Redescription and taxonomic clarification of Tricleidus svalbardensis 175. the Agardhfjellet Formation (Middle Volgian)

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1 NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of Tricleidus svalbardensis 175 Redescription and taxonomic clarification of Tricleidus svalbardensis based on new material from the Agardhfjellet Formation (Middle Volgian) Espen M. Knutsen, Patrick S. Druckenmiller & Jørn H. Hurum Knutsen, E.M., Druckenmiller, P.S. & Hurum, J.H. Redescription and taxonomic clarification of Tricleidus svalbardensis based on new material from the Agardhfjellet Formation (Middle Volgian). Norwegian Journal of Geology, Vol 92, pp Trondheim 2012, ISSN X. A partial postcranial plesiosaurian skeleton uncovered on Svalbard in the winter of was described and named Tricleidus svalbardensis Persson, However, the precise geographic location and stratigraphic unit in which the skeleton was found were not published. Recent fieldwork on Svalbard has uncovered two more plesiosaur skeletons that are demonstrated herein to be conspecific with the 1931 taxon. These new specimens, along with recently discovered historic documents produced by the excavation members of the expedition, reveal that the holotype specimen, PMO A 27745, was recovered from the Upper Jurassic Slottsmøya Member of the Agardhfjellet Formation, which is Middle Volgian in age. Collectively, the Svalbard material is neither morphologically nor stratigraphically consistent with the Callovian taxon Tricleidus Andrews, 1909 and is here referred to Colymbosaurus Seeley, 1874 from the Kimmeridge Clay Formation (Kimmeridgian to Tithonian) of the UK. Espen M. Knutsen, Natural History Museum (Geology), University of Oslo, Pb Blindern, 0318 Oslo, Norway. Present address: Haoma Mining NL, Bamboo Creek Mine, P.O. Box 2791, South Headland, 6722 WA, Australia, espenkn80@gmail.com. Patrick S. Druckenmiller, University of Alaska Museum, 907 Yukon Drive, Fairbanks, AK 99775, United States. psdruckenmiller@alaska.edu. Jørn H. Hurum, Natural History Museum (Geology), University of Oslo, Pb Blindern, 0318 Oslo, Norway. j.h.hurum@nhm.uio.no Introduction Plesiosaur remains from Svalbard have been surprisingly understudied, with the first description ever being made by Wiman (1914) of a single pliosaurid cervical or caudal vertebral centrum. The first significant find was made in the winter of by a team of American medical doctors studying the occurrence of the common flu (Figure 1) who found and collected a partially articulated postcranial skeleton, including an incomplete pelvic girdle, hind limbs and associated vertebrae. The specimen, PMO A 27745, was shipped to the Natural History Museum, University of Oslo (PMO), and later described by Persson (1962), who erected the new taxon Tricleidus svalbardensis. The assignation of the Svalbard taxon to Tricleidus Andrews, 1909 was based on morphological similarities in the propodial and epipodials of the hind limb with the type species T. seeleyi Andrews, 1909 (Persson, 1962:66). The material was later redescribed in an unpublished masters thesis (Andreassen, 2004) and tentatively referred to Tricleidia sensu O Keefe (2001). At the time of its description, the exact geographic and stratigraphic location of the holotype was imprecisely known. A label associated with the skeleton simply reads Jurassic? The mouth of Sassenfjorden, Vestspitsbergen (Persson, 1962:67). Heintz (1964) stated that the specimen was found in the area of Diabasodden, on the south side of Isfjorden, without providing any further details. New information regarding the location of the find site has recently been found at the Natural History Museum in Oslo in the form of the original field drawings, the original geological map used by the expedition (Figure 2), and an original letter dated 1938 and sent to PMO by Gunnar Aasgaard (who also took the photograph of the excavation team in Figure 1) inquiring about the specimen. The map is marked with a small X, presumably indicating the excavation site (as suggested by the information provided by the letter) just south of Diabasodden, in the top section of what is now named the Slottsmøya Member of the Agardhfjellet Formation. This map is also likely the source of information used by Heintz (1964). In 1976, an expedition from the Norwegian Polar Institute, led by Mark Edwards, recovered the remains of a large plesiosaur at Agardhfjellet (type locality for the Agradhfjellet Formation containing the Slottsmøya Member), on the east side of Spitsbergen. These remains (PMO ), which include two partial propodials (the larger articulated with the nearly complete distal limb), are housed in the collections at PMO and are listed by Andreassen (2004) as the humeri of a cryptoclidid plesiosaur. During the summer of 2009, a team from PMO excavated a partial plesiosaurian postcranium from the upper

