Drapeau Michelle S M, Bobe René, Wynn Jonathan G, Campisano Christopher J, Dumouchel Laurence, Geraads Denis
Département d'anthropologie, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, QC, H3C 3J7, Canada.
Department of Anthropology and Center for the Advanced Study of Hominid Paleobiology, The George Washington University, 2110 G St., NW, Washington, DC 20052, USA; Interdisciplinary Center for Archaeology and Evolution of Human Behavior, Universidade do Algarve, 8005-139, Faro, Portugal.
J Hum Evol. 2014 Oct;75:64-79. doi: 10.1016/j.jhevol.2014.07.001. Epub 2014 Aug 21.
Dating to more than four million years ago (Ma), the Mursi Formation is among the oldest of the Plio-Pleistocene Omo Group deposits in the lower Omo Valley of southwestern Ethiopia. The sedimentary sequence is exposed along a strip ∼35 km by 4 km, but it has received relatively little attention due to the difficult access to this area. Although expeditions to the lower Omo Valley between 1968 and 1973 focused primarily on the Usno and Shungura Formations, survey of the Mursi Formation produced a faunal collection of about 250 specimens deriving exclusively from the Yellow Sands area at the southern extent of the exposures. In 2009, we reinitiated an investigation of the formation by focusing on the most northern exposures, and a new fossil site, Cholo, was identified. Cholo is depositionally similar to the lowermost exposures at the Yellow Sands, although no stratigraphic correlation between the two localities has yet been made. The fossiliferous sediments at Cholo are capped by a prominent vitric tuff that is compositionally distinct from any other known tephra preserved in East African rift basins, including the only known vitric tuff at the Yellow Sands. The faunal assemblage of the Yellow Sands area presents interesting characteristics: the fossils generally show little weathering and include a large proportion of suids (44% of the mammalian fauna) and a small proportion of bovids (14%) compared with other Pliocene African sites. The sample is also unusual in the high frequency of deinotheres (7%). Taxon-specific stable carbon isotopic composition of the Mursi mammals tends to show generally higher proportions of C3 diets compared with other Pliocene sites in East Africa and Chad. This and the particular faunal proportions suggest that the environments represented by the Mursi Formation were more closed than those of other Pliocene sites.
穆尔西组的年代可追溯到400多万年前，是埃塞俄比亚西南部奥莫河谷下游上新世-更新世奥莫群沉积物中最古老的地层之一。该沉积层序沿着一条约35公里长、4公里宽的地带出露，但由于该地区交通不便，受到的关注相对较少。尽管1968年至1973年间对奥莫河谷下游的考察主要集中在乌斯诺组和舒恩古拉组，但对穆尔西组的调查收集到了约250个动物化石标本，这些标本全部来自露头南端的黄沙地区。2009年，我们重新开始对该地层进行调查，重点关注最北部的露头，并确定了一个新的化石地点乔洛。乔洛在沉积方面与黄沙最下部的露头相似，不过这两个地点之间尚未建立地层对比关系。乔洛的含化石沉积物之上覆盖着一层显著的玻质凝灰岩，其成分与东非裂谷盆地保存的任何其他已知火山灰都不同，包括黄沙唯一已知的玻质凝灰岩。黄沙地区的动物群组合呈现出有趣的特征：化石一般风化程度较低，与其他上新世非洲地点相比，猪科动物比例较大（占哺乳动物群的44%），牛科动物比例较小（14%）。该样本中恐象的出现频率也很高（7%），这也很不寻常。穆尔西哺乳动物特定分类群的稳定碳同位素组成与东非和乍得的其他上新世地点相比，往往显示出以C3饮食为主的比例普遍更高。这一点以及特定的动物群比例表明，穆尔西组所代表的环境比其他上新世地点的环境更为封闭。
