A New Species of Tylosaurine Mosasaur from the Upper Campanian Bearpaw Formation of Saskatchewan, Canada Paulina JimEnez-Huidobroa*, Michael W
Journal of Systematic Palaeontology, 2018 https://doi.org/10.1080/14772019.2018.1471744
A new species of tylosaurine mosasaur from the upper Campanian Bearpaw Formation of Saskatchewan, Canada Paulina Jimenez-Huidobroa*, Michael W. Caldwella,b, Ilaria Paparellaa and Timon S. Bullarda aDepartment of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada; bDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada
(Received 15 February 2018; accepted 24 April 2018)
Mosasaurs assigned to the genus Tylosaurus have been reported from the North Atlantic Circle Basin, including the Western Interior Seaway in North America and Europe, from the Turonian of Chihuahua, Mexico, to the early Maastrichtian of Belgium. The youngest record of Tylosaurus in North America is from the middle Campanian of the Pierre Shale, South Dakota. Data obtained by examination of an almost complete skull and associated postcranial elements of a large tylosaurine mosasaur from the upper Campanian of Saskatchewan supports the recognition of this specimen as a new species, Tylosaurus saskatchewanensis sp. nov. The specimen, collected from the Bearpaw Formation, southern Saskatchewan, Canada, presents unique features and combinations of features. It is recognized as a tylosaurine based on: edentulous rostrum; relatively long suprastapedial process of quadrate not reaching infrastapedial process; predental anterior extension of dentaries; and 12–13 maxillary and dentary teeth. Some of the characters that support recognition of the new species include: exclusion of prefrontal from dorsal rim of orbit by anterior process of postorbitofrontal; frontal extends anteriorly well into narial openings; well developed dorsal midline eminence of frontal; straight margins of parietal table; small infrastapedial process of quadrate located high on quadrate shaft almost touching suprastapedial process; thick tympanic ala of quadrate; 55 vertebrae anterior to chevron-bearing caudals; and rounded astragalus with big semicircular crural emargination. The new species represents the most northern occurrence of the genus and the subfamily and extends the stratigraphical distribution of the genus Tylosaurus in North America, occupying the northern Western Interior Seaway during the late Campanian. http://zoobank.org/urn:lsid:zoobank.org:pub:55338D44-B3C7-4F9C-8B51-C29DB9B99364 Keywords: mosasaur; Tylosaurus saskatchewanensis; Bearpaw Formation; Saskatchewan
Introduction pembinensis were described from the middle Campanian of the Pierre Shale of South Dakota and Manitoba, respec- The fossil record of tylosaurinae mosasaurs in North tively (Nicholls 1988; Martin 2007; Bullard & Caldwell America extends from the Turonian (Polcyn et al. 2008; 2010). Loera-Flores 2013) to the middle Campanian (Nicholls The Bearpaw Formation, in both outcrop and subcrop, 1988; Martin 2007; Bullard & Caldwell 2010). In North is located in Montana (USA) and southern Alberta and America, Tylosaurus proriger Cope, 1869, described southern Saskatchewan (Canada) (Hatcher & Stanton from the upper Santonian–lower Campanian of the Nio- 1903). It is considered to be a Campanian-aged marine brara Formation of Kansas, is also known from the upper shale, rich in marine reptiles, such as elasmosaurid and Santonian–lower Campanian of the Mooreville Chalk of polycotylid plesiosaurs (Sato 2003, 2005), chelonioid tur- Alabama (Russell & Applegate 1970; Kiernan 2002), the tles (Brinkman et al. 2006) and a diverse assemblage of middle Campanian of the Taylor Group of Texas, and the mosasaurs. Of the latter, Holmes (1996) recognized the middle Campanian of the Pierre Shale of Kansas and plioplatecarpine Plioplatecarpus primaevus from Sas- South Dakota (Russell 1967). Tylosaurus nepaeolicus was katchewan; Konishi et al. (2014) described an extraordi- described from the upper Coniacian–lower Santonian of narily well-preserved Mosasaurus missouriensis, and the Niobrara Formation of Kansas (Cope 1874), and some Konishi et al. (2011) described exceptional specimens of referred specimens were recovered from the upper Conia- Prognathodon overtoni, both from southern Alberta. An cian of the Boquillas Formation, Texas (Bell et al. 2012). additional specimen (TMP1983.126.1 on exhibit at the And, finally, Tylosaurus neumilleri and Tylosaurus Royal Tyrrell Museum) has been recovered from the
*Corresponding author. Email: [email protected]
Ó The Trustees of the Natural History Museum, London 2018. All rights reserved.
Published online 22 May 2018 2 P. Jimenez-Huidobro et al.
Bearpaw Formation although never formally described. photographic images. Measurements (in mm) were taken The latter has been mentioned as “a nearly complete tylo- using calipers and a measuring tape. saurine mosasaur” (Caldwell et al. 2005, p. 243) from the To run the phylogenetic analysis, the matrix was edited lower Bearpaw Formation and was determined to be early using Mesquite 3.03 for Mac; traditional search (heuristic late Campanian in age (T. Konishi pers. comm.). algorithm) with 1000 replicates was performed in TNT Bullard’s (2006) unpublished dissertation described the 1.1 (Goloboff et al. 2008); cladograms were recovered right side of a skull belonging to a tylosaurine mosasaur using TNT 1.1. The trees were analysed in Mesquite 3.03, from the Bearpaw Formation of Saskatchewan. More and edited using Photoshop CS6 for Mac. Assessment of recently, the preparation of the other half of the skull, the phylogenetic relationships of tylosaurine mosasaurs together with reanalysis of the postcranial material of the was conducted using the data matrix of Jimenez-Huidobro same specimen, allowed the collection of more data and et al. (2016), which itself was derived from Palci et al. supports the recognition of a new species. Indeed, the (2013). The matrix was modified by the inclusion of Tylo- specimen shows a set of unique characters that distinguish saurus saskatchewanensis. Character 7 was deleted as it from all other species of Tylosaurus. The aim of our most taxa were coded as a ‘?’. Characters 93 and 94 were research is to present the new taxon from the upper Cam- summarized in a single character, as follows: Character panian of the Bearpaw Formation of Saskatchewan (Can- (92): presacral vertebrae number: 32 or fewer (0); 33–38 ada), highlighting its differences in comparison to all the (1); 39 or more (2). Character 95 was deleted because the other tylosaurine mosasaurs. sacral vertebral number does not apply to hydropelvic mosasauroids, and a new character (129) was added, addressing the condition of the sacrum: present (0); absent Geological setting (1). The final data matrix consists of 42 terminal taxa and 129 characters (Online Supplementary Material).