2 176 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY Figure 1: 1931 photograph of the three American medical doctors and two locals standing behind the excavated holotype specimen of Tricleidus svalbardensis, PMO A Their report was published by Paul & Freese (1933). (Photo: Gunnar Aasgaard). part of the Slottsmøya Member, approximately 3 km west of the site indicated on the 1931 map. The new specimen (PMO ) includes two incomplete hind limbs and two associated caudal vertebrae that closely resemble those of PMO A New material collected since Persson s (1962) original description, along with recently discovered historic documents, provide important new morphological and stratigraphic data on this poorly known taxon. Here we redescribe the holotype of Tricleidus svalbardensis (PMO A 27745) and provide full descriptions of the 1976 (PMO ) and the 2009 (PMO ) material, and revise the taxonomic status of the taxon. Geological setting A large number of marine reptile skeletons have recently been collected in the Slottsmøya Member of the Agardhfjellet Formation. The Agardhfjellet Formation comprises part of the Adventdalen Group (Parker, 1976), a thick succession of Middle Jurassic to Lower Cretaceous sedimentary rocks that crops out extensively in central Spitsbergen. The Agardhfjellet Formation is underlain by shallow shelf to marginal marine sandstone and shale of the Wilhelmøya Formation (Kapp Toscana Group). The Rurikfjellet Formation overlies the Agardhfjellet Formation and together make up the Janusfjellet Subgroup (Parker, 1976), which consists of several hundred metres of organic-rich, clayey and silty sediment. Four named members are recognised in the Agardhfjellet Formation, of which the Slottsmøya Member is the uppermost (Dypvik et al., 1991a). The Slottsmøya Member conformably overlies the Oppdalsåta Member and is overlain by the Myklegardfjellet Bed (i.e., base of the Rurikfjellet Formation; Birkemajer 1980). The Slottsmøya Member, which ranges from 70 to 100 metres in thickness in the study area, consists of darkgrey to black silty mudstone, often weathering to paper shale, discontinuous silty beds, with local occurrences of red to yellowish sideritic concretions as well as siderite and dolomite interbeds (Dypvik et al., 1991a; Hammer et al., 2011; Collignon & Hammer, this volume). The Slottsmøya Member was deposited in an open marine environment under oxygen-deficient settings (Nagy et al., 1988; Dypvik et al., 1991b). Paleogeographic reconstructions for Svalbard during the Kimmeridgian to Valanginian interval place paleoshorelines several hundred kilometres to the north and west (Dypvik et al., 2002). The Agardhfjellet Formation ranges from the Middle Jurassic to the Lower Cretaceous based on macrofossils (mostly ammonites: Parker, 1976; Ershova, 1983; Wierzbowski et al., 2011), foraminifera (Nagy & Basov, 1998) and palynology (Løfaldli & Thusu, 1976; Bjærke, 1978). The Slottsmøya Member is largely Volgian in age, but recent work in the study area indicates that the unit becomes condensed up-section, with the top of the member lying at or close to the Volgian-Ryazanian boundary (Nagy & Basov, 1998; Hammer et al., 2011), or as young as the Boreal Valanginian (Collignon & Hammer, this volume). The specimens described here, PMO A 27745, PMO and PMO , occur in either the Dorsoplanites maximus or D. ilovaiskyi zone, corresponding to the Middle Volgian. Precise age correlations between Upper Jurassic and Lower Cretaceous units of the Boreal and Tethyan regions remain controversial (Ogg, 2004;

3 NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of Tricleidus svalbardensis 177 Figure 2: Original field map showing the small X in the blue band (labelled Jura ) in the upper right corner.

