LITHIFICATION DOES NOT NECESSARILY BIAS DIVERSITY: EXPERIMENTAL EVIDENCE FROM SYNTHETIC ROCKS

It is commonly assumed that observed biodiversity should be higher in paleontological samples collected from unconsolidated sediments than from lithified sediments (all else being equal). Fossils from unconsolidated sediments can be extracted more easily and may have experienced less diagenetic alteration. Previously, the effects of lithification on biodiversity were assessed by comparing samples from unconsolidated sediment with those from naturally lithified rocks of the same age and approximate paleoenvironment. However, these samples can differ in multiple ways (paleoenvironment, taphonomy, diagenesis), making it difficult to determine exactly why diversity differs. To tackle this problem experimentally, we created synthetic rocks from unconsolidated, fossiliferous sediment. We compared biodiversity data collected from these “rocks” with data collected from unlithified replicate samples.

We lithified samples of Pleistocene sediment from the Caloosa Shell Quarry (Fort Thompson and Bermont Formations) in Ruskin County, Florida, using industrial hydraulic cement designed to anchor bolts. When added to our mixed clastic sediment, this yielded a synthetic rock similar to highly fossiliferous, argillaceous sandstone. The fossils were not altered, so this process does not simulate diagenetic effects such as dissolution or recrystallization. Most of the synthetic rocks were well cemented, although two poorly cemented samples were included as well.

Our results indicate that in the absence of other diagenetic alteration, biodiversity derived from fossils in lithified samples is not lower than that derived from unconsolidated sediment, after correcting for sample size. In fact, the rarefaction curves for the synthetic rocks tend to be slightly higher than those for their replicate unconsolidated samples, probably because locating specimens is inherently less random in lithified samples. Other diversity indices that are not heavily influenced by sample size or species richness also indicate that the unconsolidated sediment samples do not have higher biodiversity. These results indicate that the physical entombment of fossils in rock, in and of itself, need not prevent accurate measurement of biodiversity. Other processes that can accompany cementation may be more important.