泥河湾盆地马圈沟遗址化石哺乳动物及年代讨论

Fossil Mammals from Majuangou Section of Nihewan Basin,China and Their Age

本文简述了马圈沟旧石器遗址产出的哺乳动物化石:出自马圈沟遗址Ⅲ的20个种,马圈沟遗址Ⅰ的4个种,半山遗址的7个种;分析了以Allophaiomys deucalion,Cromeromys gansunicus,Borsodia chinensis等小哺乳动物组合为代表的马圈沟遗址Ⅲ的生物地层时代为早更新世早期,绝对年龄应大于1.80Ma;讨论了该遗址生物地层时代与磁性地层年代的关系。

The Majuangou section（= MJG,40°13′31.2″E,114°39′50.9″N） lies near Cenjiawan village and on the south of the Sanggan river in the Nihewan Basin.In 1990,Wei Qi first reported a new paleoliths-bearing layer from the middle part of the Majuangou section with the name Banshan Site（Wei Qi,1994）.Two years later,the Hebei Province Institute of Cultural Relic（=HPICR） found another artefact layer named as ＂Maguangou Site＂from the lower part of the same section and did excavation in the next year（Li and Xie,1998）.In 2001,the first two authors gathered numbers of mammal remains and several paleoliths by washing the matrix collected from the＂New artefact layer＂,8 meters below the bottom of ＂Maguangou Site＂（Cai and Li,2003）.A test excavation was done and Goudi Site was named by Wei Qi（Wei,2002）.During the HPICR′s excavation seasons from 2001 to 2005,the fourth artefact layer was found and called MJG Ⅱ.The four artifact layers found in the Majuangou section are,from the top to bottom,Banshan Site, MJGⅠ（=＂Majuangou Site＂in Li et al,1998）,MJG Ⅱ and MJG Ⅲ（=＂New artefact layer＂in Cai and Li,2003 and ＂Goudi Site＂ in Wei,2002）（see Fig.1）.Zhu et al（2004） reported magnetostratigraphic results of the four artefact layers mentioned above. The magnetic age of the MJG Ⅲ given by Zhu et al（2004） is about 1.66Ma,while the biostratigraphic age estimated by Cai et al（2003） is suggested to be older than 1.80Ma.There is a small disparity between the two results.In 2005,invited by HPICR,the authors undertook the screen-washing work in the Majuangou sites and collected additional fossil mammalian specimens from MJG Ⅰ and Banshan Site.The purpose of this paper is to briefly describe the fossil mammals collected in 2001 and 2005 and to ascertain the age of the fossil-bearing beds.Further more,the potential factors interfering with the determination of magnetic chronology for these layers are discussed. By the way of washing about 3.5 tons of matrix,553 small and 6 large mammal specimens were collected from MJG Ⅲ in 2001.They are Sorex sp.,Erinaceus sp.,Ochotona youngi,Ochotonoides complicidens,Allactaga sp.,Spermophilus sp.,Cricetulus sp.,Yangia tingi,？Episiphneus sp.,Micromys sp.,Chardinomys nihowanicus,Cromeromys gansunicus,Borsodia chinensis,Allophaiomys deucalion,Canidae gen.et sp.indet.,Canis sp.,Mammuthus trogontherii,Hipparion sp.,Rhinocerotidae gen.et sp.indet.and Cervidae gen.et sp.indet.（Fig.2—4）. Only 7 poorly preserved specimens were found after washing about 850 kgs of matrix from MJGⅠin 2005.Four species,including Ochotona sp.,Cromeromys sp.,Borsodia sp.and Allophaiomys sp.can be recognized（Fig.5）. Sixty three specimens were collected from about 3 tons of matrix from Banshan artefact layer.These specimens belong to 7 species,Ochotona sp.,Ochotonoides sp.,Leporidae gen.et sp.indet.,Allocricetus ehiki,？Cricetulus sp., Borsodia chinensis and Allophaiomys pliocaenicus（Fig.6）. The co-occurrence of A.deucalion,C.gansunicus,B.chinensis,？Episiphneus sp.,Y.tingi and C.nihowanicus shows that the MJG Ⅲ mammal fossils assemblage can be correlated with those from Haiyan Formation of the Yushe Basin,Shanxi（Flynn et al,1997） and Loc.93001 of Wenwanggou,Lingtai,Gansu（Zheng and Zhang,2001）.The age of MJG Ⅲ seems to be earlier than that of the end of the Olduvai normal polarity event,and possibly falls within the range of 2.10—1.80Ma.In addition,the coexistence of rooted Episiphneus and rootless Y.tingi has only been recorded in Chinese early Early Pleistocene（Zheng,1994）. In Europe,the earliest appearance of Allophaiomys was once considered as the beginning of Biharian（Repenning,1992）.The age of the type locality of A.deucalion,Villany-5 in Hungary,however,is at present considered as from late Villanyian to early Biharian（Kowalski,2001）.In addition,the magnetic age of the layer yielding Allophaiomys in Russian Tizdar is 2.25—1.96Ma（Pevzner et al,1998）.In the loess section of Xunyi,Shaanxi,A.pliocaenicus appears in the paleosol layer S25（Xue et al,2001）,which is 1.83—1.8Ma（Ding et al,2002）. It seems to be un-doubted that A.deucalion occurs before Biharian in Eurasia. In the Donggou section,about 2 km south of the Majuangou Site,A.deucalion was found in both layers 11 and 16 with C.gansunicus,while A.pliocaenicus in layer 19 with Y.tingi.The first two layers（11 and 16） were all regarded as to be late Villanyian,while the third layer（19） to be early Biharian（Zheng et al, 2006）.It is rather closed to the magnetic result（Zhu et al,2001） that the 81.12 m depth of the 121 m Donggou section is with the age of the upper limit of the Olduvai event,which should be equivalent to the position between layers 18 and 19（Zheng et al,2006）. The mammal assemblage from MJG Ⅲ should be contemporaneous with those from layer 11 and 16 of the Donggou section because they all have rootless A.deucalion, rooted C.gansunicus and B.chinensis in common.Their ages should be earlier than 1.80Ma,rather than the magnetic result of 1.66Ma（Zhu et al,2004）. In fact,there exist a few deposit intervals in the MJG section,such as the exposure of elephants＇ footprints（Xie et al,2006） and distinct unconformity beneath the MJG Ⅲ artefact layer（Fig.7）,which may make the Olduvai normal polarity event absent.The question here comes that the normal polarity event measured from 10 m depth under MJG Ⅲ（Zhu et al,2004） is or not the real Olduvai event. Without considering the absence of strata,it is easy to make an oversight on estimating the age by calculating the average deposit velocity between the two normal polarity events.As a matter of fact,the deposit velocity of the total 63m thick section of Majuangou Site should not be homogeneous,especially in its lower part with some deposit intervals.The magnetic dating,therefore,seems to be too young.