A new high-latitude Tylosaurus (Squamata, Mosasauridae) from Canada with unique

dentition

A thesis submitted to the

Graduate School

of the University of Cincinnati

in partial fulfillment of the

requirements for the degree of

Master of Science

in the Department of Biological Sciences

of the College of Arts and Sciences

by

Samuel T. Garvey

B.S. University of Cincinnati

B.S. Indiana University

March 2020

Committee Chair: B. C. Jayne, Ph.D. ABSTRACT

Mosasaurs were large aquatic lizards, typically 5 m or more in length, that lived during

the Late Cretaceous (ca. 100–66 Ma). Of the six subfamilies and more than 70 species

recognized today, most were hydropedal (flipper-bearing). Mosasaurs were cosmopolitan apex predators, and their remains occur on every continent, including Antarctica. In North America, mosasaurs flourished in the Western Interior Seaway, an inland sea that covered a large swath of the continent between the Gulf of Mexico and the Arctic Ocean during much of the Late

Cretaceous. The challenges of paleontological fieldwork in high latitudes in the Northern

Hemisphere have biased mosasaur collections such that most mosasaur fossils are found within

0°–60°N paleolatitude, and in North America plioplatecarpine mosasaurs are the only mosasaurs yet confirmed to have existed in paleolatitudes higher than 60°N. However, this does not mean mosasaur fossils are necessarily lacking at such latitudes. Herein, I report on the northernmost

occurrence of a tylosaurine mosasaur from near Grande Prairie in Alberta, Canada (ca. 86.6–79.6

Ma). Recovered from about 62°N paleolatitude, this material (TMP 2014.011.0001) is assignable

to the subfamily Tylosaurinae by exhibiting a cylindrical rostrum, broadly parallel-sided

premaxillo-maxillary sutures, and overall homodonty. I further refer this material to Tylosaurus

based on the lack of a dorsal midsagittal ridge on the premaxilla. Unexpectedly, TMP

2014.011.0001 exhibits widely spaced, high-aspect-ratio marginal tooth crowns and low-profile

maxillae, similar to the typical juvenile condition of Tylosaurus, despite its likely adult age based

on an estimated body length of at least 6.5 m. The specimen also exhibits anterior maxillary

tooth roots covered by downward extensions of the maxillary cortical bone, a feature previously

unknown in Tylosaurinae. TMP 2014.011.0001 hints at an undiscovered, temporally more stable

ii Tylosaurus diversity in the northern latitudes of the seaway throughout the Late Cretaceous, possibly even into the latest Cretaceous and inclusive of the Arctic Circle. Analogous dental morphologies in other non-mosasaurid taxa, as well as a standard model of tooth function based on tooth morphology, indicate TMP 2014.011.0001 may have been specially adapted for piscivory. This study suggests the possible presence of a Cretaceous boreal marine community that was distinct from those across the more southern stretches of the Western Interior Seaway, in the western and southern United States.

iii iv ACKNOWLEDGEMENTS

I must first thank Dr. T. Konishi. Although not technically my committee chair, he was

my supervisor for this undertaking, and has proved an excellent guide, in both my research and

my teaching. I also wish to thank Drs. B. C. Jayne and E. J. Tepe for their thoughtful and incisive

advice, feedback, and support on my committee, with particular gratitude to Dr. B. C. Jayne for serving as my committee chair. Thanks as well to J. Jakielaszek and B. Neuman, who discovered

TMP 2014.011.0001, A. McGee at the Royal Tyrrell Museum of Palaeontology, who prepared the specimen, and B. Strilisky and Dr. D. Brinkman, of the same institution, who loaned us the specimen. The following individuals and institutions facilitated my data collection: M. Currie at the Canadian Museum of Nature, Dr. G. Storrs at the Cincinnati Museum Center, W. Simpson at the Field Museum of Natural History, C. Byrd and Dr. L. Wilson at the Sternberg Museum of

Natural History, and Dr. D. Burnham at the University of Kansas Natural History Museum. This entire endeavor was possible thanks to the scholarship and teaching assistantship offered to me by the University of Cincinnati Department of Biological Sciences. I also received further financial support from the Department, including a Wieman-Benedict Research Award, and from several Conference Travel Awards from the University of Cincinnati Graduate Student

Governance Association and a Jackson School of Geosciences Student Member Travel Grant from the Society of Vertebrate Paleontology. The photos used in Figure 11C and Figure 14A and

C were taken by Dr. T. Konishi while funded by a Natural Sciences and Engineering Research

Council of Canada Discovery Grant (no. 238458-01) to Dr. M. Caldwell. I would additionally like to thank R. Russell, R. Sanchez, and B. Strilisky, as well as Drs. J. Carter, D. Coleman, M.

