New data on the earliest known arsinoitheriid embrithopod (Mammalia, Paenungulata), Namatherium Pickford, Senut, Morales, Mein & Sanchez, 2008 from the middle Eocene of Namibia

We here describe and interpret recently discovered material of the embrithopod mammal Namatherium blackcrowense Pickford, Senut, Morales, Mein & Sanchez, 2008 from the Lutetian Black Crow locality, in Namibia, a species previously only known from the holotype. The new specimen, a distal part of a skull including the petrosals, complements the holotype well and allows a virtual reconstruction of a large part of the skull of N. blackcrowense. The CT scan study of the petrosal and labyrinth and the squamosal morphology unambiguously support referral of the new specimen to N. blackcrowense. Our comparisons and phylogenetic study support a close relationship between Namatherium Pickford, Senut, Morales, Mein & Sanchez, 2008 and the more derived Fayum genus Arsinoitherium Beadnell, 1902, and its inclusion within the family Arsinoitheriidae. This is evidenced by remarkable shared derived traits of the petrosal and inner ear such as the thick and flattened semicircular canals, the absent fossa subarcuata, the reduced secondary bony lamina, the secondary crus commune in the course of separation, and the partially fused fenestra cochleae and cochlear aqueduct. They add to previous arsinoitheriid synapomorphies reported on the holotype such as presence of nasal horns and the pseudolophodont molars. Namatherium helps to reconstruct the ancestral morphotype of the family Arsinoitheriidae, and it emphasises the advanced condition of Arsinoitherium zitteli Beadnell, 1902 with respect to Namatherium in many autapomorphic traits, especially in striking reversals in some cranial morphological traits. This implies a substantial divergent evolution of the Arsinoitherium clade between the Lutetian and Priabonian, which remains undocumented and poorly understood from an adaptive perspective.