Specimen RSM P2588.1 was found in deposits considered Institutional abbreviations. RSM, Royal Saskatchewan to belong to the Snakebite Member, one of the uppermost Museum, T. rex Discovery Centre, Eastend, Saskatche- of the 11 members of the Bearpaw Formation, near Her- wan, Canada; TMP, Royal Tyrrell Museum, Drumheller, bert Ferry, southern Saskatchewan (Fig. 1). The Bearpaw Alberta, Canada; IRScNB, Institut Royal des Sciences Formation consists of silty clays and subordinate sands Naturelles de Belgique, Brussel, Belgium. deposited in shallow-water, marine conditions during the last part of the Late Cretaceous in the Western Interior Seaway (W. G. E. Caldwell 1968); the Snakebite Member Systematic palaeontology is dominated by dark grey silty clays, including numerous bentonite seams, and limestone concretions (W. G. E. Order Squamata Oppel, 1811 Caldwell 1968). Based on ammonites, the Bearpaw For- Family Mosasauroidea Gervais, 1853 mation ranges from the upper Campanian to the lower Subfamily Tylosaurinae Williston, 1897 Maastrichtian, and the deposits of the Snakebite Member Genus Tylosaurus Marsh, 1872 span the zones of Baculites cuneatus and B. reesidei and may also include the B. jenseni zone (W. G. E. Caldwell Type species. Tylosaurus proriger (Cope, 1869), upper 1968). The outcrops near Herbert Ferry, where RSM Smoky Hill Chalk Member, Niobrara Formation, western P2588.1 was found, are included in the B. reesidei Zone, Kansas, USA. suggesting a late late Campanian age (71–72 Ma) for the rocks and thus for the specimen (cf. W. G. E. Caldwell Range. Late Turonian? to early Maastrichtian (Polcyn 1968; Kauffman et al. 1993). et al. 2008; Loera-Flores 2013; Jimenez-Huidobro & Caldwell 2016).
Diagnosis. Twelve to 13 maxillary teeth; prefrontal does Material and methods not contribute to external nares; frontal overlaps supraor- bital portion of prefrontal; frontal does not contribute to Specimen RSM P2588.1 was collected in 1995 from the the orbit; ventroposterior process on jugal present; 10–11 Snakebite Member of the Bearpaw Formation, near Her- pterygoid teeth; broad projection of dentary anterior to bert Ferry on the shore of Lake Diefenbaker in southern first dentary tooth; 13 teeth on dentary; six to seven Saskatchewan. The right side of the skull was studied by pygals, 33 to 34 intermedial caudals and 56 to 58 terminal Bullard (2006), while the left side of the skull was only caudals; scapula smaller than coracoid, convex superior recently prepared. Detailed photographs were taken using border of scapula; radial process absent in the humerus; a Canon EOS t2i camera, and edited in Photoshop CS6 for elongated radius, same length of metacarpals one and Mac. Drawings were made in Photoshop by tracing two; ischium well expanded medially at symphysis; distal A new species of tylosaurine mosasaur 3
Figure 1. Map of southern Saskatchewan. The star indicates the locality where specimen RSM P2588.1 was recovered. Stratigraphi- cally, the locality corresponds to the Snakebite Member of the Bearpaw Formation. end of femur more expanded than proximal; phalangeal the Snakebite Member, Bearpaw Fm., Saskatchewan, formula of pes 5-8-8-8- (modified from Russell 1967). Canada (Kauffman & Caldwell 1993). Etymology. Named after the Canadian province of Sas- Tylosaurus saskatchewanensis sp. nov. katchewan, where the only specimen was found. (Figs 2–6) Diagnosis. (1) Extensive overlap of premaxilla onto Material. Holotype: RSM P2588.1, moderately complete the frontal; (2) anterior process of the frontal extending and articulated skull (Fig. 2), associated with a quite com- anteriorly up to half of the length of the external naris; plete disarticulated postcranium (Figs 4, 5). The vertebral (3) frontal with a well-developed dorsal midline emi- column is almost complete, except for a few vertebrae in nence; (4) exclusion of the prefrontal from the dorsal the caudal series; among the appendicular elements only rim of the orbit by a long anterior process of the post- the phalanges are missing. Near Herbert Ferry on the orbitofrontal; (5) suprastapedial process of the quadrate southern shore of Lake Diefenbaker, upper Campanian of moderate in size; (6) infrastapedial process of the 4 P. Jimenez-Huidobro et al.
Figure 2. Tylosaurus saskatchewanensis sp. nov., RSM P2588.1, holotype, skull. A, right side of the skull in lateral view; B, left side of the skull in lateral view; C, schematic drawing of the right side of the skull; D, schematic drawing of the left side of the skull; E, dorsal view of the skull. Anatomical abbreviations: a, angular; ar, articular; d, dentary; f, frontal; ib, interanrial bar; j, jugal; m, maxilla; no, narial openings; F, schematic drawing of the dorsal view of the skull; pa, prearticular; pf, prefrontal; pm, premaxilla; pof, postorbitofron- tal; q, quadrate; sa, surangular; sp, splenial; sq, squamosal; srp, suspensorial ramus of the parietal; st, supratemporal; l and r preceding the bones indicate left or right. Scale bar 30 cm. D quadrate rounded, and located high on the quadrate bump, visible on the ventral surface, anterior to the pre- shaft, almost touching the suprastapedial process; (7) maxillary teeth. Laterally, the suture with the right maxilla tympanic ala of the quadrate thick; (8) femur longer is sinusoidal in shape, while the maxillo-premaxillary than the humerus; (9) 55 vertebrae anterior to chevron- suture is rectangular on the left side; both shapes are usu- bearing caudals; (10) rounded astragalus with large ally observed in T. proriger. The dorsal crest is moder- semicircular crural foramen. ately developed. The premaxilla bears two pairs of teeth, all fully erupted, preserved in the bone. The teeth are Description. sharp, striated, posteriorly recurved and labiolingually Cranium. Premaxilla. In RSM P2588.1 the premax- compressed, with only anterior carinae. The anterior illa is complete, and appears long and slender (Fig. 2A, carina is placed lingually at the tip of the tooth and curves B). Its anterior tip is cylindrical in cross section, while in further lingually. The tips of the crowns are asymmetrical lateral view it has a rectangular shape. An edentulous ros- in cross section: the anterior margin of the carina is acu- trum, 74 mm in length, is present anterior to the premaxil- minate while the posterior margin is rounded. The pre- lary teeth. The predental process is characterized by a maxilla contributes to the anterior half of the medial A new species of tylosaurine mosasaur 5
