Abstract
An empirical correlation between mean water temperature and test weight in shallow-wrater forms of the living echinoid Dendraster excentricus (Eschscholtz) shows that sand dollars of a given size are heavier in cold water than in warm water. The correlation is interpreted as the result of phenotypic (non-heritable) adaptation to water temperature. Changes in test weight are accompanied by changes in several morphological characters that have been used to classify members of the genus Dendraster into species and subspecies. Investigation of the value of these characters as indicators of paleotemperatures in the fossil record of Dendraster indicates that the characters have evolved over time, thus eliminating their use as indicators of absolute temperatures. Furthermore, genetic or evolutionary differences in the same characters are recognized between certain geographically separated contemporaneous populations in the Pliocene. Thus only within a small geographic area and within a short range of time can morphology be used to deduce relative differences in temperature.
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