Destruction of a (Texas) Pennsylvanian Goniatite Nursery-the East Mountain Shale

Some aspects of the depositional environment of the 1.3m black shale on East Mountain in Mineral Wells (Pennsylvanian, Desmoinesian), Texas (UTM 14SNM83923044) are unclear and even controversial. This black shale contains abundant trilobites (Ditomopyge) and scallops (Dunbarella) it is famous as one of the productive intervals for Stauffer and Plummer's (1932) conodont study. Our interpretation of the depositional environment of this shale is as an extremely shallow water flotant marsh environment similar to that of Zangerl and Richardson (1963). The uppermost ten centimetres of this shale contains a tremendous abundance of extremely juvenile goniatites, most of which have two or three camerae, totaling several hundred per kilogram. More adult specimens are present, but rare, and the combination and location of the goniatites within the column makes it almost certain that these juveniles were killed by the first fresh water influx as the beginning of the regression that terminated the marine event. Although others would put this dark shale in deep to extremely deep water and would cite the presence of goniatites as evidence for that, we are convinced these ancient cephalopods like their living ectocochliate relative, Nautilus belaunsis, lived in deeper water for most of their lives, but migrated into shallow water to spawn as documented in Landman, et al. (1994). The spawning females may have laid their eggs attaced to floating vegetation. Microgastropods were also found in the sample and we also believe them to have been attached to the flotant.