Day, Z. Johanson, H. Street, and D. Surge for providing information and/or photographs

v pertaining to particular specimens; R. Carr and Drs. D. D’Amore, J. Massare, and N. Morehouse for helpful discussions; D. Foffa for kindly sharing his data; and Drs. M. Friedman and S. Smith for the use of their micro-CT equipment. Finally, I would like to thank my wife, Hannah, and our family and friends for their continued love and support over the course of my graduate studies and throughout my life.

vi TABLE OF CONTENTS

Abstract ...... ii

Acknowledgements ...... v

List of Figures ...... ix

Introduction ...... 1

Institutional Abbreviations...... 3

Anatomical Abbreviations ...... 3

Geological Setting ...... 3

Materials and Methods ...... 5

Systematic Paleontology ...... 6

Description ...... 9

Premaxilla ...... 9

Maxilla ...... 11

Vomer ...... 14

Septomaxilla ...... 16

Mandible ...... 19

Marginal Dentition ...... 20

Intrageneric Comparison ...... 23

Marginal Dentition ...... 23

Maxillary Depth ...... 26

Discussion ...... 27

Diagnosis...... 27

vii Osteology ...... 31

Septomaxilla ...... 31

Downward Maxillary Extensions ...... 33

Ontogenetic Changes in Dentition ...... 33

Paedomorphosis ...... 35

Paleobiogeography ...... 36

Northernmost Tylosaurus ...... 36

Tylosaurus Distribution in Western Interior Seaway ...... 41

Food Habits and Paleoecology...... 44

Conclusions ...... 47

Literature Cited ...... 49

Figures...... 71

Appendices ...... 115

Appendix 1 ...... 115

Appendix 2 ...... 117

Appendix 3 ...... 121

Appendix 4 ...... 124

viii LIST OF FIGURES

Figure 1. Phylogenetic hypotheses of Squamata and Mosasauridae ...... 71

Figure 2. Locality and horizon of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp.

nov...... 73

Figure 3. Skull of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., in dorsal

view ...... 75

Figure 4. Skull of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., in left

dorsolateral view ...... 77

Figure 5. Skull of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., in left lateral view ...... 79

Figure 6. Skull of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., in right lateral view ...... 81

Figure 7. Skull of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., in anterior view ...... 83

Figure 8. Tylosaurus vomerine processes of the premaxilla ...... 85

Figure 9. Skull of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., in posterior view ...... 87

Figure 10. Right lower jaw of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. ..89

Figure 11. Downward extension of the maxilla in mosasaurs ...... 91

Figure 12. Tylosaurus septomaxillae ...... 93

Figure 13. Marginal dentition of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov...... 95

ix Figure 14. Juvenile and adult Tylosaurus marginal dentition ...... 97

Figure 15. Tylosaurus maxillary tooth crown aspect ratios ...... 99

Figure 16. Tylosaurus maxillary tooth crown spacing ratios ...... 101

Figure 17. Tylosaurus maxillary tooth crown dimensions ...... 103

Figure 18. Tylosaurus dentary tooth crown aspect and spacing ratios ...... 105

Figure 19. Relative height of the maxilla in Tylosaurus ...... 107

Figure 20 North American Tylosaurus paleobiogeography ...... 109

Figure 21. Range of maxillary crown aspect ratios in different Tylosaurus species and the corresponding feeding guilds ...... 111

Figure 22. Ontogenetic series of Varanus niloticus skulls ...... 113

x INTRODUCTION

The strictly Late Cretaceous (ca. 100–66 Ma) mosasaurs (Squamata, Mosasauridae) were aquatic lizards that generally possessed a large, hydrodynamic body and flippered appendages typical of secondarily aquatic tetrapods (Fig. 1). They were cosmopolitan apex predators, with most of more than 70 species being over 5 m long (e.g., Russell, 1967a). During the Late

Cretaceous, mosasaurs and other marine life flourished in the Western Interior Seaway, an epicontinental sea that covered a large swath of North America and connected the Cretaceous antecedents of the Arctic Ocean and the Gulf of Mexico, east of the emerging Rocky Mountains

(e.g., Nicholls and Russell, 1990). Today, thousands of mosasaur fossils have been excavated in western North America, where this seaway existed, from Mexico to Arctic Canada (Russell,

1967b, 1988; Nicholls and Russell, 1990). Given the remote and less hospitable nature of many fieldwork locales in the higher latitudes of the Western Interior Seaway, the majority of mosasaur specimens from North America have historically come from the southern region of the seaway, in the western and southern United States, particularly Kansas and Alabama. Despite thousands of mosasaur specimens from Kansas alone (Russell, 1988), very few specimens have been found in the upper reaches of the seaway, above 60°N paleolatitude (Russell, 1967b;

Nicholls and Russell, 1990; Bell et al., 2014). Aside from a single mosasaur centrum referred to the parafamily Russellosaurina, which encompasses four mosasaur subfamilies—Tylosaurinae,

Plioplatecarpinae, Tethysaurinae, and Yaguarasaurinae—the few previously known mosasaur specimens from above 60°N paleolatitude in North America are all assignable to the subfamily

Plioplatecarpinae (Russell, 1967b; Nicholls and Russell, 1990; Polcyn and Bell, 2005a; Palci et al., 2013; Bell et al., 2014; Fig. 1B).

