Clinal variation, heterochrony, and facies in the TrentonianSowerbyellaLineage (Ordovician, New York State)

Abstract

Clinal variation is systematic, morphologic variation within a species along a geographic or ecologic gradient. While common among today's biotas, it has rarely been well documented in the fossil record. It is, however, clearly displayed among the large populations of the Trentonian brachiopod genusSowerbyella. In general, shallow agitated waters were populated by specimens with flat or round pedicle valve exteriors and ornately sculptured brachial valve interiors. Deeper, quieter facies were populated by forms with medial folds on pedicle valve exteriors and plain brachial valve interiors. This clinal variation was apparently the product of heterochrony with either the shallow-water forms having been peramorphic or the deeper water forms having been paedomorphic.The distributions of theSowerbyellaclines are directly related to the facies developed within approximately eight million years of continuously deposited strata of the Trenton Limestone. Evolution of the Trentonian species ofSowerbyellaappears to have been phyletic and locally adaptive. New clinal variants evolved by orthoselection during episodes when new environments were becoming available for occupation. The lower Trentonian transgression and the middle Trentonian shallowing were the major examples. Some clinal variants were eliminated when they failed to pass through facies bottlenecks in the lower and upper Trentonian. Thus, the history of theSowerbyellalineage of the Trenton Group is the product of the facies-controlled production and selection of clinal variants. Clinal variation appears to be a central factor of phyletic evolution.Sowerbyella kayin. sp. is described.