Geochemical investigation of the taphonomy, stratigraphy, and palaeoecology of the mammals from the Ouled Abdoun Basin (Paleocene-Eocene of Morocco)

https://doi.org/10.1016/j.palaeo.2021.110523Get rights and content

Highlights

  • Rare earth element (REE) chemistry of fossil bones traces stratigraphic origin.

  • Marine diagenesis of terrestrial remains indicated by REE, Ca/P and 87Sr/86Sr.

  • The δ18OPO4 and δ13C of mammal enamel are linked to climatic conditions.

  • Warm and dry terrestrial biotope recognized for the Paleocene-Eocene period.

  • 87Sr/86Sr ratios of marine fossils confirmed latest Paleocene and early Eocene age.

Abstract

We performed different geochemical analyses of Paleogene terrestrial mammal remains to establish their taphonomy, stratigraphic provenance, and palaeoenvironmental conditions. Rare earth element (REE) results indicate a similar diagenetic history to that of previously investigated marine taxa from these beds. Therefore, the mammal remains were initially deposited in a marine setting, and probably not long after the death of the animals their bodies were washed into the sea. The Ce/Ce* and Pr/Pr* ratios for the mammal fossils were compared with the background dataset from the phosphate mines, which varies with time. This allowed us to characterize the stratigraphic levels bearing the fossils. The provenances of fossils with known origins were confirmed, while remains with unknown origins could be assigned to certain stratigraphic horizons that are compatible with previously proposed phylogenic relationships.

Marine diagenesis affected the various skeletal tissues differently, with the largest alteration in the bone and the least or none in the enamel. This is mostly demonstrated by the high F concentration, high Ca/P, and seawater related 87Sr/86Sr in the bone/dentine samples. Enamel shows the opposite, and retained the most pristine terrestrial values. The δ18OPO4 and δ13C results from mammal enamel revealed warm (>20 °C mean annual temperature-MAT) and dry (<500 mm mean annual precipitation) conditions for Paleocene-Eocene period in the region. From the early to mid-Ypresian about +5 °C ΔMAT is recorded that might be linked to the Early Eocene Climatic Optimum.

Furthermore, the 87Sr/86Sr ratios derived from shark tooth enameloid fit the global open ocean Sr-isotope record during the latest Paleocene and early Eocene providing further evidence for the age of these phosphate beds in the Ouled Abdoun Basin. However, older marine fossils yielded higher ratios than the global Sr-isotope curve, reflecting an alteration and/or somewhat restricted conditions in the Moroccan coastal basins, possibly triggered by global sea-level changes.

Introduction

The phosphate series of the Ouled Abdoun Basin in Morocco, which is exploited in large industrial quarries, is renowned for its Upper Cretaceous-Lower Eocene vertebrate assemblages (e.g., Arambourg, 1952; Bardet et al., 2017). The fauna predominantly comprises sub-autochthonous marine aquatic organisms such as cartilaginous and bony fishes, mosasaurs, and turtles, but occasionally terrestrial remains have also been described, such as non-avian dinosaurs, birds, and mammals (e.g., Gheerbrant et al., 2003; Pereda Suberbiola et al., 2004; Bourdon et al., 2005). The mammals are rare and most of the time scientists come to know about them from private collectors living near the phosphate quarries, and often cannot determine their precise stratigraphic origin. Knowing the stratigraphic origins of the Ouled Abdoun fossils is crucial to elucidating the early Cenozoic evolution of African mammals (e.g., Gheerbrant et al., 2018). Accurate dating of the Ouled Abdoun mammals is also important for knowledge and characterization of the early Cenozoic successive African faunal assemblages.

To tackle the stratigraphic origins of vertebrate remains, geochemical taphonomy is a widely used method (e.g., Staron et al., 2001; Trueman et al., 2003; Metzger et al., 2004; MacFadden et al., 2007; Suarez et al., 2010; Herwartz et al., 2013; Botfalvai et al., 2021). This is based on trace element contents that are absent or occur only in very low concentrations (sub-ppm) in modern bones and teeth, however, after the death of the animals their concentrations increase through the influence of the sedimentary environment (e.g., Elderfield and Pagett, 1986). The most widely used elements are the rare earths (REE) and uranium, which can reach 4–5 orders of magnitude enrichment in fossil material compared to modern concentrations (e.g., Trueman and Tuross, 2002). These elements therefore could reflect the chemistry of the pore fluid in which the vertebrate remains were fossilized. There are several studies using REE chemistry of fossil bioapatite to reconstruct palaeoenvironmental conditions in terrestrial and marine depositional settings (e.g., Trueman et al., 2003; Lécuyer et al., 2004; Shields and Webb, 2004; Kocsis et al., 2009; Žigaitė et al., 2016, Žigaitė et al., 2020), which are all based on the assumption that REEs are quantitatively taken up from the burial environment (e.g., Reynard et al., 1999). During their incorporation into the bioapatite structure, however, fractionation could occur between the light and heavy REEs due to size differences along the REE series and related diffusion (e.g., Trueman et al., 2011; Herwartz et al., 2013). In addition, protracted late diagenetic modification (Kocsis et al., 2010; Herwartz et al., 2011) may further complicate environmental interpretations, especially for older samples (Kowal-Linka et al., 2014). To address these issues, a large number of samples, including different types of materials (e.g., bones, teeth, enamel, and dentine), and multiple in-situ analyses, are needed to evaluate further the origin of the REEs within the bioapatite.

