Morphologic studies of the brachiopod genus Composita

Abstract

The genus Composita consists of smooth-shelled athyroid brachiopods which in North America have a range from Late Devonian or Early Mississippian to Permian. Morphologic variation in the genus has been recognized by earlier workers, but few have tried to show exact relationships among the various forms. The present writers have attempted to trace the development of the genus through geologic time. Only North American species and varieties of Composita are treated in this paper. Forty-two species and varieties are included in a phylogenetic history of the genus. Nine other species are listed as doubtful forms. Eighteen specific and varietal names are rejected by the writers as obsolete. Morphologic intergradation among associated, synchronous forms is studied with the aid of frequency polygons. Lack of clear separation of the associated species within different maxima of the polygons indicates their dominant intergradation. These forms, previously described qualitatively, are regarded as typological species. A statistical method is used to test for differences between collections containing the same typological species. Significant differences in the slopes of reduced major axes are obtained in length-width, length-depth and depth-width plots of two collections containing Composita subtilita and Composita argentea . Significant differences also are obtained when two collections of Composita subtilita , Composita ovata and Composita elongata are compared in the same way. In spite of these results, slight nomenclatural changes are thought to be unnecessary. Use of the triangular graph as a means of distinguishing vertical subspecies is also tested. Four collections are plotted on one graph and five artificially separated groups of Composita subtilita on another. Mixed results are obtained. The method is suspect because the results are found to depend largely on the number of specimens used.