The material was collected from the foreland of the Gebel Abyad Plateau in NW Sudan, about 2.5 km east of an eastern spur of the plateau and around 150 km west of Dongola (Fig. 1A, B). The fossils were found in a saddle-shaped depression between two mounds on a north-south-trending ridge, in sediments of the upper part of the Kababish Formation (Fig. 1C, D). They had weathered out at the site and were exposed at the surface, but still embedded in the sediments. At the fossil site, the Kababish Formation consists of approximately 15.5 m of siltstones and mudstones (Fig. 2A), with intercalated lenses of sandy siltstone up to 50 cm thick and 7 m wide (Fig. 2B), and some heavily iron-cemented layers up to a few centimeters thick. The siltstones and mudstones form heterolithic beds (Fig. 2A) and upward-coarsening and upward-fining sequences, mostly 0.5–2.5 m thick (Fig. 2A). Some siltstones show current ripple or wave ripple structures, and siltstone-mudstone beds, occasionally flaser or lenticular bedding (Fig. 2C). However, most siltstone beds are massive and, particularly in the upper section, moderately to intensively bioturbated primarily by Spongelimorpha (Fig. 2D), as well as Ophiomorpha, Thalassinoides (Fig. 2E), Rhizocorallium, and Lockeia. The mudstones are predominantly horizontally laminated, often bioturbated by Thalassinoides and Rhizocorallium, and occasionally contain cm-scale mud lenses. Numerous gypsum veins and nodules, along with fossil plant remains, are present primarily in some mudstone beds (Fig. 2A). Occasionally, fossil root traces are present. The fossil ichthyofaunal remains were found in association with a Brachiosuchus kababishensis skeleton.

The Kababish Formation unconformably overlies the Wadi Howar Formation (Turonian–Santonian; Wycisk et al. 1990; Wycisk 1991). It is conformably overlain by marginal-marine dolomites and limestones of the Gebel Abyad Formation, which has been assigned to the Paleocene–Eocene, mainly based on the discovery of foraminifera (Barazi and Kuss 1987). The stratigraphic position of the Kababish Formation is further supported by correlations of low-stand and high-stand cycles with sedimentary sequences in the southern and central Western Desert of Egypt, as well as by paleontological correlations (Barazi 1985; Wycisk et al. 1990; Wycisk 1994). The shallow-marine deposits of the Kababish Formation are correlated with shallow-marine Campanian–Maastrichtian sediments in the Dakhla and Asyut Basins in Egypt (Barazi 1985; Wycisk 1994). Similar fossil plant communities occur in both the Kababish Formation and the Campanian–Maastrichtian sediments of the Western Desert of Egypt. For example, plant fossils such as Credneria, Ficophyllum, Magnoliaephyllum, and Typhaephyllum, which occur in the Kababish Formation (Barazi 1985), are also found in the Kiseiba Formation (Campanian–Maastrichtian) in southern Egypt near the border with Sudan, as well as in the Quseir Formation (Campanian–Maastrichtian) and the Mut Formation (Campanian) in the vicinity of the Dakhla and Kharga oases (Klitzsch and Hermina 1989). Lejalia leaves have been found in both the Kababish Formation (Coiffard and Mohr 2015) and the Mut Formation (Campanian) near the Dakhla Oasis (Coiffard and El Atfy 2023).

MfN SUD Museum für Naturkunde Berlin, Sudan Collection.

The predominantly fine-grained character of the sediments that contain the fossils indicates relatively low-energy depositional conditions. Sedimentary structures, including current and wave ripples, together with flaser and lenticular bedding, suggest periodic influence of currents, waves, and tidal processes (Reineck and Wunderlich 1968; Reineck and Singh 1980; Fan 2012). Similar sedimentary associations are widely interpreted as characteristic of protected tidal-flat environments with low depositional gradients, such as coastal embayment (Dashtgard et al. 2021). Within this framework, the sandy siltstone lenses are interpreted as probable tidal-channel fills, whereas the vertical succession of sandy siltstones, siltstones, and mudstones is consistent with deposition on a tidally influenced coastal mudflat. The observed fining-upward successions may represent progradational tidal-flat parasequences related to progressive shallowing (Desjardins et al. 2012).

