Effects of Past Global Change on Life (1995)
Chapter: Coal-Swamp Species and Ecomorphs
and structure are seen most clearly. Long-term trends in landscape, habitat, and species composition must be examined among coals. Each can be evaluated independently, or the relationships among them can be investigated. For example: Is the timing of change similar at all levels of observation? Are there relationships among levels of organization that are detectable only on the longer time scales? Do events or changes at one level appear to dictate or constrain those at other levels?
Climatic change is often brought about by geological phenomena that develop over millions of years, such as the movement of continental plates, the elevation of mountain ranges, or the waxing and waning of ice sheets and associated eustatic sea-level fluctuations. It is extremely difficult to correlate such events, which have their own considerable margins of error, with specific changes in community composition. Broadly based correlations can be developed, however, and there is great likelihood that widespread biotic changes are the consequence of events of global or at least regional scale. Thus, the long-term analysis of swamp patterns is a scale at which such phenomena can be examined. Certainly, glacial eustatic and climatic effects occur at time scales approximately the same as those over which we believe coal seams accumulated. However, the peat accumulation may have required a relatively narrow range of conditions, such that coal seams occur between the "events," which may mark their beginnings and ends. Consequently, it is only on the interseam level of observation that we can make a case for the climatic changes that affected the dynamics observable at lower levels.
Coal-Swamp Species and Ecomorphs
Well over 100 whole-plant species have been identified in coal balls from the upper Carboniferous of Europe and United States, across a time span of 12 to 15 m.y. Most of these can be assigned broadly to several ecomorphic groups, based on their habit, reproductive morphology, and life history attributes (Figure 8.4). The ecomorphic groups, particularly trees, conform fairly strictly to taxonomic groups or clades, that is, to groups of closely related taxa, which is the basic pattern of the Carboniferous. Much of what we know of evolution based on studies of modern plants and animals suggests that this should not be the case; ecological overlap between groups of plants with different ancestries is the rule in modern forests. It was not an expected result when we began to look for ecomorphic patterns among coal-swamp plants. The explanation for this pattern may lie partly in the ecological structure of the Carboniferous world, including the origin of that pattern.
Habits of the major ecomorphic tree groups are illustrated in Figure 8.4. Ecomorphic determination is based mainly on critical life history attributes: timing of reproduction; approximate reproductive output; morphology of, and energy investment in, disseminules; and dispersibility of disseminules. Habit and vegetative morphology also are important: size at maturity; adult shape; growth habit
FIGURE 8.4 Major tree ecomorphs in Late Carboniferous coal swamps. Left to right: Mature monocarpic lycopsid about 20 m in height; juvenile lycopsid; polycarpic lycopsid (these came in a wide range of sizes); medullosan pteridosperm (seed fern); small Psaronius typical of Westphalian swamps; large Psaronius typical of the latest Westphalian and Stephanian; cordaite reconstructed as possible mangrove; scrambling cordaite, large arboreous cordaite; sphenopsids illustrating clonal habit.
