New plesiosaur fossils from the Pliensbachian (Early Jurassic) of Bornholm, including the first juvenile specimen from Denmark

Plesiosaurs were an extremely successful, globally distributed group that thrived from the Late Triassic to the end of the Cretaceous Period. The Early/Middle Jurassic faunal turnover greatly affected plesiosaur evolution, although it remains poorly understood, partly due to the scarcity of plesiosaur fossils from the Pliensbachian age (192.0–184.2 Ma). This paper describes five fossil plesiosaur skeletal elements found in the Pliensbachian aged Hasle Formation on the Danish island of Bornholm. The five specimens are isolated remains, including a tooth, spinal fragments, a rib and a propodial. The tooth can be assigned to Pliosauridae indet. due to its robust form and ornamentation, and the spinal fragments to Plesiosauria indet. The neural spine is detached and could therefore either be from an osteologically immature individual or show signs of paedomorphism. Lastly, but most importantly, an osteologically immature propodial is described. It is the first of its kind found in Denmark.