Figure 3. Tylosaurus saskatchewanensis sp. nov., RSM P2588.1, holotype, detailed characters. A, right orbit showing the jugal at the posteroventral margin of the orbit, and prefrontal and postorbitofrontal overlapping over the dorsal margin; B, schematic drawing of the orbit, note the arrow indicating the long prefrontal process of the postorbitofrontal at the dorsal margin of the orbit; C, parietal bone in dorsal view, disarticulated from the skull; D, left quadrate in lateral view; E, right quadrate in lateral view. Anatomical abbreviations: isp, infrastapedial process of the quadrate; j, jugal; pf, prefrontal; pof, postorbitofrontal; sr, sclerotic ring; srp, suspensorial ramus of pari- etal; ssp, suprastapedial process of the quadrate. Scale bars: A, B, D, E 5 cm; C 10 cm. D D margin of the narial openings. The internarial bar invades Maxilla. Both maxillae are present (Fig. 2A–D). The deeply into the frontal, where the latter contributes to the left maxilla is 654 mm long, while the right is 616 mm. posterior half of the medial rim of the narial openings. Both left and right elements bear 12 teeth. The maxilla contributes to the lateral rim of the narial openings, which start above the posterior border of the fourth maxillary tooth and ends between the posterior border of the ninth tooth and the anterior border of the tenth; the external narial opening is five to six tooth positions in length. The posterodorsal process is located above the tenth tooth, where it projects dorsally and fits into a shallow groove on the anterior margin of the prefrontal, and contacts a small portion of the frontal; this process is subtriangular in shape, as in T. proriger (Russell 1967). The maxilla posteroventral point extends along the bottom of the orbit, medial to the jugal, for at least half of the length of the orbit. The contact between the right maxilla and the pre- frontal is crushed and badly preserved. Prefrontal. Both prefrontals are present in the specimen, Figure 4. Comparison of marginal teeth of species of Tylosau- rus. A, Tylosaurus saskatchewanensis, RSM P2588.1, in lateral although the right one is crushed at the contact with the aspect; B, Tylosaurus proriger, YPM 1302, in medial aspect; C, maxilla (Figs 2, 3A, 3B). The bone is trapezoidal in shape Tylosaurus bernardi, IRScNB 3672, in lateral aspect. Scale and slightly convex laterally, and bears a posterior process bars 2 cm. D that overlaps the postorbitofrontal above the orbit. 6 P. Jimenez-Huidobro et al.
Figure 5. Tylosaurus saskatchewanensis sp. nov., RSM P2588.1, holotype, vertebral morphology. A, atlas elements, including both left and right neural arches, atlas intercentrum, and atlas centrum or odontoid; B, anterior view of the axis; C, cervical vertebrae in anterior view; D, lateral view of dorsal vertebrae series in articulation; E, pygal vertebra in anterior view; F, intermedial caudal vertebra. Ana- tomical abbreviations: ac, atlas centrum (odontoides); ai, atlas intercentrum; ana, atlas neural arch. Scale bar 10 cm. D
However, in RSM P2588.1 the prefrontal is excluded from other species of Tylosaurus. Anterolaterally the frontal the dorsal rim of the orbit by the anterior process of the contacts the prefrontal, whereas posterolaterally it con- postorbitofrontal, unlike in any other species of Tylosau- tacts the postorbitofrontal. Posteriorly, the lateral and rus. Therefore, the contribution of the prefrontal is limited medial frontal alae are rounded in outline. to the anterior margin of the orbit; this contact surface Parietal. The parietal is incomplete and badly pre- appears smoothly rounded. Anteriorly and ventrally, the served, with the main fragment having been removed prefrontal contacts the maxilla, while dorsally it is in con- from the matrix during preparation (Fig. 3C). The bone is tact with the frontal, excluding the participation of the lat- dorsoventrally compressed, and anteroposteriorly elon- ter to the orbital rim. gated. The dorsal surface is well preserved, but the ventral Frontal. The frontal is a shield-shaped bone that is face is broken. A small fragment of the parietal is still nearly triangular and is longer than wide ( 304 mm in attached to the frontal, with the suture between the two length, and 197 mm in maximum width). Its posterior» por- bones being slightly visible, and the parietal foramen on tion is broad, while it becomes narrower anteriorly. In the parietal table close to the suture with frontal. The out- dorsal view (Fig. 2E, F), it is characterized by a well- line of the parietal table is straight instead of curved, a developed dorsal midline eminence that looks like a keel, character more pronounced in RSM P2588.1 and T. ber- even sharper than the one present in T. proriger; this emi- nardi than in T. nepaeolicus, and differing from T. pro- nence runs from the posterior half of the bone to the point riger where the outline of the parietal table is convex. The where the frontal overlaps the posterior end of the pre- suspensorial rami are dorsoventrally flattened, and diverge maxilla. The anterior processes extend up to half of the posterolaterally from the parietal table forming an angle length into the narial openings, a feature not observed in of about 70 with the longitudinal axis. Only a fragment A new species of tylosaurine mosasaur 7
Figure 6. Tylosaurus saskatchewanensis sp. nov., RSM P2588.1, holotype. A, right pectoral girdle consisting of scapula and coracoid; B, right forelimb in medial view, showing humerus, radius and ulna; C, right pelvic girdle with ischium, ilium and pubis; D, right hind limb in medial view, showing femur, tibia, fibula and astragalus. Anatomical abbreviations: as, astragalus; co, coracoid; fe, femur; fi, fib- ula; h, humerus; il, ilium; is, ischium; r, radius; sc, scapula; t, tibia. Scale bars 10 cm. D of the right suspensorial ramus is preserved, and the left reduced posteroventral process of the jugal that looks like ramus is only preserved at the base. The total length of the a short extension of the horizontal ramus, also known as parietal table is 236 mm. However, due to the incomplete- ‘striated tuberosity’ (cf. Bullard 2006), less developed ness of the suspensorial rami, it is not possible to take than that of T. proriger. measurements of the complete bone. Sclerotic ring. The sclerotic ring is only preserved on Postorbitofrontal. This bone has a rhomboid shape with the right side of the skull, visible in lateral view, and is two long processes, one extending anteriorly and the other composed of at least 12 ossicles; cf. the 14 ossicles posteriorly, and two short and broad processes, one described for the genus Tylosaurus (Yamashita et al. extending dorsally and the other ventrally (Fig. 3A, B). 2015). The overall shape of the sclerotic ring is nearly cir- The postorbitofrontal contributes to the posterodorsal cular, as it usually is in mosasauroids. The ossicles are margin of the orbit, which is rounded and smooth. The slightly convex and finely striated (Fig. 3A, B). long anterior process overlaps the posterior process of the Quadrate. Both quadrates are present: the right quadrate prefrontal, excluding the latter from the dorsal margin of is in articulation with the rest of the skull and is anteriorly the orbit, a feature unique to RSM P2588.1. Laterally, the inclined but not well preserved; the left quadrate is disar- long posterior process overlaps the squamosal; dorsally, ticulated and anteroposteriorly and dorsoventrally the dorsomedial process articulates with the frontal and crushed, also missing the suprastapedial process (Fig. 3D, parietal, whereas laterally, the ventrolateral process E). The quadrate is an ovoidal bone, lateromedially com- extends down to half of the orbit with a concave outer sur- pressed and longer than wide. The suprastapedial process face, to finally overlap the jugal; this is visible only on the in RSM P2588.1 is moderately long, extending down- right side of the skull. wards to the mid-length of the bone. The infrastapedial Jugal. Only the