In certain conglomerates and in all turbidites it is often obvious that shells have been transported, but in many shelly beds the degree of transportation may not be obvious until some careful statistical work has been carried out on the fossil populations.
Schafer (1972) has described five types of situations in which marine fossils may be preserved. At one extreme, there is a shallow water reef assemblage, where all the animal skeletons are preserved in their life positions, and there are no clear bedding planes. In moderately turbulent shallow water environments, some burrowing organisms are preserved in place, but the majority of the epifauna is transported. With high turbulence, nothing is preserved in position of growth. In quieter environments, there may be some areas where a large proportion of the benthic fauna is in place, but the nekton and plankton may make up a large proportion of the fossils. Finally, in very tranquil areas with no bottom currents, the sea floor may become stagnant and the only fossils are from the nekton and plankton.
If a collection of a living species is measured (say the length or some other character which changes with growth), most characters show variations in growth rates with ontogeny. For example, most mammals stop growing after maturity is reached; there will thus be a skewed distribution in the measurements because there will be more adult-sized individuals than juveniles in a population. In fact, different animals will show widely different size distributions according to the relationships between recruitment, growth and mortality (Craig and Oertel, 1966). By contrast, the size distributions of a transported population will be normal (provided it has not been carried wholesale in a density current); the shell size distribution will depend on the speed of the depositing current for a particular shape of shell (Boucot, 1953; Craig and Hallam, 1963; McKerrowet al. 1969).
Although this type of statistical work has not been done on many fossil communities, there is one other supporting fact to suggest that the majority of those described in this book, do, in fact, represent animals which lived in the same habitat: the same communities occur repeatedly. Although the different contributors have often approached the study of fossils in different ways, the communities described have at least this characteristic in common: they have been observed occurring many times by the author concerned, usually over a wide geographical area.
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