This post is about a concept that is a very important in the study of recent and/or fossil shells. It is "how much variation in morphology (size, shape, etc.) does a species have?" One would expect differences in size between juvenile and adult shells of a species, as well as differences in color (which would not be a concern for fossils). The over-riding question becomes, however, how much variation occurs in a population of individuals of a species. The best way to attempt to answer this question is to collect as many specimens as possible, in order to establish a baseline for what is the normal range of morphology for a particular species.
In the case of the mangrove gastropod Tympanotonos fuscatus from Casamaance in Senegal, West Africa, it would be necessary to collect at least six specimens in order to establish its morphological variation. It extremely important to know that they all lived together at a single locality. The largest specimen shown here is 49 mm in height; the smallest specimen is 36 mm in height.
The above series of specimens helps to establish that the morphology of this species shows an insensible gradation from beaded to spiny forms. Otherwise one might mistakenly believe that these specimens represent six different species, which would give erroneous information about the biodiversity of gastropods at this one locality.
The degree of morphological variation in mollusks that live in shoreline environments (for example, mangrove swamps and tidal flats) are subject to much variation in salinity, turbulence, water depth, water temperature, and so on. These kinds of environments create micro-environments, which, in turn promote morphologic variation. This kind of information, if available for a particular family or genus, should always be used in paleontological studies, so as to avoid the over naming of species.
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