SEMELPAROUS CEPHALOPOD ASSEMBLAGES THROUGHOUT THE GEOLOGIC RECORD

Semelparity, or mass mortality associated with reproduction, is exhibited by a number of modern cephalopods, most notably species of Recent squids. This phenomenon has been documented in Middle Carboniferous ammonoid occurrences in the southern midcontinent, primarily Arkansas. There, ammonoid localities have been characterized as "hot spots," that are isolated both stratigraphically and geographically, and are separated by barren intervals. Fossiliferous horizons yield populations of multiple taxa represented by abundant individuals that are of similar size and level of maturity. These occurrences cannot be explained by mass mortality due to environmental catastrophe because ammonoid taxa are not represented by all growth stages and they lack an associated fauna that would have been affected by the "kill." While not all cephalopod occurrences can be explained by this phenomenon, our expanded investigations indicate that semelparity was common in nautiloids, ammonoids and belemnoids throughout most of the geologic record. Studied nautiloid occurrences include those of Isorthoceras from the Upper Ordovician Maquoketa Shale, Iowa, Aphelaeceras from the Late Mississippian Fayetteville Shale, Arkansas, and Hercoglossa from the Paleocene Midway Formation, Arkansas. Additional investigations of ammonoid populations include Muensteroceras from the Early Mississippian of Indiana, and Phaneroceras from the Middle Pennsylvanian of Oklahoma. We have also examined occurrences of belemnoid rostra representing several horizons of Pachyteuthis from Utah. In all cases, these "hot spot" localities are represented by hundreds of individuals of a particular taxon that are of similar size and stage of maturity. These occurrences lack juveniles and associated fauna that usually suggest an environmental catastrophe.