The Plesiosaurs were a group of Marine Reptiles which appeared in the Triassic and persisted till the End of the Cretaceous. The group obtained a global distribution during the Jurassic, when the supercontinent of Pangea broke up and high global temperatures led to large areas of the world's continents being submerged. However, towards the end of this period the situation had changed, when falling sealevels led to many ocean basins becoming isolated, many groups of large Marine Reptiles becoming extinct, and faunal communities becoming provincialized in different ocean basins. Plesiosaurs are not thought to have been strongly affected by the End Jurassic Extinction Event, but their fossil record is poor during the Early Cretaceous, limiting our understanding of the group during this interval.
In a paper published in the journal Acta Palaeontologica Polonica on 2 December 2024, Lene Delsett of the Natural History Museum at the University of Oslo, adam Smith of Nottingham Natural History Museum, Stephen Ingrams of Llandudno, and Simon Schneider of the Cambridge Arctic Shelf Programme, describe a Plesiosaur from the Early Cretaceous Deer Bay Formation of Ellesmere Island in the Canadian Arctic.
The specimen was excavated in 1952 by the Danish geologist Johannes Christian Troelsen, however, while he mentioned it in a report of his expedition to the area, it was never formally described. It was sent to the University of Copenhagen, where it appears to have undergone some preparation work by a student working under the supervision of curator Eigil Nielsen, and possibly subsequent curator Niels Bonde. The remains were subsequently packed into a series of wooden crates, one of which was opened and examined by Adam Smith in 2005. This box were subsequently misplaced, possibly during the flooding which affected the museum in 2011, but was subsequently relocated by curators Bent Lindow and Arden Bashfortht, and its contents transferred to a draw s in the main fossil Vertebrate collection. Two further crates, labelled '“Reptile Creek, Troelsen’s office' were discovered in 2019, and found to contain several girdle elements of the Plesiosaur specimen. Another box was discovered in the collections of the Zoological Museum in 2020, containing what was labelled as part of a 'Scoresbysund Plesiosaur', but which clearly belong to the Elis Island specimen. Another box, labelled 'NHMD 189689' contained some fragmentary ribs which also appeared to belong to the Elis Island specimen, but in the absence of any documentation were not included in the study.
The surviving specimen comprises 22 non-consecutive vertebrae from the cervical, dorsal, and caudal regions, hundreds of rib fragments, partial girdle elements, all four propodials, and several distal limb elements. All these elements are worn, and the larger elements mostly fragmentary. Delsett et al. were able to assign the specimen to the Cryptoclidid genus Colymbosaurus, but, due to the fragmentary nature of the specimen and a limited amount of overlapping material with other specimens, were not able to determine whether it belonged to either of the two previously described species in the genus, or from a different, as yet undescribed species.
Previous known specimens of Colymbosaurus spp. have been described from Spitsbergen, southern England and western Russia, so the Elis Island specimen represents a significant range expansion for the genus. The genus was therefore present in two separate ocean basins, the Boreal Arctic and Boreal Atlantic, which were connected by two seaways, one running between Norway and Greenland beneath the modern North Atlantic, and one in present day western Russia.
The Boreal Ocean during the Late Jurassic and Early Creraceous has been considered to have been an ecologically depleted environment, with most of the described fossils being Bivalves, Ammonites, and Belemnites. However, Cryptoclidid Pleisiosaurs were large, predatory Animals, reliant on Fish for at least part of their protein intake, which implies that these must also have been present. Delsett et al. suggest that, few deposits from this interval are noted for their fossil content, at least part of the apparent absence of many Animal groups may be because most studies of these deposits have concentrated on their stratigraphy rather than their faunal diversity, and therefore useful index fossils, such as shelled Molluscs tend to have been described, whereas less stratigraphically useful fossils, such as Plesiosaurs or Fish, may have been overlooked.
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