Gypidulid Brachiopods: Their Life Position and Paleoenvironment: ABSTRACT

Abstract

Life assemblages of 2 species of gypidulid brachiopods, preserved in their original positions of growth, have been collected from Upper Silurian and Lower Devonian limestones of the Appalachian basin. The 2 assemblages inhabited the same environmental locus at different times in a transgressing carbonate sea. The earlier G. prognostica is in the Keyser Formation in Pennsylvania, Maryland, Virginia, and West Virginia, and the later G. coeymanensis is found in the Coeymans Formation of New York state. The gypidulids, in both cases, lived clustered in a beak-down position on poorly sorted skeletal sand substrates in association with a diverse faunal assemblage of brachiopods, bryozoans, crinoids, and trilobites. The specimens appear to have no pedical openings, indicating lack of pedical development in mature forms. The inner prismatic shell layer in the pedical umbo is many times thicker than the total shell elsewhere on the organism. The resulting weighting of the pedical umbo and lateral contact with other individuals in the cluster promoted an upright, posterior down orientation. Gypidulids are restricted to shallow (near, but above wave base) shelf environments. Onshore skeletal barrier sands and skeletal lagoonal muds, as well as offshore fossiliferous muds, are barren of gypidulids. The gypidulid environmental position occurs only in transgressive stratigraphic sequences. End_of_Article - Last_Page 637------------