Communities Communities Plankton

Fig. a. Modern animal communities between Denmark and Sweden (from Hedgpeth, 1957, p. 31 with permission).

in common with other recent publications (e.g. Treatise on Invertebrate Paleontology) we refer these simple organisms to a third kingdom: Protista.

There are some problems in our definition of a community. For example, are the animals living above (or below) the sea floor to be included in the same habitat as those on the sediment surface? It is quite reasonable to argue that they are not, but for the purpose of this book, we frequently show free-swimming animals in the same diagram as the benthos on the floor below. However, in Figure a (above) the epifaunal and infaunal organisms are shown as separate communities.

Within a single habitat there can be many niches. For example, on a particular patch of sea floor there may exist, side by side, epifaunal suspension-feeding bivalves attached by byssal threads to shell fragments on the sea floor, infaunal burrowing depositfeeding crustaceans, epifaunal suspension-feeding serpulid worms encrusting some of the bivalves, and an epifaunal carnivorous gastropod boring the bivalves with its rasp-like radula. Each of these animals is in a distinct niche within the same habitat.

In many fossil assemblages, the niche occupied by a particular organism can often be determined by a comparison with present day animal and plant communities, but further back in time we see fewer and fewer close relatives of living organisms, and other criteria (like the type of sediments or geographical distributions) have to be used to determine the ecology of these ancient assemblages. In some cases geologists have produced very convincing evidence for accurate interpretations of ancient environments, but in other instances there is still much uncertainty.

Many modern marine communities have sharp boundaries. At the margin of a rock outcrop in the sea, the mussels, limpets and winkles (Mytilus, Patella and Littorina) living on the rock will suddenly give way to sand-dwelling cockles and heart urchins (Cardium and Echinocardium). There is no difficulty in recognizing a different bottom-dwelling community in a case like this. A change from clean sand to muddy sand will allow a great increase in deposit feeders and a marked reduction in sessile epifauna. Here again, though the change may be more gradual, there is a reasonable boundary between the two distinct habitat communities.

Many fossil collections from Palaeozoic rocks consist largely of suspension-feeding brachiopods, and thus contain very few fossils which were not suspension feeders. Some beds show burrows (perhaps made by annelid worms or arthropods) and there must have been carnivores or scavengers around, but their fossil record is very poor by comparison with that of the brachiopods. Five Silurian communities have been described (Ziegler, 1965) living on the sea floor between the coast and the deep sea. A single sample from each community is very distinctive, but when a large number of samples are made gradations between most of the communities become evident, and some arbitrary decisions, depending on the proportions present of certain selected species, have to be made to define the community boundaries. We still do not yet know in many cases which fossil communities are grada-tional and which are not, but it must be borne in mind that those communities described here (especially those with a high proportion of suspension feeders) may be arbitrary points on an ecological gradient. ,

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