EVOLUTION AND DIVERSIFICATION OF CONODONTS PRESERVED IN PERITIDAL DEPOSITS, SIMPSON GROUP (ORDOVICIAN), SOUTHERN OKLAHOMA

The Simpson Group of southern Oklahoma preserves carbonate and siliciclastic sediments deposited in shallow marine environments. Each formation in the group represents a transgressive-regressive event. During latter stages of regression, shallow subtidal to intertidal carbonate platforms were established. The record of those shallow, carbonate platforms is represented by layers of fine-grained, fenestral limestone of, in ascending order, the Pruitt Ranch Member (Oil Creek Formation), uppermost McLish Formation, and Pooleville Member (Bromide Formation). Those three units span lower Darriwilian through middle Sandbian Stages.

Conodont faunas derived from peritidal facies of the Pruitt Ranch, McLish, and Pooleville, are linked by common species and show a trend toward increasing diversity through time. The Pruitt Ranch contains a low diversity Parapanderodus striatus-Leptochirognathus quadratus fauna. In younger McLish deposits, L. quadratus persists and is joined by Plectodina joachimensis, Erismodus typus, and Panderodus gracilis. In the Pooleville, L. quadratus is replaced by Curtognathus sp. which is accompanied by species of Plectodina, Erismodus, and Panderodus. Along with increasing diversity, peritidal conodont faunas can be divided into several skeletal morphotypes: one with coniform elements only; another with robust, differentiated skeletal components; and a third with broad, palmate elements. The former two morphotypes are represented in a variety of shallow water Simpson facies, whereas the latter is much more restricted in its distribution.