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Original article
The early/late Pliocene ichthyofauna from Koro-Toro,Eastern Djurab, Chad§
L’ichtyofaune du Pliocène inférieur/supérieur de Koro-Toro, Est Djurab, Tchad
Olga Otero a,*, Aurélie Pinton a, Hassan Taisso Mackaye b,Andossa Likius b, Patrick Vignaud a, Michel Brunet a,c
a Institut de paléoprimatologie, paléontologie humaine, évolution et paléoenvironnements (iPHEP),CNRS-UMR 6046, université de Poitiers, SFA, 40, avenue du Recteur-Pineau, 86022 Poitiers, France
b Département de paléontologie, université de N’Djaména, BP 1117, N’Djaména, Tchadc Chaire de paléontologie humaine, collège de France, 3, rue d’Ulm, 75231 Paris cedex 05, France
Received 6 July 2009; accepted 9 October 2009
Available online 1 February 2010
Abstract
This is the first extensive study of a freshwater fish fauna from the Pliocene site of Koro-Toro (Chad), aged 3.58 � 0.27 Ma. The assemblageincludes an abafish (Mormyriformes, Gymnarchidae: Gymnarchus), a tigerfish (Characiformes, Alestidae: Hydrocynus), six different catfishes(Siluriformes, Ariidae: Carlarius; Bagridae: Bagrus; Claroteidae: Clarotes and Auchenoglanis; Mochokidae: Synodontis; Clariidae: Clarias orHeterobranchus), perciform fishes (Perciformes, Latidae: Lates sp. cf. niloticus, and Cichlidae indet.), and a pufferfish (Tetraodontidormes,Tetraodontidae: Tetraodon). The diversity is relatively low when compared with other Chadian Neogene sites. This is probably mostly explained bythe wind erosion of the outcrops being responsible for the lack of minute remains. However, we recognize that the aquatic environment recordedcorresponds to open waters.# 2010 Published by Elsevier Masson SAS.
Keywords: Ichthyofauna; Early/late Pliocene; Chad; Central Africa; Continental palaeoenvironment
Résumé
Nous proposons ici la première étude complète de l’ichtyofaune d’eau douce du site tchadien de Koro-Toro, daté à 3,58 � 0,27 Ma.L’assemblage comporte un gymnarche (Mormyriformes, Gymnarchidae : Gymnarchus), un poisson-tigre (Characiformes, Alestidae : Hydro-cynus), six poissons-chats différents (Siluriformes, Ariidae : Carlarius; Bagridae : Bagrus; Claroteidae : Clarotes et Auchenoglanis; Mochokidae :Synodontis; Clariidae : Clarias ou Heterobranchus), des perciformes (Perciformes, Latidae : Lates sp. cf. niloticus et Cichlidae indet.) et un poissonglobe (Tetraodontidormes, Tetraodontidae : Tetraodon). La diversité est relativement faible lorsqu’on la compare avec celle des autres sites connusdu Néogène tchadien. C’est probablement dû à la déflation éolienne des sites, entraînant l’absence des restes fossiles de petites dimensions. Onreconnaît néanmoins une association typique des environnements aquatiques africains continentaux ouverts.# 2010 Publié par Elsevier Masson SAS.
Mots clés : Ichtyofaune ; Pliocène inférieur/supérieur ; Tchad ; Afrique centrale ; Paléoenvironnement continental
1. Introduction
The African Neogene faunas are mainly known throughmaterial from East and South Africa. The fossils collected bythe Mission Paléoanthropologique Franco-Tchadienne(MPFT) during the last two decades in Chad, Central Africa,led to enlarge the field of vertebrate palaeontology by thediscovery of four fossiliferous areas in the Chadian Saharan
Geobios 43 (2010) 241–251
§ Corresponding editor: Gilles Escarguel.* Corresponding author.
E-mail address: [email protected] (O. Otero).
0016-6995/$ – see front matter # 2010 Published by Elsevier Masson SAS.doi:10.1016/j.geobios.2009.10.003
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desert basin called Djurab (Fig. 1). Koro-Toro is the youngest ofthese fossiliferous areas. Moreover, in the sites KT12 andKT13, it has yielded the only australopithecines known outsidethe Rift Valley, notably Australopithecus bahrelghazali (Brunetet al., 1995, 1996). The associated mammal fauna gives anestimated age at about 3–3.5 Ma (Brunet et al., 1995, 1997),and cosmogenic nuclide dating provides an absolute age at3.58 � 0.27 Ma (Lebatard et al., 2008). Indeed, the Koro-Toroarea not only provides unique palaeontological data in CentralAfrica around the early/late Pliocene boundary, but it alsoprovides information on the early hominid geographicexpansion and the associated environments.
