'Tricleidus' Svalbardensis Based On

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. 175-186. Trondheim 2012, ISSN 029-196X.

A partial postcranial plesiosaurian skeleton uncovered on Svalbard in the winter of 1930-31 was described and named Tricleidus svalbardensis Pers- son, 1962. 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 1930-31 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. 1172 Blindern, 0318 Oslo, Norway. Present address: Haoma Mining NL, Bamboo Creek Mine, P.O. Box 2791, South Headland, 6722 WA, Australia, E-mail: [email protected]. Patrick S. Druckenmiller, University of Alaska Museum, 907 Yukon Drive, Fairbanks, AK 99775, United States. E-mail: [email protected]. Jørn H. Hurum, Natural History Museum (Geology), University of Oslo, Pb. 1172 Blindern, 0318 Oslo, Norway. E-mail: [email protected]

Introduction side of Isfjorden, without providing any further details. New information regarding the location of the find site Plesiosaur remains from Svalbard have been surprisingly has recently been found at the Natural History Museum understudied, with the first description ever being made in Oslo in the form of the original field drawings, the by Wiman (1914) of a single pliosaurid cervical or caudal original geological map used by the expedition (Figure vertebral centrum. The first significant find was made 2), and an original letter dated 1938 and sent to PMO by in the winter of 1930-31 by a team of American medi- Gunnar Aasgaard (who also took the photograph of the cal doctors studying the occurrence of the common flu excavation team in Figure 1) inquiring about the speci- (Figure 1) who found and collected a partially articulated men. The map is marked with a small “X”, presumably postcranial skeleton, including an incomplete pelvic indicating the excavation site (as suggested by the inform girdle, hind limbs and associated vertebrae. The speci- ation provided by the letter) just south of Diabasodden, men, PMO A 27745, was shipped to the Natural History in the top section of what is now named the Slottsmøya Museum, University of Oslo (PMO), and later described Member of the Agardhfjellet Formation. This map is also by Persson (1962), who erected the new taxon Tricleidus likely the source of information used by Heintz (1964). svalbardensis. The assignation of the Svalbard taxon to Tricleidus Andrews, 1909 was based on morphologi- In 1976, an expedition from the Norwegian Polar Insti- cal similarities in the propodial and epipodials of the tute, led by Mark Edwards, recovered the remains of a hind limb with the type species T. seeleyi Andrews, 1909 large plesiosaur at Agardhfjellet (type locality for the (Persson, 1962:66). The material was later redescribed in Agradhfjellet Formation containing the Slottsmøya an unpublished masters thesis (Andreassen, 2004) and Member), on the east side of Spitsbergen. These remains tentatively referred to Tricleidia sensu O’Keefe (2001). (PMO 218.377), which include two partial propodials (the larger articulated with the nearly complete distal At the time of its description, the exact geographic and limb), are housed in the collections at PMO and are stratigraphic location of the holotype was imprecisely listed by Andreassen (2004) as the humeri of a crypto known. A label associated with the skeleton simply reads clidid plesiosaur. “Jurassic? The mouth of Sassenfjorden, Vestspitsbergen” (Persson, 1962:67). Heintz (1964) stated that the speci- During the summer of 2009, a team from PMO exca- men was found in the area of Diabasodden, on the south vated a partial plesiosaurian postcranium from the upper 176 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Figure 1: 1931 photograph of the three Ame- rican medical doctors and two locals standing behind the excavated holotype specimen of ‘Tricleidus svalbardensis’, PMO A 27745. Their report was published by Paul & Freese (1933). (Photo: Gunnar Aas- gaard).

