Vertebrate paleontological exploration of the Upper Cretaceous succession in the Dakhla and Kharga Oases, Western Desert, Egypt

Highlights

• Vertebrate paleontological work on Qusier and Duwi formations from the Late Cretaceous of Egypt.

• The fossil bearing localities occur within the middle Campanian-early Maastrichtian.

• New age constraints on the examined rock units based on nannofossil taxa.

• Fossil remains include sharks, rays, actinopterygian and sarcopterygian fishes, turtles, and rare dinosaurs and crocodiles.

Abstract

The Campanian and Maastrichtian stages are very poorly documented time intervals in Africa's record of terrestrial vertebrate evolution. Upper Cretaceous deposits exposed in southern Egypt, near the Dakhla and Kharga Oases in the Western Desert, preserve abundant vertebrate fossils in nearshore marine environments, but have not yet been the focus of intensive collection and description. Our recent paleontological work in these areas has resulted in the discovery of numerous new vertebrate fossil-bearing localities within the middle Campanian Qusier Formation and the upper Campanian-lower Maastrichtian Duwi Formation. Fossil remains recovered from the Campanian-aged Quseir Formation include sharks, rays, actinopterygian and sarcopterygian fishes, turtles, and rare terrestrial archosaurians, including some of the only dinosaurs known from this interval on continental Africa. The upper Campanian/lower Maastrichtian Duwi Formation preserves sharks, sawfish, actinopterygians, and marine reptiles (mosasaurs and plesiosaurs). Notably absent from these collections are representatives of Mammalia and Avialae, both of which remain effectively undocumented in the Upper Cretaceous rocks of Africa and Arabia. New age constraints on the examined rock units is provided by 23 nannofossil taxa, some of which are reported from the Duwi Formation for the first time. Fossil discoveries from rock units of this age are essential for characterizing the degree of endemism that may have developed as the continent became increasingly tectonically isolated from the rest of Gondwana, not to mention for fully evaluating origin and diversification hypotheses of major modern groups of vertebrates (e.g., crown birds, placental mammals).

Introduction

Post-Cenomanian deposits from the Late Cretaceous of continental Africa are scarce (Haughton, 1963, Dingle et al., 1983, Mateer et al., 1992) and limited to only a few geographically restricted localities. Whereas the expanding terrestrial/freshwater vertebrate record from the ‘Middle’ Cretaceous (∼Aptian-Cenomanian) has improved considerably in recent years (e.g., Sereno et al., 2004, Gomani, 2005, Sereno and Brusatte, 2008, Gottfried et al., 2009, Cavina et al., 2010, O'Connor et al., 2010, Gorscak et al., 2014, Sertich and O'Connor, 2014), our understanding of biotic dynamics leading up to and through the K-Pg boundary in Africa and Arabia remains extremely limited. Moreover, the few vertebrate fossils that are known from the Late Cretaceous of Africa derive from temporally restricted stratigraphic intervals, precluding direct comparison of faunas. This issue is particularly problematic for the latest Cretaceous, where the rate of new discoveries pales in comparison with earlier parts of the Cretaceous record.

The Late Cretaceous is of great interest for paleontologists as it coincides with a number of significant global-level faunal changes, and culminates in the extinction of many vertebrate (e.g., nonavian dinosaurs, marine reptiles) and invertebrate (e.g., ammonites) groups. The final two stages of the Cretaceous (Campanian and Maastrichtian) represent a time when Gondwanan fragmentation culminated in the near-complete isolation of most of the southern landmasses and Africa in particular (e.g. Müller et al., 1993, Smith et al., 1994, Scotese, 1998, Hay et al., 1999, Pletsch et al., 2001 and Scotese, 2001), offering a significant mechanism that could have profoundly influenced the evolutionary trajectories of numerous terrestrial vertebrate clades. However, our working knowledge of the African terrestrial biosphere and vertebrate diversity during this critical period of isolation is currently too limited to allow for meaningful comparisons of faunal patterns that have been documented in the Late Cretaceous on other landmasses. Moreover, the current sampling bias also precludes a critical evaluation of hypotheses positing the origin of major vertebrate groups prior to the Cretaceous–Paleogene boundary (e.g., Ericson et al., 2006, Meredith et al., 2011, Jetz et al., 2012, Jarvis et al., 2014; although see Prum et al., 2015 for a recent synopsis regarding the temporal origin of modern birds).