So far, the palaeontological studies of the Koro-Torofaunas concentrated on the assemblages, mainly themammals, of the sites KT12 and KT13 that yielded theaustralopithecine remains from Chad (respectively Brunetet al., 1995, 1997). These two similar assemblages arecharacterized by a certain provincialism illustrated in severalmammal groups (e.g., primates, bovids, carnivores) andconfirmed by systematic reviews of the hippos (Boisserieet al., 2003) and bovids (Geraads et al., 2001). Moreover,these two studies provide information on the terrestrialenvironment in Koro-Toro. The scarcity of hippos may belinked to a rather dry environment (Boisserie et al., 2003).Among bovids, the open country grazers largely predominate,although the members of reduncines show that the grasslandwas not dry and the wood cover reduced (Geraads et al.,2001). Indeed, the mammal fauna assemblage as a wholeindicates rather open and dry environments in a regionbordering a lake (Brunet et al., 1997), with supremacy of C4plant feeders (Zazzo et al., 2000). Associated with thisterrestrial environment, the presence of aquatic environmentis ascertained by sedimentary data (Schuster, 2002) and byvarious taxa, notably birds such as a swan or a goose, and aduck (Louchart et al., 2004), by turtles, crocodiles, otters,
hippos, and also fishes (Brunet et al., 1997). Concerning thelatter, the fish diversity in KT has long been under-estimated.Preliminary faunistic lists record large catfishes (Siluri-formes) in KT12 (Brunet et al., 1995) and one Lates (the NilePerch) and two catfishes (Siluriformes: Ariidae and Bagridae)in KT13 (Brunet et al., 1997). Indeed, an accurate review ofthe material reveals the presence of at least ten different fishtaxa in Koro-Toro, where catfish remains largely predomi-nate. This provides original information on the fish diversityin Central Chad around the early/late Pliocene boundary. Thismay also inform on the aquatic environment that existed atthat time in the Djurab region whereas the terrestrialenvironment showed a certain drought trend.
In this paper, we describe and determine the fish remainsfrom the Koro-Toro area (Fig. 1). This extensive study concernsall the fish fossils collected in the sites of the area. It follows andcompletes a first attempt to study the evolution of the Chadianfish diversity that concentrates on two taxa that are absent inChad today, the catfish Carlarius and the perciform Semli-kiichthys (Otero et al., 2009a). After estimating the taphonomicbias, the diversity observed is analysed through the comparisonwith more ancient Chadian fish assemblages – the slightly olderearly Pliocene assemblage from Kolle, dated at 3.96 � 0.48 Ma(Otero et al., 2009b), and the Late Miocene assemblage fromToros-Menalla, dated at 7.04 � 0.18 Ma (Otero et al., 2006,2007, 2008, in press) – and with the modern ichthyofauna thatinhabits the Chadian basin. The ecological features of the taxaconstrain the reconstruction of the aquatic environment inKoro-Toro.
2. Geological context
The formation in Koro-Toro consists mainly of siliciclasticseries dominated by weakly lithified sandstone interbeddedwith argillaceous mudstone and diatomite; they correspond tofluvio-lacustrine environments which may have occupied alarge part of both north and south sub-basins of the Chad-Charibasin (Schuster, 2002). About 40 sites were identified in thefossiliferous area among which are two hominid-bearing sites(KT12 and KT13) that have been described in Brunet et al.(1995, 1997). The associated mammal fauna first gives anestimated age at about 3–3.5 Ma in KT12 (Brunet et al., 1995,1997). Then, cosmogenic nuclide dating of the argileous pelitesthat yielded the holotype fossil of Australopithecus bahrel-ghazali provided an age of 3.58 � 0.27 Ma in KT12 (Lebatardet al., 2008). Fish fossils were collected at eight sites in theKoro-Toro area (KT2, 4, 10, 12, 13, 15, 27, 40).
3. Material
The list of the Koro-Toro fish material is available inAppendix A. The specimen number refers to the fossiliferoussite and to the year of collection: e.g., KT12-96-05 is the fifthspecimen collected in 1996, at site 12 of Koro-Toro. Afterexamination, the fish fossils collected by the MPFT will behoused in the collection of the Centre national d’appui à larecherche (CNAR, N’Djaména, Republic of Chad).
