian ariid material from Kossom Bougoudi, Kolle and KoroToro is described for the first time here (so far, their presence has onlybeen noted in the area of Kossom Bougoudi (Brunet et al., 2000)). These

Fig. 1. Age and location of the Neogene Chadian fossiliferous area and of the other fossiliferous outcrops cited in the study. Definition of the ichthyological provinces according toRoberts (1975). Datings of the Neogene Chadian fossiliferous area from Lebatard et al. (2008).

135O. Otero et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 134–139

three fossiliferous areas are located in the Eastern part of the Djurabdesert (Fig. 1), and have yielded rich vertebrate faunas includingmammals, squamats, turtles, crocodiles, birds and fish (Brunet et al.,1997,1998, 2000). The fossiliferousdeposits consistmainlyof siliciclasticseries dominated by weakly lithified sandstone interbedded withargillaceous mudstone which corresponds to marginal deposits of afluvio-lacustrine system that may have occupied a great part of bothnorth and south sub-basins of the Chad Chari basin; they are locallycovered by lacustrine diatoms resulting from lake transgression(Schuster, 2002; Vignaud et al., 2002). Cosmogenic nuclide dating ofthe three formations has dated Kossom Bougoudi at 5.26±0.23 Ma,Kolle at 3.96±0.48Ma and Koro Toro at 3.58±0.27 Ma (Lebatard et al.,2008).

The rest of the material mentioned in our paper comes fromseveral basins included in the Nilo-Sudan province, and has beendescribed and figured. They include an ariid basioccipital from the Eo-

Fig. 2. Anatomical characters that enable to attribute Semlikiichthys material. Neurocraniummaxillae in labial views; palatine in (g) median and (h) ventral views; (i, j) dentaries in labMenalla, Chad; Otero et al., 2008, modified), (c, e, i, k) belong to S. rhachirhinchus (Lake ARG17.575–577 (reverse); i: part of lot of fragments of dentaries from point 1; k: part of lot

Sahabi deposits in Libya (Gaudant, 1987), and Semlikiichthys materialfrom Toros Menalla in Chad (Otero et al., 2008), from Lower Niledeposits in Wadi Natrun, Egypt (Greenwood, 1972), from Lake Albertand Edward deposits in the D. R. Congo (Greenwood and Howes,1975;Van Neer, 1992; Otero and Gayet, 1999), and from Turkana basindeposits in Kanapoi and Lothagam, Kenya (Stewart, 2003a,b).

3. The record of Semlikiichthys perciform fishes

Semlikiichthys fishes are only known from Mio-Pliocene Africansites. They were long thought to belong to the genus Lates(Perciformes: Latidae). Phylogenetic study demonstrates that thespecies belongs neither to the genus Lates nor to the family Latidae(Otero, 2004), and so far it remains Perciformes incertae sedis (Oteroand Gayet, 1999). Semlikiichthys is distinguished by several osteolo-gical characters, some of which preserve well in fossil specimens;

in (a) left and (b) ventral views; (c) vomer in ventral view; (d) premaxilla and (e, f)ial views and (k) first centrum in anterior view. (a, b, d, f–i) belong to S. darsao (Toros-lbert–Lake Edward rift region; b: paratype, part of RG17.510–512; e: paratype, part ofof 1st vertebrae from point 2).

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characters include: vomer with an anterior crest (Fig. 2a), basiocci-pital with facets for the Baudelot's ligament in ventral position (Fig.2b), premaxilla with frontward projecting ascending process (Fig. 2d),the position of the articular process (Fig. 2e) and the tooth patch onthe palatine (Fig. 2g and h), the dentary with a ventrally-directedventral plateau (Fig. 2i; broken in Fig. 2j), and the shape of thearticular facets of the first vertebra (Fig. 2k).

