BRACHIOPOD COMMUNITY EVOLUTION ACROSS THE SILURIAN-DEVONIAN BOUNDARY, HENRYHOUSE AND HARAGAN FORMATIONS, ARBUCKLE MOUNTAINS, OKLAHOMA

The brachiopod dominated fossel assemblages of the Henryhouse (Ludlovian) and Haragan (Gedinnian) formations represent two separate depositional episodes within essentially identical environments. The Henryhouse contains the Rhipidomellid-Atrypa Community which is homologous to the North Silurian Realm Dicaelosia-Skenidioides Community while the Haragan contains the Dicaelosia-Kozlowskiellina Community which is homologous to the Appohimichi Subprovince Dicaelosia-Hedeina Community. These communities are considered to be in direct lineage (Boucot, 1975). Taxonomic similarities between the communities of the Henryhouse and Haragan indicate evolution within one continuous community. Taxonomic differences indicate extinction of earlier taxa and introduction of new taxa through either migration or evolution within an increasingly isolated faunal province.;The abrupt faunal change observed at the Henryhouse-Haragan contact defines an unconformity between them during which the community changed in composition but not structure. Speciation during this period of regression fits the allopatric model of Eldridge and Gould (1973).