Fig. 1. Age and location of the fossiliferous areas of Central Africa on atopographical map. TM: Toro-Menalla; KB: Bossom Bougoudi;, KL: Kolle.
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The material for comparison is composed of late Mioceneand early Pliocene fossils from Chad, described in Oteroet al. (2006, 2007, 2008, 2009a, in press), and of prepareddry-skeletons: the catfish Clarotes laticeps from thereference collection of Wim Van Neer (Institut des SciencesNaturelles, Brussels); the catfishes Clarotes laticeps andHeterobranchus sp., from the dry-skeleton collection in theNatural History Museum (Zoology Department, London);and mormyriform (Gymnarchus niloticus), characiforms(Hydrocynus brevis), siluriforms (Arius thalassinus, Bagrusbajad, Auchenoglanis biscutatus, A. occidentalis, Synodontisclarias, S. schall, Clarias sp. cf. gariepinus), perciforms(Lates niloticus, Oreochromis niloticus) and tetraodontiform(Tetraodon lineatus), from the reference collection of OlgaOtero.
4. Systematic palaeontology
Order MORMYRIFORMESFamily GYMNARCHIDAEGenus Gymnarchus Cuvier, 1829Gymnarchus niloticus Cuvier, 1829Gymnarchus sp. cf. niloticusFig. 2Material: One tooth from KT12 (Appendix A).Description and attribution: The teeth of Gymnarchus
(abafish) vary in shape from incisiform at the symphysis tocaniniform distally on the jaw. The single tooth collected inKoro-Toro might thus be symphysal. The lingual face is slightlymore flattened than the convex labial one (Fig. 2(A)). At thecrown, both faces join in a delicately serrated cutting edge(Fig. 2(B)); the cross section is oval at the base of the tooth. Inall these characteristics, the tooth is similar to the single livingspecies of the family, Gymnarchus niloticus, and is thusattributed to this taxa.
Comments: Living abafishes are endemic to Nilo-Sudanesestreams. These big fishes frequently reach over 1 m in length.They are carnivorous hunters and require swampy areas forreproduction (Bailey, 1994). From its dimensions, the abafishtooth from Koro-Toro corresponds to a specimen with a totallength greater than 1 m.
Order CHARACIFORMESFamily ALESTIDAEGenus Hydrocynus Cuvier, 1817Hydrocynus sp.Fig. 3Material: About two dozen Hydrocynus teeth have been
collected in KT12 and KT13 (Appendix A).Description and attribution: Several Hydrocynus
(tigerfish) teeth have been collected in Koro-Toro. They varyfrom about 5 mm to about 15 mm in height. They are typicallyunicuspidate, pointed, with a labio-lingually compressed crownand an enlarged base showing a festooned outline in proximalview (Fig. 3). The teeth also typically present a small notch atthe base of the cutting edge. The morphology of the teeth doesnot allow any specific determination.
Comments: Tiger-fishes are the only African alestid fisheswith unicuspidate pointed teeth organized in one row. Sevenmodern Hydrocynus species inhabit the African fresh waters ofthe Nilo-Sudanese, Zaire, East coast and Guinean provinces(Froese and Pauly, 2008), including three species in the Chad-Chari basin (Paugy, 1990). These pelagic fishes, known for theirvivacity and voracity, are strictly ichthyophagous predators(ibid.). The bigger Hydrocynus fishes in Koro-Toro may havebeen over 1 m in length according to their teeth size.
Order SILURIFORMESFamily ARIIDAEGenus Carlarius Marceniuk and Menezes, 2007cf. Carlarius sp.Fig. 4Material: About a dozen large bony elements of ariid fishes
have been collected. There are neurocranial fragments,basioccipital/Weberian apparatus complexes and fin spines(Appendix A).
Description and attribution: Several skull bones andneurocranial fragments, together with pectoral and dorsalspines belong to ariid catfishes. The neurocranial fragmentsshow constant characters of the ariid catfishes. In dorsal view,the neurocranium is notably characterized by the presence offive pairs of bones connected with the parieto-supraoccipitalinstead of three or four in other African catfishes (Fig. 4(A, C)).In ventral view, the otic region is largely inflated (Fig. 4(B)).