4 178 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY Hammer et al., 2011; Gradstein et al., in press). However, the Middle Volgian age assigned to this specimen correlates to an informally defined mid Tithonian or early Portlandian age of the Tethyan region (Gradstein et al., in press). The geographic location of the type locality of PMO A has now been ascertained to correspond to outcrops in the area southwest of Diabasodden that are now termed the Slottsmøya Member of the Agardhfjellet Formation. The excavated discovered specimen PMO was found approximately 13 metres below the Dorsoplanites Bed. The stratigraphic occurrence of the 1976 specimen (PMO ) was also unknown. δ 13 C analyses of shale collected along with the specimen were compared with the δ 13 C curve for the entire Slottsmøya Member (see Collignon & Hammer, this volume). The results of this analysis indicate that this specimen was either found between metres or metres below the Dorsoplanites Bed (Collignon & Hammer, this volume). Institutional abbreviations: NHMUK C.M.N PMO Natural History Museum, London, UK Castle Museum, Norwich, UK University of Oslo Natural History Museum, Norway Systematic Paleontology SAUROPTERYGIA Owen, 1860 PLESIOSAURIA de Blainville, 1835 PLESIOSAUROIDEA Welles, 1943 Genus Colymbosaurus Seeley, 1874 Type species Colymbosaurus trochanterius (Owen, 1840) Brown, 1981 Holotype - NHMUK 31787, an isolated right humerus. Horizon - Kimmeridge Clay Formation, upper Kimmeridgian to Tithonian. Diagnosis - as given by Brown, Colymbosaurus svalbardensis (Persson, 1962) comb. nov. Holotype - PMO A 27745, dorsal, sacral and caudal vertebrae; partial pelvic girdle; one nearly complete, and one partial hind limb. Type locality East slope of Konusdalen, southeast of Deltaneset, Spitsbergen, Svalbard, Norway (approximate coordinates: N E). Type horizon and age Upper part of the Slottsmøya Member, Agardhfjellet Formation, Janusfjellet, Middle Volgian (Tithonian; Nagy & Basov, 1998; Collignon & Hammer 2012; Gradstein et al. in press). Diagnosis - Differs from C. trochanterius in having a triangular postaxial ossicle, relatively shorter epipodials, and a longer femoral facet for the postaxial ossicle (distal postaxial flange); posterior margin of ischium is abruptly squared-off and broad. Referred specimens - PMO : Two partial hind limbs; two nearly complete caudal vertebrae; unprepared pelvic fragments and ribs. PMO : Two partial limbs (possibly fore and hind) and fragmentary vertebral remains. Redescription of holotype Axial skeleton PMO A preserves a total of 37 vertebrae including 5 dorsals, 4 sacrals, 26 caudals and two unidentifiable centra. Vertebral centra diminish in length and height, and become relatively wider in articular view posteriorly. The concavity of the sacral and caudal centra is slightly more pronounced than the dorsals and all of the neural arches are fused to the vertebral centra, indicative of osteological maturity (Brown, 1981). The dorsal vertebral centra (Figure 3) are nearly circular in outline. The neural canal displays a characteristic keyhole-shaped outline. The transverse processes are inclined dorsally from the neural arch at approximately a 135-degree angle. The dorsal rib facets are oval in outline and angled in an anterodorsal to posteroventral direction. None of the sacral centra are complete, but based on the position of the rib and neural arch facets there appear to be a minimum of 4 vertebrae in this region. The caudal series is incomplete and is missing the posterior-most vertebrae. Caudal rib facets lay in the top half of the verte bral centrum, making the upper half wider than the lower half in articular view. Persson (1962) noted that the rib facets lay closest to the posterior margin, which is clearly seen in many of the centra, while being more ambiguous in others. Caudal ribs are not fused to the caudal centra. The chevron facets are triangular in outline and are present on caudal centra three to twentysix. All preserved centra share chevron facets between adjacent vertebrae (Figure 4), some equally, and some bear only one-third of the facet on the anterior centrum. The chevron facets also vary in size and surface texture. Each chevron facet is raised relative to the ventral surface of the centrum and bears a ridge that stretches ventromedially. As centrum length decreases posteriorly, the ridges from the anterior and posterior chevron facets

