PALEOECOLOGY: PLANTS IN THEIR ENVIRONMENT
Paleoecology, the study of past environments, is a rapidly changing field that involves the integration and synthesis of both botanical and geological information. In recent years there has been a concerted effort by many paleobotanists to understand the paleoenvironment of fossil land plants more completely. For example, Bateman and Scott (1990) examined the famous late Tournaisian (lowermost Carboniferous) plant-bearing deposits at Oxroad Bay, Scotland, from a number of different perspectives, including an analysis of the geologic history and sedimentology of the site, as well as the paleoenvironment and paleoecology of the plants. Their studies indicate that the Oxroad Bay flora is found at eight levels in five successive facies, and these facies show the increasing influence of nearby volcanoes in the ocean-marginal setting. Details of the depositional environments through time at this site make it possible to understand plant adaptations to a rapidly changing, lowland environment, and to better understand both the biological and evolutionary importance of the floras.
Much paleoecological work initially focused on analyses of the swamp vegetation that contributed to extensive coal deposits in the Midcontinent of North America during the Carboniferous (Phillips and Peppers, 1984). The data used in these early analyses came primarily from the study of Pennsylvanian plants in coal balls—nodular concretions of limestone that contain permineralized peat (FIG. 1.5; see section on "Preservation"), coupled with a precise stratigraphic framework for the coal ball deposits based on palynology and careful field observations and measurements. Through the pioneering efforts of Phillips, DiMichele, and cowork-ers, we now possess an excellent understanding of many aspects of the paleoecology of coal-swamp vegetation during the Carboniferous (Wagner and Diez, 2007). Analysis of the plants preserved at different levels in these deposits not only documents the partitioning of the habitat among the different plant groups along ecological lines (Fig. 1.7), but also records changes in the depositional environment through time (DiMichele and Phillips, 1994; Wagner and Mayoral, 2007).
In one study on the peat flora just above the Mahoning coal in Ohio (Conemaugh Group, Pennsylvanian), DiMichele et al. (1996), utilizing macrofossils, palynomorphs, and coal petrology, concluded that the lepidodendrid trees (lycops-ids—see Chapter 9) in this succession were flooded once, recovered, and then finally drowned by another flood event. This type of information can be utilized to recognize regional (Phillips et al., 1985) and global responses of plant communities to climate change (DiMichele et al., 2001a; Wagner and Mayoral, 2007). Understanding the interplay between these swamp-inhabiting plants and a variety of environmental parameters has now made it possible to interpret large-scale ecological shifts (e.g., the role of sea level fluctuations) in the community structure of the swamps, and to examine evolutionary questions within these habitats (DiMichele et al., 1985; DiMichele et al., 1996). These studies in turn have stimulated interdisciplinary research focused on broader questions, for example the evolution of major terrestrial ecosystems through time (Behrensmeyer et al., 1992; DiMichele and Hook, 1992).
Paleoecological studies are very important in revealing the diversity of fossil communities inhabiting a geographic area (horizontal variation in floras) at the same time. Wing et al. (1993) examined fossil floras preserved in an ash fall and in fluvial deposits from the Upper Cretaceous (mid-Maastrichtian) of Wyoming, USA. They found that ferns (49%), along with palmettolike palms (25%), dominated the landscape, and that other angiosperms (Chapter 22), mostly herbaceous dicots, were dominant only in disturbed areas close to streams (fluvial deposits). The flowering plants were more diverse—constituting 61% of the species present—but they represented only 12% of the vegetational cover in this area. This study, and other similar ones (e.g., McElwain et al., 2007), have detailed the difference between the diversity of plants in fossil floras and the dominance of particular taxa within the paleoecosystem. Of course, it is only possible to fully comprehend the fossil assemblages, or taphocoenoses, by comparison with extant plants in various depositional environments (Spicer, 1981; Burnham, 1989, 1997) and by being aware of the taphonomy of fossil plants (Spicer, 1989) (see section on "Preservation"). Understanding and interpreting the sedimentological nature of the fossil assemblage, whether based on megafossils or microfossils, is only one of several aspects required in determining the diversification of plants through geologic time (Wing and DiMichele, 1995; Lupia, 1999).
Paleoecologists use many of the same statistical methods used in contemporary ecological studies, including a variety of multivariate methods (Spicer and Hill, 1979; McCune and Grace, 2002). These tools, and many others, now make it possible to examine the evolutionary and ecological processes that governed the plant communities which we now document as the fossil record (Jackson and Erwin, 2006). For a more in-depth approach to the study and methodologies of plant paleoecology see DiMichele and Wing (1988), Gastaldo (1989), and Jones and Rowe (1999).
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