5.8 Myr old Mimosoideae fossil woods from Ethiopia and comparison with African Albizia

This paper presents a description of fossil wood materials from the late Miocene Adu-Asa Formation of the Middle Awash, located in the Afar Rift of Ethiopia. The three fossilized wood fragments were collected on the surface of outcrops of the upper Adu Dora Member, at Adu Dora north. The Adu Dora Member is dominated by lacustrine and diatomite strata overlain by the Ladina Basaltic Tuff dated at 5.8 Myr. The silicified fossil wood samples are thus slightly older than 5.8 Myr. They show preserved microscopic structures. Diagnostic characters including axial parenchyma paratracheal vasicentric or aliform and confluent, homocellullar rays with procumbent cells, intervascular pits vestured and prismatic crystal in chambered parenchyma allow an attribution of the three samples to the Mimosoideae subfamily in the legume family. One specimen is attributed to Mimosoxylon sp. (Mimosoxylon MULLER-STOLL & MADEL, type species: M. tenax (FELIX) MULLER-STOLL & MADEL) whereas the two others specimens are identified as Albizia. The Middle Awash fossil woods are compared with three Pliocene Albizia fossil woods found in the Shungura Formation from the Omo valley, and with one Miocene fossil specimen from the Djerem basin in Cameroon. The fossil wood structure is also compared with that of other genera among the extant Mimosoideae. Although all the African Albizia species could not be checked, a great resemblance between the Middle Awash sample and A. gummifera is found. Based on this, some palaeoenvironmental implications are given. In tropical Africa, most of the 150 Albizia species mostly occur in humid forests. The finding of A. gummifera in the Middle Awash region therefore indicates that climatic conditions 5.8 Myr ago were significantly more humid than today.