Predicting body length and assessing the shape of tail-propelled Mesozoic marine reptiles

Body length is a crucial ecological predictor in vertebrates, yet total body length proxies have seldom been assessed for ancient marine top predators. Here, we test the strength of phylogenetic imputation and 23 linear measurements, sampling both broad skeletal regions and frequently fossilised elements (such as vertebral centra), in predicting the body length of the main clades of tail-propelled Mesozoic marine reptiles (Ichthyosauria, Mosasauridae, and pelagic thalattosuchians). We embed this marine reptile sample within a comparative framework with raptorial cetaceans, and analyse the evolution of body proportions in these clades. We find that trunk length and centrum dimensions are strong predictors of body length, opening up the possibility to build vast datasets of body length estimations for Mesozoic marine reptiles from minimal preserved remains. We provide body length calculation equations for all traits and all clades. Proxies fared much better and often had distinct slopes when applied clade-wide rather than when applied to the global dataset. We also show that body length in Mesozoic marine reptiles is more labile than their skeletal architectures, rendering phylogenetic imputation methods less effective than skeletal proxies for assessing body lengths.