5 NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of Tricleidus svalbardensis 179 Figure 3: Dorsal centrum of PMO A in articular (A) and right lateral (B) view. Note that the apparent anterior lip on the ventral margin of the centrum is a result of taphonomic deformation, and is not observed in other dorsal centra. Figure 4: Caudal centrum of PMO in posterior (A), right lateral (B), and ventral (D) view and a neural spine of PMO in right lateral view (C). A caudal centrum of PMO A 7745 in ventral (E) view is presented for comparison. Note the fusion of the caudal ribs to the centrum in PMO meet medially and form what Persson (1962) referred to as a ventral ridge. In articular view, the otherwise rounded ventral half of the caudal centra appears more angular and squared off due to the presence of this ridge. Pelvic girdle The pelvic bones of PMO A (Figure 5) are preserved in articulation with each other and with the propodials of the hind limbs. Although somewhat fragment ed, the girdle is almost complete with the exception of the left ilium and right pubis. Due to its slightly fragmentary nature the descriptions are based on a complementary study of the left and right pelvic girdles. The pubis (Figure 5) is approximately equally long anteroposteriorly as it is wide mediolaterally (Table 1) and has an anteriorly convex border. It possesses no

6 180 E.M. Knutsen et al. preserved posteromedial facet for articulation with the ischium, but the preservational state of the ischium makes it uncertain if there was a pelvic bar. The lateral margin is only partially preserved, but it appears that at least a moderately developed anterolateral cornu is present. Medially, the two pubes contact along a dorsoventrally thickened, straight symphysis. Only the posterior part of the medial symphysis is preserved in both pubes. The pubis as a whole is dorsoventrally thickened in its posterior half, with the area of the glenoid being the thickest, and the anterior half being thinnest. The ischium (Figure 5) is dorsoventrally thickest anteriorly. Its total anteroposterior length is approximately NORWEGIAN JOURNAL OF GEOLOGY equal to its maximum mediolateral width, and its anterior width is greater than its posterior width (Table 1). The anterior border of the ischium is concave, mirroring the concave posterior border of the pubis. The posterior margin of the ischium does not taper gradually to a point, but rather is abruptly squared-off and mediolaterally broad in dorsal view. The ischia articulate medially along a thickened midline symphysis that extends almost the entire medial length of the bones contra Persson (1962). The ilium (Figure 5 A) is incomplete but its general form can be discerned based on complementary parts of the left and right elements. Its proximal end is not fully Figure 5: Right pubis and ischium of PMO A in ventral view and ilium in medial (A1) and anterior (A2) views. The reconstructed outline is based on complementary data from the left and right pelvis. The ilium is a composite of the fragments of the left (orange) and mirrored right (blue) ilia.

7 NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of Tricleidus svalbardensis 181 Table 1: Selected measurements (in mm) for PMO A 27745, PMO and PMO PMO A PMO PMO Remarks Pubis NA NA Width (acetabulum-midline) Maximum anteroposterior length Length of acetabular facet ~ Length of ischial facet Ischium NA NA Width (acetabulum-midline) Maximum anteroposterior length Length of acetabular facet ~ Length of pubic facet Length of ilial facet ~ Ilium NA NA Proximal width Distal width Missing part of the distal margin in PMO A Maximum proximodistal length Humerus NA NA Distal width PMO : distal margin incomplete. Length ulnar facet PMO : distal margin incomplete. Length radial facet Femur (right) (right) Shaft missing in PMO Proximal width Distal width PMO : distal margin incomplete. Width mid-shaft 90 - Maximum proximodistal length Missing proximal-most part in PMO A Length of tibial facet Length of fibular facet Distance from postaxial margin of fibular facet to end of distal postaxial flange Tibia NA NA Maximum width Maximum length Fibula Maximum width Maximum length Dorsal centrum Coll. No. 185 NA NA Width Height Length Length of rib facet 45 - Width of rib facet 30 - Largest caudal centrum Coll. No. 025 NA Somewhat deformed in PMO A Width Height Length preserved but is strongly mediolaterally compressed and appears to be asymmetrically expanded anteroposteriorly. The shaft of the ilium is not as anteroposteriorly broad as the proximal end and there does not appear to be any clear indication of a tubercle at midlength, although this area is not well preserved. The lateral face of the ilium is proximodistally longer than the medial face due to the relative positions of the condyles at the acetabulum. The distal end is well preserved, is both mediolaterally and anteroposteriorly expanded relative to the shaft, and is approximately triangular in outline in distal view.

