Articulate Brachiopods of the St. Clair Limestone (Silurian), Arkansas, and the Clarita Formation (Silurian), Oklahoma

Abstract

The St. Clair Limestone and Clarita Formation contain large, Upper Silurian (Wenlockian) brachiopod faunas. The St. Clair Limestone crops out in two areas in north-central Arkansas; one is the type area near Batesville, in parts of Independence, Stone, and Izard Counties, the other is near Gilbert, Searcy County. The Clarita Formation, Fitzhugh Member, crops out in the Arbuckle Mountain region and Criner Hills of south-central Oklahoma. The articulate brachiopod fauna of the St. Clair Limestone comprises 45 species and subspecies referred to 35 genera and subgenera, and that of the Clarita Formation, Fitzhugh Member, comprises 23 species and subspecies referred to 22 genera and subgenera. The combined St. Clair-Clarita fauna totals 51 species and subspecies, of which 17 are new; 40 genera and subgenera are represented, of which 9 are new:Orthostrophella, Onychotreta(Eilotreta),Onychotreta(Lissotreta),Placotriplesia, Boucotides, Leangella(Opikella),Virginiata, Dicamaropsis, andHomoeospirella.The faunas of the St. Clair and Clarita are remarkably similar; all but six of the Clarita species are present in the St. Clair, and the two formations are closely related in age. Strata of this age are probably widely distributed in the Midcontinent area of the United States, although the brachiopod evidence for this is meager. The generic suite represented in the St. Clair-Clarita brachiopod faunas is similar to that of the Wenlock of Great Britain and the Slite and Mulde Marlstones of Gotland; the stage of development represented by the St. Clair species ofDicoelosiais also similar to that ofD. bilobafrom the Wenlock Limestone and Slite and Mulde Marlstones. The St. Clair-Clarita brachiopod fauna does not show any marked similarity to brachiopod faunas known to be in the Silurian of Bohemia, and, in fact, correlation with the Czechoslovakian Silurian section is at present uncertain.