In relation to the Moroccan phosphate series in the Ouled Abdoun and Ganntour basins, the trace element chemistry of nearly 200 marine samples (fish teeth, bones, coprolites) was published in Kocsis et al. (2016). Fractionation and late diagenetic overprint were completely ruled out. All the samples yielded similar, typical oxic seawater REE distributions with heavy REE enrichment and a negative Ce-anomaly. Importantly, however, the Ce-anomalies show a distinct change with stratigraphic level (Kocsis et al., 2016), a trend that is used in this work to assign a stratigraphic age for unprovenanced fossils. Such an approach was successful for the Embrithopoda mammal taxa of Stylolophus minor and S. major in the Ouled Abdoun Basin (Gheerbrant et al., 2018).

The main aim of this study is to further investigate the validity of the trace element provenancing method for the rare mammal fossils with known and unknown origins in the Ouled Abdoun Basin. To help better assess the taphonomy of these remains, major and other trace elements, and strontium isotope ratios (87Sr/86Sr) of selected mammal fossils, were compared with marine end-members such as data from shark teeth. Enameloid 87Sr/86Sr data derived from the latter also provide a proxy for the age of the phosphate beds, through comparing their ratios with the global 87Sr/86Sr record. In addition, some mammal fossils were measured for their stable oxygen and carbon isotopic compositions to get an insight into the ecological conditions of some of these early African placental mammals.

Section snippets

Geological and stratigraphical settings

The Ouled Abdoun Basin is one of the ancient epicontinental marine basins along the Atlantic coast of Africa, where large amounts of phosphorite accumulated during the late Cretaceous - early Eocene period. The basement rocks are covered by Cenomanian-Turonian carbonates followed by yellow marls with possible age within the Coniacian-Campanian interval (El Assel et al., 2013), which were eventually succeeded by the phosphorite series. The top of the succession is either covered by Lutetian

Investigated materials

Several mammal species and/or other fossils from the mammal-related sediment matrix were involved in our research. Some of the fossils have known stratigraphic positions, while the origins of the others are questionable or completely unknown (Table 1). We focused primarily on the trace element compositions of the fossils (Table 1). However, some specimens were also analyzed for the presence of major elements (Ca, P, F), stable oxygen and carbon isotopes, and radiogenic strontium isotopes. The

Major element compositions

The remains of four mammal taxa, Daouitherium rebouli (PM65), Phosphatherium escuilliei (MNHN.F PM25), Hyracoidea indet (MNHN.F PM52), and Ocepeia grandis (PM66) were analyzed for CaO, P2O5 and F content (Table 2, Fig. 2). The Ca/P ratio ranges from 1.70 to 2.02, with the values increasing from enamel to dentine to bone. The F content varies between 0.73 and 3.95 wt% and a similar trend to the Ca/P ratio can be seen, as the lower values come from the enamel, and the highest ones from the bone

Terrestrial, marine, and diagenetic signals

Hard tissues (i.e., bones, teeth) of terrestrial and marine animals record the chemical and isotopic signatures of their respective environments, hence when these remains are fossilized the ancient ecological conditions can be traced in the case of no alteration (e.g., Tütken et al., 2006; Kocsis et al., 2007; Domingo et al., 2009; Reynard and Balter, 2014; Akhtar et al., 2020). Generally, freshwater (e.g., terrestrial drinking sources) has a lower F and Sr content than seawater, while Ba is

Conclusions

The rare but diverse terrestrial mammal fossils from the Ouled Abdoun phosphorite are proven to have a similar diagenetic history to that of the marine fossils. This is reflected by a similar REE distribution and stratigraphy matched Ce/Ce* and Pr/Pr* ratios. The Ce/Ce* variation along the phosphate series either confirmed previous known origins or helped to estimate the stratigraphic provenance of the Ouled Abdoun mammal specimens whose origin was unknown. Exchanges with marine pore fluid

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.

Acknowledgements

The research was conducted under L.K.'s Ambizione Research grants (Nr. PZ00P2_126407 & PZ00P2_145115/1 – Switzerland), but in the later phase of the research LK also received support from his URC Grant from Universiti Brunei Darussalam (UBD/PNC2/2/RG/1(325)). The help from both funding agencies is much appreciated. This study benefited from the paleontological “Phosphapal” collaborative Agreement with the Ministère de l'Energie, des Mines, de l'Eau et de l'Environnement (MEMEE), the Office

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