The influence of molluscan taxon on taphofacies development over a broad range of environments of preservation: The SSETI experience

Abstract

We utilize information from a suite of molluscan species deployed in a variety of Caribbean and Gulf of Mexico environments of preservation (EOP) by SSETI (Shelf and Slope Experimental Taphonomy Initiative) to examine to what degree interpretation of the taphofacies is influenced by the mix of species presented to the observer. The environment of preservation was the overwhelming determinant of taphonomic state at SSETI sites. Species mix played a subsidiary role. A few consistent trends were observed: for example, Arctica islandica, Strombus luhuanus, Mytilus edulis, and Mercenaria mercenaria tended to be more highly dissolved than others. Mytilus edulis consistently was fragmented more than other species. Otherwise, most species behaved uniquely over a range of EOPs: significant species by EOP interactions were the norm. A taphofacies model is developed to examine further the role of species on the degradational state of the assemblage. Two metrics are described: a taphonomic degradation score describes the overall degradational state of the assemblage from pristine to highly degraded; and a taphonomic signature describes the relative importance of a suite of taphonomic processes. Model simulations show that the impact of species on degradational state is low initially because all shells are in relatively pristine condition shortly after death. The impact is high at intermediate degradational states because factors inherent to the species modulate the time course of taphonomic decay. The impact is low again at highly degraded assemblages, as the taphonomic process brings shell condition to a common denominator of relatively intense degradation regardless of species. Fossil assemblages almost certainly express this continuum in some degree. The consequence is that better preserved assemblages are ones in which species can affect taphofacies character and so similar EOPs may be judged distinctive due to differential dominance by one species or another. Teasing out characteristics that might forewarn the observer in this instance is an important goal. Simulations show that low evenness is an important warning sign.