A New Species of Tylosaurine Mosasaur from the Upper Campanian Bearpaw Formation of Saskatchewan, Canada Paulina JimEnez-Huidobroa*, Michael W
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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