MIDDLE PERMIAN (LATEST GUADALUPIAN) AGE GASTROPODS FROM THE REEF TRAIL MEMBER OF THE BELL CANYON FORMATION, TEXAS

Ten species of silicified gastropods are recognized from the latest Middle Permian age Reef Trail Member of the Bell Canyon Formation. The gastropods represent faunas present immediately before the end-Guadalupian extinction event.

The Reef Trail Member of the Bell Canyon Formation of West Texas represents the youngest richly fossiliferous Permian beds in the Delaware Basin (late Capitanian). Their deposition preceded a global fall in sea level and deposition of evaporitic rocks of Late Permian (Lopingian) age. Only one occurrence of younger Permian gastropods (Rustler Formation) is known from cratonic North America. A mollusk-rich bed in the upper Reef Trail Member also contains abundant sponges, pelecypods, and large scaphopods including Plagioglypta, significantly fewer brachiopods, bryozoans, and chitons, and rare tabulate corals and rostrochonchs. Silicified gastropods present include Shwedagonia elegans Batten, Soleniscus variabilis Erwin, Retispira, Discotropis, Apachella, Naticopsis, undescribed genus aff. Naticopsis, Pseudozygopleura, Holopea, and an undescribed species of Lamellospira. The small shell size of most of the abundant gastropod fauna suggests they were foliage grazers in a mollusk dominated community.

Some of the genera reported here (Shwedagonia, Lamellospira, and Discotropis), appear to go extinct at the end of the Guadalupian and this occurrence eliminates some “back smearing” of their last known occurrence, because previous studies of gastropods in the Permian Basin and the American Southwest are from older Permian rocks. The Permian Basin specimens of the undescribed genus aff. Naticopsis are very similar to the Late Permian N. (N.) shizishanensis Pan and Erwin from China. The small size (less than 1 cm) demonstrates that micromorphic gastropods can dominate a fauna and not represent a repopulation following an extinction event.