Fig. 3. Alestidae from Koro-Toro (part of KT12-98): (A) labial, (B) lateral and(C) lingual views of the tooth of a Hydrocynus fish.
Fig. 2. Gymnarchidae from Koro-Toro (part of KT12-98): (A) lateral and (B)lingual views of the tooth of a Gymnarchus fish.
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Moreover, the basioccipital fused with the Weberian apparatus isdistinguished by a deep hollow and a prominent ventral spur(Fig. 4(B, D)). The vertebral complex is ventrally covered by abony flange so that the aortic gutter only opens posteriorly(Fig. 4(D)). The second dorsal fin spine of ariid fishes isdistinguished by a large foramen on a bulbous prominentarticular median process and a double tubercle row at the anterioredge of the body of the spine (Fig. 4(E, F)). In all their preservedcharacters, these fossils resemble Carlarius gigas (previouslyArius gigas, for taxonomic details see Marceniuk and Menezes,2007) in its description by Jousse and Van Neer (2009). This isalso the case of a fragmentary cleithrum (Fig. 4(G)) whichnotably exhibits rows of tubercles and a pectoral spine whichshows a deep and wide proximal process (Fig. 4(H, I)).
Comments: Today, in continental Africa, ariid fishes areonly known by the endangered species Carlarius gigas, thegiant catfish, endemic to the Niger inner Delta Basin. Owing toits scarcity, very little is known about the ecology of this
freshwater fish (Jousse and Van Neer, 2009). Body lengths ofabout two metres have been reported (Daget et al., 1988), andthe present study will show that it also reached that size in thepast. The largest cranium from Koro-Toro corresponds to aspecimen of about 1 m long according to the regressionformulae used in Jousse and Van Neer (2009).
Family BAGRIDAE sensu Mo, 1991Genus Bagrus Bosc, 1816cf. Bagrus sp.Fig. 5Material: A single fragment of neurocranium (Appendix A).Description and attribution: Only one fossil can be
referred to this family. It consists of a neurocranial fragmentshowing in dorsal view the posterior part of the right frontal anda thin fragment of the left one (on both sides of the posterior endof the anterior fontanel and of the anterior tip of the posteriorone), and the anterior half of the right sphenotic. The fibrous
Fig. 4. Ariidae from Koro-Toro: (A) posterior fragment of a neurocranium with the Weberian centra connected and the supraoccipital process lacking, in dorsal view(KT13-96-66); (B) posterior fragment of a neurocranium in ventral view (KT12-95-66); (C) parieto-supraoccipital in dorsal view (KT13-96-467); (D) ventral part of aWeberian apparatus in left lateral view (KT13-96-48); (E, F) second dorsal spine in anterior and lateral view (KT13-96-380); (G) articular and humeral part of a leftcleithrum in lateral view (KT13-96-109); (H, I) articular head of a left pectoral spine in anterior and posterior views (KT12-95-66). All belong to Carlarius fishes.
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ornamentation observed on the flat skull roof bones is typical ofthe genus Bagrus (Fig. 5(A)). In lateral view, most of theposterior half of the parasphenoid is preserved, showingmarked surface for ligaments at the back and horizontal flangesbelow the pterosphenoid (Fig. 5(B)).
Comments: Bagrus is an African genus of the Afro-Asianfamily Bagridae. Among the ten modern species, two areknown in the modern Chad-Chari basin (Risch, 1992).Depending on species, Bagrus fishes are demersal orbenthopelagic, but they are all carnivorous predators, withadults feeding mainly on fishes.
Family CLAROTEIDAE sensu Mo, 1991Subfamily CLAROTEIDINAE sensu Mo, 1991Genus Clarotes Kner, 1855cf. Clarotes sp.Fig. 6(A–C)Material: A near complete neurocranium and pectoral
spines are referred to Clarotes; other pectoral spines areattributed to ?Clarotes (Appendix A).
Description and attribution: A ragged neurocranium andnine pectoral spines are attributed to large claroteidin fishes,probably Clarotes (Fig. 6(A–C)). Among others, the roof showsthe typical tubercle ornamentation, the mesethmoid hasdeveloped cornua (Fig. 6(A)), and the basioccipital is notfused with the Weberian apparatus. The pectoral spinestypically exhibit a large inner fossa, a small axial processand a projecting proximal process (Fig. 6(B and C)).