Semlikiichthys fishes are known in four basins of the Nilo-Sudanprovince during the Mio-Pliocene. The species S. rhachirhinchus wasfirst identified in Lake Albert and the Semliki Region deposits dating tothe Late Miocene and Pliocene, and from an outcrop doubtfullyattributed to the Early Pleistocene (Greenwood and Howes, 1975; VanNeer, 1992; Otero and Gayet, 1999). The same taxon is also recognizedin Upper Miocene and Pliocene deposits of Lothagam and Kanapoi,Kenya (Stewart, 2003a,b). A second species, S. darsao, has beendescribed in the Upper Miocene deposits from Toros-Menalla (Oteroet al., 2008). The two species differ in a few osteological characters,such as the extension of the vomerine tooth patch which is large inS. rhachirhinchus and reduced in S. darsao (Fig. 2b and c); theornamentation and position of the surface for the levator muscle onthe maxilla (Fig. 2e and f); and the lateral position of the anterioropening of the dentary canal which is lateral in S. rhachirhinchus andventral in S. darsao (Fig. 2i and j). A dentary found in Kolle (Pliocene,Chad) is attributed to S. sp. cf. darsao. Semlikiichthys material fromother fossiliferous areas (including one vertebra and one fragmentarypremaxilla from Kossom Bougoudi [Lower Pliocene, Chad, OO pers.obs.], and one palatine from the Lower Pliocene ofWadi Natrun, Egypt,described by Greenwood (1972)) is referred to Semlikiichthys sp.,because specific characters are not preserved. For the same reason, wealso refer to Semlikiichthys sp. some Early Miocene elements from R. D.Congo that Greenwood and Howes (1975) attribute to Semlikiichthyssp. cf. rhachirhinchus (for details, see Otero et al., 2008).

Fig. 3. Anatomical characters that enable to attest the presence of Arius fishes. (a) posterior papparatus in left lateral view; (d) posterior half of a dorsicranium in dorsal view; (e–g) dorsal sp48, d: KB7-97-44, e: KT13-96-382, f: KL2-99-025, g: KB3-97-108) and (b) from Sahabi (Gauda

4. The record of freshwater ariid catfishes in Africa

Siluriformes, or catfishes, preserve relatively well as fossils,primarily because of their skull bones, vertebrae and pectoral anddorsal spines which may be diagnostic at least at a familiar level.The family Ariidae includes 10 genera with about 120 species. Mostof them inhabit New and Old World coastal waters and are marine(and often brackish tolerant) as were Eocene ariids identified fromNigeria (White, 1926), Congo and Angola (Darteville and Casier,1942; Casier, 1957). Today in continental Africa, freshwater ariidcatfishes are represented only by the endangered species Calariusgigas (previously Arius gigas, for taxonomic details see Marceniukand Menezes, 2007), which is endemic in the Niger basin (Daget,1992). The freshwater ariid African fossil record is limited to veryLate Miocene and Pliocene deposits in Chad and to Neogenesediments in Libya (Gaudant, 1987). Ariid catfishes are distinguishedby the basioccipital fused with the Weberian apparatus (the catfishcharacteristic anterior vertebral complex), behind a deep hollow anda prominent ventral spur (Fig. 3a and b) which is apomorphic forthe subfamily Ariinae (Acero and Betancur, 2007); the vertebralcomplex is ventrally covered by a bony flange so that the aorticgutter only opens posteriorly (Fig. 3c); the otic region is largelyinflated (Fig. 3a); in dorsal view, the neurocranium is notablycharacterised by the presence of five pairs of bones connected withthe parieto-supraoccipital (Fig. 3d), instead of three or four inother African catfishes; the second dorsal fin spine is distinguishedby a large foramen on a bulbous prominent articular medianprocess and a double tuberculated row at the anterior edge ofthe body of the spine (Fig. 3e–g).

In Kossom Bougoudi and Koro Toro several skull fragments,Weberian apparatus and fin spines confirm the presence of ariidfishes (Fig. 3a,c–e and g), whereas in Kolle only one weathered

art of a neurocranium and (b) basicranium in ventral views; (c) lower half of a Weberianines in anterior views. (a, c–g) fromChadian fossiliferous areas (a: KB4-97-104, c: KT13-96-nt, 1987, modified).