1 In North America, fossils of Tylosaurinae—a clade of large russellosaurines (ca. 6–12 m in length) with a prow-like, edentulous premaxillary rostrum—range stratigraphically from the

Turonian (Polcyn et al., 2008; Loera Flores, 2013) to the upper Campanian (Jiménez-Huidobro et al., 2018) and geographically from Mexico (Loera Flores, 2013) and Texas (Bell et al., 2013) to southern Alberta (Caldwell, 2005) and Saskatchewan (Jiménez-Huidobro et al., 2018). Four nominal species have been recognized, all representing Tylosaurus—T. proriger, T. nepaeolicus,

T. pembinensis, and T. saskatchewanensis—with a previously established fifth species,

Hainosaurus neumilleri, recently designated as a nomen dubium and reassigned to Tylosaurus sp. (Martin, 2007; Bullard and Caldwell, 2010; Jiménez-Huidobro, 2016; Jiménez-Huidobro et al., 2018; Jiménez-Huidobro and Caldwell, 2019). The status of Hainosaurus as a valid genus has been questioned, as four of its five established species worldwide have been reassigned to

Tylosaurus (Lindgren, 2005; Bullard and Caldwell, 2010; Jiménez-Huidobro and Caldwell,

2016; Jiménez-Huidobro and Caldwell, 2019). Jiménez-Huidobro and Caldwell (2016) suggest that Hainosaurus as a whole be considered a junior synonym of Tylosaurus, although they stop short of including such a systematic revision.

Herein I describe a newly discovered high-latitude tylosaurine mosasaur specimen, consisting mainly of a partial, articulated muzzle. The specimen, TMP 2014.011.0001, came from the Puskwaskau Formation (ca. 86.6–79.6 Ma, Santonian–lower middle Campanian) exposed north of Grande Prairie, northwest Alberta, Canada, at about 62°N paleolatitude. I will first assign the new material to the genus Tylosaurus and, further, establish a new species based on a set of autapomorphies, including aberrant dentition. I will then reassess current understanding of North American tylosaurine paleobiogeography, with TMP 2014.011.0001

2 providing compelling new context. Finally, I will explore potential ecological implications of

TMP 2014.011.0001’s unexpected tooth morphology.

Institutional Abbreviations—AMNH, American Museum of Natural History, New

York, New York, USA; BMNH, Natural History Museum, London, UK; CMN, Canadian

Museum of Nature, Ottawa, Ontario, Canada; FFHM, Fick Fossil and History Museum, Oakley,

Kansas, USA; FHSM, Sternberg Museum of Natural History, Hays, Kansas, USA; FMNH,

Field Museum of Natural History, Chicago, Illinois, USA; IGF, University of Florence Museum of Natural History, Geology and Paleontology Section, Florence, Italy; IRScNB, Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium; KU, University of Kansas Natural

History Museum, Lawrence, Kansas, USA; MNHN, Muséum National d'Histoire Naturelle,

Paris, France; RMM, McWane Science Center, Birmingham, Alabama, USA; RSM, Royal

Saskatchewan Museum, T. rex Discovery Centre, Eastend, Saskatchewan, Canada; SDSM,

Museum of Geology, South Dakota School of Mines and Technology, Rapid City, South Dakota,

USA; SGO.PV, Área de Paleontología, Museo Nacional de Historia Natural, Santiago, Chile;

TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UAM, University of Alaska Museum of the North, Fairbanks, Alaska, USA; YPM, Yale Peabody Museum of

Natural History, New Haven, Connecticut, USA.

Anatomical Abbreviations—d, dentary; D1, 3, dentary teeth 1, 3; inb, internarial bar;

m, maxilla; M1–3, 6, 7, 9, maxillary teeth 1–3, 6, 7, 9; pm, premaxilla; PM2, premaxillary tooth

two; sm, septomaxilla; spl, splenial; v, vomer; vp, vomerine process of premaxilla. When sides

are distinguished, l/L and r/R preceding abbreviations indicate left and right.

GEOLOGICAL SETTING

3

TMP 2014.011.0001 came from the Puskwaskau Formation, at coordinates

55°36’34.45”N, 118°22’42.85”W. It was found as a loose concretion along the streambed of

Kakut Creek, northeast of Grande Prairie, Alberta, Canada, and was likely washed downstream

to where it was discovered (Fig. 2A). Based on an approximate age of 85 Ma, this locality had a

paleolatitude of about 62°N. The Puskwaskau Formation resulted from the deposition of muddy

sediments offshore in likely less than 50 m of water, and represents a Santonian–early middle

Campanian time span (Hu and Plint, 2009; Bell et al., 2014). As the concretion containing the

specimen was found loose in the streambed, its exact provenance along the creek is unknown.

