Distribution of three key Early Permian fossil groups in western USA and northern Mexico and their relevance to interpretation of paleotectonic features along the southwestern margin of Laurentia

Three species groups, including two groups of corals and one of fusulinids, delineate the zone of favorable marine environments for these animals along the shelves bordering southwestern Laurentia during the Cisuralian (Early Permian). The three species groups are: the coral Protowentzelella group of late Asselian to early Sakmarian age, the fusulinid Eoparafusulina linearis group of late Sakmarian age, and the coral Pararachnastraea illipahensis group of late Artinskian to Kungurian age. Occurrences of these three species groups clearly outline most of the major paleotectonic features that were present along the southwestern margin of the Laurentian shelf at that time. The paucity of data in Mexico, however, leaves open the question of large-scale displacement on the Mojave–Sonora megashear, a feature proposed to cut across northern Mexico and southwestern USA, although the data presented here could be construed to suggest lack of significant displacement in post-Pennsylvanian time.