论长辛店组的地质时代

SUPPLEMENTARY REMARKS ON THE AGE OF CHANGXINDIAN FORMATION

长辛店组(旧称长辛店砾石层)的地质时代,廿多年前曾有第三纪末期、始新世、第四纪初期等种种不同的看法。1953年,周明镇同志讨论过这个问题,他根据岩性和产状,特别是根据一些残破而只能粗略地鉴定的古动物化石,肯定“砾石层的时代应为第三纪初期,最新不能晚于渐新世后期”。

Changxindian Formation (=Changsintien Gravel, named by Hsieh, 1933) is athick series of tilted conglomates and red clays cropping out sporadically in the vi-cinity of Changxindian, some 20 km S. W. of Beijing. For more than half a centurythe age of Changxindian Formation has been much debated. At first, it was regar-ded as of late Tertiary or early Pleistocene. From its tilted position and lithologicalcharacter, J G. Anderssoll (1923) comparied it with Kuanchuang Series, ShantungProvince in default of any fossil evidence. Young Chung-ehien (1934) based on apremolar of ungulate which determinated with hesitation as ?Eudinoceras sp., consi-dered it to be of Eocene-Oligocene age. Later, Chow Minchen (1953) collected somefragments of mammal teeth as well as invertebrate remains. He briefly reviewed onthe subject and reached the conelusion that the Formation is certainly early Tertiaryin age. This conclusion which we considered neyond question is not generally accep-ted by the geologists. In the/Spring of 1976, a small collection was made by commune members atGaodian, 1km east of Changxindian. At least four distinct forms can be recogifizedin the collection. They are: Imequincisoria sp., a small fostereooperid; Hypsimylusbeijingensis (gen. et sp. nov., vide infra), an advanced eurymyloid; a miacid and acanid carnivores. On the basis of these fossils as well as those definately deter-minated by Chow (1953), some supplementary remarks on the age of ChangxindianFormation are made and a late Eocene age is tentatively assigned to the fossil bear-ing beds in this paper. Hypsimylus beijingensis, represented by a fragment of left lower jaw and anisolated talonid of a milk premolar. The lower jaw is badly damaged and only amolar and a milk premolar are preserved after preparation. The crown of the milkpremolar is rather low and with a well developed third lobe. It is similar to theimtermediate cheek teeth of Rhombomylus. The molar (M_1) probably rootless or withonly tiny roots. Though larger and much higher it is, the structure and relativelocation of lobes and grooves are essentially the same as that of Rhombomyluts. It istherefore suspected that Hypsimylus is probably a descendant of eurymyloid.