Fig. 4. Ariid and Semlikiichthys fishes distribution through time in the African basins (references in the text).

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basioccipital and one ragged dorsal spine were recovered (Fig. 3f).Ariid fishes therefore are present since at least the very Late Miocene/Early Pliocene times in the Chadian basin (Kossom Bougoudi is datedat 5.26±0.23). No ariid remains were recovered from Toros-Menalla,and they were probably absent in the Chadian basin at that time(7 Ma). This absence cannot be explained by ecological characteristicsas similar depositional environment that yielded ariid in the Plioceneare present in Toros-menalla (OO pers. obs.). The absence of ariids atToros-Menalla is also not due to fewer remains as many more fishfossils were collected than in the Pliocene areas.

Outside Chad, the only other ariid fossil record is a basioccipitalfrom Sahabi, in Libya, attributed by Gaudant (1987) to ?Arius. Thisbasioccipital (Fig. 3b) belongs to an ariin because it carries all thetaxonomic characters. However, in our opinion the skull roof fragmentthat Gaudant (1987) attributed to the Ariidae may belong to anothercatfish. The Sahabi ariid was certainly a freshwater fish for tworeasons. First, it was associated with strictly freshwater fish (Gaudant,1987; Boaz, 1987a). Second, there are no marine or brackish ariidfishes in the Mediterranean Sea, neither living nor fossil, so that theappurtenance of the Sahabi fish to the freshwater African ariid lineageappears muchmore parsimonious. The age of the Sahabi ariid is underdiscussion and still unsolved; for Boaz it is extremely Late Miocene orvery Early Pliocene (Boaz, 1987b).

So far, freshwater ariid fishes have not been collected inPleistocene and Holocene sites outside the Niger basin (Jousse andVan Neer, in press).

Fig. 5. Palaeogeographical sketch proposed to explain the Semlikiichthys and freshwater ariiprovinces according to Roberts (1975).

5. Discussion

5.1. LateMiocene split of the ancient Trans-African hydrographical network

At 7 Ma, Semlikiichthys sister species are present in Chad and ineastern Africa (Fig. 4). This distribution implies, first, that the tworegions were previously hydrologically connected and, second, thatthis ancestral area was subsequently disrupted (Fig. 5A). The firstassertion is supported by the great similarity of the ichthyologicalassemblages of Toros Menalla (Chad, Vignaud et al., 2002, OO. persobs.) and contemporaneous levels of Lothagham at Lake Turkana,Kenya (Stewart, 2003a,b) and of the Semliki River that links LakeAlbert and Edward in the eastern Congo (Greenwood and Howes,1975). Concerning the second assertion, a vicariance event slightlybefore 7 Ma between the eastern and western part of the Nilo-Sudanprovince is confirmed for fishes and also for mammalian taxa, notablypotamophilous ones, with different species of hippos in the two sub-basins (Boisserie et al., 2005). The existence of separated, eastern andwestern, areas at 7 Ma is also supported by the presence ofanthracotheres at Toros-Menalla whereas they are absent in con-temporaneous levels in eastern Africa (Lihoreau et al., 2006).

This evidence, that an ancient hydrographical system existedbetween the Chad basin region and eastern Africa, and that it splitduring the Miocene, is consistent with the assertion of McCauley et al.(1982, 1986). According to these researchers, an east–west flowinghydrological system developed during the Late Eocene and Oligocene,

d fossil record. A, B, C are explained in the text. Definition of the modern ichthyological

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with high productivity during the Early Miocene. They hypothesise anorigin along the western margin of the Red Sea in Sudan and Ethiopia,and across possibly to the Atlantic via Lake Chad. That drainage systemwas separated from the Congolese region by the Central African ThrustZone, which existed at least since the Early Cenozoic according toStankiewicz and de Wit (2006). McCauley et al. (1986) also postulatethat the system gradually disrupted during the Late Miocene becauseof intraplate doming.

