Abstract
BACKGROUND: Plesiosaurs were a clade of Mesozoic aquatic reptiles exhibiting high diversity in neck length. Although their body sizes have long attracted scientific and public attention, mass estimates grounded in rigorous skeletal reconstructions are notably lacking. Existing data often rely on historical museum mounts or outdated illustrations from old literature, casting doubt on their reliability. The body masses of many species also remain unknown to date. METHODS: To bridge this knowledge gap, the present study employs a hybrid paradigm: (1) establish a standardized protocol for accurate skeletal reconstruction of plesiosaurs, with particular emphasis on deriving ribcage morphology from dorsal rib orientation; (2) uniformly apply this protocol to build multiple plesiosaur models spanning multiple clades, whose body masses are estimated using the cross-sectional method, a recently developed volumetric-density approach for mass estimation; and (3) identify reliable skeletal mass proxies by means of regression analysis. Regression models were also employed to estimate the dimensions of unpreserved skeletal elements. Model fit and predictive accuracy were assessed using sample-size corrected Akaike Information Criterion values and per cent prediction errors, respectively. RESULTS: Rigorous body reconstructions were developed for 27 taxa covering all major plesiosaur clades, with body mass estimates ranging from 79 kg to 12,824 kg. This range sufficiently captures the size diversity seen across most plesiosaurs. These reconstructions were used to evaluate the performance of various skeletal elements as body size proxies. Among the tested metrics, trunk length and the mean volume of dorsal vertebral centrum were identified as the most robust predictors of plesiosaur body mass. DISCUSSION: Critiques of the hybrid approaches often focus on inconsistent modeling criteria and limited taxonomic sampling. The present framework seeks to mitigate these concerns through uniform reconstruction standards applied across multiple plesiosaur clades. Potential uncertainties in soft-tissue restoration and limitations of the mass equations are explicitly addressed. By providing both rigorous skeletal reconstructions and practical mass-estimation tools, this work narrows the knowledge gap regarding the body masses of Mesozoic aquatic reptiles, and facilitates subsequent biomechanical and macroevolutionary studies on plesiosaurs.