机会（r—选择）和均衡（Ｋ—选择）遗迹化石的现代生态学解释

MODERN ECOLOGICAL INTERPRETATION OF OPPORTUNISTIC (R-SELECTED) AND EQUILIBRIUM (K-SELECTED) TRACE FOSSILS

介绍地质记录中在不稳定的沉环境下，由于生活条件的恶化（盐度的变,严酷的温度，低含氧量及移动的沉积基底等）以及沉积事件（浊流沉积和风暴沉积）所产生的机会(r-选择）遗迹化石和均衡（Ｋ－选择）遗迹化石及其特点以及机会行为方式的古生物证据。对现代底栖生物的生态学研究，揭示现代机会生物均衡生物的生态特征（居群策略）描述由于破坏事件导致动物群消除后重新移居的现代实例，有助于了解地质记录中机会(r-选择）遗迹化石和均衡（Ｋ－选择）遗迹化石的成因及其古生态特征。

The opportunistic (r-selected) trace fossils produced under the unstable depositional environments and serious ecological conditions (the fluctuating salinity and temperature, low-oxygen and shifty substrates et al. ) and the equilibrium (K-selected) trace fossils produced under the stable and predictable depositional conditions in the geological histories have attracted much attention from ichnologists and sedimentologists for a long time. Seilacher noted that the trace fossils in the black shale of lower Jurassic Pasidonienschiefer are mainly composed of highly branched Chondrites which is related to the low-oxygen sedimentary environments. The pellet-walled burrow Ophiomorpha and Granularia are considered to be opportunistic trace fossils in relation to the shifty sandy substrate and the sedimentary conditions with high depositional rate. Miller and Johnson (1981) described an opportunistic burrow Spirophyton deposited under environmentally harsh fluvialtidal conditions, where temperatures, salinities and even intervals of subaerial exposures were quite variable and unpredictable.The opportunistic (r-selected) trace fossils with respect to event deposits (turbidite and tempestites) have also been noticed. Seilacher (1962) and Kern (1980) recognized the pre-turbidite equilibrium (Kselected) trace fossils consisting of Palaeodictyon, Desrnograpton , Cosrnorhaphe , Spirohaphe ,Nereties and regular Scolicia and the post-turbidite opportunistic (r-selected) trace fossils consisting of Granularia, Phycosiphon and irregular Scolisia. The trace fossil assemblages in the terrigenous tempestites of the Upper Cretaceous Cardium Formation are described by Pemberton et al. (Pemberton and Frey, 1984; Pemberton et al., 1992; Pemberton et al. in press). The opportunistic (r-selected) trace fossils consisting of Skolithos ichnofacies members include Arenicolites , Diplocraterion , Ophiomorpha , Palaeophycus and Skolithos. The equilibrium (Kslected) trace fossils consisting of Cruziana ichnofacies members include Anconichnus, Asterosoma, Chondrites , Helminthopsis , Palaeophycus , Planolites, Rosselia, Teichichnus, Terebellina and Thalassinoides. The trace fossil assemblages in the storminfluenced carbonate deposits found in the Gushan Formation of the Upper Cambrian from eastern North China Platform are somewhat similar to those mentioned above in the Cardium Formation. The prestorm equilibrium ( K-selected ) trace fossils Phycodes, Thalassinoides , Palaeophycus and Planolites are composed of Cruziana ichnofacies memebers. The post-storm opportunistic (r-selected) trace fossils Diplocraterion, Arenicolites and Skolithos are composed of Skolithos ichnofacies members ( Zhou Zhicheng et al. , in press).The characteristics of an opportunistic body fossil assemblage were summarized by Pemberton and Frey (1984) as (1) of limited aerial distribution (Waage, 1968); (2) occurring in a continuous, thin isochoronous horizon (Waage, 1968); (3) abundant in several otherwise distinct faunal assemblages (Levinton, 1970);(4) having great abundance in a facies with which it is not generally associated (Levinton, 1970); and (5) numerical domination of one species within the fossil assemblage (Levinton, 1970).In recent ecological studies of benthic organisms, equilibrium (K-selected) species have been distinguished from opportunistic (r-selected) species. The studied results are important for understanding the paleoecologic characteristics of opportunistic (r-selected) trace fossils and equilibrium (K-selected) trace fossils in the geological histories. In general, opportunistic species can respond rapidly to an open or unexploited niche and are characterized by (1) a lack of equilibrium population size, (2) a density-dependent mortality, (3) the ability to increase abundance rapidly, (4) a relatively poor competitive ability, (5) high dispersal ability, and (6) a high proportion of resources devoted to reproduction (Glassle and Grassle, 1974). Opportunistic organisms display a rstrategy in population dynamics, emphasizing rapid growth rate (r), whereas equilibrium species adopt a K-strategy, based on the carrying capacity of the environment (K) (Boesch and Rosenberg, 1981 ). Short generation span is the most important mechanism for increasing population size in a r-strategy; therefore life spans of opportunistic species are shorter, and sexual maturation is reached earlier (Rees et al., 1977). Broad environmental tolerance and generalized feeding habits facilitate rapid colonization of open niches (Pianka, 1970; Wolff, 1973).Currently, ecologists involving organisms are realizing the importance of hydrodynamics in larval settlement, and a new hypothesis, termed passive deposition, is gaining wide acceptance. Butman (1987) summarized the two main points of view regarding passive deposition as the following: either (1) larvae are deposited over broad areas but differently survive only in hospitable abult habitats or (2) species-specific larval fall velocities correspond with particular sediment fall velocities, such that hydrodynamically similar particles and larvae are deposited in the same environment. Episodic depositional events, like storms and turbidites, therefore can have a powerful effect on the redistribution of the larvae of benthic organisms. In most event deposits, initial larval settlement, initial larval settlement may be a function of the hydrodynamics of the event bringing both larvae and sediment into the area.At present, most biologists believe that larval settlement involves a complex interaction between active and passive processes. Competent planktonic larvae initially reach the seafloor at sites where passively sinking particulate, having fall velocities similar to the larvae, initially settle (Hannan, 1984). Other biological or physical processes may then redistribute them. In most event deposits, therefore, initial larval settlement may be a function of the hydrodynamics of the event bringing both larvae and sediment into the area. After initial settlement, exploitation of the open niche becomes more a function of the reproductive characteristics of the individual species. In most cases, because of their efficient reproductive cycles, opportunistic species quickly dominate the initial stages of recolonization.Studies of recolonization rates of stable and unstable (e. g. fluctuating ecological parameters, such as salinity, sedimentation rate, and temperature) modern environments show that organisms in stable environments are more adversely affected by physiological stress. Species present in unpredictable environments (e. g. , estuaries and river-dominated deltas), usually have broad environmental tolerance and can recover from disturbances quickly (Jenelov and Rosenberg, 1976). For instance, the benthic population of relatively stable deep-sea environments can take more than 2 years to recover completely, while the benthic population of relatively unstable estuarine environments may recover in less than 11 months (Dauer and Simon, 1976; Grassle, 1977).