最古老的后生动物痕迹化石?--对北京十三陵前寒武系常州沟组充填管状构造的探讨

前言 地球上最古老的生命现象在35亿年前形成的岩石中已经发现一些踪迹(Sohopf)2 6]。但是,目前比较公认的最古老的动物化石是出现在7亿年前后的沉积地层中,被称之为伊迪克拉后生动物化石(Edicaran Metazoan fossils)。在此以前是否有更古老的后生动物化石问题,一直为古生物学家、地层学家和沉积学家所注意。Walcott(1899)27]最先在上前寒武系中部大约10-11亿年的地层(Belt Supergroup)中,描述过两种古老的痕迹化石，分别定名为:Pldnol i tes和Helminthoidichnites。后来，Cloud14]和W alter28]等对这些痕迹化石进一步研究后，认为Walcott描述的痕迹化石或者是藻类，或者是一些孤立的波痕或结核。以后，Sabrodin(1971)在苏联的上前寒武系里菲组中，发现了一种规则的管状充填痕迹化石，直径1.3毫米，管壁边界清楚，伴生有团粒(粪粒?)，用K-Ar法测得这些团粒为12.7亿年的同位素年龄值，并将这种充填管状痕迹化石定名为Rugoinfractus. Palij(1974)23]并论述T这一痕迹化石，但根据地层产状对其年代有怀疑。

ARE THESE THE OLDEST METAZOAN TRACE FOSSILS ALSO? -A PROBE INTO BURROW-LIKE TUBE FILLING STRUCTURES IN CHANGZHOUGOU FORMATION OF THE UPPER PRECAMBRIAN IN MING TOMBS DISTRICT,BEIJING

There is a set of tidal flat deposits, with the correlated geochronological data about 1950 m.y., made of sand-silty mudstone interlayers in Changzhougou Formation of Changcheng -System in the lower part of the Upper Precambrian strata in Ming Tombs, Beijing. Many shallow water sedimentary structures can be found in the strata, such as: ripple mark, mud crack, ice model(?), runzel,rain print model, herringbone cross-bedding, flaser and lenticular bedding, etc. showing tidal flat depositional environment. It is worth noticing that among the interlayers of sand-silty mudstone there are so many burrow-like tube filling structures (See Fig.l, Fig.7, Plate I-1,2,3 and 5, Plate II 3 and 5), which have been compared by the authors with folded mud crack filling structures, ball and pillow structure, load deformation, frozen crack filling structre, sandbody liquidization structure and water escape structure, etc., but all of those sedimentary structures are helpless to interpret the genesis of the burrow-like tube filling structures in this paper. But there are many obvious similarities between the burrow-like tube filling structures and the so-called oldest Metazoan trace fossils or dubiofossils discovered by E.G Kauffman and J. R. Steidtmann in Medicine Peak Quartzite, Wyoming, U. S.A. The Medicine Peak Quartzite dated at 2000 m.y.B.P*, which is roughly coincident with the geological age of Changzhougou Formation of the Upper Precambrian Strata in Ming Tombs, Beijing. So, are these the oldest Metazoan trace fossils also? And there is also one more question- had some catastrophic events happened since 2000 m.y.B.P.to Phanerozoic so as to eliminate these oldest lives in such a long period? The authors have paid attention to the comments on the reports about the so-called oldest Metazoan trace fossils made by P.E. Cloud, H. Clemmey, J.W. Durham, and C.W. Byers, et al. It is supposed that if there was not any possibility for the existence of the oldest Metazoan trace fossils in sediments at 2000 m.y. B.P., there must be an extreme situation for the genesis of burrow-like tube filling structures either in Medicine Peak Quartzite, Wyoming or in Changzhougou Formation in Ming Tombs, Beijing. However, it is necessary for us to deal with the kinetic mechanism of burrow-like tube filling structures continuously. Fortunately, we have found a kind of traces distributed on the mudy siltstone bed surface in the middle part of Changzhougou Fr. overlaid on the layers, envoling burrow-like tube filling structures in the tidal deposits of the lower part of the Upper Precambrian Strata as mentioned above. We try to judge by the appearances of those traces as a kind of the oldest Metazoan trace fossils, according to the regular narrow grooves with two widges. The diameter of grooves is about 3mm and the widge width about 1.2mm. Those traces can be slightly straight or curved, they look very like crawled trails by some fine earthworms, however it was a long past story before 1900m.y A Canadian famous geologist, Dr.R.A. Rahmani had a look of the oldest Metazoan trace fossil on the outcrop of Changzhougou Fr. in the Ming Tombs District, when he was invited for making lectures in Beijing. Dr. R. A.Rahmani considered that was a kind of true trace fossil characterized by very regular grooves as the trace fossil Gyrochortae in Cretaceous Strata. It will be a definite support to Dr. E. G. Kauffman and Dr. J. R.Steidtmann's theory about the existence of the oldest Metazoan trace fossils in the early Proterozoic period, if the real oldest Metazoan trace fossil in the Changzhougou Fr. of the lower part of Upper Precambrian Strata (about 1900m. y. BP) in the Ming Tombs District, Beijing, China, is demonstrated.