Osteology and relationships ofColymbosaurusSeeley, 1874, based on new material ofC. svalbardensisfrom the Slottsmøya Member, Agardhfjellet Formation of central Spitsbergen

Abstract

ABSTRACT-Colymbosaurus is a genus of long-necked plesiosaurian represented by two valid species: C. megadeirus from the Upper Kimmeridge Clay Formation (Kimmeridgian-Tithonian) of the United Kingdom and C. svalbardensis from the Slottsmøya Member of the Agardhfjellet Formation (Tithonian-Berriasian) of Svalbard, Norway. Due to the lack of complete and articulated skeletons and a near absence of cranial material, Colymbosaurus has been problematic to characterize morphologically. Here, we describe and conduct a phylogenetic analysis on an informative new specimen referable to C. svalbardensis from the Slottsmøya Member, preserving a large portion of the axial and appendicular skeleton. The new material contributes important new osteological data for the species and together with an extensive examination of congeners in British museums, clarifies the diagnostic characters of the genus. We provide two new diagnostic characters of the epipodials for the genus and reevaluate the utility of an anteroposteriorly oriented bisecting ridge on the distal end of the propodials. We also present two new diagnostic features for C. svalbardensis regarding the neural canal and femoral morphology. A phylogenetic analysis recovers a monophyletic and well-supported Colymbosaurus. The new specimen of C. svalbardensis confirms that this species is not synonymous with other described Slottsmøya Member plesiosauroids, demonstrating considerable diversity of the clade at high latitudes close to the Jurassic-Cretaceous boundary.

Peer Review #1 of "A new plesiosaurian from the Jurassic–Cretaceous transitional interval of the Slottsmøya Member (Volgian), with insights into the cranial anatomy of cryptoclidids using computed tomography (v0.1)"

“…Manuscript to be reviewed Distally, there is little to no preaxial expansion. A large, posteriorly expanded postaxial flange is present, although not to the same degree as seen in Colymbosaurus svalbardensis (Knutsen, Druckenmiller and Hurum, 2012c;Roberts et al, 2017). PMO 214.248 lacks an anteroposteriorly oriented bisecting ridge on the distal epipodial facets, as observed in some specimens of Colymbosaurus megadeirus (Benson and Bowdler, 2014).…”

“…The surface of the glenoid is slightly undulated posteriorly and dorsoventrally deep, being over half the dorsal-ventral height of the mandible. This is distinct from the shallow and smooth articular facet of Colymbosaurus, Cryptoclidus eurymerus, Muraenosaurus leedsii and Kimmerosaurus langhami (Brown, 1981;Roberts et al, 2017).…”

“…obs. AJR, NHMUK R2616) and Colymbosaurus megadeirus (Benson and Bowdler, 2014;Roberts et al, 2017). The medial symphysis continues posteriorly throughout the preserved medial margin of the coracoid (Fig.…”

“…The distal facet morphology in Ophthalmothule cryostea differs from the two distal facets seen in Lower Jurassic plesiosauroids, Microcleidus spp. and Plesiopterys wildi (Bardet, Godefroit and Sciau, 1999;O'Keefe, 2004) and the Middle Jurassic taxon Muraenosaurus leedsii and (Andrews, 1910), the three seen in Colymbosaurus (a single postaxial ossicle facet; Roberts et al, 2017) and the four seen in 'Plesiosaurus' manselii (two postaxial facets; Hulke, 1870) and Tricleidus seeleyi (Andrews, 1910).…”

“…The majority of the specimens derive from the Oxford and Kimmeridge Clay Formations of the UK. The recent recovery and description of numerous cryptoclidid specimens from the Slottsmøya Member Lagerstätte of the Agardhfjellet Formation (central Spitsbergen), now constitute a major component of overall Boreal plesiosaurian richness from the Tithonian-Berriasian interval (Benson and Druckenmiller, 2014;Roberts et al, 2017).…”