关于辽西孔子鸟层时代的讨论

A DISCUSSION ON THE AGE OF THE FEATHERED DINOSAURS BEARING BEDS OF LIAONING, CHINA

SwisherⅢ等报道的辽西孔子鸟层同位素年龄 ( 1 2 4 .6Ma)的准确性存在可疑处 ,其取样点地层受后期侵入岩床的烘烤 ,因此“巴列姆期”的时代结论不可靠 ;罗清华等的取样点和数据 ( 1 47Ma)较为可信。对义县组动物群与欧洲Solnhofen层动物群的对比讨论 ,表明两生物群基本同时代 (Tithonian期 ) ,并不是“超过 2 0Ma”的“长寿动物群”

A dating result of 124.6Ma for the age of the feathered dinosaurs bearing beds (or Confuciusornis beds) of west Liaoning, Northeast China, was recently reported (Swisher Ⅲ et al., 1999). The age was correlated with the “middle Barremian” of mid Early Cretaceous. However, the authors of this paper hold, based on the investigations, that the reliability of the dating basement is dubious, and that the age conclusion is seriously contradictory with the palaeontologic data. The problems are discussed in this paper.\;1. On the samples We have faith to the analyses, but doubt the suitability of their sampling. At their two sampling sites there existed a widely distributed basalt sill. The sill might have strongly baked the sedimentary layers to form thermal or chemical metamorphic zones. Therefore, we consider that, the dating result of 124.6Ma indicates a changed age of the metamorphous event caused by the intrusion rather than a sedimentary age. Oppositely the age dated by Lo et al. (1999) is more reliable, because the samples come from a site non influenced by the sill.\;2. On the “long lived chronofauna” The fauna of the lower Yixian Formation was said “at least 20Ma younger than” the Solnhofen fauna, and so that, it was “a part of a long lived chronofauna”. Actually the bird fauna of the lower Yixian Formation presents a feature of a radiation stage in the early bird evolution. It contains various charactors, “advanced” jointing “primitive”, occurring in the same horizons. Therefore, the Confuciusornis may not be much younger than the Archaeopteryx .\;3. On the Jehol biota The authors of the present paper example some invertebrate faunas to prove an age, near the Jurassic/Cretaceous boundary, of the Jehol biota of the lower Yixian Formation. A conchostracan assemblage from the Dabeigou Formation, which lies conformably beneath the Yixian Formation, can be correlated with that of the Kimmeridgian Morrison Formation of North America. An ostracod Cypridea assemblage from the lower Yixian Formation is compared with the C.dunkeri Zone and the C.granulosa Zone of the lower Purbeck Group of England. A bivalve assemblage from a horizon, several hundred meters above the lower Yixian Formation, is correlated with the Valanginian assemblage of Japan. An insect charactor, Aeschnidium, of the Yixian Formation very closely resembles a species of the same genus from the Solnhofen Limestone of Germany.