part of the Slottsmøya Member, approximately 3 km west Member conformably overlies the Oppdalsåta Member of the site indicated on the 1931 map. The new specimen and is overlain by the Myklegardfjellet Bed (i.e., base (PMO 216.838) includes two incomplete hind limbs and of the Rurikfjellet Formation; Birkemajer 1980). The two associated caudal vertebrae that closely resemble Slottsmøya Member, which ranges from 70 to 100 those of PMO A 27745. metres in thickness in the study area, consists of dark- grey to black silty mudstone, often weathering to paper New material collected since Persson’s (1962) origi- shale, discontinuous silty beds, with local occurrences of nal description, along with recently discovered historic red to yellowish sideritic concretions as well as siderite documents, provide important new morphological and and dolomite interbeds (Dypvik et al., 1991a; Hammer stratigraphic data on this poorly known taxon. Here we et al., 2011; Collignon & Hammer, this volume). The redescribe the holotype of Tricleidus svalbardensis (PMO Slottsmøya Member was deposited in an open marine A 27745) and provide full descriptions of the 1976 (PMO environment under oxygen-deficient settings (Nagy et 218.377) and the 2009 (PMO 216.838) material, and al., 1988; Dypvik et al., 1991b). Paleogeographic recon- revise the taxonomic status of the taxon. structions for Svalbard during the Kimmeridgian to Valanginian interval place paleoshorelines several hundred kilometres to the north and west (Dypvik et al., Geological setting 2002).

A large number of marine reptile skeletons have recently The Agardhfjellet Formation ranges from the Middle been collected in the Slottsmøya Member of the Agard- Jurassic to the Lower Cretaceous based on macrofos- hfjellet Formation. The Agardhfjellet Formation com- sils (mostly ammonites: Parker, 1976; Ershova, 1983; prises part of the Adventdalen Group (Parker, 1976), a Wierzbowski et al., 2011), foraminifera (Nagy & Basov, thick succession of Middle Jurassic to Lower Cretaceous 1998) and palynology (Løfaldli & Thusu, 1976; Bjærke, sedimentary rocks that crops out extensively in central 1978). The Slottsmøya Member is largely Volgian in age, Spitsbergen. The Agardhfjellet Formation is underlain by but recent work in the study area indicates that the unit shallow shelf to marginal marine sandstone and shale of becomes condensed up-section, with the top of the mem- the Wilhelmøya Formation (Kapp Toscana Group). The ber lying at or close to the Volgian-Ryazanian boundary Rurikfjellet Formation overlies the Agardhfjellet For- (Nagy & Basov, 1998; Hammer et al., 2011), or as young mation and together make up the Janusfjellet Subgroup as the Boreal Valanginian (Collignon & Hammer, this (Parker, 1976), which consists of several hundred metres volume). The specimens described here, PMO A 27745, of organic-rich, clayey and silty sediment. PMO 216.838 and PMO 218.377, occur in either the Dor- soplanites maximus or D. ilovaiskyi zone, corresponding Four named members are recognised in the Agardh- to the Middle Volgian. Precise age correlations between fjellet Formation, of which the Slottsmøya Member is Upper Jurassic and Lower Cretaceous units of the Boreal the uppermost (Dypvik et al., 1991a). The Slottsmøya and Tethyan regions remain controversial (Ogg, 2004; 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. 178 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Hammer et al., 2011; Gradstein et al., in press). However, Type horizon and age – Upper part of the Slottsmøya Member, the Middle Volgian age assigned to this specimen cor- Agardhfjellet Formation, Janusfjellet, Middle Volgian relates to an informally defined mid Tithonian or early (Tithonian; Nagy & Basov, 1998; Collignon & Hammer 2012; Portlandian age of the Tethyan region (Gradstein et al., Gradstein et al. in press). in press). Diagnosis - Differs from C. trochanterius in having a The geographic location of the type locality of PMO A triangular postaxial ossicle, relatively shorter epipodials, and a 27745 has now been ascertained to correspond to out- longer femoral facet for the postaxial ossicle (distal postaxial crops in the area southwest of Diabasodden that are flange); posterior margin of ischium is abruptly squared-off now termed the Slottsmøya Member of the Agardhfjel- and broad. let Formation. The excavated discovered specimen PMO 216.838 was found approximately 13 metres below the Referred specimens - Dorsoplanites Bed. The stratigraphic occurrence of the PMO 216.838: Two partial hind limbs; two nearly complete 1976 specimen (PMO 218.377) was also unknown. δ13C caudal vertebrae; unprepared pelvic fragments and ribs. analyses of shale collected along with the specimen were PMO 218.377: Two partial limbs (possibly fore and hind) and compared with the δ13C curve for the entire Slottsmøya fragmentary vertebral remains. Member (see Collignon & Hammer, this volume). The results of this analysis indicate that this specimen was either found between 31-22 metres or 15-11 metres Redescription of holotype below the Dorsoplanites Bed (Collignon & Hammer, this volume). Axial skeleton

