Late Miocene mammalian fauna from the Middle Awash Valley, Ethiopia

This study presents a descriptive and comparative analysis of the late Miocene mammalian fauna from the Middle Awash, Ethiopia. The Middle Awash has yielded fossils spanning the last six million years. Although vertebrate fossils have been collected from the late Miocene sites along the western margin of the study area since the mid-1970s, none of these fossils was comprehensively studied until this work. The geology and geochronology of the late Miocene sites are now refined and a reliable chronostratigraphy has been established by 40Ar/39Ar radiometric dating. The Adu-Asa Formation is divided into four members distinguished from each other by marker horizons, most of which are dated basaltic tuffs. Radiometric dating has confined the age of the Adu-Asa Formation to between 5.2–5.8 Ma. These dates are also supported by paleomagnetic results.; More than 2,000 fossil specimens were collected from the Middle Awash late Miocene deposits between 1992 and 2000. These fossils document 64 mammalian species in 32 genera, 23 families, and 8 orders. This assemblage includes a number of new taxa. Included in the assemblage are First and Last Appearance Dates (FADs and LADs) of some groups, including the earliest record of hominidae, with the earliest known Ardipithecus, A. ramidus kadabba. Most of the taxa from the west margin localities indicate a predominance of mesic and wooded habitat during the deposition of the Adu-Asa Formation. In these deposits, colobines, viverrids, mustelids, bovines, boselaphines, and tragelaphines are abundant, whereas alcelaphines are absent. Quantitative analyses of biogeographic relationships of the Middle Awash late Miocene mammalian fauna indicate stronger relationships with other African sites than with faunas from Eurasian sites.; The Middle Awash late Miocene deposits have generated critical dataset for analytical works on past environments, biogeographic relationships, and African vertebrate evolution. Moreover, the geographic position of the Middle Awash, coupled with its tight calibration and chronological span, make this site a key to our understanding of past faunal interchanges and immigration patterns between Africa and Eurasia.