Assessing the effect of sampling proxies on plesiosaur taxic diversity with comments on implications for Mesozoic sampling design

The fossil record of plesiosaurs, while extensive, is strongly affected by geological and anthropogenic biases that may obscure true diversity patterns. This study evaluates the influence of rock availability and sampling effort on taxic diversity estimates (TDE) of plesiosaurians throughout the Mesozoic period and proposes a method for determining priority stages for future fossil sampling. Two primary proxies—the number of fossiliferous marine formations (FMF) and fossil marine collections (FMC)—are considered. Our findings indicate that neither FMF nor FMC alone accounts for observed diversity peaks and troughs in plesiosaurian TDE. To address this, we integrate phylogenetic diversity estimates (PDE) into our framework, using the difference between PDE and TDE (i.e., phylogenetic residuals) to identify stratigraphic stages where recorded diversity likely underrepresents true lineage richness. This combined approach reveals that some previously recognized declines in TDE—such as during the Berriasian—are consistent with low sampling proxy values and may reflect genuine sampling bias. In contrast, other intervals, particularly the Aalenian–Bathonian, Valanginian and Coniacian ages, exhibit low TDE despite limited rock and collection data but high phylogenetic residuals, highlighting them as high-priority targets for future paleontological fieldwork. These stages represent critical intervals for testing the consistency between diversity models based on sampling proxies and phylogenetically inferred diversity trends.