Paleoecology

Pebbles and shells on the beach give us clues about their sources. Paleontologists can reconstruct ancient lifestyles and ancient scenes based on such limited information, and this is the basis of paleoecology. Paleoecology is the study of the life and times of fossil organisms, the lifestyles of individual animals and plants together with their relationships to each other and their surrounding environment. We know a great deal about the evolution of life on our planet but relatively little about the ways organisms behaved and interacted. Paleoecol-ogy is undoubtedly one of the more exciting disciplines in paleontology; reconstructing past ecosystems and their inhabitants can be great fun. But can we really discover how extinct animals such as the dinosaurs or the graptolites really lived? How did the bizarre animals of the Burgess Shale live together and how did such communities adapt to environmental change?

It is impossible to journey back in time to observe extraordinary ancient communities, so we must rely on many lines of indirect evidence to reconstruct the past and, of course, some speculation. This element of speculation has prompted some paleontologists to exclude paleoecology from mainstream science, suggesting that such topics are better discussed at parties than in the lecture theatre. Emerging numerical and statistical techniques, however, can help us frame and test hypotheses - paleoecology is actually not very different from other sciences.

More recently, too, paleoecology has developed much wider and more serious significance in investigations of long-term planetary change; ecological data through time now form the basis for models of the planet's evolving ecosystem. The influential writings of James Lovelock have extravagantly echoed the suspicions of James Hutton over two centuries ago, that Earth itself can be modeled as a superorganism. The concept of Gaia describes the planet as a living organism capable of regulating its environment through a careful balance of biological, chemical and physical processes. Ecological changes and processes through time have been every bit as important as biodiversity changes; these studies form part of the relatively new discipline of evolutionary paleoecology.

Paleoecological investigations require a great deal of detective work. It is relatively easy to work out what is going on in a living community (Fig. 4.1). Ecologists are very interested in the adaptations of animals and plants to their habitats, the interactions between organisms with each other and their environment, as well as the flow of energy and matter through a community. Ecologists also study the planet's life at a variety of levels ranging through populations, communities, ecosystems and the biosphere as a whole. By sampling a living community, ecologists can derive accurate estimates of the abundance and biomass of groups of organisms, the diversity of a community and its trophic structure. But fossil animals and plants commonly are not preserved in their life environments. Soft parts and soft-bodied organisms are usually removed by scavengers, whereas hard parts may have been transported elsewhere or eroded during exposure (see Chapter 3). In a living nearshore community (Fig. 4.1) the soft-bodied organisms, such as worms, would rapidly disappear together with the soft parts of the bony and shelly animals, for example the fishes and the clams; the multiskeletal organisms such as the bony fishes would disaggregate and animals with two or more shells would disarticulate. Fairly quickly there would only be a layer of bones and shells left with possibly some burrows and tracks in the sediment. Moreover, some environments are more likely to be preserved than others; marine environments survive more commonly than terrestrial ones.

Although fossil assemblages suffer from this information loss, paleoecological studies must, nevertheless, have a reliable and sound taxonomic basis - fossils must be properly identified. And although much paleoecologi-cal deduction is based on actualism or unifor-mitarianism, direct comparisons with living analogs, some environments have changed through geological time as have the lifestyles and habitats of many organisms. For example, some ecosystems such as the "stromatolite world" - sheets of carbonate precipitated by cyanobacteria (see p. 189) - existed throughout much of the Late Precambrian, returning during the Phanerozoic only after some major extinction events and only for a short time (Bottjer 1998). Nevertheless, a few basic principles hold true. Organisms are adapted for, nektonic mobile nektonic mobile

Figure 4.1 Life modes of marine organisms in a living offshore, muddy-sand community in the Irish Sea with a range of bivalves (a-e, l), gastropods (f), scaphopods (g), annelids (h, j), asterozoans (i), crustaceans (k, r), echinoids (m, n) and fishes (o-q). Insets indicate large and small burrowers. (From McKerrow 1978.)

Figure 4.1 Life modes of marine organisms in a living offshore, muddy-sand community in the Irish Sea with a range of bivalves (a-e, l), gastropods (f), scaphopods (g), annelids (h, j), asterozoans (i), crustaceans (k, r), echinoids (m, n) and fishes (o-q). Insets indicate large and small burrowers. (From McKerrow 1978.)

and limited to, a particular environment however broad or restricted; moreover most are adapted for a particular lifestyle and all have some form of direct or indirect dependence on other organisms. These principles are valid also for the study of the ecology of ancient animals and plants.

