ABSTRACT
Plesiosaurs are an iconic group of Mesozoic marine reptiles, uniquely equipped with four flippers of the same size and shape. Plesiosaurs first appeared in the latest Triassic and became extinct near the end of the Cretaceous. Morphology and geological context indicate adaptation to the open sea. Despite a long history of study, a comparative perspective on plesiosaur bone histology has only recently become available. Like their body plan, plesiosaur bone histology is highly distinctive and uniform across diversity and through time. Plesiosaur cortical bone histology is characterized by a peculiar kind of fibrolamellar tissue in juveniles to early adults, termed plesiosaur radial fibrolamellar bone (pFLB). Later in ontogeny this tissue was remodeled into dense Haversian bone and secondary trabecular bone, resulting in an apparent ontogenetic mass decrease of bone. A low number of growth cycles indicates that plesiosaur growth was extremely fast: juveniles reached over 60% of final cortical thickness by the end of the first year and skeletal maturity in less than five years. Histomorphometry indicates cortical apposition rates and inferred resting metabolic rates in the range of extant birds, suggesting that plesiosaurs were endotherms.
