Cranial anatomy of the hypercarnivore <i>Bastetodon syrtos</i> gen. nov. (Hyaenodonta, Hyainailourinae) and a reevaluation of <i>Pterodon</i> in Africa

During the Paleogene in Afro-Arabia, most terrestrial mammalian carnivores belonged to Hyaenodonta, an extinct lineage bearing a pair of carnassials between each set of molars. The Fayum Depression of Egypt preserves multiple lineages of hyaenodonts across the Eocene–Oligocene boundary. Here, we describe one of the most complete hyaenodont crania ever recovered from the lower Oligocene of the Fayum (Jebel Qatrani Formation, Quarry I, ∼30 Ma). The cranium is about the size of a hyena’s and preserves the complete upper tooth row. The long, shearing metastyle and reduced and mesially shifted protocones indicate a hypercarnivorous diet, and robust zygomatic arches suggest strong chewing muscles. Based on dental comparisons, we refer the cranium to <i>Bastetodon syrtos</i> (previously “<i>Pterodon</i>” <i>syrtos</i>). <i>Bastetodon syrtos</i> has three premolars and two molars—a reduced dental formula compared with other Fayum hyainailourines such as <i>Akhnatenavus</i> and European hyainailourines such as <i>Pterodon</i>. The cranium preserves the clover-shaped lambdoidal crest and long pharyngeal tube that unites Hyainailouroidea (Apterodontinae, Teratodontinae, and Hyainailourinae). The new Fayum cranium allows us to reevaluate the genus <i>Pterodon</i> and explore characters that distinguish Afro-Arabian and Eurasian species placed in this genus. Our phylogenetic analysis recovers a paraphyletic <i>Pterodon</i>, with <i>B. syrtos</i> as the sister taxon of <i>Falcatodon schlosseri.</i> We erect <i>Sekhmetops</i> to describe the large Fayum hyainailourine species <i>S. phiomensis</i> and <i>S. africanus</i>. This effort clarifies the biogeographic history of the clade that includes <i>Pterodon</i>, revealing multiple Tethys Seaway dispersals during the Paleogene followed by endemic radiations in Eurasia and Afro-Arabia.