Morphology and phylogenetic relationships of the earliest known hippopotamids (Cetartiodactyla, Hippopotamidae, Kenyapotaminae)

The earliest known hippopotamids, attributed to the subfamily Kenyapotaminae, are known essentially from dental remains of two species. The first was found in the middle Miocene of Kenya, the second at the beginning of the upper Miocene in eastern and northern Africa. The exact affinities of the Kenyapotaminae are critical for resolving the long debated origin of the Hippopotamidae, as part of the wider question of cetacean affinities within artiodactyls. We performed the first detailed comparative description, character by character, of kenyapotamines, using the following putatively related taxa: Hippopotaminae, Suoidea, and Anthracotheriidae. The development of an improved nomenclature for the dentition facilitates comparisons amongst a wide array of cetartiodactyls. This has permitted the first assessment of the phylogenetic position of Kenyapotaminae using a cladistic analysis. This work provides an emendation of kenyapotamine taxonomy and diversity. Kenyapotaminae are indeed closely related to Hippopotaminae and should be kept within Hippopotamidae, the late Miocene kenyapotamines not necessarily being the forerunners of the first known hippopotamines. Hippopotamidae are deeply nested within anthracotheriids, with crown bothiodontines as sister group, and any close affinities with Suoidea should be rejected. This reinforces a scenario linking cetaceans to Hippopotamoidea (Hippopotamidae + Anthracotheriidae), possibly via other early Palaeogene artiodactyls. RESUME Morphologie et phylogenie des premiers hippopotamides (Cetartiodactyla, Hippopotamidae, Kenyapotaminae). –Les plus anciens hippopotamides, attibues a la sous-famille Kenyapotaminae, sont connus essentiellement par les restes dentaires de deux especes, connues dans le Miocene moyen du Kenya et a la base du Miocene superieur d’Afrique orientale et septantrionale. Les affinites exactes des Kenyapotaminae doivent etre connues pour resoudre le long debat sur l’origine des Hippopotamidae, partie integrante de la question plus large des relations entre cetaces et artiodactyles. Nous avons effectues la premiere comparaison detaillee, caractere par caractere, des kenyapotamines avec les Hippopotaminae, Suoidea, et Anthracotheriidae, grâce au developpement d’une nomenclature dentaire amendee. Sur cette base, nous avons conduit une analyse cladistique de la position phylogenetique des Kenyapotaminae. Nous proposons une emendation de leur taxonomie et de leur diversite. Ils apparaissent proches des Hippopotaminae, et devraient etre conserves au sein des Hippopotamidae, mais les kenyapotamines du Miocene recent ne sont pas necessairement les precurseurs des premiers hippopotamines. Les Hippopotamidae apparaissent profondement enracines auseines Anthracotheriidae, leur group frere incluant les bothriodontines terminaux. Une relation etroite avec les Suoidea est rejetee. Ces resultas renforcent le scenario liant Cetacea et Hippopotamoidea (Hippopotamidae + Anthracotheriidae), et possiblement d’autres artiodactyles du Paleogene ancient. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158, 325–366.