Department of Anthropology and Archaeology, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada.
Department of Anthropology, University of Michigan, Ann Arbor, MI, 48109, USA.
J Hum Evol. 2018 Mar;116:95-107. doi: 10.1016/j.jhevol.2017.12.001. Epub 2018 Feb 6.
Field expeditions to Bukwa in the late 1960s and early 1970s established that the site had a small but diverse early Miocene fauna, including the catarrhine primate Limnopithecus legetet. Initial potassium-argon radiometric dating indicated that Bukwa was 22 Ma, making it the oldest of the East African early Miocene fossil localities known at the time. In contrast, the fauna collected from Bukwa was similar to other fossil localities in the region that were several million years younger. This discrepancy was never resolved, and due to the paucity of primate remains at the site, little subsequent research took place. We have collected new fossils at Bukwa, reanalyzed the existing fossil collections, and provided new radiometric dating. Ar/Ar incremental heating ages on lavas bracketing the site indicate that the Bukwa fossils were deposited ∼19 Ma, roughly 3 Ma younger than the original radiometric age. Our radiometric dating results are corroborated by a thorough reanalysis of the faunal assemblage. Bukwa shares taxa with both stratigraphically older localities (Tinderet, Napak) and with stratigraphically younger localities (Kisingiri, Turkana Basin) perfectly corresponding to our revised radiometric age. This revised age for Bukwa is important because it indicates that significant faunal turnover may have occurred in East Africa between 20 and 19 Ma. Bukwa samples immigrant taxa such as large suids, large ruminants, and ochotonids that are absent from stratigraphically older but well-sampled localities in the region, such as Tinderet (∼20 Ma) and Napak (20 Ma). Further age refinements for Bukwa and the entire East African early Miocene sequence will help to constrain the timing of this faunal turnover event, of particular importance in paleoanthropology since this temporal sequence also provides us with what is currently our best window into the early evolution of cercopithecoid and hominoid primates.
20 世纪 60 年代末和 70 年代初对布瓦进行的实地考察表明，该遗址有一个小型但多样化的早中新世动物群，包括原始灵长类动物 Limnopithecus legetet。最初的钾氩放射性测年表明，布瓦的年龄为 2200 万年，使其成为当时已知的东非最早的中新世化石产地。相比之下，从布瓦采集的动物群与该地区其他年轻几百万年的化石产地相似。这一差异从未得到解决，由于该遗址灵长类动物遗骸稀少，因此很少进行后续研究。我们在布瓦采集了新的化石，重新分析了现有的化石收藏，并提供了新的放射性测年数据。对该遗址周围熔岩的氩/氩增量加热年龄表明，布瓦的化石沉积于约 1900 万年前，比原始放射性年龄年轻约 300 万年。我们的放射性测年结果得到了对动物群组合的彻底重新分析的支持。布瓦与年代更老的地点（廷德尔特、纳帕克）和年代更年轻的地点（基塞林吉里、图尔卡纳盆地）共享分类群，这与我们修正后的放射性年代完全吻合。布瓦年龄的修正非常重要，因为它表明在 2000 万至 1900 万年前，东非可能发生了重大的动物群更替。布瓦的样本中存在移民分类群，如大型猪、大型反刍动物和食虫目动物，而这些分类群在该地区年代更老但采样良好的地点（如廷德尔特（约 2000 万年）和纳帕克（2000 万年））中则不存在。对布瓦和整个东非早中新世序列的进一步年龄细化将有助于限制这一动物群更替事件的时间，这在古人类学中尤为重要，因为这个时间序列也为我们提供了目前观察到的灵长类和人科灵长类早期进化的最佳窗口。
