A common question concerning paleozoological data among ecologists, naturalists, and/or biogeographers who work with modern or historic period data is how can paleozoologists determine the absence of species in fossil and subfossil faunal assemblages? This is an important question that receives extensive consideration in zooarchaeology and paleontology because the presence or absence of particular species in prehistoric contexts potentially provides deep temporal information on environmental change in biological communities. Regarding mussel remains, research tends to focus on contingencies of preservation in marine fossil beds. We develop a predictive conceptual model using density and morphological characteristics to determine which species should leave remains that are resilient to destructive taphonomic processes. The predictive model is applied to five assemblages of late Holocene unionid remains from four stream systems in North Texas.
Proportional taxonomic abundance in freshwater mussel assemblages from North Texas is accurately predicted by our model. Shell size has little to no impact on preservation, but shell morphology and density appear to prevent post depositional destruction. Mussel species that are spherical in shape with moderate densities have higher numbers of identifiable remains. Species that are neither dense nor spherical were not predicted to be preserved nor were they observed in the paleozoological assemblages. We suspect that spherical morphology mitigates both crushing from overlying sediment and breakage from when shells are in direct contact. These results echo those of vertebrate and marine shellfish taphonomic studies that emphasize the importance of shape and structural density in preventing fragmentation. Temporal shifts in the presence or absence of particular species for purposes of paleobiogeography and paleoenvironmental reconstruction should take into account that some species are not likely to preserve well, thus their absence from paleozoological assemblages is not evidence of absence from the landscape. Presence or absence of species that exhibit morphology resistant to destruction holds more meaning for studies using freshwater mussel remains to illustrate changes in local unionid biodiversity as a result of modern impacts. We provide a late Holocene paleobiogeographic example from North Texas.