This geological context helps to explain the homogenous fish faunadistributed throughout the Nilo-Sudan province, based on bothmodern (e.g. Worthington andWorthington,1933; Poll, 1957; Roberts,1975; Beadle, 1981; Greenwood, 1983; Lévêque, 1997) and fossil data(e.g. Stewart, 2001; Otero and Gayet, 2001). The disruption in easternand western regions of the hydrographical system, before 7 Ma,encouraged speciation in the newly separated areas. Further isolationof the Chadian basin from the eastern African region is suspectedbecause of endemism in several mammal groups in the Chad basin,such as, for instance, hippos and aardvarks (Boisserie et al., 2003;Lehmann et al., 2004, 2005).

5.2. A Pliocene trans-Saharan ichthyological region

In the western and central zones of the Nilo-Sudan region and inNorth Africa, the Neogene fish record is scarce; therefore, the Chadianfossil areas offer a unique data set in Central Africa. Some older dataexist in Tunisia (Greenwood, 1973; Mannai-Tayech and Otero, 2005).There is no Neogene fossil record in the Niger basin. And finally at leastpart of the northern Libyan deposits from Sahabi contains fossil fishand is nearly contemporaneous with Chadian fossil areas (see above).The Nilo-Sudan affiliation of these fishes was noted by Gaudant(1987), but their origin was unknown. The presence of exclusivelyfreshwater ariid fishes in the modern Niger, in the three PlioceneChadian fossiliferous area, and at Sahabi (Fig. 4) suggests: (1) devel-opment of an Atlantic ariid lineage in the Niger freshwaters(connected or not with Chad) before the Mio-Pliocene boundary,(2) probable regular connections between both the Niger and Chadianbasins through the Pliocene (Fig. 5B), and (3) the existence of aconnected drainage system between the Chadian and Libyan basins atabout the Mio-Pliocene boundary (Fig. 5C).

The first point of the scenario proposed, which suggests a uniqueSouth-Atlantic origin for the African freshwater ariid fish, is preferredto separate marine origins, one Atlantic and one Mediterranean.Indeed, the absence of any living ariid in the Mediterranean Sea andof any fossil in Mediterranean deposits favours the hypothesis of anAtlantic origin for the modern freshwater Calarius gigas. Thishypothesis is also supported by their presence in the Chadian Mio-Pliocene and Pliocene deposits. The freshwater ariids may haveoriginated in the Atlantic-related Niger basin and then entered theChad basin via connections that may have been frequent or evenpermanent during humid periods. Sporadic connections between thetwo drainage systems still exist today in the Mayo-Kébi (Lake Léréarea). Earlier, ariids were apparently absent in Chad before the end ofthe Late Miocene. This might be due to the lack of a connection atthat time between the two basins, or more simply to the absenceof these catfishes themselves in the African freshwater before thevery end of the Miocene. Little is known about the ecology of themodern and endangered C. gigas (Daget, 1992). However from theirpalaeoecological study, Jousse and Van Neer (in press) concludethat like many other large predatory fish, they would not toleratevery shallow and stagnant waters. They may prefer open waters andwould only occasionally enter marginal waters during the floodseason. This is consistent with the habitat preference of the fishesfound in fossil assemblages with C. gigas in the Chadian Plioceneareas and with the environmental reconstruction at Sahabi. This wouldimply that the river system used had open waters with permanentflow.

Concerning the second point of the scenario, further frequentconnections between the Chad and Niger drainage system may havehelped to maintain the presence of Chadian ariid populations throughthe Pliocene (Fig. 5B), at least from 5.26±0.23 Ma (KossomBougoudi) to 3.58±0.27 Ma (Koro Toro). However, the Chadian ariidsmay have been isolated for 2Myr from the Niger, whether connectionsexisted or not.