8 182 E.M. Knutsen et al. Hind limb The right hind limb of PMO A (Figure 6 A) is preserved almost in its entirety, missing only the proximal -most portion of the femur and the postaxial accessory ossicle (which is evident from the gap between the propodial and mesopodials). The left hind limb consists of fragmentary material including a partial femur and various tarsals and phalanges. The distal propodial, and epipodial and mesopodials (except for the tibiale and first distal tarsal) appear to have undergone NORWEGIAN JOURNAL OF GEOLOGY dorsoventral compression as they are much thinner than the phalanges. The femoral shaft constricts in diameter distally from the capitulum (which is not preserved) and then gradually expands anteroposteriorly one-third of the way along the shaft distally. At its distal end, the posterior margin of the femur forms a prominent postaxial flange that is dorsoventrally thinner than the preaxial edge. In postaxial view, the femoral profile tapers distally and evenly in dorsoventral thickness. The femur bears three distinct Figure 6: The right hind limb of PMO A in anterior (A1), ventral (A2), and distal (A3) views. The left hind limb of PMO in proximal (B1) view, and the right hind limb in ventral (B2), anterior (B3), and distal (B4) views. The reassembled hind limb of PMO in proximal (C1), dorsal or ventral (C2), and distal (C3) views.

9 NORWEGIAN JOURNAL OF GEOLOGY distal facets for elements of the epipodial row (contra Persson, 1962). The tibial and fibular facets are approximately equal in length and extend approximately twothirds of the total width of the distal end of the femur, and the remaining third facet accommodates the postaxial element and postaxial flange. Both the tibial and fibular facets face directly distally, while the third postaxial facet is angled posterodistally. The tibia and fibula are almost equal in their overall dimensions, both are dorsoventrally thickest medially, and they are both anteroposteriorly wider than they are proximodistally long (Figure 6 B, Table 1). An epipodial foramen is absent (contra Persson, 1962). The tibia exhibits four facets; a long femoral facet, two short facets for the fibula and intermedium, and a long facet for the tibiale. The fibula is broadly triangular in shape and possesses five facets; a long femoral facet, two short facets for the tibia and postaxial ossicle, and two equally long facets for the intermedium and fibulare distally. Six mesopodial elements are preserved in PMO A (Figure 6 A). The fibulare is hexagonal and possesses four facets that form one proximal and one distal apex, and two short pre- and postaxial sides. The intermedium is anteroposteriorly oblong and has six articular facets. The tibiale is rectangular with five facets. The first tarsal is square and exhibits three facets, of which the proximal is longer than the distal. The second tarsal has two angled, equally long distal facets, a long proximal facet, a longer postaxial facet that is longer than the preaxial facet, and a very short facet for the tibiale. The third tarsal has five facets. Its preaxial margin is curved in contrast to its straight postaxial edge. Proximally the third tarsal has two angled facets, and distally one long facet. Metapodials III and IV bear bifaceted articular surfaces Redescription and taxonomic clarification of Tricleidus svalbardensis 183 at their proximal end. The phalanges are robust with flat to convex articular surfaces, and they diminish in size distally. The first digit possesses the relatively shortest and widest phalanges. As preserved, the phalangeal formula is Ribs Five fragmentary dorsal ribs are preserved and are unremarkable. Description of 1976 and 2009 material PMO is partly preserved in a siderite concretion and consists of two partial hind limbs, a fragmentary pelvic girdle, and two caudal vertebrae. The left femur has been slightly displaced by faulting, and its posterodistal flange was lost during preparation. PMO preserves one incomplete front and hind limb. Axial skeleton The two caudal centra preserved in PMO (Figure 4 A-D) are identical to those in PMO A 27745, with the exception of the caudal ribs being fused to the centra in the former. Their size, rib placement and presence of chevron facets on only the posteroventral margin, suggests that the centra are from the anterior part of the caudal series. The caudal neural spine is slightly angled caudally and is shifted posteriorly in relation to the base of the neural arch, so that the posterior margin of the base of the arch is in line with the midline of the neural spine. The right proximal portion of one chevron remains in articulation with Figure 7: The left (?) humerus of PMO in proximal (A), ventral (B), distal (C), and anterior (D) views.

