Biodiversity and paleoenvironments of vertebrate-rich Eocene marine deposits (Lutetian) of the tropical western Neotethys: New insights from the Arabian Platform

The Eocene deposits of Saudi Arabia (Dammam Formation) are characterized by intertidal to shallow marine carbonate rocks interbedded with meter-scale shale members (Midra and Saila shales). The latter are dated from the middle Eocene and include large numbers of marine vertebrates, dominated by elasmobranch (sharks, skates, and rays) teeth. The rich fossil assemblage comprises at least 13 elasmobranch taxa (including 8 different sharks and 3 batomorph taxa) and at least 7 bony fish taxa. This assemblage is described for the first time from the Arabian Platform (western margin of the Neotethys) and is critically important to understand the changes in circum Tethyan marine biotas during the Eocene. The larger elasmobranchs are dominated by lamniform sharks, such as “Carcharias” koerti, Brachycarcharias cf. lerichei and Striatolamia cf. macrota, along with the tiger shark Galeocerdo eaglesomei, the hammerhead shark Sphyrna sp., and myliobatoids. Teleost fishes appear less diversified in the area and are dominated by Eotrigonodon and Pycnodus species. A low-diversity benthic fauna including bivalves, gastropods, echinoids and corals was found along with the neritic elasmobranch and teleost taxa. The fish assemblage found in the present study is consistent with other middle Eocene faunas worldwide, especially in the tropical Neotethys seaway. Biostratigraphic analyses and strontium isotope ratio measurements from tooth enameloid confirm a Lutetian age (possibly refined to mid-Lutetian). The variety of taxa recovered in Saudi Arabia enables a preliminary reconstruction of the likely interactions between the various fossil groups and their broad paleoenvironments. Apex predators characterized by large lamniform and carcharhiniform sharks probably hunted in shallow near shore waters, where other fish (including eagle rays, carpet sharks, sawfish, and bony fish) were dwelling.