Recently-developed Gondwana-wide (e.g., Sereno et al., 2004, Krause et al., 2006, Ali and Krause, 2011) and Africa-specific (e.g., O'Connor et al., 2006, Sertich and O'Connor, 2014, Gorscak and O'Connor,) models related to Cretaceous biogeography can only be tested and refined through intensification of paleontological work in the uppermost Cretaceous deposits exposed on the continent. One of the most promising regions where such data may be collected corresponds to the sparsely vegetated circum-Saharan areas that preserve varied depositional settings (e.g., fluviatile, estuarine, near-shore marine, etc.) like the Western Desert of Egypt. Rigorously characterizing biotas (e.g., Claeson et al., 2014) that are unearthed from these units should provide insight into physical processes at both local and regional scales (e.g., timing of Gondwanan fragmentation, subdivision of large terrestrial landforms by marine transgressions, etc.) thought to influence terrestrial/freshwater communities during this time.

The Upper Cretaceous deposits along the Abu Tartur Plateau (Fig. 1), and in particular near the Dakhla and the Kharga Oases, hold great promise for the recovery of fossil vertebrates that are necessary for characterizing latest Cretaceous African faunas. New discoveries are essential for formally evaluating biogeographic models or characterizing the relative endemicity/increasing provincialism (e.g., Sereno et al., 1994, Sereno et al., 2004) that may have existed on the African continent more generally during the Late Cretaceous. The Dakhla Oasis is situated in the southern part of the Western Desert about 150 km west of the Kharga Oasis, south of the Abu Tartur Plateau. A number of vertebrate clades have been noted from Cretaceous rocks in this region, either having been documented directly (e.g., Churcher, 1995; Rauhut and Werner, 1997, Lapparent de Broin and Werner, 1998, Churcher et al., 2006, Lamanna et al., 2004, Smith and Lamanna, 2006) or referred to indirectly as part of geological reports (e.g., Awad and Ghobrial, 1965, Klitzsch et al., 1979, Hendriks et al., 1984).

It has already been documented that sediments of the middle Campanian Quseir Formation exposed along the Abu Tartur Plateau (Fig. 1) preserve remains of freshwater gastropods, reptiles, dinosaurs, and terrestrial plant matter (Hendriks et al., 1984). The Quseir Formation reaches a maximum thickness of 70–90 m near the Dakhla Oasis and consists primarily of variegated shale (Hermina, 1990). Thus far, very little focused paleontological work has been conducted in this area due in large part to its remote location. Previous reconnaissance work in the eastern part of the Dakhla Oasis has led to the recovery and identification of crocodyliforms (cf. Dyrosaurus), dinosaurs (cf. Spinosaurus), turtles (“Podocnemis” aegyptiacus), lungfish (Ceratodus and Protopterus), bony fish, and sharks from the Quseir Formation (Churcher and de Iuliis, 2001, Claeson et al., 2014). With the exception of lungfish, however, none of these fossils have been described in any additional detail.

The upper Campanian/lower Maastrichtian Duwi Formation (Fig. 2) overlies the Quseir Formation and underlies the Maastrichtian-Paleocene Dakhla Formation (Tantawy et al., 2001), and contains phosphate beds in a sequence of alternating claystone, sandstone, siltstone, and conglomerate (Hermina, 1990). Dinosaur material (e.g., a single theropod tooth) from the Duwi Formation has been reported, but with unknown provenance other than it was recovered near Idfu in the Nile Valley (Smith and Lamanna, 2006). The Maastrichtian/Paleocene Dakhla Formation consists mainly of shales with sandstones, siltstones and marls, and preserves vertebrates and fossil wood (Tantawy et al., 2001). A femur of a sauropod dinosaur was collected from the Maastrichtian subunit (Ammonite Hill Member) of the Dakhla Formation just west of Mut by German geologists decades ago (Rauhut and Werner, 1997), but no further collecting has since been undertaken by vertebrate paleontologists.

Given the potential of these units for yielding vertebrate remains, the Mansoura University Vertebrate Paleontology (MUVP) project was initiated to carry out exploratory fieldwork in the poorly-sampled vertebrate-bearing strata of Egypt's Western Desert. Six expeditions conducted to date have identified numerous fossil-bearing localities, with specimens ranging from fragmentary material to complete cranial and postcranial skeletons.

Institutional abbreviations: MUVP, Mansoura University Vertebrate Paleontology Center at the Geology Department, Faculty of Science, Mansoura University, Egypt.