10 184 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY one of the caudals, and it is mediolaterally flat with its anterior edge angled medially. Front limb The smaller limb preserved in PMO contains a distal propodial fragment which agrees in size with a smaller proximal fragment (Figure 7). This proximal fragment possesses a tuberosity that is postaxially offset relative to the capitulum consistent with its identity as a left humerus. The capitulum has a rugose surface and meets the humeral shaft at a well-demarcated, sharp junction, in contrast to the tuberosity, which has a less pronounced and more rounded junction with the humeral shaft. The distal fragment is missing what is interpreted to be the postaxial flange that is seen in the femora of PMO A and PMO The articular facets for the radius and ulna are of equal length, the latter being angled slightly postaxially. Hind limbs The appendicular material of PMO and the slightly larger PMO (Figure 6 B, C) are interpreted as hind limbs based on the relative positions of the trochanter and capitulum, with the former lying directly dorsal to the latter in proximal view. This is also consistent with the association of caudal vertebrae in PMO Only the femora of PMO preserve the complete capitulum and trochanter. In this specimen they are only slightly separated by a constriction of smooth, periosteal bone. As in PMO A 27745, the propodial shaft of PMO is markedly constricted immediately distal to the capitulum. The left femur of PMO possesses a large ventrally-situated fossa approximately 15 cm distal to the proximal margin. At its distal end, the femur is expanded anteroposteriorly, and bears a prominent postaxial flange that is dorsoventrally thinner than the preaxial edge. The postaxial flange is more pointed in PMO than in PMO A The three distal femoral facets of PMO are equal in shape and size to those in PMO A PMO does not preserve a distal postaxial flange, but its tibial and fibular facets appear similar to those of the other two specimens, apart from being somewhat more defined. Neither PMO nor PMO preserves a tibia. The fibulae of both specimens have five facets similar to PMO A 27745: a long femoral facet, two short facets for the tibia and postaxial ossicle, and two equally long facets for the intermedium and fibulare (Figure 6 B2, C2). The postaxial ossicle of PMO and PMO possesses three distinct facets: two long facets for the femur and the fibulare, and a short fibular facet. Six mesopodial elements are preserved in both PMO and PMO , all of which are very similar in their morphology (Figure 6 B2, C2). The fibulare has six discrete facets and is equally broad as long, while the intermedium is anteroposteriorly oblong and has six articular facets. The tibiale is rectangular and bears five facets. As preserved the minimum phalangeal formula in the hind limb of PMO is: (Figure 6 B2). Some disarticulated phalanges were found near the limb, but have not been taken into account here. The size of the distal phalanges in the articulated limb suggests that there are some missing from digits 3, 4 and 5. Due to the limb being reassembled from a field drawing, it is also possible that the phalangeal formula is somewhat inaccurate. Discussion Based on their morphological similarities and stratigraphic co-occurrence, we interpret that all three specimens described above are conspecific. The most convincing similarities are those found in the hind limbs. In addition to being of similar adult size (Table 1), the femora all show the same triple-faceted and postaxially flared distal end. Distally, the articulated epipodial and mesopodial elements in all three specimens are very similar, apart from being dorsoventrally thinner in PMO A than in the other two specimens. However, this is most likely attributable to dorsoventral compression as some elements of PMO A are as thick as those seen in PMO and PMO PMO A and PMO also exhibit slightly different phalangeal formulas, although this variation likely relates to how the specimens were originally collected and subsequently reassembled. The caudal vertebral morphologies of PMO A and PMO are also nearly identical, apart from the fusion of the ribs to the caudal centra in PMO , suggesting a more advanced ontogenetic state in the latter. Comparisons of the newly acquired material with the holotype also show that the distal part of the hind limb previously had been incorrectly re-assembled, resulting in erroneous interpretations in the description (see Persson, 1962, fig. 1). Persson (1962) based his generic referral of the type material of Tricleidus svalbardensis largely on the basis of the shape of propodial and epipodial elements seen in T. seeleyi. However, a morphological re-examination of the type specimen of T. svalbardensis augmented by new data from recently discovered material described above, clearly demonstrates that this taxon is not referrable to Tricleidus. In T. Seeleyi, the epipodial facets occupy almost the entire distal end of the femur (Andrews, 1910; pers. obs by EMK), whereas they only occupy two-thirds in PMO A 27745, PMO and PMO Furthermore, the distal postaxial flange of the femur of T. seeleyi is abruptly truncated and forms a distinct posteriorly-facing facet (Andrews, 1910; pers. obs. by EMK). In contrast, the postaxial flange of the

