The selachian fauna (sharks and rays) of the phosphate series of Ndendouri-Ouali Diala (Matam, Western Senegal): Dating and paleoenvironmental interests

Abstract

The phosphate series of Matam (located at the NorthEast of Senegal) consists of a 10 meters alternation of soft phosphate levels, sand horizons and phosphatic limestones. While they were only mentioned in some preliminary geological works, isolated teeth of selachians are nevertheless common in these phosphate deposits. Detailed fieldwork of 9 new well-drillings allowed us to establish the first exhaustive list of the Matam selachian fauna with their biostratigraphical distribution through five main fossiliferous levels spanning the Thanetian-Early Lutetian period. Their importance for biochronological, paleoenvironmental purposes and correlations with the other Eocene phosphate deposits worldwide is noteworthy. The Matam selachian is represented by 38 species belonging to 34 genera, 16 families and 6 orders, which allow us to define different biochronological associations. The first association (e.g. characterized by Eotorpedo hilgendorfi and E. jaekeli) is associated with the sands of the top of Gorgol Fm. that constitute the ground level of exploited phosphate deposits and testify of a clear marine influence during the Late Thanetian. The three following associations were recovered in the main levels of exploited phosphate of the Matam Fm. They contain a richer and more diversified fauna dated to ?Lower Ypresian and are characteristic of neritic environments too. The last association (top of the Matam Fm.) overlies the phosphate layers and is characterized by a drastic decrease in selachian richness and diversity. Dated to the Lower Lutetian, this last faunal association may be indicative of a marine embayment in Matam area and contrary to western Senegal where phosphate deposits begin. If all these associations appear typical of the eastern margins of central Atlantic, the light observed faunal turnover across the ten-meter sequence suggests productivity changes in phosphate depositions during the Late Thanetian-Early Lutetian period.

Contribution to the reappraisal of the mid Paleogene ichtyofauna of Western Africa with three new enigmatical elasmobranchs from Thanetian–Lutetian of Senegal

“…Gossas, Bargny). We report here, three new taxa of elasmobranchs with unambiguous dental morphologies that support biostratigraphical correlations between Formations from two main Senegalese areas: Matam (Pascal, 1987;Sambou et al, 2017;Sambou, 2018) and Kaolack in Sine-Saloum region (Tabuce et al, 2019). These new taxa contribute to the reappraisal of mid Paleogene fauna of Western Africa and improve understanding the diversification of modern elasmobranchs.…”

“…Most of the specimens were found in the phosphate ore of Ndendouri-Ouali Diala near Matam (sub-prefecture of Orkadiéré, Matam region, approximatively 15°18'N, 13°01'W, see Sambou et al, 2017 for details) located on the left riverbank of the Senegal River, northeasternstern part of Senegal (Fig. 1).…”

“…and following a NW direction (see Sambou, 2018). Sambou et al (2017) recently revised the faunas from the Gorgol and Matam Fm. outcropping in the phosphate ore of Matam as previously reported in Pascal (1987).…”

“…outcropping in the phosphate ore of Matam as previously reported in Pascal (1987). 38 species of elasmobranch have been recognized in the different levels of the 20 well-drilling and 33 different samples along the Matam series (see Sambou et al, 2017 : Tab.1 for details). These faunas include several determined and undetermined species of Brachycarcharias, " Cretolamna", Protoginglymostoma, Ginglymostoma, Nebrius, Rhizoprionodon, Physogaleus, Triakis, Foumtizia, Rhynchobatus, Pristis, Anoxypristis, Eotorpedo, Lophobatis, Myliobatis, Rhinoptera, Leidybatis, Garabatis, Coupatezia, Ouledia and the three new taxa reported here.…”

Diversity and renewal of tropical elasmobranchs around the Middle Eocene Climatic Optimum (MECO) in North Africa: New data from the lagoonal deposits of Djebel el Kébar, Central Tunisia

“…Only one species of Arechia is known to date: A. arambourgi Cappetta, 1983 from the Late Ypresian of Morocco. However, several reports of the genus indicate a more widespread geographical and stratigraphical ranges (e.g., Cappetta and Traverse, 1988;Tabuce et al, 2005;Cappetta, 2012;Sambou et al, 2017). The KEB-1 dental material is comparable to that figured by Cappetta (1983 pl. 2), but it seems somewhat different from Arechia arambourgi in having a higher cutting transversal keel with a slight ornamentation on its inner border, prefiguring the appearance of a real large cusp.…”