Institutional abbreviations: PMO A 27745 preserves a total of 37 vertebrae including 5 dorsals, 4 sacrals, 26 caudals and two unidentifiable NHMUK Natural History Museum, London, UK centra. Vertebral centra diminish in length and height, C.M.N Castle Museum, Norwich, UK and become relatively wider in articular view posteriorly. PMO University of Oslo Natural History The concavity of the sacral and caudal centra is slightly Museum, Norway more pronounced than the dorsals and all of the neural arches are fused to the vertebral centra, indicative of osteological maturity (Brown, 1981). Systematic Paleontology The dorsal vertebral centra (Figure 3) are nearly circu- SAUROPTERYGIA Owen, 1860 lar in outline. The neural canal displays a characteris- PLESIOSAURIA de Blainville, 1835 tic keyhole-shaped outline. The transverse processes are PLESIOSAUROIDEA Welles, 1943 inclined dorsally from the neural arch at approximately a 135-degree angle. The dorsal rib facets are oval in out- Genus Colymbosaurus Seeley, 1874 line and angled in an anterodorsal to posteroventral direction. Type species – Colymbosaurus trochanterius (Owen, 1840) Brown, 1981 None of the sacral centra are complete, but based on the position of the rib and neural arch facets there appear to Holotype - NHMUK 31787, an isolated right humerus. be a minimum of 4 vertebrae in this region. The caudal series is incomplete and is missing the posterior-most Horizon - Kimmeridge Clay Formation, upper Kimmeridgian vertebrae. Caudal rib facets lay in the top half of the to Tithonian. vertebral centrum, making the upper half wider than the lower half in articular view. Persson (1962) noted that Diagnosis - as given by Brown, 1981. 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 Colymbosaurus svalbardensis (Persson, 1962) comb. nov. caudal centra. The chevron facets are triangular in outline and are present on caudal centra three to twenty- Holotype - PMO A 27745, dorsal, sacral and caudal vertebrae; six. All preserved centra share chevron facets between partial pelvic girdle; one nearly complete, and one partial hind adjacent vertebrae (Figure 4), some equally, and some limb. bear only one-third of the facet on the anterior centrum. The chevron facets also vary in size and surface texture. Type locality – East slope of Konusdalen, southeast of Each chevron facet is raised relative to the ventral sur- Deltaneset, Spitsbergen, Svalbard, Norway (approximate face of the centrum and bears a ridge that stretches ven- coordinates: 78.33930°N 15.92190°E). tromedially. As centrum length decreases posteriorly, the ridges from the anterior and posterior chevron facets NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of ‘Tricleidus’ svalbardensis 179

Figure 3: Dorsal centrum of PMO A 27745 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 deforma- tion, and is not observed in other dorsal centra.

Figure 4: Caudal centrum of PMO 216.838 in posterior (A), right lateral (B), and ventral (D) view and a neural spine of PMO 216.838 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 216.838.

meet medially and form what Persson (1962) referred propodials of the hind limbs. Although somewhat to as a ventral ridge. In articular view, the otherwise fragmented, the girdle is almost complete with the excep- rounded ventral half of the caudal centra appears more tion of the left ilium and right pubis. Due to its slightly angular and squared off due to the presence of this ridge. fragmentary nature the descriptions are based on a complementary study of the left and right pelvic girdles. Pelvic girdle The pubis (Figure 5) is approximately equally long The pelvic bones of PMO A 27745 (Figure 5) are pre- anteroposteriorly as it is wide mediolaterally (Table served in articulation with each other and with the 1) and has an anteriorly convex border. It possesses no 180 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY preserved posteromedial facet for articulation with equal to its maximum mediolateral width, and its ante- the ischium, but the preservational state of the ischium rior width is greater than its posterior width (Table 1). makes it uncertain if there was a pelvic bar. The lateral The anterior border of the ischium is concave, mirroring margin is only partially preserved, but it appears that the concave posterior border of the pubis. The posterior at least a moderately developed anterolateral cornu is margin of the ischium does not taper gradually to a present. Medially, the two pubes contact along a dorso- point, but rather is abruptly squared-off and mediolat- ventrally thickened, straight symphysis. Only the pos- erally broad in dorsal view. The ischia articulate medi- terior part of the medial symphysis is preserved in both ally along a thickened midline symphysis that extends pubes. The pubis as a whole is dorsoventrally thickened almost the entire medial length of the bones contra Pers- in its posterior half, with the area of the glenoid being the son (1962). thickest, and the anterior half being thinnest. The ilium (Figure 5 A) is incomplete but its general The ischium (Figure 5) is dorsoventrally thickest ante- form can be discerned based on complementary parts of riorly. Its total anteroposterior length is approximately the left and right elements. Its proximal end is not fully