Comments: Two modern Clarotes species inhabit the riversof the Nilo-Sudanese province. They are demersal fishes andfeed on crustaceans, insects, molluscs and fishes. Since thefossil material often belongs to bigger fishes, it is generallyattributed to C. laticeps, which however only reaches 80 cmstandard length in living specimens.
Subfamily AUCHENOGLANIDINAE sensu Mo, 1991Genus Auchenoglanis Günther, 1865cf. Auchenoglanis sp.Fig. 6(D–G)
Material: A few pectoral and dorsal spines (Appendix A).Description and attribution: A few pectoral and dorsal
spines probably belong to Auchenoglanis fishes (Fig. 6(D–G)).The dorsal spine (Fig. 6(D and E)) resembles A. occidentalis,because of the triangular head associated with a few reduceirregular tubercles on the anterior edge (Otero et al., 2007). Themedial process is pointed, which is only observed inAuchenoglanis (Fig. 6(D)). The pectoral spine is characterizedby irregular tubercles on the anterior edge, a bulky articularhead with reduced inner fossa and a deep and wide proximalprocess (Fig. 6(F and G)), allowing differentiation from theresembling Synodontis.
Comments: The genus Auchenoglanis is represented by twoliving species both present in the Nilo-Sudanese fresh waters(e.g., Geerinckx et al., 2004). They are benthic insectivores thatmainly feed on aquatic insect larvae and eventually on smallmolluscs, alevins, and swimming insects (Lauzanne, 1988;Paugy, 1994). These feeding habits should also enable them towithstand a relatively wide range of ecological conditions(Paugy, 1994).
Family MOCHOKIDAEGenus Synodontis Cuvier, 1816Synodontis sp.Fig. 7Material: Several pectoral spines (one articulated in
the cleithrum fragment), 1 dorsal spine and 1 vertebra(Appendix A).
Description and attribution: The presence of Synodontisfishes is born out by a few dorsal and pectoral spines and afragment of a cleithrum with the head of the pectoral spinearticulated. At least one dorsal spine is attributed to the genus(Fig. 7(A and B)), due to its rounded medium-sized articularforamen, the longitudinal striae along the body of the spineand its anterior crest (Pinton and Otero, in press). Tworelatively well-preserved pectoral spines (Fig. 7(C–E)) areconfidently attributed to Synodontis because of a developedaxial process, a smooth articular plateau, a medium-sizedinternal fossa with a stout ventral wall, a ventro-dorsally
Fig. 5. Bagridae from Koro-Toro: dorsal (A) and left lateral (B) views of a neurocranium fragment (KT10-94-14b) of a Bagrus fish.
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compressed body of the spine, and stout conical tubercles atthe inner edge (Pinton et al., 2006). Some of these charactersare observed on the articular head in position in a cleithralfragment (Fig. 7(F)). This latter shows the typicallydeveloped humeral plate and its ornamentation (Pinton andOtero, in press).
Comments: Synodontis is a diverse catfish genus endemic toAfrica. It lives in different types of freshwater environmentsand displays various feeding habits depending on the species.Eleven modern species are present in the Chad-Chari basin.
Family CLARIIDAEGenus Clarias Scopoli, 1777Genus Heterobranchus St Hilaire, 1808cf. Clarias sp. and/or Heterobranchus sp.Fig. 8Material: Dozens of remains belong to clariid catfish. This
abundance is notably due to the presence of a specimen founddisarticulated (and somewhat ragged). Most frequent remainsare skull roof elements (Appendix A).
Description and attribution: The remains attributed toclariid fishes are a fragmentary neurocranial snout, a pectoralspine, an isolated supraoccipital process and a dozen of raggedbones from KT40 that may belong to a single neurocranium.Among others, the main characteristics observed on thesefossils are the forked mesethmoid overlying a wide toothedvomer (Fig. 8(A and B)), the triangular supraoccipital processthat lacks any articulation surface (Fig. 8(C)), and the pectoralspine that shows a reduced proximal process and striae on thearticular plateau (Fig. 8(D–F)) like observed by Gayet and VanNeer (1990) on clariid spines and by Otero and Gayet (2001)and following personal observation of two of us (OO, AP).
Comments: Clariids are euryhalin fishes that inhabit variousenvironments in Asia and Africa. Five clariid species live in theChad-Chari basin, i.e. two C. (Clarias), one C. (Claroides) andtwo Heterobranchus species (Teugels, 1992). They all have anair-breathing system allowing them to cope with poorlyoxygenated water, and even to survive out of the water for somehours and to migrate from one pond to another. However, theyprefer to live in the well-oxygenated shallow waters of riversand lakes (Teugels, 1986). There is at least one clariid species inKoro-Toro, possibly two.
