Johan Lindgren

The rarity of preserved soft tissues in the fossil record has limited our understanding of the life appearance of extinct vertebrates. However, through examination of various osteological features and comparisons with extant ecomorphological analogues, we can predict some of the external characteristics of ancient taxa. Specifically, for the Cretaceous mosasaurians (a group of seagoing squamates), extant sharks serve as suitable modern analogues because of their comparable caudal vertebral morphologies and ecologies. In this study, we build a novel framework by combining two-block partial least squares (2B-PLS) regression analysis and geometric morphometrics to investigate the relationship between tail fin shape and the underlying musculoskeletal morphology in sharks and mosasaurians. Our analysis reveals a strong correlation between the “tail fin expanded soft tissue” (TFEST) and shape of the associated caudal skeleton/musculature. Moreover, the covariation pattern between these two structures is remarkably similar in sharks and mosasaurians. Based on these findings, we then develop a predictive model that reconstructs the tail fin in mosasaurian taxa without known soft tissues. Our model indicates that all hydropedal forms possessed a bilobed, hypocercal (downturned) fluke, while plesiopedal species lacked a well-developed fleshy dorsal lobe. The variation in fluke morphologies between the four different evolutionary lineages of derived mosasaurians (mosasaurines, halisaurines, plioplatecarpines, and tylosaurines) suggests multiple independent origins of a bilobed tail fin. This study thus highlights the complexity of aquatic adaptations in mosasaurians and demonstrates the utility of predictive models when reconstructing the life appearance of extinct animals.