The specimen has veins of yellowish calcite crystals running through it, and similar crystals were

found in two concretions about 155 m upstream (55°36’35.42”N, 118°22’51.67”W) from where

the specimen was recovered (T. Konishi, pers. comm., 2017). Across the creek from these

concretions were freshly exposed shale layers with concretions evident above. However, no

further mosasaur remains have been found there or elsewhere in the creek, aside from a single

previously discovered but undescribed mosasaur jaw fragment from the creek (Collom, 2001).

The Puskwaskau Formation is composed of five members. From oldest to youngest, they

are the Dowling, Thistle, Hanson, Chungo, and Nomad (Fig. 2B). The Dowling and Thistle are

Santonian in age, with the Hanson straddling the Santonian–Campanian boundary, and the

Chungo and Nomad being early–early middle Campanian (Hu and Plint, 2009; Bell et al., 2014).

Aside from the Chungo Member, which is largely sandstone, the formation is predominantly mudstone and siltstone (Hu and Plint, 2009). The member of origin for TMP 2014.011.0001 is unknown, but the Chungo Member can be excluded as the specimen is encased in gray mudstone. The specimen is likely from below the Chungo, as the lower three members form the

4 majority of the rest of the formation. It is possible, however, that it came from the uppermost

Nomad Member. Given these parameters, the specimen is no younger than early middle

Campanian, about 79.6 Ma, and no older than Santonian, about 86.6 Ma (Bell et al., 2014).

According to Collom (2001:fig. 2-37), the bank of the Smoky River at 55°33'02.20"N,

118°11'51.16"W, about 200 m upstream from the mouth of the Bad Heart River, encompasses only the Dowling and Thistle Members from the Puskwaskau Formation, both exclusively

Santonian in age, directly overlain by Quaternary deposits. The discovery site of TMP

2014.011.0001 in Kakut Creek, which flows into the Bad Heart River, is just over 13 km away from the outcrop described by Collom (2001:fig. 2-37). At the mouth of the Puskwaskau River, where it meets the Smoky River about 8 km south of the mouth of the Bad Heart River, Hu and

Plint (2009:fig. 7) report a dip cross section where all present members of the Puskwaskau

Formation are older than Campanian. Thus the exposure of the Puskwaskau Formation in the banks of Kakut Creek, where TMP 2014.011.0001 was discovered, could be similarly restricted to the Santonian portion of the formation, as in these nearby exposures along the Smoky River.

MATERIALS AND METHODS

Size is only an indirect indicator of age (e.g., Brinkman, 1988; Hone et al., 2016).

However, in the absence of other well-established indicators in Tylosaurinae, I used relative size as a proxy for relative maturity among Tylosaurus specimens examined. I estimated midline skull length (anterior tip of the rostrum to posterior margin of the occipital condyle) as a proxy for body size (Appendix 1). All measurements used to calculate the relative metrics for my comparative study of Tylosaurus dental and maxillary morphology were taken in lateral view.

5 Data were collected preferentially from the right maxilla and maxillary teeth, with left maxillary measures included only when I could not reliably determine a particular measurement on the right maxilla of an individual. Measurements were made either with Vinca model DCLA-1205 digital calipers or with the ruler tool in Adobe Photoshop CC 2018 (19.1.5) using photographs taken with a Nikon D5000 digital SLR camera, with either a Nikon AF-S DX NIKKOR 18-

55mm f/3.5-5.6G VR zoom lens or a Tamron G005 macro lens. Where necessary, I corrected my measurements for taphonomic distortion. I did not include any measurements where the damage to the specimen seemed too extensive for a reliable estimate. I treated CMNFV 40930, 40937, and 40943 as one individual, and the same for BMNH R3626 and R3627. In both cases, this decision was based on similarity in size, morphology, appearance, and provenance (both location and year of collection/acquisition). All figures were created in Adobe Photoshop CC 2018

(19.1.5). I used Microsoft Excel 2016 to perform regression analyses and to create the data plots presented in Figures 15–19 and 21. I calculated paleolatitude estimates using www.paleolatitude.org (van Hinsbergen et al., 2015).

SYSTEMATIC PALEONTOLOGY

Class REPTILIA Linnaeus, 1758

Order SQUAMATA Oppel, 1811

Family MOSASAURIDAE Gervais, 1852

Parafamily RUSSELLOSAURINA Polcyn and Bell, 2005a

Subfamily TYLOSAURINAE Williston, 1897

6 Tylosauridae Marsh, 1876:59.

“mosasaurines megarhynques” Dollo, 1890:163.

Tylosauridae Williston, 1895:169.

Tylosaurinae Williston, 1897:177.

Included Genera—Tylosaurus Marsh, 1872b; Taniwhasaurus Hector, 1874; and

Hainosaurus Dollo, 1885; Kaikaifilu Otero et al., 2017.

Emended Diagnosis—The following are newly proposed diagnostic characters for

Tylosaurinae sensu Russell (1967a). Premaxilla broad between maxillae, the premaxillomaxillary sutures nearly parallel in dorsal view, trending only slightly medially until the termination of the sutures at the anterior ends of the narial openings; marginal dentition largely homodont.