right jugal is preserved in the holotype process is located high on the quadrate shaft, about one- (Fig. 3A, B). This ‘L’- shaped bone contributes to the third of the height of the bone, while in other tylosaurines, posteroventral margin of the orbital notch. The ascending such as T. proriger and T. bernardi, it is usually lower on ramus is wide and laterally compressed, thicker than in T. the shaft, closer to the mandibular condyle (Fig. 3E). The proriger; it overlaps dorsally with the ventral process of tympanic ala is quite thick in comparison to that of T. pro- the postorbitofrontal. The horizontal ramus is thin in com- riger, and similar to that of T. nepaeolicus; it extends ven- parison to the ascending one and the process articulates trally to almost contact the mandibular condyle. The anteriorly with the maxilla. The angle between the vertical tympanic rim is thin and sharp. Unfortunately, the stape- ascending and horizontal rami is about 95, similar to that dial notch is not recognizable, as the right quadrate is par- in other species of the genus Tylosaurus. There is a very tially covered by sediment, and probably a calcified 8 P. Jimenez-Huidobro et al. extracolumella, and the left one is missing. The stapedial section, and higher than wide. Posteriorly, the articulation pit is visible in the left quadrate, rectangular in shape and surface with the angular is slightly concave, similar to elongated dorsoventrally, as is typical in the genus Tylo- that of T. proriger. Dorsally, it articulates with the den- saurus. The dorsal rim is saddle shaped, and characterized tary, and contributes to the intramandibular joint by the presence of a crest similar to that of T. proriger. (Fig. 2A–D). No significant differences were found with The posterodorsal surface of the suprastapedial process of T. proriger. the quadrate (cephalic condyle) articulates with the ven- Angular. Both angulars are preserved, although the tral face of the squamosal. The mandibular condyle is con- right one is posteriorly broken. The right angular is vex and contacts the concave glenoid formed mostly by 316 mm in length, and the left one is not well enough pre- the surangular. The right quadrate is 183 mm high, while served to take measurements. The anterior portion of the the left bone is 181 mm. bone is elliptical in cross section, being higher than wide, Squamosal. No significant differences were recovered while posteriorly it narrows and is mediolaterally flat- for this bone between RSM P2588.1 and T. proriger. The tened. Anteriorly, the articulation surface with the splenial right squamosal is complete in RSM P2588.1, while the is slightly convex. It articulates dorsally with the surangu- left one is fragmented, missing the posterior portion lar and posteriorly with the anterior border of (Fig. 2). The bone curves downwards along an anteropos- the articular. It also contributes to the intramandibular terior axis, and the total length is 257 mm. Anteriorly, the joint (Fig. 2A–D). It does not show differences from bone overlaps the postorbitofrontal on a long articular sur- T. proriger. face. The posterior portion of the bone is nearly ovoidal in Surangular. Both surangulars are preserved in the speci- shape, with a convex dorsal surface and a concave ventral men. The right surangular is well preserved, while the left surface. The posteroventral surface of the squamosal artic- one is incomplete and crushed (Fig. 2A–D). The surangu- ulates ventrally with the quadrate, medially with the sus- lar is an elongated bone, mediolaterally flattened, and pensorial ramus of the parietal, and with a small portion articulates anterodorsally with the coronoid, posteriorly of the supratemporal, although the latter is barely visible. with the articular, ventrally with the angular, and anteri- Marginal dentition. The marginal dentition is moder- orly with the posterior margin of the dentary. The coro- ately well preserved. All the teeth are relatively large and noid buttress is low and thick with a dorsal border slightly robust, labiolingually compressed, finely striated, and rounded, as in T. proriger. The right surangular is gently curved posteriorly, as in T. proriger (Fig. 4). The 438 mm in length, whereas the left one is 391 mm long, fifth, sixth and seventh maxillary teeth preserved on the although incomplete and pushed forward underneath the right maxilla were measured from the base of the crown left dentary postmortem. Dorsoposteriorly, the postero- to the apex: their lengths are 54, 55 and 54 mm, respec- dorsal portion of the surangular contributes to most of the tively. The first four maxillary teeth have only the anterior glenoid fossa, which articulates with the mandibular con- carina, while on the fifth tooth both anterior and posterior dyle of the quadrate. The glenoid fossa appears strongly carinae are present. In the posterior portion of the dentary, concave. all the teeth are bicarinate. The carinae of the dentary and Articular. This is the most posterior bone of the lower maxillary teeth are serrated, with small and slender den- jaw. The right articular is present, moderately well pre- ticles that contribute to the cutting effectiveness of the served and in articulation with the rest of the skull, teeth. The interdental space on both maxillae and the den- whereas the left articular is badly preserved in the matrix. taries is quite tight, being about 9 mm between each tooth. The articular looks nearly rectangular in shape. Anteri- Dentary. Both left and right dentaries are present, and orly, it contacts with a small portion of the angular; ante- moderately well preserved (Fig. 2A–D). Posteroventrally, rodorsally it articulates with the surangular, and dorsally the dentary contacts the splenial, and posteriorly the sur- with the quadrate. The articular looks horizontal in this angular. Both left and right dentaries have 13 tooth posi- specimen, following the same horizontal axis as the suran- tions. In lateral view, the anterior tip of the bone is gular. This feature differs from the condition in T. pro- rectangular in outline, with a projection that extends ante- riger where the articular follows a curve and tilts riorly to the first tooth; the same process is also present in downwards at the posterior end of the bone; however, it T. proriger and all other species of Tylosaurus. The left resembles the articular of T. bernardi. Dorsoanteriorly, predental process is 64 mm long, while the right process the articular contributes to the most posterior portion of is 61 mm before the first tooth. The total length of the left the glenoid fossa to articulate with the quadrate. Posteri- dentary is 818 mm, and the right dentary is 766 mm, orly, the retroarticular process twists about 90, and it pos- though the latter is posteriorly fragmented. sesses a present but poorly preserved foramen. Splenial. Both splenials are preserved. The left splenial is 450 mm in length, while the right bone is 426 mm. The Axial skeleton. The atlas complex is completely pre- anterior portion of this bone is mediolaterally flattened served (Fig. 5A): left and right neural arches, atlas inter- and becomes thicker posteriorly, elliptical in cross centrum and atlas centrum (i.e. odontoid). Ventrally, the A new species of