The diversity of fossils recorded by paleontologists from the geological record is the product of two factors -- original biodiversity and the completeness of the geological rock record that survives today to be sampled.
Andrew B. Smith and Roger B.J. Benson show that species diversity of Cretaceous echinoids can be predicted with a high degree of precision from proxies for marine sedimentary bedrock area and habitat diversity that is captured by the rock record.
However, they found no simple relationships to link surviving bedrock area to the original area over which marine sediments were deposited, or the surviving lithofacies diversity to the range of habitats that once existed.
Rather, the completeness of the rock record varies markedly during large-scale sea level cycles. The ability to sample fairly across a broad range of habitats is as crucial as the ability to sample fairly within those habitats for the accurate estimation of paleodiversity, but large-scale sea level cycles clearly hinder the former.
Paper: Andrew B. Smith (corresponding) and Roger B.J. Benson, doi: 10.1130/G33773.1