Fig. 6. Claroteidae from Koro-Toro: (A) dorsal view of a near completeneurocranium (KT13-96-252); (B, C) proximal fragment of a right pectoralspine in posterior and ventral views (KT12-95-04); (D, E) proximal fragment ofa second dorsal spine in anterior and left lateral views (KT40-01-26); (F, G)proximal fragment of a left pectoral spine in anterior and posterior views (part ofKT10-94-02). (A–C) belong to Clarotes fishes and (D–G) belong to Auche-noglanis fishes.
Fig. 7. Mochokidae from Koro-Toro: (A, B) anterior and left lateral views of asecond dorsal spine (KT40-01-25); (C, D) anterior and posterior views of aproximal fragment of a right pectoral spine (part of KT10-94-02); (E) posteriorview of a proximal fragment of a left pectoral spine (part of KT10-94-02); (F, G)outer and inner views of a fragment of cleithrum showing the articular zone andpart of the humeral plate (KT40-01-21). All belong to Synodontis fishes.
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Family LATIDAEGenus Lates Cuvier and Valenciennes, 1828Lates niloticus (Linnaeus, 1758)cf. Lates niloticusFig. 9Material: Latid remains are the most abundant in the Koro-
Toro area. The most frequent are the vertebrae, but a widevariety of skull bones have been collected also, including awell-preserved near complete neurocranium (Appendix A).
Description and attribution: Accurate descriptions of theseveral latid species are available from Greenwood (1976) andOtero (2004). The remains collected in Koro-Toro fit in all thedetails that are preserved with the living specimens of Latesniloticus. Some diagnostic characters are clearly observed onthe following fossils (Fig. 9). On the neurocranium, the vomertooth patch exhibits a straight or convex posterior border,shapes that both exist in L. niloticus (type 1, Van Neer, 1989)but not in the other Lates species (Otero, 2004). The vomerinetooth sockets are similar to each other in size and shape and arenumerous as in most latid fishes including L. niloticus. Thefrontward projections of the mesethmoid (Fig. 9(A)) arecharacteristic of the family and are particularly stout, which isremarkable in L. niloticus (Otero, 2004). The projectingposterior process of the pterotic as well as low fronto-parietaland a developed supraoccipital crests are also typical of Lates(Fig. 9(B)). As usual on the maxilla of L. niloticus, the lateralplate on which the palatine ligaments insert is extendedanteriorly (Fig. 9(C)). The dentary is a robust bone with a large
and globally horizontal tooth patch of undifferentiated teeth(Fig. 9(D)), and a medially directed ventral plate where themandibular sensory canal runs in a bony tube with fouropenings (Fig. 9(E, F)). The glenoid cavity of angulo-articularis deep and placed anterior to the retro-articular bone(Fig. 9(G)). The quadrate is triangular in shape with a stoutposterior edge (Fig. 9(H and I)). The second centrum exhibitslateral fossa on each side of the articulation surface (Fig. 9(J));it is short in lateral view (Fig. 9(K)) and the posterior articularfacet is dorso-ventrally flattened (Fig. 9(L)) whereas theanterior is roughly squared in outline (Otero, 2004). Themedian fin spines of L. niloticus have a stout articular head andbody; the body of the spine exhibits longitudinal asymmetricaldepressions; the articular head shows marked triangular lateralsurfaces anteriorly and the foramen is small (Fig. 9(M)).
Comments: In Africa, living Lates fishes inhabit the freshwaters of the Nilo-Sudanese province, with a few endemicspecies in certain great lakes, notably Lake Tanganyika. TheNile perch, alias Lates niloticus, is widespread in many basinsof the province, including the Chad-Chari basin. Largespecimens need deep and well-oxygenated waters, whereasjuveniles are able to swim in the alluvial plain and prefershallow waters (Van Neer, 1994). They are predators and largespecimens are ichthyophagous.
Family CICHLIDAECichlid indet.Fig. 10Material: A few spines and vertebrae (Appendix A).Description and attribution: Perciform median fin spines
and vertebrae that do not belong to Lates niloticus are attributedto indeterminate cichlids. The median fin spines are character-ized by the width of their bodies in anterior view (Fig. 10(A)),their thinness in lateral view (Fig. 10(B)) and the high positionof articular facets on both sides of a rather large foramen inposterior view (Fig. 10(C)). It resembles tilapia fin spines, butfurther attribution is questionable (see below). One of the centrabears the well-developed parapophyses that occur in abdominalvertebra in cichlids (Fig. 10(D and E)).