The trace fossil association corresponds to a mixture of the Skolithos and Cruziana ichnofacies, which is typical of shallow marine nearshore-to-offshore shelf environments (Seilacher 2007; Pemberton et al. 2012; Knaust 2017). The dominance of deep- to medium-tier traces produced by crustaceans (Spongelimorpha, Ophiomorpha, Thalassinoides; Seilacher 2007; Knaust 2017) and polychaetes (Rhizocorallium; Knaust 2013), but also by bivalves (Lockeia; Seilacher and Seilacher 1994) is consistent with a wave-influenced tidal flat environment (e.g. Desjardins et al. 2012; Mángano and Buatois 2015). The iron-cemented beds may have formed during temporary subaerial exposure of the mudflat. Fossil plant remains occurring in several mudstone beds, as well as occasional fossil root traces, also point to a coastal, and at times even emergent environment. The gypsum veins and nodules in some mudstone beds were likely formed by diagenetic gypsum, possibly from pyrite (e.g., Bain 1990; Liu et al. 2018), which may have formed in the presence of organic matter and euxinic conditions during early diagenesis (Berner 1984). Overall, the deposition of the Kababish Formation at the fossil site appears to have occurred in a sheltered, low-energy, tide-influenced marginal-marine setting, likely a protected embayment or lagoon. Our interpretation of the fossil-bearing layers largely agrees with the interpretation of Barazi (1985) for the Kababish Formation. Based on the occurrence of trace fossils like Rhizocorallium and Thalassinoides, sedimentary structures such as flaser bedding, and rare foraminifera, Barazi (1985) interpreted the depositional environment of the Kababish Formation as shallow-marine, foreshore-to-nearshore.

The ecological preferences of the recovered taxa are consistent with a shallow marginal-marine ecosystem that probably supported abundant benthic invertebrates. Bathymetric constraints derived from the presence of Nebrius, which inhabits waters shallower than about 50 m, suggest a shallow marine to nearshore interpretation (Carrillo-Briceño et al. 2016). This is consistent with sedimentological and ichnological indicators that point to tropical coastal settings influenced by tides (Barazi 1985; Seilacher 2007; Pemberton et al. 2012; Knaust 2017). Pycnodontids are generally interpreted as specialized durophagous fishes adapted to crushing hard-shelled prey typical of coastal mudflats and lagoons (Kriwet 2001; Poyato-Ariza and Wenz 2002; Poyato-Ariza 2005), whereas Hybodontiform sharks are interpreted to have occupied a broader range of ecological roles, with some taxa potentially occupying a similar durophagous niche (Rees and Underwood 2006, 2008; Lane 2010). The robust tuberculated dorsal fin spines have been interpreted as defensive structures against predators (Maisey 1978; Parker 2008; Cowen 2019), although direct evidence of predator-prey interactions in the Kababish ecosystem is lacking.

From a paleobiogeographic perspective, the Kababish ichthyofauna is compatible with regional geological and paleontological evidence for marine influence extending into north-central Sudan during the Campanian–Maastrichtian. Sedimentological, ichnological, and paleontological observations suggest that the Kababish Formation was deposited in a marginal-marine setting that may have formed within a southern embayment of the Tethys. The occurrence of pycnodontids, whose evolutionary history is closely associated with Tethyan marine environments (Cawley and Kriwet 2024), provides an additional line of evidence consistent with, but not by itself demonstrating, this interpretation.

Faunal similarities between the Kababish assemblage and coeval deposits in Egypt and Morocco suggest possible faunal connectivity along the southern Tethyan margin. These similarities are consistent with broader regional paleobiogeographic patterns, including the occurrence of comparable shallow-marine assemblages (Cappetta 2012; Cawley and Kriwet 2024), and may reflect episodic dispersal of shallow-marine fishes into north-central Sudan during the Late Cretaceous. The occurrence of Nebrius, pycnodontids, and hybodontiforms in a marginal-marine setting is also consistent with ecological preferences documented for these groups elsewhere and suggests that estuarine and coastal environments may have provided suitable habitats along the southern margin of the Tethys. Accordingly, the Kababish Formation preserves a localized record of a Late Cretaceous coastal ecosystem that contributes to our understanding of vertebrate faunal distribution in northeastern Africa. However, because the present assemblage is based on a limited number of isolated specimens, these paleobiogeographic interpretations should be regarded as working hypotheses supported by the currently available geological, sedimentological, and paleontological evidence rather than as definitive conclusions.