Eccentricity in the clypeasteroid echinoid Dendraster : environmental significance and application in Pliocene paleoecology

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Abstract
The echinoid Dendraster lives in the northwest Pacific in environments ranging from open exposed coast to protected embayment. Water energy and the abundance and quality of food along this environmental gradient are correlated with test size and eccentricity of the apical area, so that larger and more eccentric specimens occur on the open coast than in bays. Study of Dendraster in Pliocene strata of the Coalinga California [USA] region indicates that these morphologic characteristics provide valuable information for paleoenvironmental reconstruction. Lateral and temporal environmental gradients within a broad Pliocene embayment and the location and size of the bay entrance can be determined on the basis of Dendraster eccentricity and size. Several species and subspecies of Dendraster in the Pliocene of the Coalinga region, described largely by differences in eccentricity, are probably not valid taxa but are merely ecophenotypic expressions of the laterally and temporally variable environment.