The Mosasaur Fossil Record Through the Lens of Fossil Completeness

The quality of the fossil record is essential for interpreting the significance of macroevolutionary patterns. Palaeodiversity is filtered through geological and human processes, and efforts to correct for these biases are part of a debate concerning the role of sampling proxies and standardisation in models of biodiversity. Here, we analyse the fossil record of mosasaurs in terms of fossil completeness as a measure of fossil quality, using three novel metrics of fossil completeness for a compilation of 4,083 specimens. All metrics correlate with each other. A new qualitative measure of character completeness (QCM), correlates with the phylogenetic character completeness metric. Mean completeness by species decreases with specimen count, and average completeness by substage varies significantly. Mean specimen completeness is higher for species-named fossils than those identified to genus and family. The effect of tooth-only specimens is analysed. Importantly, we find that completeness of species does not correlate with completeness of specimens. Completeness varies by palaeogeography, North American specimens showing higher completeness than those from Eurasia and Gondwana. These metrics can be used to identify exceptional preservation, with specimen completeness varying significantly by both formation and lithology. The Belgian Ciply Formation displays the highest completeness, and clay lithologies show higher completeness values than others. Neither species diversity nor sea level correlates significantly with fossil completeness. A GLS analysis using multiple variables agrees with this result. However, GLS shows that two variables have significant predictive value for modelling averaged diversity, namely sea level and mosasaur- and plesiosaur-bearing formations, the latter of which is shown to be redundant with diversity. Mosasaur completeness is not driven by sea level, nor is completeness limiting the diversity signal in the mosasaur record.