Quantitative histological models suggest endothermy in plesiosaurs

Area of Science:

• Paleontology
• Vertebrate Biology
• Evolutionary Biology

Background:

• Plesiosaurs were highly successful marine reptiles with a distinct body plan, existing from the Late Triassic to the Late Cretaceous.
• Accumulating evidence, including bone histology and oxygen isotope analysis, suggests plesiosaurs may have been endothermic (warm-blooded).
• Quantitative estimations of metabolic and bone growth rates in plesiosaurs have not been previously performed.

Purpose of the Study:

• To quantitatively estimate metabolic rates and bone growth rates in plesiosaurs.
• To test the hypothesis of endothermy in plesiosaurs using phylogenetic comparative methods.
• To explore the relationship between physiological traits and the evolutionary success of plesiosaurs.

Main Methods:

• Utilized phylogenetic eigenvector maps to estimate trait values, accounting for evolutionary relationships.
• Measured vascular density (primary osteon density) in bone histology of fossil and extant taxa as a predictor variable.
• Quantified bone growth rate (local bone apposition rate) and resting metabolic rate (RMR) as response variables.

Main Results:

• Estimated bone growth rates and RMRs for plesiosaurs fall within the range observed in modern birds.
• These findings suggest that plesiosaurs were endothermic.
• Even basal eosauropterygians showed estimated metabolic rates comparable to or higher than mammals.

Conclusions:

• Statistically robust inferences indicate fast growth and high metabolism in plesiosaurs, supporting the hypothesis of endothermy.
• Endothermy could explain plesiosaurs' survival through the end-Triassic extinction and their subsequent global radiation.
• Limitations include potential data biases due to a lack of comparative data for large marine amniotes.