A mammalian fauna from the late oligocene of Northwestern Kenya

Mammalian fossils have been collected over several decades by teams associated with the Kenya National Museums from two sites in northwestern Kenya, Losodok and Benson's Site. Geological work in the area has determined that Losodok is Late Oligocene in age (Boschetto et al. 1992); only one mammal from Losodok has been previously discussed in light of the age determinations, a catarrhine primate (Leakey et al. 1995). In this paper, we describe the rest of the mammals known from these sites, which proves to be a fauna consisting of at least 21 species in eight orders. Recent field work during the summer of 2007 yielded more mammals from both of these sites; additional mammal localities were found near Benson's Site, prompting us to designate the new region Nakwai. Several mammalian species are shared between the Losodok sample and that from Nakwai, indicating that they are about the same age. The mammalian faunas are distinctly Oligocene in their taxonomic composition, consisting of African groups previously known from the Early Tertiary such as hyracoids, arsinoitheres, primitive proboscideans, thryonomyoid rodents, proviverrine and pterodontine creodonts, catarrhine primates, and anthracotheres. Immigrant mammals characteristic of the African Early Miocene are conspicuously absent except for a small true carnivore of the genus Mioprionodon, the earliest fossil record of Carnivora in Africa. We formally describe two new genera (Losodokodon, a proboscidean, and Mlanyama, a creodont), and 8 new species (of the genera Brachyhyrax, Thyrohyrax, Meroëhyrax, Diamantomys, Mioprionodon, and the two new genera). Several other species are informally described and discussed but not named in this paper, and we also make new comparisons regarding Kamoyapithecus, the only species from Losodok previously named (Leakey et al. 1995). The new fauna is transitional between earlier Oligocene mammal faunas of Africa (known from Ethiopia, Egypt, and several other countries) and the Early Miocene faunas of East Africa. Several taxa are morphological intermediates, and potential phylogentic links, between early Tertiary forms and Miocene taxa; these include the proboscidean Losodokodon and the primate Kamoyapithecus. Others represent the earliest record for African groups otherwise known in the Miocene {Brachyhyrax, Afrohyrax, Meroëhyrax, Prodeinotherium, Diamantomys). Still others represent the latest occurrence of taxa previously known from the Early Tertiary (pachyhyracine hyracoids, Thyrohyrax, Arsinoitherium). All taxa in the fauna represent forms that could have evolved in situ in Africa except for the new species of Mioprionodon, which is an immigrant from the north. The new discoveries highlight the fact that the faunal transition between archaic endemic mammals of the Early Tertiary and the more modern Neogene faunas occurred during a very short time interval at or near the Oligocene-Miocene boundary. We discuss the nature and importance of this transition, which we designate the African Mid-Tertiary Event (AMTE).