The Pliocene sites of the Djurab region (Chad, Africa) have yielded 19 avian remains. They belong to seven different taxa: a cormorant, Phalacrocorax cf. carbo, a darter, Anhinga cf. melanogaster, a heron (Ardeidae), a Ciconiidae of the tribe Leptoptilini, the size of the extant African species of marabou stork or Saddlebill, an extinct marabou stork (or adjutant) of large size – Leptoptilos sp. B, an Anserinae (swan or goose), and a duck (Anatinae). These birds indicate freshwater environments, a slow river or a lake. Also represented in the landscape were open as well as forested areas, like gallery forest. The avifaunas are thus well congruent with other paleoenvironmental results obtained at these sites, particularly regarding the humid character of the region in the Pliocene, and generally of the climate for that period in a part of the Sahel which is now desertic.
The researches led in the Djurab desert (northern Chad, Africa) by the MPFT (Mission Paléoanthropologique francotchadienne) yielded major contributions in paleoanthropology as well as in other fields of paleontology for the African Mio-Pliocene [Brunet et al., 1995, 1996, 1997, 2002]. Some bird remains were found between 1996 and 2000 in Koro Toro (KT) and Kossom Bougoudi (KB) areas. These are described here, and some interpretations are given regarding paleoenvironment and paleobiogeography.
Geological and biochronological context
KT and KB are both situated about 700 km N-E of N’Djamena and Lake Chad, and are dated biochronologically at respectively 3–3.5 Ma and about 5 Ma [Brunet et al., 1995, 1997 ; Brunet and MPFT, 2000]. They belong to the « paleo-Lake Chad » system, and the fossils were mainly found in fluviatile sandstones [Duringer et al., 2000].
Pelecaniformes – Phalacrocoracidae –Phalacrocorax cf. carbo
Distal left ulna, KB24-97-019 (pl. I, fig. 1). Morphologically this fragment resembles the genus Phalacrocorax, and its size indicates the species P. carbo, the only living cormorant of this size in Africa and most of Eurasia. None of the extinct cormorants or shags described from Africa reaches the size of P. carbo.
Anhingidae – Anhinga cf. melanogaster
Distal left carpometacarpus, KT12-00-002 (pl. I, fig. 2), proximal left carpometacarpus, KT12-00-003 (pl. I, fig. 3), distal right ulna, KT13-96-389, distal left ulna, KT13-96-534 (pl. I, fig. 4), and four bones found together and almost certainly belonging to a single individual, all labelled KB3-98-099: a fragment of a right coracoid, a fragment of a left scapula, a right distal humerus (pl. I, fig. 5), and a left distal carpometacarpus. Morphologically all of them match the genus Anhinga, and the distal ulnae are especially close to the living Old World species A. melanogaster. They also correspond in size to this species. All extinct species of anhingas are morphologically different, and/or larger in size than A. melanogaster and the Chadian fossils.
Family incertae sedis Ardeidae
A fragment of coracoid, KB24-97-021, indicates the presence of a heron, but no diagnostic feature is preserved that could allow a more precise identification.
Ciconiiformes – Ciconiidae –Leptoptilini indet. – sp. A
A fragmentary distal part of a right tibiotarsus, KB3-97-315 (pl. I, fig. 6) belongs to either Leptoptilos or Ephippiorhynchus, and in size is similar to the same element of the African L. crumeniferus or E. senegalensis. Leptoptilos sp. B
This species is represented by an incomplete left tibiotarsus, KB3-97-161 (pl. I, fig. 7). The robust proportions indicate the genus Leptoptilos, as opposed to Ephippiorhynchus, and the specimen is larger than any of the living species at genus. Other material from the Pliocene of Ethiopia allows a more precise identification as an extinct species that is also represented in Asia (Louchart et al. in prep.). cf. Leptoptilos sp. B
Two parts of a left carpometacarpus, the proximal, KT13-96-504 (pl. I, fig. 8), and the distal end, KT13-98-004 (pl. I, fig. 8), probably belong to the same species as the KB3-97-161 tibiotarsus, but the incompleteness of these remains does not allow exclusion of the genus Ephippiorhynchus. Leptoptilos sp. indet.
An incomplete juvenile right tibiotarsus, KT 13.00.012 (pl. I, fig. 9), belongs to the genus Leptoptilos, as indicated by its relatively robust proportions. However, the juvenile condition and the fragmentary preservation prevent more precise identification.
Anseriformes – Anatidae – Anserinae
A proximal left humerus, KT13-96-518 (pl. I, fig. 10), belongs to a fairly large species of the Anserinae. Anatinae indet.
A fragmentary distal left ulna, KB24-97-020 (pl. I, fig. 11), belongs to a duck, about the size of the Gadwall, Anas strepera or the Pintail, A. acuta. However, the bone is too incomplete to allow even a generic identification.
Paleoenvironment and paleobiogeography
Except Leptoptilos, all the birds of KT and KB (tabl. I) are strictly aquatic. The best represented, the darter Anhinga cf. melanogaster, preferentially inhabits freshwater slow streams or lakes and eats fish. The cormorant Phalacrocorax cf. carbo indicates open fresh or marine water and feeds mainly on fish in shallow water. The heron indicates a humid environment. The Leptoptilini indet. – sp. A – of KB may indicate dry savanna or swamps. Leptoptilos sp. B may also indicate open country. The juvenile Leptoptilos sp. indet. of KT indicates breeding nearby, probably in trees as for the genus today. Duck or goose-like taxa also most likely required aquatic environments.
Today the spatially closest populations of Phalacrocorax carbo and Anhinga melanogaster are found around Lake Chad [Del Hoyo et al., 1992]. These differences of distributions from today are due to the moistness of the area in the Pliocene. Studies of other groups (especially mammals, reptiles and fish) and the sedimentology indicate for KB and KT mixed environments of gallery-forest, savanna with open herbaceous areas, and areas of temporary streams occasionally becoming lakes [Brunet et al., 1995, 1997 ; Brunet & MPFT, 2000 ; Duringer et al., 2000]. The aquatic element of this environment is representative of the paleo-Lake Chad system, which was considerably more extensive than the Lake Chad of today [Duringer et al., 2000]. The avifauna is therefore in accordance with previous studies, indicating the moistness of the Pliocene climate in this part of Sahel, which is desert today.
The birds of the Djurab Pliocene are essentially aquatic. They belong to modern genera, and two or three are indistinguishable from extant species, whereas one is extinct: a Leptoptilos marabou stork. Their ecological requirements indicate a humid environment, lake or slow stream. For Koro Toro, the presence of trees is suggested, beside dry or humid open areas. All of this is in accordance with other results. The birds are especially indicative of the humid regional climate which allowed the considerable extension of paleo-Lake Chad at that period.