The convention used in the construction of the diagrams has been devised to show the relations between the rock containing fossils as observed and the mode of life of the organisms illustrated (interpretation).
Most diagrams have three sections: the sea, a block of soft sediment, and a block (protruding at the base) of sedimentary rock. The latter block represents the sediment as it now occurs in the field, and it is capped by a bedding plane which shows the fossils as they may be seen today.
Above this projecting fossil bed, the sediment is shown as it may have existed shortly after deposition, and the infauna in this soft sediment is shown, with soft parts, as it may have been when the animals were alive. The upper part of each diagram represents the sea, and the organisms in the sea (nekton and plankton) and on the sea floor (epifauna) are also reconstructed to show how they may have lived. Some empty shells may also be shown if they form a significant part of the habitat.
It is important to stress that the reconstructions of the soft parts and of the mode of life of extinct forms are interpretations, which are based mainly on comparisons with the animals' modern relatives. For some extinct groups, however, the mode of life has to be deduced from a study of the function of those hard parts preserved, a study of the traces left on the sea floor (e.g. trilo-bites), or a study of quite unrelated animals which appear to resemble the fossils.
The key to understanding these diagrams is the large time gap present above the projecting fossiliferous bed: the fossil bed and the sediments below it are as they appear today, while the sediments and the organisms above this bed are interpretations showing how they must have appeared at the time the bed was laid down and the animals were alive.
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