tylosaurine mosasaur 9 two neural arches contact the atlas intercentrum, forming inclined downwards thus creating a large space between a ring where the centrum contacts the arches at the mid- the neural arch and each transverse process, presumably point. The spinous processes of the atlas neural arches are for epaxial muscle attachments. The transverse processes separated, and never in contact with each other. The lat- do not have a surface for rib attachment, so they are elon- eral processes of the neural arch appear like a small protu- gated, and thin at the tips. The neural spines are long, berance, nearly rectangular in shape. Posteriorly, the anteroposteriorly wide and posteriorly inclined. The ven- surface to articulate with the atlas centrum is concave. tral face of the centrum is smooth and convex. The atlas intercentrum is prism shaped and is wider than Posterior to the pygals, there are 18 intermediate caudal tall; this bone is dorsally concave to receive the ventral vertebrae preserved (Fig. 5F); the complete number is not surface of the centrum. The atlas centrum, or odontoid, known. The centra of the intermedials range from triangu- has a dorsoventrally compressed cone shape (cf. Russell lar to ovoid in shape, and are taller than wide, opposite to 1967), and is wider than tall. The anterior surface of the the shape of the cervicals. The transverse processes are centrum, or cotyle, is concave to articulate with the occip- still facing downwards. The dorsal face of the centra hosts ital condyle, while the posterior face (or condyle) is the neural spines, posteriorly inclined, while the ventral convex. face exposes the articulation surface for haemal arches, The axis attaches posteriorly to the atlas and is a robust although there are no haemal arches preserved. vertebra (Fig. 5B). The centrum of the axis is rounded, The last vertebrae in the series are the terminal caudals. almost circular. Laterally, two transverse processes Only 13 terminals were found, although the posterior por- extend, both are short, wide and extend horizontally, to tion of the tail is missing. The size of these vertebrae occupy the whole lateral face of the centrum. The neural decreases posteriorly. Condyles and cotyles are ovoid, arches are fused to the centrum anteroposteriorly elon- and are taller than wide. Terminals have posteriorly gated and lateromedially flattened. Ventrally, there is a inclined neural spines as well. The ventral face has an tubercle pointing posteriorly, the hypapophysis, that con- articulation face for the haemal arches, though none are tacts the third cervical vertebra. preserved, and they all lack transverse processes. There are likely seven cervicals though the ribs are lost and it is not possible to determine exactly which vertebra Appendicular skeleton. Scapula. Both left and right in the axial column possessed a rib articulating with the scapulae are quite well preserved and are about half of the pectoral girdle. All of the cervicals are robust and moder- size of coracoid, a character common to the genus Tylo- ately well preserved and have an ovoid centrum that is saurus. The lateral face of the scapula is slightly convex, wider than tall (Fig. 5C). Transverse process arises from while the medial face is flat (Fig. 6A). The posterior emar- the lateral face of the centrum and extends upwards. The gination of the scapula is gently concave. The scapula neural arch is almost complete in all cervicals, although width is about 1.3 times the length: the left scapula is they are laterally deflected to the left side. The hypapoph- 90 mm in length and 115 mm in width, while the right one yses are all preserved, though the peduncles/intercentra is 88 mm long and 118 mm wide. The suture between are missing. Pre- and postzygaphophyses are only pre- scapula and coracoid does not show any interdigitation. served in a few of the cervicals, and zygosphenes and zyg- The lateral and medial faces near the margin of the scap- antra are absent, as in other Tylosaurus (Russell 1967). ula are covered by radial striations. The ventral edge bears Dorsal vertebrae are located posterior to the cervicals in a pair of roughened oval articulations, a posterior glenoid the vertebral series. There are 39 dorsal vertebrae (i.e. facet and a ventromedial coracoid facet, separated from from presumed last cervical to first pygal). The anterior each other by a slight mediolateral constriction. The cora- dorsals have an ovoid centrum shape and are wider than coid facet is continuous with a roughened and unfinished tall; more posterior dorsals become more rounded, and margin along the anterior and superior borders, represent- finally nearly triangular, with a wide ventral surface. The ing the point of attachment of a large cartilaginous supra- anterior dorsals have transverse processes that face scapula (Osborn 1899). The circumference of the bone is upwards, arising from the lateral face of the centrum; finished by a smooth, flexed posterior margin that lies these processes become more horizontal, and posteriorly nearly vertical above the glenoid facet. From the anterior they incline downwards with a robust surface for rib tip of the coracoid facet, this rough margin arcs backward attachment (Fig. 5D). The ventral faces of those vertebrae about 180 about the centre of the bone, terminating are smooth and slightly convex, and the dorsal face hosts above the glenoid facet. The very same phylogenetic trend the long and wide posteriorly inclined neural spines. of antero-posterior elongation of a scapula is found in Posterior to the dorsal vertebrae, nine or 10 pygals are Plioplatecarpinae, where the scapular blade evolved to found (Fig. 5E). These vertebrae correspond to the base of become longer from Plesioplatecarpus planifrons to Pla- the tail. They are robust and large, with a centrum shape tecarpus tympaniticus to Latoplatecarpus spp. to Pliopla- that is nearly triangular. The transverse processes arise tecarpus spp (Konishi & Caldwell 2011). While the bony ventrally from the lateral face of the centra and are component of Tylosaurus scapulae is relatively small, 10 P. Jimenez-Huidobro et al. when the cartilaginous suprascapula is included, the shape humeral shaft is compressed, similar to the humerus of T. difference (i.e. 120 vs. 180) of the bony scapular blade proriger. would make a significant overall ( bone cartilage) Radius. Both left and right radii are present. The right morphological difference, which wouldD certainlyC have radius is well preserved, while the left is crushed and bro- had a functional significance given the increased surface ken from the middle of the bone to the distal end. The area for the muscle groups bracing the scapulae against radius is slender and thin, and spreads out anterodistally the trunk; compare the horizontally elongate, semicircular into a wide fan (cf. Russell 1967). The left and right radii scapulae of aquatic mammals today, such as whales and are 135 mm and 137 mm in length, respectively. The ecte- sea lions, with those of terrestrial mammals that typically picondylar condyle is dorsoventrally thicker than the dis- exhibit vertically elongate scapular morphology, as in tal condyle. The shaft constricts at the midpoint of the pigs and camels (Gingerich et al. 2001; T. Konishi pers. bone (Fig. 6B). comm.). Ulna. Both ulnae are complete and well preserved. The Coracoid. Both coracoids are present and moderately ulna is a slender bone, lateromedially flattened and elon- well preserved. The coracoidisfan-shaped,dorsome- gated (Fig. 6B). The proximal and distal condyles are dially flattened, and much larger than the scapula, about slightly anteroposteriorly expanded. Laterally, the ulna is twice the size of the latter (Fig. 6A). Anteriorly, the cora- flat and smooth. Anteroproximal on the medial face, the coid possesses a foramen, which is the insertion channel ulna presents a moderately well-developed olecranon pro- for the nerve of the m. supracoracoideus (Russell 1967). cess, where the M. triceps muscle inserts. The left ulna is The coracoid forms about half of the glenoid fossa, along 124 mm long while the right is 122 mm long. with the scapula. The glenoid fossa in the coracoid is Ilium. Both ilia are well preserved: the left ilium is slightly concave and presents a roughened surface, con- 301 mm in length, whereas the right one is 299 mm trary to what Russell (1967)describedforthegenusTylo- long, measured from the tip of the anterodorsal (or supra- saurus.Thelateralfaceofthisboneisalsogently cetabular) iliac process to the post-acetabular region. The concave. The medial margin of the coracoid looks con- ilium is characterized by a slender and elongated anterior vex, while the anterior border is slightly concave, and the supracetabular process, anterodorsally oriented, as in typi- posterior margin is stronglyconcave.Thereisaslight cal hydropelvic mosasaurs (cf. Caldwell & Palci 2007) notch along the medial edge, but not an emargination (a (Fig. 6C). The anterodorsal process is mediolaterally com- feature visible in some Tylosaurus specimens (Osborn pressed for most of its length, while more proximally, 1899)). The left coracoid is 176 mm in length and close to the iliac shaft, its cross section becomes more 177 mm in width, whereas the right coracoid is 179 mm rounded. The acetabular facet on the iliac shaft for articu- long and 180 mm wide. lation of the femur appears slightly convex and quite Humerus. Both left and right humeri are moderately roughened, suggesting the attachment of soft tissue. Both well preserved. The right humerus is complete, whereas the pubic and ischial facets on the ilium are quite straight the left one is missing the posterior margin of both the gle- in lateral/medial view, and about equal in length. noid and distal condyles. The humeri are slender, elon- Pubis. Both pubes are present and complete. The left gated and longer than the maximum width, similar to the pubis is 216 mm in length and the right is 209 mm long. humerus of T. proriger. However, they differ in the The dorsal head is characterized by a gently convex artic- amount of constriction-expansion along the humeral shaft: ular surface that bears a small facet for articulation with the proximal and distal ends are relatively more expanded the ilium, and a posteriorly oriented facet to articulate and their articular surfaces are more strongly developed with the ischium (Fig. 6C). The proximal head of the and flexed, creating arched anterior and posterior margins pubis is anteroposteriorly expanded while the rest of the in medial view (Fig. 6B). The right humerus is 188 mm shaft is basically rod-like and broadly ovoid in cross sec- long, while the left is 197 mm in length. Medially, the tion. The distal end is slightly convex and about as large humerus bears a pectoral crest that extends down half the as the mid-shaft, with no terminal expansion, similar to length of the bone. Laterally, each humerus presents a dis- that of some mosasaurines. There is no evidence of a tal foramen, or ectepicondylar groove, that hosts the ecte- pubic process along the anterior margin of the shaft like in picondylar nerve. The glenoid condyle is gently convex. other Tylosaurus species. The obturator foramen is pres- Anterior to the glenoid condyle the humerus presents an ent and visible on the anterolateral surface of the bone, anterior tuberosity, and posterior to the condyle a poorly located very high on its proximal head. developed postglenoid process is preserved on the right Ischium. Both ischia are present and well preserved. humerus. The radial facet/condyle is rather thick latero- The left ischium is 173 mm in length, while the right medially (Fig. 6B). The distal condyle is lateromedially ischium is 168 mm long. The proximal head of the compressed and is divided into two condyles, one for the ischium bears two distinct facets that form a continuous radius and the other for the ulna. The midlength of the roughened articulation surface: a posterior one for articu- lation with the ilium, and an anterior one to articulate with A new species of tylosaurine mosasaur 11 the pubis (cf. Russell 1967). The iliac facet is large and 112 mm and the right fibula is 110 mm in length. No sig- slightly convex, while the pubic facet is quite small and nificant differences from that of T. proriger were found. convex. The distal end of the bone becomes dorsoven- Autopodium. The well-preserved astragalus is medio- trally flattened and anteroposteriorly expanded. The laterally flattened and discoidal in shape, and with a proxi- ischiadic tubercle is present as a wide triangular projec- mal emargination for the crural foramen facing the tibia, tion facing posteriorly and arising from above the mid- different to the astragalus without emargination of T. pro- length of the shaft. The anterior margin is deeply concave, riger, described by Russell (1967). The crural emargina- while the posterodorsal one is straight (Fig. 6C). tion is quite regular in shape, with a semicircular outline, Femur. Both femora are present and both are frag- making the crural space large (Fig. 6D). The diameter of mented and glued; the right femur is slightly twisted. The this bone is 53 mm. Some phalanges are also preserved. femur is a slender and elongated bone, approximately They are elongated and cylindrical elements that are twice as broad distally as it is proximally (Fig. 6D). The mediolaterally compressed. They are slightly expanded femur presents a trochanter at the anteroproximal surface anteroposteriorly, at the proximal and distal margins. of the medial face; this trochanter looks similar to the crest that extends downwards to a point just above the mid-length of the shaft. Proximally, the condylar head is thick and slightly convex. Distally, the bone becomes Phylogenetic analysis dorsoventrally convex, and spreads to articulate with the carpals. The left and right femora are 201 mm and In order to assess the phylogenetic position of Tylosaurus 193 mm in length, respectively. The femur is only slightly saskatchewanensis, a traditional search (tree bisection and longer than the humerus, similar to Prognathodon (Rus- reconnection [TBR] algorithm) was performed. A total of sell 1967) and unlike T. proriger and T. bernardi, in which 26 most parsimonious trees (MPT) were retained, with a the femur is considerably longer than the humerus (Rus- consistency index (CI) of 0.37 and retention index (RI) of sell 1967; Jimenez-Huidobro & Caldwell 2016). 