Comments: Modern cichlids are very diverse in Africanfreshwater today notably in certain Eastern African lakes wherethey show the ability to rapidly speciate and form speciesflocks. Because of this high diversity and the correlated lack ofdatabase of African cichlid fin spines and vertebrae, we cannotreasonably attribute the scarce remains to a taxon or another. InChad, there are eleven extant species distributed in sevengenera, some of which (such as Oreochromis aureus, the BlueTilapia) show large ecological tolerance in both temperatureand salinity. The fossil material from Chad belongs to largespecimens over 30 cm in standard length, which is currentlyreached in many species, for instance the blue tilapia.
Order TETRAODONTIDORMESFamily TETRAODONTIDAEGenus Tetraodon Linnaeus, 1758Tetraodon sp.Fig. 11
Fig. 8. Clariidae from Koro-Toro: (A, B) ethmovomerian region in ventral anddorsal views (KT10-94-04a); (C) posterior half of a parieto-supraoccipitalshowing the triangular supraoccipital process in dorsal view (KT13-98); (D–
F) anterior, dorsal and posterior views of a right pectoral spine (KT40-01-23).All belong to Clarias or to Heterobranchus fishes.
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Material: Half a dozen tooth plates, corresponding to thedisarticulated jaw elements of the quadripartite beak ofTetraodon (Appendix A).
Description and attribution: Units of quadripartite beaktypical of tetraodontids are found as separate elements. Theyare formed as stacks of elongated tooth plates. They resemblethe only living Chadian species Tetraodon lineatus. In theabsence of any study of the variability in beak dimensions andgrowth in the living species, no further attribution of thismaterial is possible (Stewart, 2003).
Comments: Two modern puffer-fishes inhabit WesternAfrican fresh waters, including Tetraodon lineatus which ispresent in the Chad-Chari basin (Lévêque, 1992). It lives in
open waters with weed beds and vegetated fringes, and feeds onmolluscs (Bailey, 1994).
5. Fish diversity and associated aquatic environment
With a list of eleven taxa, the diversity recognized in Koro-Toro around the early/late Pliocene boundary is relatively lowwhen compared with other Neogene sites from the Djurabdesert, notably with the Miocene area of Toros-Menalla, thatyielded 20 taxa in the single site TM266 (Otero et al., in press)and with the early Pliocene area of Kolle, that yielded 19 taxa inthe single site KL2 (Otero et al., 2009b). Moreover, most of thefish taxa in the several sites of Koro-Toro are identified at a
Fig. 9. Latidae from Koro-Toro: (A, B) sub-complete neurocranium in lateral and dorsal views (KT13-96-65); (C) left maxilla in lateral view (KT13-96-261); (D–F)left dentary in occlusal, lateral and ventral views (KT13-96-360); (G) lateral view of the posterior fragment of a right angulo-articular articulated with the retro-articular (KT13-96-280); (H, I) posterior and lateral views of a left quadrate (KT13-96-112); (J–L) anterior, left lateral and posterior views of a second abdominalvertebra (KT4-094-14c); (M) median fin spine in anterior view. All belong to Lates sp. cf. niloticus fishes.
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generic or a familial level. Two factors combine to explain therelative low diversity and preservation quality of theassemblage in Koro-Toro. They are, first, the Aeolian weath-ering and the strong erosion of the Koro-Toro outcrops and,second, the absence of screening below 5 mm when sampling(for the impact of screening sampling methods on the fossildiversity, see Otero et al., 2009b, in press). The other alternateexplanation of the relatively low fossil diversity in Koro-Torowould have been that it corresponds to a real low diversity in theopen waters of the Eastern Djurab at the early/late Pliocene.This hypothesis is undoubtfully rejected because the fossilfishes present correspond to members of regular water streamsor lakes where highly diverse communities live (for instance,the presence of Hydrocynus and Lates), and not to taxafrequenting relict environments with low diversity commu-
nities. Moreover the fossil taxa missing are those with fragileand minute remains (such as Heterotis and Alestes/Brycinus).They are the first affected by high energy in the depositionalenvironment and in the modern outcrop erosion.
