Constrained body mass evolution and decoupled morphological rates in plesiosaurs

Abstract

Body size, through its links to various physiological traits, has often been hypothesized to influence evolutionary rates. Negative body size-rate correlations have been reported in the morpho-logical or molecular evolution of several extant vertebrate groups, including mammals, birds, reptiles, and teleost fishes. In this study, we estimated body masses for 89 species of plesiosaurs, a clade of Mesozoic aquatic reptiles, and found that their body size evolution conforms to a three-regime Ornstein-Uhlenbeck process, indicative of constrained evolution. Rates of morphological evolution, inferred using the skyline fossilized birth-death process and the variable-rates model, show minimal support for a correlation with body size in this clade. Our results thus serve as a counterexample, suggesting that the negative body size-rate relationship is not a universal vertebrate pattern, but rather a trend restricted to certain lineages.