Figure 5: Right pubis and ischium of PMO A 27745 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. NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of ‘Tricleidus’ svalbardensis 181

Table 1: Selected measurements (in mm) for PMO A 27745, PMO 216.838 and PMO 218.377. PMO A 27745 PMO 216.838 PMO 218.377 Remarks Pubis NA NA Width (acetabulum-midline) 325 - - Maximum anteroposterior length 335 - - Length of acetabular facet ~100 - - Length of ischial facet 60 - -

Ischium NA NA Width (acetabulum-midline) 310 - - Maximum anteroposterior length 390 - - Length of acetabular facet ~100 - - Length of pubic facet 65 - - Length of ilial facet ~50 - -

Ilium NA NA Proximal width 75 - - Distal width 60 - - Missing part of the distal margin in PMO A 27745. Maximum proximodistal length 210 - -

Humerus NA NA Distal width - - 185 PMO 218.377: distal margin incomplete. Length ulnar facet - - 85 PMO 218.377: distal margin incomplete. Length radial facet - - 90

Femur (right) (right) Shaft missing in PMO 218.377. Proximal width 110 125 - Distal width 230 225 190 PMO 218.377: distal margin incomplete. Width mid-shaft 90 - Maximum proximodistal length 420 440 - Missing proximal-most part in PMO A 27745. Length of tibial facet 95 95 100 Lengthg of fibular facet 85 90 85 Distance from postaxial margin of fibular facet to end of distal 95 95 - postaxial flange

Tibia NA NA Maximum width 85 - - Maximum length 65 - -

Fibula Maximum width 95 85 85 Maximum length 65 45 58

Dorsal centrum Coll. No. 185 NA NA Width 90 - - Height 80 - - Length 65 - - Length of rib facet 45 - Width of rib facet 30 - Largest caudal centrum Coll. No. 025 NA Somewhat deformed in PMO A 27745. Width 88 75 - Height 73 60 - Length 43 45 -

preserved but is strongly mediolaterally compressed and of the ilium is proximodistally longer than the medial appears to be asymmetrically expanded anteroposte- face due to the relative positions of the condyles at the riorly. The shaft of the ilium is not as anteroposteriorly acetabulum. The distal end is well preserved, is both broad as the proximal end and there does not appear mediolaterally and anteroposteriorly expanded relative to be any clear indication of a tubercle at midlength, to the shaft, and is approximately triangular in outline in although this area is not well preserved. The lateral face distal view. 182 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY

Hind limb dorsoventral compression as they are much thinner than the phalanges. The right hind limb of PMO A 27745 (Figure 6 A) is preserved almost in its entirety, missing only the The femoral shaft constricts in diameter distally from proximal-most portion of the femur and the postaxial the capitulum (which is not preserved) and then gradu- accessory ossicle (which is evident from the gap between ally expands anteroposteriorly one-third of the way along the propodial and mesopodials). The left hind limb the shaft distally. At its distal end, the posterior margin consists of fragmentary material including a partial of the femur forms a prominent postaxial flange that is femur and various tarsals and phalanges. The distal pro- dorsoventrally thinner than the preaxial edge. In post- podial, and epipodial and mesopodials (except for the axial view, the femoral profile tapers distally and evenly tibiale and first distal tarsal) appear to have undergone in dorsoventral thickness. The femur bears three distinct