Remarks—As noted by Jiménez-Huidobro and Caldwell (2019), Kaikaifilu hervei, recently established by Otero et al. (2017) as a new genus and species within Tylosaurinae, is problematic. The holotype and only known specimen, SGO.PV.6509, from the upper

Maastrichtian of the Antarctic Peninsula, exhibits no exclusively tylosaurine features. Instead, many of its characteristics actually contradict the tylosaurine diagnosis (e.g., Russell, 1967a;

Jiménez-Huidobro, 2016; Jiménez-Huidobro and Caldwell, 2016; Jiménez-Huidobro et al., 2016;

Otero et al., 2017).

Genus TYLOSAURUS Marsh, 1872

Macrosaurus Cope, 1869:123.

7 Liodon Cope, 1870:184.

Rhinosaurus Marsh, 1872a:17.

Rhamphosaurus Cope, 1872:141.

Tylosaurus Marsh, 1872b:47.

Generic Type—Tylosaurus proriger (Cope, 1869)

Emended Diagnosis—In addition to Jiménez-Huidobro and Caldwell (2019), the following diagnostic character is added herein: premaxilla lacking median dorsal ridge.

TYLOSAURUS BOREALIS, sp. nov.

(Figs. 3–13)

Holotype—TMP 2014.011.0001, a semi-articulated partial muzzle and right mandible

fragment.

Type Locality and Horizon—Kakut Creek, about 55 km northeast of Grande Prairie,

Alberta, Canada. From an unknown member of the Puskwaskau Formation, most likely

Santonian in age based on the siltstone nature of the concretion and the extent of the formation

reported in nearby exposures (Collom, 2001; Hu and Plint, 2009; Bell et al., 2014; Fig. 2).

Etymology—From the Latin word borealis, meaning northern, referring to the high-

latitude northern locality of the new species.

Diagnosis—Anterior four maxillary teeth with crown aspect ratio greater than 2.0, crown aspect ratio above 1.7 in other maxillary teeth; marginal tooth crowns with mesio-distal basal diameter consistently less than mesio-distal space between adjacent crown bases; height of

8 maxilla at posterior end of premaxillomaxillary suture equal to combined height of fully exposed root and crown of fourth maxillary tooth; downward extension of maxillary cortical bone covering roots of up to three anteriormost maxillary teeth.

DESCRIPTION

Premaxilla—Dorsal and anterior to the anterior margins of the maxillae, the premaxilla has been crushed laterally and dorsally, evidenced by numerous fissures in the bone, dorsomedial exposure of cancellous bone, and circumferential constriction of the bone (Figs. 3–7). The portion of the premaxilla anterior to the second premaxillary teeth is missing (Fig. 7). The root of the right second premaxillary tooth is in place, although all but the base of the crown is broken off and missing. The left second premaxillary tooth is present only as a broken and displaced crown, with the root lost in the breaking of the premaxilla. Although any predental rostrum of the premaxilla has been lost postmortem, the robustness and round cross section of the dentigerous premaxilla seen in anterior view are typical of tylosaurines and indicative of the presence of a typical tylosaurine predental rostrum (Russel, 1967a; Fig. 7).

Also of note are the vomerine processes of the premaxilla, which are clearly visible in anterior view as two narrow projections extending downward from the ventral midline of the bone, between the second premaxillary teeth (Fig. 8A). The bifurcating ridge morphology of the vomerine processes has been described previously in mosasaurs, including Tylosaurus (e.g.,

Williston, 1898; von Huene, 1910; Russell, 1967a; Street and Caldwell, 2017). TMP

2014.011.0001 exhibits a clear gap of several millimeters between these two ridges, which taper distally (Figs. 7, 8A). Although obscured by the matrix, these processes presumably continue

9 posteriorly, each appressed between the respective maxilla and vomer, as seen in other mosasaur specimens (e.g., Merriam, 1894:pl. 1 fig. 3; Caldwell and Bell, 2005:fig. 2C; Konishi and

Caldwell, 2011; Street and Caldwell, 2017; pers. observ.; Fig. 8). Anteriorly, these two ridges would have joined together between the first premaxillary teeth, anterior to the point where the premaxilla has been broken (e.g., Merriam, 1894:pl. 1 fig. 3; Williston, 1898:pl. 24 fig. 2;

Lingham-Soliar, 1995:fig. 7B; Caldwell and Bell, 2005:fig. 2C; Everhart, 2005:fig. 8E; Street and Caldwell, 2017:fig. 2B; pers. observ.; Fig. 8). By contrast, Caldwell et al. (2005:fig. 5B–D) describe a single median vomerine process in Taniwhasaurus oweni, but this apparent morphology is likely an artifact of preservation. Being counter to the morphology established in at least mosasaurine, plioplatecarpine, and tylosaurine mosasaurs, it would require the vomers to articulate on the lateral surfaces of this single process, rather than on the medial surfaces of the expected paired processes (e.g., Williston, 1898; Russell, 1967a; Konishi and Caldwell, 2011;

Street and Caldwell, 2017; Fig. 8C, D).