0.72, and with a tree length of 447 steps. The resulting Tibia. Both tibiae are present; the left tibia is well pre- strict consensus tree is shown in Figure 7, including served, but the right one is fragmented and reconstructed at Bremer support indices and bootstrap values > 50%. the midpoint of the shaft. This bone is slightly longer than Tylosaurus saskatchewanensis appears nested within the clade Tylosaurinae (Bremer 6; bootstrap 96%), as wide, considerably broader anteroposteriorly than the fib- D D ula. The proximal articular surface is an anteroposteriorly sister group of the late Coniacian–early Santonian mosa- saur T. nepaeolicus (weakly supported by Bremer 1), elongate oval, slightly concave and irregularly roughened. D The shaft is constricted at its midpoint, slightly dorsoven- and both emerge as the sister group of the clade T. pro- riger T. bernardi (Bremer 2; bootstrap 67%). The trally but strongly anteroposteriorly, giving it an hourglass C D D profile (Fig. 6D). The posterior margin of the shaft, with a convex border with the crural foramen, is broadly rounded, while the anterior margin thins uniformly into an elongate flange on the distal end of the shaft. Anteriorly, the margin of the tibia is convex in proximal and distal sections, and concave at the middle. This anterodistal flange has a rough- ened margin, which merges seamlessly into the distal artic- ular surface. The distal articular surface is roughly diamond shaped. Its irregular surface is strongly arched anteroposteriorly, with a strong posterior deflection for the astragalar facet. The left and right tibiae are 132 mm and 136 mm in length, respectively. Fibula. Both fibulae are preserved in good condition. The fibula is a slender and elongated element, with a con- striction at the mid shaft, and is slightly expanded at the proximal and distal ends. Proximally, this bone has a slightly flat articulation surface to contact the femur, oval in shape, and lateromedially thicker than the distal mar- Figure 7. Strict consensus tree (CI 0.37; RI 0.72) of 26 D D gin. Distally, the fibula is thicker than the mid-shaft and most parsimonious trees of 447 steps, for the data matrix of 41 convex at the border. Dorsoventrally, the distal end of the taxa and 129 characters. Bremer support values of greater than 1 and bootstrap support values greater than 50% are reported bone is thinner than the proximal, with a very elongated below the branches (Bremer/bootstrap). The box highlights Tylo- oval shape (Fig. 6D). The posterior border of the shaft is saurus saskatchewanensis and its relationship with the other spe- more convex than the anterior border. The left fibula is cies of Tylosaurus. 12 P. Jimenez-Huidobro et al. clade Tylosaurus is weakly supported (Bremer 1). The T. nepaeolicus. For the European T. bernardi, Lingham- relationship between species of TaniwhasaurusDimproves Soliar (1992) described > 49 precaudals, and Lindgren with a Bremer value of 4, and bootstrap values 88%. (2005) counted 47, while Jimenez-Huidobro & Caldwell There is a clear separation of the group TylosaurinaeD (2016) noted > 39 vertebrae anterior to the chevron-bear- (bootstrap value 96%, Bremer value 6), related to the ing caudals. However, the holotype of T. saskatchewanen- other mosasauroid clades. The clade Plioplatecarpinae sis is a larger specimen with a higher vertebral count, of appears differentially separated by a Bremer value of 3. 55 vertebrae anterior to the chevron-bearing caudals. The The clade Tethysaurinae Yaguarasaurinae is well femur of T. saskatchewanensis is slightly longer than the defined, supported by BremerC 2, the subfamily Mosa- humerus, differing from T. proriger and T. bernardi, in saurinae is supported by BremerD 3, and the subfamily which the humerus and femur are virtually the same Halisaurinae is supported by a bootstrapD value of 85%. length. The astragalus possesses a semicircular emargina- When the synapomorphies are mapped onto the clado- tion on the distal face, contributing to the crural space. gram, the characters that define T. saskatchewanensis This emargination is quite large in comparison to the include: character 69[1], articular retroarticular process astragalus of other tylosaurines, such as Tylosaurus pro- with extreme inflection, almost 90; character 88[1], con- riger (Russell 1967; Caldwell 1996), and different from dyle of anteriormost trunk vertebrae essentially equidi- other mosasauroids, where the astragalus does not show a mensional. The clade T. saskatchewanensis T. large emargination (Caldwell 1996). nepaeolicus is defined by character 45[1], quadrate poster-C One character not accounted for in the data matrix is the oventral ascending tympanic rim high, with an elongate elongated anterior process of the postorbitofrontal, which triangular crest; and character 46[1], quadrate tympanic excludes the prefrontal from the dorsal margin of the ala thick; both characters have been described for adults orbit. In all other tylosaurines, both the prefrontal and of T. nepaeolicus (Jimenez-Huidobro et al. 2016). The postorbitofrontal contribute to the dorsal edge of the orbit, genus Tylosaurus is defined by character 91[1], cervical overlapping at about the midpoint of the dorsal margin, a vertebrae almost equal in length to the other vertebrae. character that was described by Russell (1967). Another The clade Tylosaurinae is the sister group of Plioplatecar- unique character of Tylosaurus saskatchewanensis is the pinae, in accordance with previous phylogenies (e.g. LeB- greater contribution of the frontal to the narial opening in lanc et al. 2012; Palci et al. 2013). comparison to other tylosaurines: in T. proriger and T. bernardi the frontal contributes to a third, or less, of the posteromedial border of the narial openings (Russell Discussion 1967; Jimenez-Huidobro & Caldwell 2016), while in T. saskatchewanensis the anterior process of the frontal Phylogenetic relationships of the new taxon extends half the length of the narial openings, and the The type specimen of Tylosaurus saskatchewanensis, internarial bar of the premaxilla extends far posteriorly, RSM P2588.1, represents a large tylosaurine mosasaur overlapping the frontal midline. This character is, how- with a skull length of 132 cm. The specimen is character- ever, unknown in Taniwhasaurus oweni and T. antarcti- ized by a unique suite of features that distinguish it from cus. In the phylogeny, Tylosaurus saskatchewanensis is all the other species of Tylosaurus. Usually, the cervical the sister group of T. nepaeolicus, based on the morphol- vertebrae are somewhat dorsoventrally compressed in ogy of the quadrate (i.e. the similar thickness of the tym- tylosaurines; however, in T. saskatchewanensis the con- panic ala, and the tall quadrate posteroventral ascending dyle/cotyle of the cervicals are quite rounded, essentially tympanic rim). The length of the suprastapedial process of equidimensional. The articular in tylosaurines is usually the quadrate is similar to that of T. proriger and T. slightly laterally turned, as in T. proriger and T. nepaeoli- nepaeolicus. However, the infrastapedial process is cus; however, in T. saskatchewanensis the bone is located higher up on the quadrate shaft than in the other extremely turned laterally, from