A fossil terrestrial fauna from Tobène (Senegal) provides a unique early Pliocene window in western Africa
Graphical abstract
Introduction
Hypotheses on early hominins temporal and spatial distribution across Africa were formulated on the basis of geographically restricted paleontological data. The informal concept of “cradle of humankind” – repeatedly recognized in different regions– perfectly exemplifies how grey areas in the fossil record can bias our understanding of early human history. Although Pliocene localities are common and intensively surveyed in eastern and southern Africa (Werdelin, 2010), this time period remains poorly documented in the rest of the continent, notably in northern and central Africa (Arambourg, 1979, Brunet et al., 1995, Brunet et al., 1998, Brunet et al., 2000, Werdelin, 2010, Zouhri et al., 2017). Researchers however acknowledge important biases in the fossil record when proposing new biogeographic hypotheses or conceptual models (e.g. Joordens et al., 2019), especially the lack of vertebrate remains in western Africa for this time period. Outcrops exploitable for fossil discoveries are rare in this area, mostly due to Cenozoic lateritic weathering, vegetation, and the widespread Quaternary sand cover along the southern fringe of the Sahara (e.g., Michel, 1973, Beauvais et al., 2008).
Recent prospections undertaken as part of the PaleoSen project between 2012 and 2015 uncovered several vertebrate fossils in the Taïba Ndiaye quarry, Senegal (Fig. 1). Most of these remains were found in limestone deposits embedded in the Lutetian-Bartonian phosphate ore formation, and belonged to marine vertebrates (Hautier et al., 2012, Hautier et al., 2014, Vautrin et al., 2019). Remains have also been excavated from the overlying continental deposits intercalated between the Eocene phosphate ore and the Quaternary sand cover in the Tobène pit of the Taïba Ndiaye quarry (Hautier et al., 2014).
This paper provides a description of the Tobène site, which hosts the only known Pliocene fauna from Senegal and well-beyond classical African Pliocene faunas, being 2,400 km distant from the nearest known Pliocene fossil site of Lissasfa, Morocco (Raynal et al., 1999) and Kossom Bougoudi and Kollé, Chad (Brunet et al. 1998, 2000) (Fig. 1). The Tobène locality, although poor in fossils relative to other Pliocene localities, is key to documenting the distribution of Pliocene ecosystems in Africa. Its age estimation was based on the combined study of biochronological markers and lateritic weathering events documented regionally and dated using geochronology (Beauvais et al., 2008). The taxonomic richness of the locality and the abundance of carnivorans is encouraging for future discoveries.
Section snippets
Geological context
The PaleoSen project is a collaborative framework between Senegalese and French paleontologists from the University Cheikh Anta Diop of Dakar and the Institut des Sciences de l’Evolution of Montpellier. In addition to numerous field expeditions led in Senegalese Paleogene outcrops, three fieldtrips (2012, 2013, and 2015) focused on prospecting the upper part of the Tobène pit in the Taïba Ndiaye quarry, i.e., in strata overlying the exploited phosphate ore (Hautier et al., 2014).
During ore
Description of cf. Osteolaemus
Class Reptilia Laurenti, 1768
Order Crocodylia Gmelin, 1789
Family Crocodylidae Cuvier, 1807
cf. Osteolaemus
Age of the fossiliferous conglomerate
The age of the Tobène fossiliferous locality is first discussed from the perspective of paleoweathering chronology, then from the perspective of biochronology.
Cenozoic climate changes over Sahelian western Africa led to punctuated erosion/weathering history of its landscape. During the Neogene, three main pediment systems (erosion and transportation land surfaces) formed under a dominantly semi-arid climate (Chardon et al., 2018). Each of these surfaces was weathered/duricrusted under
Conclusions
Most Sahelian sediments of putative late Cenozoic age are overlain by Quaternary sands that preclude surveys for fossil remains. In Senegal, Pliocene continental sediments are only exposed in mining quarries. The Tobène pit fauna shows that the latest iron duricrusts formed in West Africa in the late Pliocene may cap (and protect) continental strata that have the potential to fill a major gap in our understanding of African fossil faunas. Furthermore, the conspicuous and widespread occurrence
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank Mr El Hadji Mansour Sambe and Mr Ibrahima Gaye, authorities of the Industries Chimiques du Sénégal of the quarry of Taïba Ndiaye, for their logistic help and authorization. We also thank Faye Lassy of the ICS for his help in the field. The work benefited from the support of the Geology Department of the University Cheikh Anta Diop (Prof. E. Sow and Dr. Faaye Cisse S.). We are indebted to colleagues who allowed us accessing their collection: dir. Desalegn Abebaw, and T.
Funding
This work was supported by the CNRS PICS Grant; the International Exchange Scheme of the Royal Society; the National Geographic Society’s Global Exploration Fund (Northern Europe); the Sidney Sussex College (Cambridge, UK) and the IRD.
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