11 NORWEGIAN JOURNAL OF GEOLOGY Svalbard material has more of a rounded projection, and is not squared-off. The fibula of T. seeleyi differs from PMO A 27745, PMO , and PMO in the length of the postaxial margin, which is longer than its medial margin (pers. obs. by EMK). Further, the two distal fibular facets are equal in length in PMO A 27745, PMO , and PMO , whilst the postaxial facet is longer than the preaxial facet in T. seeleyi (pers. obs. by EMK). Although the tibiae of both taxa are fairly similar, an epipodial foramen is present in T. seeleyi, but absent in the PMO material. It is also noteworthy that a signifi cant temporal gap exists between Tricleidus (Oxford Clay Formation; Callovian) and the Svalbard material (Agardh fjellet Formation; Volgian/Tithonian). Whilst differing markedly from Tricleidus, the hind limb morphology of all three Svalbard specimens is very similar to that described and figured for the Kimmeridgian taxon Colymbosaurus Seeley, The axial skeleton of Colymbosaurus is similar in the number of sacral and caudal vertebrae (~4 and ~25, respectively), and in the occurrence of chevron facets from caudal centrum four posteriorly. Hind limbs figured and discussed by Owen (1869, pl. 4, NHMUK 40640) and Brown (1981, fig. 40, M.M.N (2)) are both of similar size to the Svalbard specimens, and show the presence of three epipodial elements. The propodial of Colymbosaurus also bears a postaxial flange resembling that found in PMO A 27745, PMO and PMO Further, the reconstructions of the fibula of Colymbosaurus in both Owen (1869) and Brown (1981) exhibit the straight preand postaxial sides and the angled facets for the intermedium and fibulare, producing a distally directed apex. Stratigraphically, Colymbosaurus trochanterius is known from the Kimmeridge Clay Formation (Kimmeridgian to Tithonian; Brown, 1981), and is therefore very close in age to the Agardhfjellet Formation. Thus, based on both morphological and stratigraphic grounds, the holotype (PMO A 27745) and referred material (PMO and PMO ) of Tricleidus svalbardensis is here assigned to Colymbosaurus Seeley Although generically similar, the Svalbard taxon does exhibit distinct differences from Colymbosaurus trochanterius. The tibia and fibula of C. svalbardensis are proximodistally much shorter than those described for C. trochanterius, and the fibulare (figured by Owen 1869, pl. 4) has a very different overall shape to those preserved in all three Svalbard specimens. The postaxial accessory ossicle in C. trochanterius is only known in NHMUK 40640, and only from an impression in the surrounding matrix. The outline clearly shows that the bone was proximodist ally oblong with an almost rectangular shape. This contrasts to the triangular shape seen in the postaxial ossicles of PMO and PMO Another autapomorphy of C. svalbardensis mentioned by both Persson (1962) and Andreassen (2004) was the medio laterally flat posterior margin of the ischium Redescription and taxonomic clarification of Tricleidus svalbardensis 185 in the holotype. Unfortunately, pelvic material of C. trochanterius is unknown (Brown, 1981) so comparisons are not possible. At present, Colymbosaurus svalbardensis is recognised primarily on the basis of its hind limb skeleton, and is known from only three, incomplete, postcranial skeletons. In contrast, Colymbosaurus (both C. trochanterius and C. megadeirus) is known from several relatively more complete postcranial specimens in the UK and is diagnosed on the basis of other features, including the presence of 42 cervical vertebrae and a distinctive pectoral girdle. Pending the discovery of new and more complete material of C. svalbardensis and/or systematic revision of the British material, it may become necessary to reassess the taxonomic status of this taxon. Conclusions The taxon previously known as Tricleidus svalbardensis (Persson, 1962) is, based on the redescription of the holotype and description of new material, incompatible with the type material for this genus, but shares character istics of the hind limb with the Kimmeridgian to Tithonian genus Colymbosaurus, to which it is referred. C. svalbardensis is one of several new plesiosaurian taxa now recognised from the highly fossiliferous Slottsmøya Member of the Agardhfjellet Formation, which collectively provide important new data on the diversity of the clade during this poorly known interval. Further, C. svalbardensis and its contemporaries inhabited marine settings at or near the paleo-arctic Circle (Hammer et al., 2011 ; Knutsen et al., 2012(a), 2012(b), and 2012(c)), and therefore present a rare opportunity during the entire Mesozoic to study paleoecology and life history strategies of plesiosaurs at high paleolatitudes. Acknowledgements First and foremost, we thank Øyvind Enger, Magne Høyberget, Stig Larsen, Lena Kristiansen and Tommy Wensås for spending their summer holidays excavating marine reptiles on Svalbard every summer during the past seven years, and May-Liss Knudsen Funke and other volunteers for preparing PMO , without whom this work would have been impossible. We also thank financial sponsors ExxonMobil, Fugro, OMW, Spitsbergen Travel, Power shop and National Geographic for funding the 2009 excavation, which led to this study. Thanks also go to reviewers Glenn Storrs and Marta Fernandez. 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