All the fishes of Koro-Toro inhabit the modern Nilo-Sudanzone as fossil or modern (when not extinct). Indeed, the extanttaxa identified in Koro-Toro still have living members in theChad-Chari basin except the Ariidae which are now confined tothe Niger Inner Basin but that inhabited the Chadian watersduring the early Pliocene (Otero et al., 2009a). The lowerapparent diversity when compared with the other fossiliferousareas from Central Africa (Fig. 1) is explained by taphonomicalbias towards robust elements of chiefly larger fish. When weconsider remains equivalent by the dimension and bone type inother outcrops and the same fraction in the modern Chadianichthyofauna, Koro-Toro perfectly takes place in an apparentichthyofaunic continuum documented sporadically since thelate Miocene in Toros-Menalla (Fig. 1). The basin probablysuffered isolation at certain time (e.g., Toros-Menalla) butalways preserved a frank Nilo-Sudan imprint (Otero et al.,2009a, in press), as observed in Koro-Toro. Indeed, the badpreservation in the latter outcrop prevents the recognition ofspecies and thus the identification of endemism. Moreover, thepresence of Carlarius might indicate strong water connexionwith the Niger basin if we exclude the hypothesis of a newspecies. The hydrographical system in Koro-Toro area at theearly/late Pliocene boundary clearly belongs to the Chaddrainage system and cannot be interpreted like isolatedtemporary systems. Indeed, it might have been also stronglyconnected with the rest of the Nilo-Sudan waters, notably at theWest (Otero et al., 2009a).
The palaeoenvironmental information is reduced because ofthe low precision in the determination and because the remainsmay have been somewhat transported before deposit. Theassemblage cannot be interpreted like a precise ecosystem thatfossilized in situ. However, it might be confidentiallyinterpreted at a large scale. All the fishes of the Koro-Toroarea show high affinities with open waters, includingGymnarchus and Clarias, despite that the first one needsswamps for reproduction and the second can stand anoxicwaters (but prefer oxygenated environments when possible).So, the fossil diversity in Koro-Toro corresponds to open watersof streams or lakes with swampy grassy margins orembayments. Such inferred paleoenvironmental conditions fitwith sedimentological evidences of a large fluvio-lacustrinesystem that may have occupied a large part of both north andsouth sub-basins of the Chad-Chari basin at that time (Schuster,2002).
6. Conclusion
Eleven fish taxa were identified in the Chadian Plioceneassemblage from Koro-Toro, aged around 3.5 Ma and locatedin the Eastern Djurab. The strong Aeolian weathering affectsmost of the fossils and is responsible for the low systematiclevel determination and lack of many taxa in the sample. Thislevel of determination prevents discussion on the evolution and
Fig. 11. Tetraodontidae from Koro-Toro. (A, B) left dentary, in inner and outerviews (part of KT13-98); (C, D) right dentary, in inner and outer views (part ofKT12-98, reversed). All belong to Tetraodon sp.
Fig. 10. Cichlidae from Koro-Toro: (A–C) median fin spine in anterior, leftlateral and posterior views (part of KT10-94-02); (D–E) abdominal vertebra inanterior and left lateral views (part of KT10-94-02).
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possible endemicity of the Chadian ichthyofauna in the early/late Pliocene, whereas a certain provincialism was illustrated insome mammal groups, such as bovids (Geraads et al., 2001) andhippos (Boisserie et al., 2003). However, the review of the fishecological preferences indicates that aquatic environmentsregistered correspond to open waters with a frank Nilo-Sudanimprint: the open water belongs to a larger Chadian fluvio-lacustrine drainage system and cannot be considered like anisolated one. This is important data to evaluate the drainagesystem size and connectivity at the time of climate drought.
Acknowledgements
We thank Chadian authorities (MENR: Université deN’Djaména, CNAR) and French authorities (MESR: CNRSUniversité de Poitiers; MAE: DCSUR, Paris; FSP and SCAC,Ambassade de France à N’Djaména). The MPFT fieldwork inKoro-Toro area in the late 1990s was also supported by theCNRS ‘‘PeH’’ program and the study by ANR (Project ANR05-BLAN-0235) and NSF (RHOI). We extend our gratitude toWim Van Neer for access to his comparison collection and alsoto all members of the MPFT who participated to the fieldmissions in the Eastern Djurab. Photographs by O. Otero.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.geobios.2009.10.003.
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