Figure 6: The right hind limb of PMO A 27745 in anterior (A1), ventral (A2), and distal (A3) views. The left hind limb of PMO 216.838 in proximal (B1) view, and the right hind limb in ventral (B2), anterior (B3), and distal (B4) views. The reassembled hind limb of PMO 218.377 in proximal (C1), dorsal or ventral (C2), and distal (C3) views. NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of ‘Tricleidus’ svalbardensis 183 distal facets for elements of the epipodial row (contra at their proximal end. The phalanges are robust with flat Persson, 1962). The tibial and fibular facets are approx- to convex articular surfaces, and they diminish in size imately equal in length and extend approximately two- distally. The first digit possesses the relatively shortest thirds of the total width of the distal end of the femur, and widest phalanges. As preserved, the phalangeal for- and the remaining third facet accommodates the post mula is 4-9-12-11-8. axial element and postaxial flange. Both the tibial and fibular facets face directly distally, while the third post- Ribs axial facet is angled posterodistally. Five fragmentary dorsal ribs are preserved and are unre- The tibia and fibula are almost equal in their overall markable. dimensions, both are dorsoventrally thickest medially, and they are both anteroposteriorly wider than they are proximodistally long (Figure 6 B, Table 1). An epipo- Description of 1976 and 2009 material dial 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 PMO 216.838 is partly preserved in a siderite concretion tibiale. The fibula is broadly triangular in shape and pos- and consists of two partial hind limbs, a fragmentary sesses five facets; a long femoral facet, two short facets pelvic girdle, and two caudal vertebrae. The left femur for the tibia and postaxial ossicle, and two equally long has been slightly displaced by faulting, and its postero- facets for the intermedium and fibulare distally. distal flange was lost during preparation. PMO 218.377 preserves one incomplete front and hind limb. Six mesopodial elements are preserved in PMO A 27745 (Figure 6 A). The fibulare is hexagonal and possesses Axial skeleton four facets that form one proximal and one distal apex, and two short pre- and postaxial sides. The intermedium The two caudal centra preserved in PMO 216.838 is anteroposteriorly oblong and has six articular facets. (Figure 4 A-D) are identical to those in PMO A 27745, The tibiale is rectangular with five facets. The first tar- with the exception of the caudal ribs being fused to sal is square and exhibits three facets, of which the prox- the centra in the former. Their size, rib placement and imal is longer than the distal. The second tarsal has two presence of chevron facets on only the posteroventral angled, equally long distal facets, a long proximal facet, margin, suggests that the centra are from the anterior a longer postaxial facet that is longer than the preaxial part of the caudal series. The caudal neural spine is facet, and a very short facet for the tibiale. The third tar- slightly angled caudally and is shifted posteriorly in sal has five facets. Its preaxial margin is curved in con- relation to the base of the neural arch, so that the trast to its straight postaxial edge. Proximally the third posterior margin of the base of the arch is in line with tarsal has two angled facets, and distally one long facet. the midline of the neural spine. The right proximal Metapodials III and IV bear bifaceted articular surfaces portion of one chevron remains in articulation with

