The effect of synsedimentary substrate modification on the composition of paleocommunities; paleoecologic succession revisited

Three kinds of paleoecological succession have been recognized: 1) long-term (through a considerable stratigraphic interval), 2) reef succession, and 3) short-term (usually recorded in a single bed). The third kind must have occurred under more or less constant conditions of external environment, and over a short time interval. It is, then, the only type of fossil succession which is directly comparable to modern ecological successions. The nature and causes of short-term succession are the subject of this study. We believe short-term paleoecological successions were often controlled by gradual change in the nature of the substratum during or shortly after deposition. We call these processes of substratum change synsedimentary modification; three kinds of processes are recognized. First, water loss from the substratum produces gradual change in its consistency from fluid to soft, and ultimately to firm. The nature and relative timing of these consistency changes can be discerned by observing burrowlsediment interrelationships. Fossil assemblages commonly reflect these changes by containing a mix of morphotypes, each of which was adapted to a different substratum consistency. The second modification process involves the changing mix of substratum niches as successive taxa colonize the environment. The most common example is the paving of initially soupy substrata by sediment floaters, and the subsequent use of the skeletons of the pavers as settlement sites for later population. Finally, sediment may undergo substantial change early in its history by marine cementation. The process is patchy in threedimensional distribution, so at any given time the substratum consists of scattered areas of hardground interspersed with areas of loose sediment. Again the ultimate result is the mixing in one bed of morphotypes adapted to these divergent substratum types. Subsequent lithification may encrypt the relative timing of various cements, but petrographic analysis allows its resolution. The examples given here are drawn from the Middle Ordovi-