LITHIFICATION AND THE MEASUREMENT OF BIODIVERSITY – IS MISSING ALPHA STUCK BETWEEN A ROCK AND A HARD PLACE?

It is well established that the fossil record is influenced by various biases, distorting apparent patterns of past biodiversity and ecological change. These biases, which may result from sampling, environmental and preservational factors, have the potential to strongly influence perceptions of alpha (within-community) diversity. Recent research has corroborated the long-held view that alpha diversity has increased from the Paleozoic to the Cenozoic as much as three to four-fold, after mitigating for such biasing influences as secular variation in time-averaging, and environmental coverage, and latitudinal variation through the Phanerozoic. However, these recent efforts have not accounted for the considerable increase in the availability of unlithified fossiliferous sediments in strata of Late Cretaceous and Cenozoic age.

By analyzing a large dataset of census counts from over 300 bulk-sampled fossil assemblages in the Miocene-Pleistocene of New Zealand I address the problem of how to estimate loss of taxonomic information associated with the lithification bias in the fossil record. Analyses have been restricted to transgressive shell bed assemblages to minimize taphonomic variability and to allow for comparison of relatively consistent environments. Sampling standardization analyses indicate that unlithified assemblages yield up to twice the genus richness for the same number of sampled specimens as assemblages from lithified sediments. This appears to relate to under-representation of smaller-sized and apparently fragile skeletonized Mollusca and Brachiopoda in lithified samples. The implication is that a large component of the difference in estimates of within-community diversity between Paleozoic and Cenozoic assemblage may be related to the increased availability of unlithified sediments representing the Cenozoic. The bias imposed by lithification could be as great, if not greater, than that related to time-averaging, latitudinal or environmental factors.