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Colymbosaurus is a genus of cryptoclidid plesiosaur from the Late Jurassic (Callovian-Tithonian) of the UK and Svalbard, Norway. There are two currently recognized species, C. megadeirus and C. svalbardensis.
In the meantime, other plesiosauroid remains were being described from the Kimmeridge Clay by independent workers. The species Plesiosaurus megadeirus was coined for two partial postcranial specimens in a publication cataloging Mesozoic tetrapod specimens in the collections of the Sedgwick Museum of Earth Sciences at the University of Cambridge.H. G. Seeley. 1869. Index to the Fossil Remains of Aves, Ornithosauria, and Reptilia, from the Secondary System of Strata, Arranged in the Woodwardian Museum of the University of Cambridge. Deighton, Bell, and Co, Cambridge 1-143 The name Pliosaurus portlandicus was coined for a partial hindlimb from Dorset,R. Owen. (1869). Monographs on the British Fossil Reptilia from the Kimmeridge Clay III: contains Pliosaurus grandis, Pliosaurus trochanterius, and Pliosaurus portlandicus. Palaeontographical Society Monographs 22(98):1-12 while John Whitaker Hulke erected the species Plesiosaurus manselii for an incomplete postcranial skeleton ( NHMUK PV OR 40106) from Kimmeridge, Dorset.R. Owen. (1869). Monographs on the British Fossil Reptilia from the Kimmeridge Clay III: contains Pliosaurus grandis, Pliosaurus trochanterius, and Pliosaurus portlandicus. Palaeontographical Society Monographs 22(98):1-12 Harry Govier Seeley came to recognize Plesiosaurus megadeirus as generically distinct, and coined the name Colymbosaurus for the species.H. G. Seeley. 1874. Note on some generic modifications of the plesiosaurian pectoral arch. Quarterly Journal of the Geological Society of London 30:436-449 He later referred Pliosaurus portlandicus and Plesiosaurus manselii to Colymbosaurus and considered them distinct from C. megadeirus based on the morphology of the epipodials.H. G. Seeley. 1892. The Nature of the Shoulder Girdle and Clavicular Arch in Sauropterygia. Proceedings of the Royal Society of London 51:119-151
A review of all nominal species of plesiosauroids from the Late Jurassic of England resulted in the recognition of one species of Colymbosaurus, C. trochantericus. P. megadeirus, P. portlandicus, and P. manselii were synonymized with C. trochantericus. Because no skull remains were known, Colymbosaurus was considered possibly a senior synonym of Kimmerosaurus based on comparisons with referred material of Kimmerosaurus.
The distribution of Colymbosaurus was extended beyond the UK when the Norwegian "Tricleidus" svalbardensis was recognized as a valid species of Colymbosaurus, C. svalbardensis, based on the holotype, discovered in the winter of 1930-31 in the Agardhfjellet Formation,Persson, P.O. (1962): Plesiosaurians from Spitsbergen. Norsk Polarinstitutt, Årbok 1961, p. 62-68.
Oslo University Press, 169 pp.
and more remains from the same formation described in 2012. This assignment, along with the discovery of new plesiosauroids from Svalbard, prompted a new revision of Kimmeridge Clay cryptoclidids. ''Plesiosaurus trochantericus'' was declared a ''nomen dubium'', being referable to ''Colymbosaurus'' but indeterminate at the species level, while ''Kimmerosaurus'' and ''Plesiosaurus manselii'' were recognized as distinct. Because of this revision, ''megadeirus'' was resurrected as the epithet of the type species of ''Colymbosaurus''.Benson RBJ, Bowdler T. (2014). Anatomy of Colymbosaurus (Reptilia, Plesiosauria) from the Kimmeridge Clay Formation of the U.K., and high diversity among Late Jurassic plesiosauroids. ''Journal of Vertebrate Paleontology'' 34(5), 1053-1071.
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