The premaxillomaxillary sutures in TMP 2014.011.0001 begin anteriorly immediately posterior to the second premaxillary teeth. The sutures rise dorsally, and although an artificial gap separates these sutural contacts, on the right side in particular, an interdigitating double buttress outline is discernable in these rising sutures, as is often observed in tylosaurines (e.g.,

Lingham-Soliar, 1992:pl. 7B; Lindgren and Siverson, 2002; Martin, 2007; Fernández and

Martin, 2009:fig. 2B; Jiménez-Huidobro et al., 2016:fig. 4B; Jiménez-Huidobro et al., 2018:fig.

2B; Figs. 4, 6). After this dorsal rise, the sutures turn abruptly and extend posteriorly in a nearly parallel fashion with each other, resulting in a characteristically tylosaurine, broadly parallel- sided premaxilla between the maxillae (Fig. 3). The dorsal surface of the premaxilla here is smooth. The posterior termini of the premaxillomaxillary sutures define the anteriormost points

10 of the nares, although the portion of the premaxilla that would have formed the posteriormost ~1 cm of the left premaxillomaxillary suture is missing. The entire right sutural margin of the premaxilla is preserved and accurately represents the posterior extent of the premaxillomaxillary suture. Accounting for the postmortem separation between the premaxilla and the maxillae, the suture terminates above the posterior margin of the fourth maxillary tooth crown (Figs. 3, 4, 6).

Among Tylosaurus, this premaxillomaxillary sutural length is consistent with T. pembinensis, T. proriger, and T. saskatchewanensis, while that suture in T. nepaeolicus sensu Jiménez-Huidobro et al. (2016) ends between the third and fourth maxillary teeth (Russell, 1967a; Bullard and

Caldwell, 2010; Jiménez-Huidobro et al., 2016, 2018).

Posterior to the premaxillomaxillary sutures, the margins of the premaxilla abruptly curve medially and then extend posteriorly, forming the internarial bar (Fig. 3). There is a small postmortem gap in the internarial bar, immediately posterior to which the internarial bar reaches its narrowest width of 14 mm and then widens slightly. The posterior preserved stretch of the internarial bar exhibits some damage and displacement and ends in a transverse break associated with a vein of calcite crystals (Fig. 9). Despite being somewhat deformed, the internarial bar at this break exhibits a characteristic mosasaurid triangular cross section (Russell, 1967a).

Maxilla—Both maxillae are present, but not entirely. Out of the 12−13 maxillary teeth in

Tylosaurus (Russell, 1967a), the right maxilla of TMP 2014.011.0001 as preserved shows the first nine maxillary tooth positions, while the left maxilla is preserved only to the sixth maxillary tooth position, with the seventh tooth position adhered separately to the posterolateral portion of the right dentary fragment (Figs. 3–6, 9, 10). When the dentary is placed as originally preserved on the ventral surface of the block of matrix containing all other preserved components, this piece of the left maxilla fits into the posterior end of the left maxilla where it was broken off at

11 the sixth maxillary tooth position. There is no reason to suspect that the total number of

maxillary teeth in TMP 2014.011.0001 differs from the expected 12 or 13 seen in Tylosaurus for

the following reasons: (1) the maxillary tooth positions relative to the premaxillomaxillary

sutures, vomers, and right septomaxilla are consistent with other congeners; and (2) the maxillae

lack any evidence of articulation with the palatines at or anterior to the ninth maxillary teeth, also

consistent with other congeners (Merriam, 1894; Russell, 1967a; Bullard and Caldwell, 2010;

pers. observ.).

In each maxilla, the teeth are rooted along a bony buttress that extends along the length of

the element and forms the floor of the nasal vestibule. This bony projection is partially preserved

in the right maxilla, but is not discernible in the left maxilla (Figs. 3, 4). A vertical lamina of

bone rises along the lateral edge of the buttress immediately anterior to the first maxillary tooth

to form the premaxillomaxillary suture. The posterior terminus of the premaxillomaxillary suture

coincides with the anteriorly tallest part of the maxilla in lateral aspect (Figs. 3, 4, 6).

Interestingly, compared to other Tylosaurus, the lateral profile of the maxilla here in TMP

2014.011.0001 appears low relative to both the height of the maxillary teeth and preserved length of the maxillary tooth row (see below). Posterior to the terminus of the premaxillomaxillary suture, the dorsal margin of the maxillary lamina is broadly emarginated to form the lateral margin of the naris. In Tylosaurus, the lamina rises again posteriorly to meet the frontal at the posterior terminus of the narial opening, precluding the prefrontal from the margin of the naris in dorsal aspect (Russell, 1967a; Bell, 1997:fig. 6C), but neither maxilla in TMP 2014.011.0001 is preserved far enough posteriorly to show this feature.