vertical to horizontal, North American tylosaurine mosasaurs, where the process about 90 compared to the axis of the surangular. The is closer to the mandibular condyle, near to the ventral frontal of T. saskatchewanensis has a very well-developed end of the suprastapedial process, and forms a small stape- dorsal midline eminence, sharper than in T. proriger and dial notch, although the two processes do not contact each T. bernardi, and more similar to that of Taniwhasaurus other, as in Platecarpus (Russell 1967). oweni and T. antarcticus (Novas et al. 2002; Caldwell et al. 2005; Martin & Fernandez 2007); the frontal dorsal midline eminence is absent in Tylosaurus nepaeolicus. Pelvic girdle of Tylosaurus Russell (1967) described the vertebral formula of Tylo- The excellent three-dimensional preservation of the saurus as 36–37 precaudals (cervicals dorsals appendicular bones of specimen RSM P2588.1 allow us pygals), probably based only on T. prorigerC as there areC to make some considerations about the modification of no known complete vertebral series for any specimens of the pelvic bones in Tylosaurus in comparison to other A new species of tylosaurine mosasaur 13
Figure 8. Comparison between the pelvic bones in lateral view of RSM P2588.1 (A, C, E) and Mosasaurus hoffmannii (B, D, F). A, ilium of RSM P2588.1; B, ilium of Mosasaurus hoffmannii; C, pubis of RSM P2588.1; D, pubis of Mosasaurus hoffmannii; E, ischium of RSM P2588.1; F, ischium of Mosasaurus hoffmannii. Photographs of Mosasaurus hoffmannii courtesy of H. Street. Anatomical abbreviations: appr, anterior pubic process; pispr, posterior ischial process; sipr, supracetabular iliac process. Scale bar 5 cm. D 14 P. Jimenez-Huidobro et al. mosasauroids (Fig. 8). As compared to mosasaurines, both Park in Alberta, Canada (Caldwell et al. 2005). Although the ilium and pubis in Tylosaurus species seem to lose the new tylosaurine taxon is not the youngest species of most of the bony processes that usually represent attach- Tylosaurus, as T. bernardi has been recognized from the ment surfaces for muscle ligaments (e.g. Snyder 1954). early Maastrichtian of Belgium (Dollo 1885), T. saskatch- The tip of the supracetabular process of the ilium in Tylo- ewanensis extends the stratigraphical range of the genus saurus is rounded and does not seem to bear an articular in the Western Interior Seaway. facet, while in Mosasaurus (e.g. M. conodon and M. hoff- mannii) the distal end of the process is sub-rectangular in shape with a small, dorsoventrally oriented facet in the Conclusions anterolateral edge (Fig. 8A, B). This facet may serve as a surface for ligament attachment, and as we know for other The tylosaurine specimen RSM P2588.1 from the upper lizards, there is an iliopubic ligament that usually runs Campanian of the Bearpaw Formation, southern Saskatch- from the supracetabular process of the ilium to the ante- ewan, presents a unique set of characters that allows rior process of the pubis (see Snyder 1954). In Mosasau- assignment to a new species: Tylosaurus saskatchewanen- rus the pubis has a well-developed process on its anterior sis. The new taxon represents a large tylosaurine mosasaur margin, while in Tylosaurus it seems that the anterior mar- characterized by: exclusion of the prefrontal from the dor- gin of the pubis is straight and does not have any projec- sal rim of the orbit by a long anterior process of the post- tions (Fig. 8C, D). The fact that in Mosasaurus both orbitofrontal; the frontal extending anteriorly well into the attachment surfaces for the iliopubic ligament are pres- narial openings; the dorsal midline eminence of the frontal ent – on the tip of the anterodorsal iliac process, and on being well developed; the parietal table straight in shape; the anterior pubic process – while in Tylosaurus they the tympanic ala thick; 55 vertebrae anterior to the chev- seem to be lost likely implies a major difference in the ron-bearing caudals; and a rounded astragalus with a large locomotion abilities between tylosaurines and mosasaur- semicircular crural emargination. ines within Mosasauridae. In Tylosaurus, the pelvic girdle In terms of phylogenetic relationships, T. saskatchewa- is not only floating in soft tissues that remain as the only nensis is nested within the monophyletic genus Tylosaurus, connection between the appendicular and axial skeletons, as the sister group of T. nepaeolicus. In comparison with all but has also lost flexibility between its elements, as if the other tylosaurines, the two species share the presence of a movement of the posterior appendages becomes less cru- thick tympanic ala, and a high posteroventral ascending cial for the swimming style of these forms. A less flexible tympanic rim of the quadrate. The clade formed by T. sas- and more stiffened appendage (girdle limb), showing katchewanensis and T. nepaeolicus appears to be the sister simplification of the proximal elementsC (in our case the group of the clade including T. proriger and T. bernardi. pelvic elements missing bony projections or facets for The Tylosaurus group is the sister group of the monophy- muscles/ligaments attachment) suggests that the fins no letic genus Taniwhasaurus, forming the clade Tylosaurinae. longer represent an active locomotor organ, and only The stratigraphical range for the genus Tylosaurus in function as stabilizers during swimming or at most for North America is now from the upper Turonian of Chi- propulsion at low speeds (Webb & Blake 1985; Buchholtz huahua, Mexico (Loera-Flores 2013) to the upper Campa- 2001). From our observations on the morphology of the nian of the Pierre Shale of South Dakota and Manitoba pelvic bones, this tendency to reduction and simplification (Nicholls 1988; Martin 2007; Bullard & Caldwell 2010). is more advanced in Tylosaurus than in Mosasaurus. Tylosaurus bernardi (formerly Hainosaurus bernardi Dollo, 1885) from the Ciply Phosphatic Chalk of Belgium Temporal and geographical distribution of extends the temporal range of Tylosaurus to the early Maastrichtian, showing a North Atlantic Circle Basin pat- Tylosaurus tern of distribution for the genus (Jimenez-Huidobro & The Campanian is the longest age of the Late Cretaceous, Caldwell 2016). from 70.6 to 83.5 Ma (Gradstein et al. 2004). The new species T. saskatchewanensis represents the latest North American tylosaurine that occupied the Western Interior Seaway during the late Campanian; indeed, before its dis- Acknowledgements covery, the genus Tylosaurus in North America was known only until the middle Campanian (Nicholls 1988; The authors thank T. Tokaryk from the T. rex Discovery Martin 2007; Bullard & Caldwell 2010). The new species Centre for providing access to the specimen. We thank T. described here represents the most northern occurrence of Konishi for his valuable comments and critical reviews. the genus and the subfamily. The previous published We also thank H. Street for providing photographs of record holder is TMP 1983.126.1 from the Bearpaw mosasaur material. This research was funded by the fol- Fmormation of Iddesleigh, east of the Dinosaur Provincial lowing grants: NSERC Discovery Grant (#23458), A new species of tylosaurine mosasaur 15
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