ISOTOPIC CONSTRAINTS ON THE ECOLOGY AND DISPERSAL OF LUTETIAN EMBRITHOPODS OF BALKANATOLIA

Abstract Embrithopods are an extinct clade of Paleogene mammals that are phylogenetically nested among paenungulate afrotheres. Embrithopods originated in Africa, but unlike other afrotheres, they successfully colonized the northern shores of the Neotethys reaching the Balkanatolian archipelago by (at least) the early Lutetian (ca. 47.8 Ma ago). Nevertheless, how embrithopods dispersed out of Africa remains controversial. Embrithopod postcranial morphology and paleoenvironmental studies led to the hypothesis that they may have been either semi- or fully-aquatic, potentially explaining their ability to disperse across Neotethys. Semi-aquatic mammal species display a lower oxygen isotope composition in their tooth enamel compared to co-occurring fully terrestrial species. However, isotopic studies of African early Oligocene embrithopods do not display this pattern, suggesting a fully terrestrial lifestyle for these taxa. Here, we evaluated the isotopic composition of tooth enamel in Lutetian embrithopods from Balkanatolia, to test whether these older and phylogenetically more basal embrithopods may have been terrestrial, semi-aquatic, or fully-aquatic. Specifically, we compared the oxygen (δ18O) and carbon (δ13C) isotope compositions of tooth enamel in embrithopod specimens from the upper Lutetian deposits of the Orhaniye Basin (central Anatolia) with those of a fully terrestrial mammal that co-occurs in the same beds, the archaic ungulate Hilalia. Results show that Balkanatolian embrithopods have systematically higher δ18O values than sympatric Hilalia. Despite a substantial difference in body size in these clades, these results suggest that embrithopods habitually occupied a fully terrestrial environment rather than a semi-aquatic one. These findings conflict with the hypothesis that early Balkanatolian embrithopods were semi-aquatic, thereby challenging a previously compelling paleobiological hypothesis for their early colonization of Balkanatolia. Instead, they suggest that the substantial body mass of embrithopods may have imparted passive buoyancy, enabling limited short-distance drift events across the Neotethys.