Osteohistology of the Maastrichtian, small‐bodied elasmosaurid <i>Kawanectes lafquenianum</i> (Sauropterygia; Plesiosauria)

Plesiosauria is a clade of marine reptiles that thrived from the Late Triassic to the end of the Cretaceous. Among plesiosaurs, the elasmosaurids are particularly notable for their diverse body sizes. Small-sized specimens of Kawanectes lafquenianum exhibit fully closed neuro-central sutures indicating skeletal maturity. To independently assess whether they are indeed small-bodied adults, we analysed the paleohistology of long bones of three differently sized Kawanectes lafquenianum specimens recovered from the La Colonia Formation, Chubut Province, Argentina. The microanatomy of the bones revealed differences in the architectural organization of the hindlimbs and forelimbs, with the humerus being highly remodelled as compared to the femur. Extensive secondary reconstruction in the propodials and ribs suggests a high rate of calcium mobilization, perhaps related to biomechanical stresses. Despite the extensive secondary reconstruction in all the bones studied, primary bone tissue comprising parallel-fibred bone and lamellar bone was identified at the outermost cortex, along with an external fundamental system in the propodial bones of one of the specimens. Overall osteohistology features indicate an adult stage for all the specimens, although it appears that our sample represents individuals at different levels of maturity. Lines of arrested growth were also registered, giving a minimum age of 11 and 14 years for the specimens MPEF-PV 1155 and MPEF-PV 11545, respectively. These results indicated that K. lafquenianum is a small-bodied elasmosaurid (3.8 m-4.2 m in body length). Additionally, the bone microstructure of the postcranial elements reflects interelemental variability. Further research involving additional specimens is needed to explore other palaeobiological aspects of Kawanectes lafquenianum, such as growth dynamics, sexual dimorphisms, locomotion, among others.