A taphonomic investigation of a fossil assemblage near Coleman, Texas, U.S.A., has revealed a rarely seen part of the cephalopod taphonomic pathway. The cephalopods occur in a brown shale with a low-diversity, gastropod-dominated fauna that is mixed together with a conifer-seed fern-dominated terrestrial flora. All of the cephalopod shells (n=2000+) are damaged. The brown shale is interpreted as representing a brackish-water or quiet, muddy beach environment, based in part on the delicate three-dimensional preservation of the terrestrial plant debris and the absence of stenohaline benthic marine fauna.
Most upper Paleozoic cephalopods (especially ammonoids) are thought to have lived in offshore (middle to outer shelf) environments. The environmental interpretation and the massive damage observed on the cephalopod shells support the interpretation that this cephalopod fauna has been transported from one or more distant offshore biotopes. These cephalopods had shells with positively buoyant conchs that floated after death onto the mud beach, where they were trapped in a Konzentrat fossil-Lagerstätte with the indigenous gastropod fauna and a well-preserved terrestrial flora.
The significance of this taphonomic analysis is the documentation of a transported cephalopod fauna from an offshore biotope to a paleoshoreline environment. By studying the Coleman cephalopods, a specific criterion for assessing the type and degree of shell damage for beached fossil assemblages has been developed; e.g., predominance of fragmentary shells (Type D). The criterion can be used to evaluate the taphonomy of other cephalopod faunas around the world.