The right maxilla is preserved in its original orientation relative to the premaxilla while the left maxilla is rotated such that its lateral face is now oriented more dorsally (Figs. 3–6). Four

12 foramina are visible above teeth one and two of the left maxilla, and one is discernable above the

midpoint between teeth one and two of the right maxilla. A second, larger foramen is present

further back on the right maxilla, above the midpoint between maxillary teeth five and six.

In the first three left maxillary teeth and the first right maxillary tooth, the maxillary

cortical bone extends downward over what would normally be exposed cementum tooth roots

(Figs. 4, 6, 11A, B). Similar downward extension of the maxilla has been suggested for

Goronyosaurus nigeriensis (Lingham-Soliar, 2002) and personally noted in the holotype of

‘Platecarpus’ somenensis (MNHN 1895-7), but it has not been reported in Tylosaurinae (Fig.

11). The surfaces of these exposed roots in TMP 2014.011.0001 are indistinguishable from the maxillary cortex in color and texture and are unlike the other preserved roots. The same condition may well have also been present in the second right maxillary tooth, but the erupted root of that tooth is missing postmortem. On the left maxilla, what appears to be a natural division between the maxilla and the anterior two tooth roots is actually a fracture in the cortical bone surface (Figs. 4, 11B). Somewhat similarly, two discontinuous postmortem grooves are also present along the base of the first right maxillary tooth (Fig. 6). On the distal portion of the first left maxillary tooth root and the posterior portion of the first right maxillary tooth root, the veneer of maxillary cortical bone has likely been worn away, revealing a more typical root surface underneath (Fig. 11A, B).

The relatively intact posterior 1 cm of the premaxillomaxillary sutural surface of the right maxilla shows the usual longitudinal grooves (Russell, 1967a). Where visible, the medial surfaces of the maxillary laminae are smooth posterior to the termini of the premaxillomaxillary sutures (Figs. 4, 9). While some wear and damage are evident, the dorsal margins of the vertical laminae of both the left and right maxillae are essentially intact to a point above the respective

13 fifth maxillary tooth, each exhibiting a gentle downward slope posterior to the premaxillomaxillary suture. Above the fifth maxillary tooth, the margin of the left lamina is crushed dorsally, with accompanying damage to the lateral face of the lamina below it (Figs. 4,

5). This crushing damage extends posteriorly to where the left maxilla has been broken off at the sixth maxillary tooth. A force from behind likely buckled the posterior broken end of the left maxilla, displacing the posterior half of the sixth maxillary tooth upward.

The crushing event evidenced in the dorsal margin of the left maxillary lamina is part of a series of deformations, all presumably resulting from the same taphonomic event, that can be traced transversely across the specimen, including breaks in the left vomer, internarial bar, and right septomaxilla, to a divot in the dorsal margin of the right maxillary lamina above the fifth maxillary tooth (Fig. 3). Posterior to this divot, there is an uneven break in the lamina, beyond which the lamina is missing. The buttress of the right maxilla continues posteriorly sans lamina until it too ends in a transverse break at the ninth maxillary tooth (Figs. 3, 4, 6, 9).

Vomer—Both vomers are partially preserved, with their anterior extents concealed or obliterated in the matrix below the premaxilla and their posteriormost portions lost. As seen in other mosasaur specimens, the vomers of TMP 2014.011.0001 presumably originate anteriorly at their insertion in the median cleft between the paired ventral vomerine processes of the premaxilla (e.g., Merriam, 1894:pl. 1 fig. 3; Caldwell and Bell, 2005:fig. 2C; Konishi and

Caldwell, 2011; Street and Caldwell, 2017; pers. observ.; Fig. 11C, D), and would have interfaced posteriorly with the palatines (Russell, 1967a). In TMP 2014.011.0001, the vomers are present as elongate splints of bone that extend longitudinally along either side of the palatal midline, diverging posteriorly in parallel with the maxillary buttresses (Figs. 3–5, 9).

14 The vomers are exposed dorsally in the narial openings, although the right vomer is obscured by the right septomaxilla in the anterior portion of the right naris (Fig. 3). Anteriorly in the left naris, the left vomer appears as two vertical lamellae of bone, forming the lateral and medial cortical surfaces of the vomer and separated by a gap of about 4 mm along the length of the bone, with an assumed ventral connection between the two lamellae buried in the matrix. The top of the vomer here is about 1.5 cm below the dorsal surface of the adjacent internarial bar.

There is a break in the middle of the left vomer associated with breaks in the other preserved skull bones, and posterior to this break the two lamellae have been somewhat crushed and the bone trends posteroventrally before leveling off again at a second break posterior to the sixth maxillary tooth (Fig. 3). The posteriormost preserved ~1 cm of the left vomer, slightly posterolateral to the broken end of the internarial bar, is a segment of slightly concave, horizontally oriented lamella, possibly representing the ventral portion of the vomer. This piece of the left vomer is about 3 cm below the dorsal surface of the internarial bar (Figs. 3–5, 9).

