CONODONT BIOSTRATIGRAPHY OF LOWER AND MIDDLE PENNSYLVANIAN STRATA IN THE TYPE AREA OF THE DERRYAN SERIES, SOUTHERN NEW MEXICO

This study describes conodonts from three Lower and Middle Pennsylvanian sections in southern New Mexico, including the type section of the Derryan Series. We use the conodonts for evolutionary and biofacies studies and for correlation. The units consist of lime mudstone and lime wackestone. Idiognathodus dominates the fauna from all sections. Adetognathus followed by Neognathodus, Hindeodus, and Idiognathoides are common. Idioprioniodus and Diplognathodus are rare. Idiognathoides and Neognathodus atokaensis indicate an Atokan age where they are present, and the distribution ofNeognathodus indicates general agreement with Atokan and Desmoinesian fusulinid zones.

Some idiognathodontids exhibit discontinuous transverse ridges on the oral surface of the Pa element, a characteristic commonly associated with Streptognathodus. These transitional morphologies are present throughout the Desmoinesian, and we consider them to represent reiterative evolution in response to stress conditions. Hindeodus, Idioprioniodus, and Adetognathus are common where transitional idiognathodid morphologies are present. They are elevated in numbers in some units that do not contain transitionals, but they are otherwise generally rare.

Conodont biofacies associations reflect a shallow shelf, marine environment of generally low to moderately elevated energy. Elevated numbers of Hindeodus reflect localized energy variations, and locally elevated numbers of Adetognathus, Idioprioniodus, and Idiognathoides indicate stress conditions.