Plesiosaur limb bones of four specimens from the Late Jurassic of Svalbard have been studied to map gross internal structure and microstructure and compare this to extant marine reptiles and mammals. Two specimens; one juvenile (Djupedalia engeri) and one subadult (Colymbosaurus svalbardensis) are from the Janusfjellet locality in the Adventdalen Group, Janusfjellet Subgroup, Agardhfjellet Formation, Slottsmøya Member, dated as MidVolgian; one adult (Colymbosaurus svalbardensis) from the Agardhfjellet locality, also dated to MidVolgian; and one juvenile (sp.indet.) of unknown stratigraphical age. The bones examined are propodials, phalanges, mesopodials and metapodials.
This study is the first to describe the microstructure of the latter two in plesiosaurs. The inner bone structure fits that of an active marine animal. Many of the features in the present material are often found in animals with rapid growth and high metabolism, including secondary osteons, high vascularisation, pits on the outside of the epiphysis, and woven and possibly fibro-lamellar bone in some areas. The long bones have two endochondral cones in a periosteal sheath, with a small medullary cavity. The propodials have a defined and quite compact cortex in the subadult, a finding that rejects the view that all plesiosaur bones became more porous through ontogeny. There is a microstructural difference between bones from different ontogenetic stages: juvenile bones lack remodelling and completely ossified endochondral cones. One of the bones has a circumferential vascular orientation with rings made of trabeculae, maybe resulting from cyclic growth caused by seasonality, migration or ontogeny.