A community is a group of organisms living together. The term is sometimes applied to organisms which depend on each other in some way (such as for food, or for protection), but in palaeontology it is normally only applied to organisms living in the same habitat.

At any one time there are different communities of animals living in different environments. The study of animals in relation to each other and to their habitats is known as ecology. Modern communities can be examined to determine not only where animals and plants live, but how they interact, especially in providing food and protection for each other. In looking at fossil associations, the palaeoecologist can describe those forms which occur together, but he can only speculate as to their interactions. Biologists argue as to whether a community should be defined according to the animals' habitat or according to their interactions. There is no doubt that, in the study of fossils, only the habitat definition can be used. The definition of "Community" used throughout this book is thus: a group of animals living in the same habitat.

The communities in this book are all known to be recurring assemblages; they are not just random collections of fossils. Some of the faunas illustrated have been analysed statistically in great detail, but many others have only been qualitatively assessed. To some extent the diagrams must be considered as cartoons in that they emphasize those elements which are most easily preserved as fossils (by having hard parts or recognizable burrows). The soft parts of extinct organisms are, of course, conjectural. The reconstructions presented here are based on the best information available, but they are deduced largely from similar organisms alive today, so there is a considerable element of informed guesswork in many of the illustrations.

It may well be that further work will indicate the presence of other animals and plants in these communities. On the other hand, it may eventually be seen that some of the communities described in this book irj, fact have members which occupied slightly different habitats. In the geological record, it is not always easy to separate fossils which occur together in a deposit, but which lived at slightly different times. Most fossil collections include those animals which have lived in one area over a period of several years, or possibly several centuries. So, even if the shells have not been transported after death, there may be a greater variety of animals on a square metre of a bedding plane than could have existed at any one time while that bedding plane was exposed on the sea floor.

It should also be stressed that the communities illustrated have been selected either because they are the most commonly occurring or because they are of particular interest. The vast majority of ancient communities still await description, and other palaeontologists would certainly make a different selection.

Modern aquatic organisms can be classified according to where they live (Figure a). Those which float or drift are known as plankton and are dominantly pelagic, that is, they live mostly on or near the surface of the sea. Actively swimming animals, like many fish and cephalopods, are called nekton. Bottom-dwelling organisms (benthos) can be placed in one of two categories: epifauna which live on the substrate, which may be soft sediment, rock or vegetation, and infauna which live within the substrate (burrowers or borers).

Plankton can be subdivided according to size (microplankton are those organisms which can only be studied under a microscope); they can also be classified according to their methods of obtaining energy: whether they manufacture their foodstuffs by photosynthesis (like most plants) or whether they feed on organisms or organic debris (like most animals). In many simple small organisms, the difference between the animal and plant kingdoms becomes blurred. In groups of apparently closely related organisms, some may employ photosynthesis and some not, and

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