Finally, a freshwater connection between the Chadian and the Libyanbasins may have existed about the Mio-Pliocene boundary and allowedthe passage of ariids from Chad to the Syrte basin (Fig. 5C). The SahabiNilo-Sudan ichthyofauna has thus at least a partial trans-Saharan origin(ones cannot exclude a mix with former inhabitants of the Syrtan basin).According to Griffin (2002), the Eo-Sahabi River originated on the easternslopes of the Tibesti Mountains, and may have connected to the largefluviolacustrine system of palaeo-Lake Chad through the low elevatedcorridor between the western Tibesti and the southeast Ennedi. Drakeet al. (2008) also suggest a possible route along the western part of theTibesti, through the Libyan Fazzan basin and possibly toward the Syrtebasin. Lihoreau et al. (2006) demonstrate the existence of a Chado-Syrtanmammalian province about 7 Ma, by the presence of a singleanthracotheriid species (Libycosaurus petrochii) in both the site of Toros-Menalla (7.04±0.18 Ma) and contemporaneous levels at Sahabi. Theexistence of this province implied at least sporadic hydrographicalconnections between the north Libyan zone and the south Chadian one,because of the aquatic ecology of this species. Our data support theexistence of hydrographical connections later in time, at least until the riseof ariid catfishes in theChadianbasinand their spreadseaward to theSyrtebasin, at about the Mio-Pliocene boundary. These connections must havebeen open and reasonably deep waters, as Calarius gigas could not haveinhabited shallow,marginalwaters (Jousse andVanNeer, inpress). At last,we have to note that our hypothesis isweakened by the poor preservationof the Sahabi ariid fossil and its controversial dating (LateMioceneor EarlyPliocene). Further sampling in Libya with precise stratigraphic positionwould enable to confirm our hypothesis and precise the hydrographicalconnections with the Chad basin through time.

6. Conclusion

Our study confirms the existence of a Nilo-Sudan ichthyologicalprovince first evidenced on modern taxa (e.g. Roberts, 1975; Green-wood, 1983; Lévêque, 1997). In the fossil, the province is welldocumented for the Late Neogene (see Stewart, 2001) and wasdiscussed for the Early Miocene by Otero and Gayet (2001) whoobserve its extent onto the Arabic plate at this time. Here, thisprovince is demonstrated to have occupied during an earlier part ofthe Miocene a trans-African region that may correspond to thedrainage system described by McCauley et al. (1982, 1986). Prior to7 Ma allopatric speciation in the Chadian and East African areassuggests a split of this drainage system (Figs. 4 and 5A), in associationwith rifting. Moreover our study demonstrates that connectionsbetween the Niger and Chad systems existed since at least the veryend of the Miocene (Figs. 4 and 5B). We are also able to extend theSyrto-Chadian hydrographical connections from 7 Ma until about theMio-Pliocene boundary at 5.3 Ma (Figs. 4 and 5C). The disappearanceof ariid catfishes in the Chadian basin after the end of the Pliocenemight be due to the intensification of arid climatic phases that affectedthe Chadian basin. The Niger Savannah river systemmay have acted asa refuge during periods climatic of drying (Lévêque, 1997).

Acknowledgements

We thank Chadian authorities (MENR: Univ. N'Djaména, CNAR)and French authorities (NESR: CNRS; Univ. Poitiers; MAE: DCSURParis; French Embassy in Ndjamena, Chad; FSP, Project no.2005-54 ofthe Franco-Chadian cooperation). The MPFT fieldwork in Kolle area inthe late 90s was also supported by the CNRS “PeH” program. Current

139O. Otero et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 134–139

research and fieldwork are supported by ANR (Projet ANR 05-BLAN-0235), ECLIPSE (CNRS), and NSF (RHOI). We extend gratitude to D.Baudet and L. Tack for access to the palaeontological collections in theMRAC (Belgium), to O. Crimmen for access to the ichthyologicalcollections at the NHM (UK), to Wim Van Neer for access to his owncollection of dry skeletons, and also to all members of the MPFT andGhislaine Florent and Carine Noël for administrative guidance. FinallyI wish to thank the anonymous reviewer and Alison Murray forreviewing this paper. Drawings by O. Otero. The English wasextensively reviewed by L. Foley-Ducrocq.

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