There are two main areas of paleoecologi-cal research: paleoautecology is the study of the ecology of a single organism whereas paleosynecology looks at communities or associations of organisms. For example, aut-

ecology covers the detailed functions and life of a coral species, and synecology might be concerned with the growth and structure of an entire coral reef, including the mutual relationships between species and their relationship to the surrounding environment. The autecology of individual groups is discussed in the taxonomic chapters. In most studies the functions of fossil animal or plants are established through analogies or homologies with living organisms or structures or by a series of experimental and modeling techniques. Geo logical evidence, however, remains the main test of these comparisons and models. In this chapter we focus on the community aspects of paleoecology (synecology), reviewing the tools available to reconstruct past ecosystems and see how their organisms socialized.

Taphonomic constraints:

sifting through the debris_

As noted above, most fossil assemblages have been really messed about before being buried and preserved in sediment. The decay and degradation of animal and plant communities after death results in the loss of soft-bodied organisms, while decay removes soft tissue with the disintegration of multiplated and multishelled skeletal taxa (see Chapter 3). If that were not enough, transport and compaction add to the overall loss of information during fossilization. On the other hand, areas occupied by dead communities may be recolo-nized and animal and plant debris may be supplemented by material washed in from elsewhere. This process of time averaging can thus artificially enhance the diversity of an assemblage over hundreds of years. But can we rely on fossil assemblages to recreate ancient communities with any confidence and accuracy? Paleoecologists know we can, with varying degrees of precision.

The similarity of a death assemblage to its living counterpart, its fidelity, can be assessed in different ways. In a series of detailed studies of the living and dead faunas of Copana Bay and the Laguna Madre along the Texas coast, George Staff and his colleagues (e.g. Staff et al. 1986) discussed the paleoecological significance of the taphonomy of a variety of nearshore communities, sampled over a number of years. Most animals in living communities are not usually preserved, nevertheless the majority of animals with preservation potential (mainly shelled organisms) are in fact fossilized. More were actually found in death assemblages than in living assemblages, where the effects of time averaging were clearly significant. Suspension feeders and infaunal organisms were the most likely to be preserved (Fig. 4.2). Measurements of biomass and taxonomic composition rather than those of numerical abundance and diversity are the best estimates of the structures of communities, and counts of the more stable adult pop-

O • living assemblage A ▲ potential death assemblage □ ■ death assemblage detritivores/herbivores predators/parasites

Figure 4.2 The transition from a living assemblage to a death assemblage. Relative proportions of different types of organism change in two living marine assemblages off the Texan coast. Living assemblages are dominated numerically by detritivores and herbivores, death assemblages by suspension feeders. (Based on Staff et al. 1986.)

ulations are the most realistic monitors of community structure.

Another method to estimate taphonomic loss involves a census of an extraordinarily preserved Lagerstätte deposit. Whittington (1980) and his colleagues' detailed reinvestigation of the mid-Cambrian Burgess Shale fauna revealed a community dominated by soft-bodied animals with very few of the more familiar skeletal components of postCambrian faunas such as brachiopods, bryo-zoans, gastropods, bivalves, cephalopods, corals and echinoderms. More importantly, the deep-water Burgess fauna is quite different from more typical Cambrian assemblages with phosphatic brachiopods, simple echi-noids and mollusks together with trilobites. Although the Burgess fauna has many other peculiarities (see Chapter 10), the high proportion of, for example, annelid and priapulid worms, adds a different dimension to the more typical reconstructions of mid-Cambrian communities (Fig. 4.3).

These important taphonomic constraints must be addressed and built into any paleo-ecological analysis and may be partly countered by a careful selection of sampling methods. A variety of methods involving the suspension feeders suspension feeders

O • living assemblage A ▲ potential death assemblage □ ■ death assemblage detritivores/herbivores predators/parasites

Figure 4.2 The transition from a living assemblage to a death assemblage. Relative proportions of different types of organism change in two living marine assemblages off the Texan coast. Living assemblages are dominated numerically by detritivores and herbivores, death assemblages by suspension feeders. (Based on Staff et al. 1986.)

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