Figure 7: The left (?) humerus of PMO 218.377 in proximal (A), ventral (B), distal (C), and anterior (D) views. 184 E.M. Knutsen et al. NORWEGIAN JOURNAL OF GEOLOGY one of the caudals, and it is mediolaterally flat with its the intermedium is anteroposteriorly oblong and has six anterior edge angled medially. articular facets. The tibiale is rectangular and bears five facets. Front limb As preserved the minimum phalangeal formula in the The smaller limb preserved in PMO 218.377 contains hind limb of PMO 218.377 is: 7-12-14-9-8 (Figure 6 B2). a distal propodial fragment which agrees in size with a Some disarticulated phalanges were found near the limb, smaller proximal fragment (Figure 7). This proximal but have not been taken into account here. The size of fragment possesses a tuberosity that is postaxially off- the distal phalanges in the articulated limb suggests set relative to the capitulum consistent with its iden- that there are some missing from digits 3, 4 and 5. Due tity as a left humerus. The capitulum has a rugose sur- to the limb being reassembled from a field drawing, it is face and meets the humeral shaft at a well-demarcated, also possible that the phalangeal formula is somewhat sharp junction, in contrast to the tuberosity, which has inaccurate. 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 Discussion femora of PMO A 27745 and PMO 216.838. The articular facets for the radius and ulna are of equal length, the Based on their morphological similarities and strati- latter being angled slightly postaxially. graphic co-occurrence, we interpret that all three specimens described above are conspecific. The most con- Hind limbs vincing similarities are those found in the hind limbs. In addition to being of similar adult size (Table 1), the The appendicular material of PMO 216.838 and the femora all show the same triple-faceted and postaxi- slightly larger PMO 218.377 (Figure 6 B, C) are inter- ally flared distal end. Distally, the articulated epipo- preted as hind limbs based on the relative positions of the dial and mesopodial elements in all three specimens trochanter and capitulum, with the former lying directly are very similar, apart from being dorsoventrally thin- dorsal to the latter in proximal view. This is also con- ner in PMO A 27745 than in the other two specimens. sistent with the association of caudal vertebrae in PMO However, this is most likely attributable to dorsoventral 216.838. Only the femora of PMO 216.838 preserve compression as some elements of PMO A 27745 are as the complete capitulum and trochanter. In this speci- thick as those seen in PMO 216.838 and PMO 218.377. men they are only slightly separated by a constriction of PMO A 27745 and PMO 218.377 also exhibit slightly smooth, periosteal bone. As in PMO A 27745, the propo- different phalangeal formulas, although this variation dial shaft of PMO 216.838 is markedly constricted imme- likely relates to how the specimens were originally col- diately distal to the capitulum. The left femur of PMO lected and subsequently reassembled. The caudal verte- 216.838 possesses a large ventrally-situated fossa approx- bral morphologies of PMO A 27745 and PMO 216.838 imately 15 cm distal to the proximal margin. At its distal are also nearly identical, apart from the fusion of the end, the femur is expanded anteroposteriorly, and bears ribs to the caudal centra in PMO 216.838, suggesting a prominent postaxial flange that is dorsoventrally thin- a more advanced ontogenetic state in the latter. Com- ner than the preaxial edge. The postaxial flange is more parisons of the newly acquired material with the holo- pointed in PMO 216.838 than in PMO A 27745. The type also show that the distal part of the hind limb pre- three distal femoral facets of PMO 216.838 are equal in viously had been incorrectly re-assembled, resulting in shape and size to those in PMO A 27745. PMO 218.377 erroneous interpretations in the description (see Pers- does not preserve a distal postaxial flange, but its tibial son, 1962, fig. 1). and fibular facets appear similar to those of the other two specimens, apart from being somewhat more defined. Persson (1962) based his generic referral of the type material of ‘Tricleidus’ svalbardensis largely on the Neither PMO 216.838 nor PMO 218.377 preserves a basis of the shape of propodial and epipodial elements tibia. The fibulae of both specimens have five facets simi- seen in T. seeleyi. However, a morphological re-exam- lar to PMO A 27745: a long femoral facet, two short fac- ination of the type specimen of ‘T’. svalbardensis aug- ets for the tibia and postaxial ossicle, and two equally mented by new data from recently discovered material long facets for the intermedium and fibulare (Figure 6 described above, clearly demonstrates that this taxon is B2, C2). The postaxial ossicle of PMO 216.838 and PMO not referrable to Tricleidus. In T. Seeleyi, the epipodial 218.377 possesses three distinct facets: two long facets facets occupy almost the entire distal end of the femur for the femur and the fibulare, and a short fibular facet. (Andrews, 1910; pers. obs by EMK), whereas they only occupy two-thirds in PMO A 27745, PMO 216.838 and Six mesopodial elements are preserved in both PMO PMO 218.377. Furthermore, the distal postaxial flange 216.838 and PMO 218.377, all of which are very simi- of the femur of T. seeleyi is abruptly truncated and forms lar in their morphology (Figure 6 B2, C2). The fibulare a distinct posteriorly-facing facet (Andrews, 1910; pers. has six discrete facets and is equally broad as long, while obs. by EMK). In contrast, the postaxial flange of the NORWEGIAN JOURNAL OF GEOLOGY Redescription and taxonomic clarification of ‘Tricleidus’ svalbardensis 185

