Cretaceous age for the feathered dinosaurs of Liaoning, China
Cretaceous age for the feathered dinosaurs of Liaoning, China
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作者：
Swisher, CC;Wang, YQ;Wang, Y
在辽宁省东北部中国地区义县组下部的古湖床中，发现了各种保存完好的化石：有羽毛的恐龙中华龙鸟(1个)、原始祖鸟和尾羽龙(2个)、原始鸟类孔子鸟(3个)和辽宁鸟(4个)、哺乳动物张合氏(5个)以及据说是最古老的开花植物古龙科植物(6个)。在湖床中保存得同样完好的还有各种各样的植物、昆虫、双壳类、甲壳类、介形类、腹足类、鱼类、火蜥蜴、海龟、蜥蜴、青蛙(7)和翼龙(1，8)。这种保存独特的化石组合为长期存在的关于鸟类与恐龙关系(1，2)、鸟类早期多样性(3，9，10)以及开花植物的起源和进化(6)的争议提供了新的见解。尽管这一化石组合很重要，但从侏罗纪晚期到白垩纪早期，对其地质年龄的估计存在很大差异。在这里，我们首次明确地将40Ar/39Ar年龄与义县组下部的主要化石层位联系在一起，从而首次提供了这一重要动物群的准确年龄定标。测年结果表明，义县下部化石层位不是侏罗纪，而是至少年轻20Myr，属于早白垩世中期。
The ancient lake beds of the lower part of the Yixian Formation, Liaoning Province, northeastern China, have yielded a wide range of well-preserved fossils: the 'feathered' dinosaurs Sinosauropteryx(1), Protarchaeopteryx and Caudipteryx(2), the primitive birds Confuciusornis(3) and Liaoningornis(4), the mammal Zhangheotherium(5) and the reportedly oldest flowering plant, Archaefructus(6). Equally well preserved in the lake beds are a wide range of fossil plants, insects, bivalves, conchostracans, ostracods, gastropods, fish, salamanders, turtles, lizards, the frog Callobatrachus(7) and the pterosaur Eosipterus(1,8). This uniquely preserved assemblage of fossils is providing new insight into long-lived controversies over bird-dinosaur relationships(1,2), the early diversification of birds(3,9,10) and the origin and evolution of flowering plants(6). Despite the importance of this fossil assemblage, estimates of its geological age have varied widely from the Late Jurassic to the Early Cretaceous. Here we present the first 40Ar/39Ar dates unambiguously associated with the main fossil horizons of the lower part of the Yixian Formation, and thus, for the first time, provide accurate age calibration of this important fauna. The results of this dating study indicate that the lower Yixian fossil horizons are not Jurassic but rather are at least 20 Myr younger, placing them within middle Early Cretaceous time.