Idiognathodus and Streptognathodus species from the lost branch to Dewey sequences (middle-upper Pennsylvanian) of the midcontinent basin, North America

A function-based taxonomic method was developed to revise Idiognathodus and Streptognathodus species from the Lost Branch Sequence (latest Desmoinesian) to the Dewey Sequence (Middle Missourian). The new method relies on characters significant to the food-processing function of the P1 element. The chief controls over food processing efficiency are surface area and surface roughness. The two primary morphological features affecting surface area are the shape and size of the rostral lobe and the shape and size of the caudal lobe. Ornamentation of the ventral and dorsal platform affects surface roughness. Use of these three features as the major characters to discriminate species resulted in the description of twenty-four species of Idiognathodus and five species of Streptognathodus.

A regional zonation for the Midcontinent Basin (North America; Barrick et al. 2004) was revised based on the new taxonomic scheme for Swadelina, Idiognathodus and Streptognathodus. Five zones and five subzones were erected within the study interval. The Swadelina nodocarinata zone of Barrick et al. (2004) includes the Lost Branch Sequence. The new Idiognathodus sulciferus zone includes the Hepler Sequence. The I. eccentricus zone comprises the Shale Hill and Hertha sequences. The revised I. cancellosus zone (two subzones) includes the Swope, Mound Valley, and Dennis sequences. The S. gracilis zone (three subzones) comprises the Hogshooter, Cherryvale, and Dewey sequences.

The merit of three proposals for levels characteristic of the boundary between the global Moscovian and Kasimovian Stages are discussed. The first level is the traditional base of the Missourian (near the traditional base of the Kasimovian) based on the disappearance of the last species of Swadelina. The second level is based on the first appearance of I. sagittalis (not found in the Midcontinent Basin), which can be approximated using the appearance of distinctive forms of I. swadei, I. turbatus, or I. eccentricus (near the redefined base of the Missourian). The third level is at the highstand in the Swope Sequence and marks the first co-occurrence of I. cancellosus and I. biliratus, two of the few species that may have had global distribution.