贵州中寒武世凯里化石库的埋藏学和生态学初步研究

TAPHONOMY AND PALEOECOLOGY OF THE EARLY MIDDLE CAMBRIAN KAILI LAGERSTTTE IN GUIZHOU, CHINA

在详细的地层剖面和化石标本野外观察和室内研究的基础上 ,初步分析了贵州中寒武世凯里化石库的埋藏环境、化石保存状态和保存方式、以及矿物成分。认为凯里化石库形成于较深水的陆棚环境 ,海底位于风暴浪基面之下。由于有频率较高的间歇性沉积事件的侵入 ,沉积速率高 ,海底附近的生物群落的发展经常被沉积事件所中断 ,因此 ,化石库内很少有群落较长时间演替而形成的多类别生物尸体堆积的硬底层。正常堆积的化石层通常由一些发育时间较短的群落演替序列构成。一些能量较强的沉积事件对一些逃跑能力差的生物或原位快速掩埋 (如始海百合类、水母状动物等 ) ,或被卷入沉积物流体内后被快速埋藏 (如浮游型的三叶虫、水母状动物等 ) ,为凯里化石库中特异化石的形成提供了先决条件。沉积事件普遍较弱是凯里化石库内特异化石种类单调的最重要原因。底内生物的缺乏、极细颗粒的粘土沉积物对封闭在泥内的生物的完整保存起到了保护作用。早期成岩矿物 (黄铁矿 )对阻止软体腐烂和复制软体组织起到了关键性的作用。

Since the 1980's numerous fossil lagersttten containing extraordinary well preserved skeletal and soft bodied fossils (Burgess Shale type fossils) have been discovered world wide, particularly from the Early and Middle Cambrian. Except for the famous Burgess Shale Lagersttte (Middle Cambrian, British Columbia), the most important locality is the Chengjiang Lagersttte from the Early Cambrian of eastern Yunnan, China. Extraordinary well preserved skeletal and soft bodied fossils in other lagersttten, for example the Sirius Passet fauna (Early Cambrian, Greenland; Conway Morris, 1989; Budd, 1993), the Big Gully fauna in the late Early Cambrian Emu Bay Shale of Australia (Nedin, 1995; Briggs and Nedin, 1997), the Spence Shale fauna (Middle Cambrian, Utah; Robison, 1991), are not as abundant as those of the Chengjiang Lagersttte. However, some of the fossils from these lagersttten provide unique phylogenetic and anatomical information for those Cambrian problematic fossils. For instance, completely preserved Halkieria from the Sirius Passet fauna (Conway Morris and Peel, 1995) not only shows articulated skeletons or spicules whose elements usually are isolated in other sediments, but also provides the phylogenetic evidences for the Cambrian mollusk like or brachiopod like shells (Conway Morris and Peel, 1995). Similarly, extraordinary well preserved skeletal and soft bodied fossils in the Kaili Lagersttte from the Kaili Formation of Guizhou are not highly diverse, but in some cases (e.g. medusiform fossils; Zhao and Zhu, 1994; Dzik et al. , 1997) they also provide a great deal of important anatomical and paleoecological information. In addition to their importance in revealing body plans and phylogenetic affinities, extraordinary well preserved skeletal and soft bodied fossils from Cambrian fossil lagersttten also provide evidences bearing on the evolution and paleobiogeography of early metazoans. For example, Conway Morris (1989) attempted to explain the ultimate fate of Burgess Shale type fossils based on their distribution and paleoenvironments, proposed that these early metazoans underwent displacement toward offshore environments. Because most of these Cambrian lagersttten have not yet been studied thoroughly, this hypothesis should be tested through extensive research on these Cambrian lagersttten, such as the Kaili Lagersttte. Additinonal work also is needed on problems of why fossil lagersttten in Cambrian are so numerous and how soft bodied fossils in these deposits were preserved (Allison and Briggs, 1993a,b,1994; Pickrill, 1994; Aronson, 1992,1993; Butterfield, 1995,1996; Towe, 1996; Orr et al. , 1998). In recent years, extensive field work and improved preparation techniques have led to discoveries of additional Cambrian Burgess Shale type fossils (e.g. Butterfield, 1994; Butterfield and Nicholas, 1996; Steiner et al., 1993; Ivantsov, 1998), it is now clear that Burgess Shale type fossils in sediments are not so rare and that their discovery is not simply a matter of chance. Therefore, understanding the preservational environments and taphonomic processes of those fossil lagersttten is critical. In the the remainder of this paper, we report on our investigation on the preservation of fossils in the Kaili Lagersttte. Our aim is to provide new information that may be useful in addressing some of the more general questions outlined above.