Svalbard material has more of a rounded projection, in the holotype. Unfortunately, pelvic material of C. and is not squared-off. The fibula of T. seeleyi differs trochanterius is unknown (Brown, 1981) so comparisons from PMO A 27745, PMO 216.838, and PMO 218.377 are not possible. in the length of the postaxial margin, which is longer than its medial margin (pers. obs. by EMK). Further, At present, Colymbosaurus svalbardensis is recognised the two distal fibular facets are equal in length in PMO primarily on the basis of its hind limb skeleton, and is A 27745, PMO 216.838, and PMO 218.377, whilst the known from only three, incomplete, postcranial skele- postaxial facet is longer than the preaxial facet in T. see- tons. In contrast, Colymbosaurus (both C. trochanterius leyi (pers. obs. by EMK). Although the tibiae of both and C. megadeirus) is known from several relatively taxa are fairly similar, an epipodial foramen is pres- more complete postcranial specimens in the UK and ent in T. seeleyi, but absent in the PMO material. It is is diagnosed on the basis of other features, including also noteworthy that a significant temporal gap exists the presence of 42 cervical vertebrae and a distinctive between Tricleidus (Oxford Clay Formation; Callovian) pectoral girdle. Pending the discovery of new and more and the Svalbard material (Agardhfjellet Formation; complete material of C. svalbardensis and/or systematic Volgian/Tithonian). revision of the British material, it may become necessary to reassess the taxonomic status of this taxon. Whilst differing markedly from Tricleidus, the hind limb morphology of all three Svalbard specimens is very similar to that described and figured for the Kimmeridg- Conclusions ian taxon Colymbosaurus Seeley, 1874. The axial skeleton of Colymbosaurus is similar in the number of sacral The taxon previously known as Tricleidus svalbarden- and caudal vertebrae (~4 and ~25, respectively), and in sis (Persson, 1962) is, based on the redescription of the the occurrence of chevron facets from caudal centrum holotype and description of new material, incompat- four posteriorly. Hind limbs figured and discussed by ible with the type material for this genus, but shares Owen (1869, pl. 4, NHMUK 40640) and Brown (1981, characteristics of the hind limb with the Kimmeridg- fig. 40, M.M.N. 15.72(2)) are both of similar size to the ian to Tithonian genus Colymbosaurus, to which it is Svalbard specimens, and show the presence of three epi- referred. C. svalbardensis is one of several new plesio- podial elements. The propodial of Colymbosaurus also saurian taxa now recognised from the highly fossilifer- bears a postaxial flange resembling that found in PMO ous Slottsmøya Member of the Agardhfjellet Formation, A 27745, PMO 216.838 and PMO 218.377. Further, the which collectively provide important new data on the reconstructions of the fibula of Colymbosaurus in both diversity of the clade during this poorly known interval. Owen (1869) and Brown (1981) exhibit the straight pre- Further, C. svalbardensis and its contemporaries inhab- and postaxial sides and the angled facets for the inter- ited marine settings at or near the paleo-Arctic Circle medium and fibulare, producing a distally directed apex. (Hammer et al., 2011 ; Knutsen et al., 2012(a), 2012(b), Stratigraphically, Colymbosaurus trochanterius is known and 2012(c)), and therefore present a rare opportunity from the Kimmeridge Clay Formation (Kimmeridgian to during the entire Mesozoic to study paleoecology and life Tithonian; Brown, 1981), and is therefore very close in history strategies of plesiosaurs at high paleolatitudes. age to the Agardhfjellet Formation. Thus, based on both morphological and stratigraphic grounds, the holotype (PMO A 27745) and referred material (PMO 216.838 Acknowledgements – First and foremost, we thank Øyvind Enger, and PMO 218.377) of ‘Tricleidus’ svalbardensis is here Magne Høyberget, Stig Larsen, Lena Kristiansen and Tommy Wensås for spending their summer holidays excavating marine reptiles on assigned to Colymbosaurus Seeley 1874. Svalbard every summer during the past seven years, and May-Liss Knudsen Funke and other volunteers for preparing PMO 216.838, Although generically similar, the Svalbard taxon does without whom this work would have been impossible. We also thank exhibit distinct differences from Colymbosaurus trochan- financial sponsors ExxonMobil, Fugro, OMW, Spitsbergen Travel, terius. The tibia and fibula of C. svalbardensis are proxi- Powershop and National Geographic for funding the 2009 excavation, which led to this study. Thanks also go to reviewers Glenn Storrs and modistally much shorter than those described for C. tro- Marta Fernandez. This work is part of a Ph.D. study by EMK funded by chanterius, and the fibulare (figured by Owen 1869, pl. the University of Oslo, Norway. 4) has a very different overall shape to those preserved in all three Svalbard specimens. 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