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Phosphatherium escuilliei, from the Thancetian of the Ouled Abdoun basin , oldest known Proboscidean from Africa


, : Phosphatherium escuilliei, from the Thancetian of the Ouled Abdoun basin , oldest known Proboscidean from Africa. Geobios (Lyon) 31(2): 247-269

We detail here the study of Phosphatherium escuilliei from the Paleocene of the Ouled Abdoun Basin, Morocco. This fossil is both the oldest proboscidean and the oldest modern ungulate. The species, known by two specimens, is dated as Thanetian on the basis of the foraminifera and the selachians identified in the matrix. P. escuilliei shows several derived proboscidean features, especially the true lophodont pattern of the molars, but also remarkable primitive features such as a vestigial dilambdodonty and a very small size, the smallest reported in the order. The vestigial dilambdodonty is interpreted here as a reminiscence of the pantomesaxonian supergroup. Primitive tethythere features are also noticed, especially the orbit in anterior position and the occurrence of a postentoconule. Numidotherium koholense from El Kohol (late Early Eocene of Algeria) is the most closely related proboscidean to P. escuilliei, and the two species are included in the same family Numidotheriidae. However, the available material shows only one possible synapomorphy: the absence of lingual cingulum in the upper molars. P. escuilliei is the most primitive known lophodont proboscidean. The dP4/ of P. escuilliei strongly recalls the single known tooth of the enigmatic species Khamsaconus bulbosus from the Ypresian site of N'Tagourt 2, Morocco. Shared features such as the occurrence of a postentoconule and the incipient bilophodonty indicate affinities of K. bulbosus with tethytheres, and others such as the absence of conules are even possibly suggestive of affinities with lophodont proboscideans. The K. bulbosus morphology remains however clearly more primitive than that of P. escuilliei, especially in being more bunodont. Phosphatherium, Numidotherium, Barytherium and even Khamsaconus raise the problem of a lophodont ancestral proboscidean morphotype, which is in accordance with the recent hypothesis of Court (1995) regarding Moeritherium as more derived than Numidotherium. P. escuilliei supports an early age of the radiation of modern orders of mammals, which was earlier than the late Paleocene, as well as its "explosive" pattern if it is closely connected to the extinctions of the K/T boundary. It shows also, more clearly than the taxa from the Adrar Mgorn fauna (late Paleocene of Morocco), the old age of the eutherian presence in the Arabo-African domain. It also supports the African origin of true proboscideans.

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