THE SKELETAL COMPLETENESS OF MESOZOIC LIZARDS AND SNAKES: QUANTIFYING THE EFFECTS OF BIAS ON GLOBAL SQUAMATE DIVERSITY THROUGH TIME

Studies of phenotypic evolution and diversification patterns among the 10,671 extant species of squamates (lizards, snakes, and amphisbaenians) can be augmented considerably by incorporating skeletal morphological data from the >242 million-year squamate fossil record. However, spatiotemporal changes in fossil specimen completeness can bias our understanding of the evolutionary history of a given group. The inherent biological, geological, and anthropogenic processes that limit our understanding of the fossil record require a holistic examination of the effects of bias on the fossil squamate skeletal record. This study uses taxonomic, geological, and geographic data downloaded from the Paleobiology Database (PBDB), and an established metric of fossil skeletal completeness, the Character Completeness Metric (CCM2), to calculate the percentage of phylogenetic characters that can be scored for 230 fossil lizard and snake species from the Middle Triassic (Anisian) to the K-Pg Boundary. Preliminary results showcase higher mean CCM2 values (range: 47.9%- 84.3%) in poorly-sampled stratigraphic stages (Anisian, Oxfordian, Tithonian), in which the few squamate species that are known largely originate from hypoxic lacustrine and/or brackish depositional settings with higher skeletal and soft tissue preservation potential. As sampling intensity, taxonomic abundance, and the diversity of terrestrial depositional settings containing squamate fossils increase in the Cretaceous, mean CCM2 values per stage are comparatively lower overall (range: 15.6%- 48.4%). The decoupled relationship between fossil squamate skeletal completeness and species-level diversity throughout the Mesozoic showcases the pronounced influence that both Lagerstätten-type deposits and sampling intensity have on our understanding of squamate evolution in deep time. Future work will expand sampling to incorporate data from the Cenozoic-Recent to assess squamate fossil record quality throughout the Phanerozoic.