The distinction between the right vomer and the right septomaxilla in the anterior portion of the right naris is not entirely clear. Alongside the anteriormost part of the internarial bar and about 1 cm below the level of the dorsal surface of the same, the right vomer is covered dorsally by the right septomaxilla, although downward compression of the septomaxilla has revealed the lateral edge of the vomer therein (Figs. 3, 12A). What is clearly part of the right vomer extends posteriorly from a point even with the broken end of the internarial bar and ends with the break in the right maxilla at the ninth maxillary tooth. This part of the right vomer is a ventromedially oriented lamella of bone, likely the ventrolateral cortex of the vomer. It is concave at its anterior end, and emerges from the matrix at a level about 3 cm below the dorsal surface of the internarial

15 bar (Figs. 3–5, 9). The matrix in the right naris from which this stretch of vomer emerges shows a vaguely ovoid cross section of the right vomer.

Septomaxilla—The right septomaxilla is a thin sheet of bone, positioned in the right naris about 1–1.5 cm below the bony margin of the external narial opening, broad anteriorly where it covers the floor of the naris and narrowing posteriorly (Figs. 3, 12A). In dorsal view, the right septomaxilla completely fills the space between the internarial bar and the right maxilla from the anteriormost extent of the right naris back to the posterior margin of the fifth maxillary tooth, where there is a break in the septomaxilla (Fig. 3). Posterior to this break, the right margin of the septomaxilla shifts medially before continuing posteriorly so that the bone no longer fully bridges the narial opening. The posteriorly extending bone splits longitudinally, and the medial

edge is separated from the internarial bar. However, these posterior features of the septomaxilla

result from an indeterminate combination of natural morphology and postmortem deformation to

both the septomaxilla and the internarial bar, rendering the preserved posterior morphology of

the septomaxilla unreliable. The posteriormost portion of the right septomaxilla has been lost at a

break at the seventh right maxillary tooth position.

In Varanus—historically thought of as a close relative to mosasaurs, although the relationship may not be as close as previously assumed (Caldwell et al., 1995; Caldwell, 1999b,

2012)—the extensive septomaxilla slopes ventrolaterally (Bellairs, 1949; pers. observ.). The septomaxilla in TMP 2014.011.0001 was depressed postmortem, resulting in a broken and

uneven surface, with the lateral edge being deeper than the medial edge, relative to the

surrounding bones (Figs. 3, 9, 12A). Given the slope of the septomaxilla in Varanus, this

elevational difference in TMP 2014.011.0001 could reflect the natural inclination of the bone.

The depression and plastic deformation of the septomaxilla in TMP 2014.011.0001 have

16 revealed the outline of the lateral edge of the right vomer, covered dorsally by the thin veneer of the septomaxilla, alongside the anteriormost extent of the internarial bar (Figs. 3, 12A [white arrowheads]). A longitudinal concavity of the septomaxilla just lateral to the vomer extends posteriorly, but it is unclear whether this is an original anatomic feature or an artifact of taphonomy. Konishi and Caldwell (2007b:66) describe the septomaxilla in Plesioplatecarpus planifrons as “gently concave,” although otherwise different in gross morphology from that of

TMP 2014.011.0001.

The septomaxilla, which covers the Jacobson’s organ (Lapage, 1928; Bellairs, 1949;

Lingham-Soliar, 1995; Caldwell, 1999a), is infrequently preserved and rarely described in

Mosasauridae. Two different general morphologies are found in the literature. The septomaxilla is an elongate bone that does not fully cover the anterior floor of the naris in Plotosaurus bennisoni (Camp, 1942), Mosasaurus hoffmannii (Lingham-Soliar, 1995), Plesioplatecarpus planifrons (Konishi and Caldwell, 2007b), and Platecarpus tympaniticus (Konishi et al., 2012).

Alternatively, it is broad and shield-like, forming a floor in the anterior of the narial opening, in

Eonatator coellensis (Páramo-Fonseca, 2013), Plioplatecarpus primaevus (Holmes, 1996),

Tethysaurus nopcsai (Bardet et al., 2003), Yaguarasaurus columbianus (Páramo, 1994; Páramo-

Fonseca, 2000), and a basal mosasaurid (Bell and Polcyn, 2005b). TMP 2014.011.0001 represents this second, broad type of septomaxilla.

The literature record is quite limited regarding the septomaxilla in tylosaurines. Merriam

(1894) reports on the septomaxillae of Tylosaurus micromus (T. proriger sensu Russell, 1967a) as thin bones covering the anterior thirds of the nares, a description that matches closely with the broad, shield-shaped morphology of the septomaxilla present in TMP 2014.011.0001. However, he only illustrates the septomaxillae in palatal view, where the described anterior broadness is

17 not discernable (Merriam, 1894:pl. 1 fig. 3). Von Huene’s (1910) description of T. dyspelor

(most likely T. proriger sensu Russell, 1967a) is unclear regarding the septomaxillae. He observes that the right septomaxilla is present medially and anteriorly in the right narial opening, but in the respective figure the septomaxilla nearly covers the anterior portion of the naris, and its lateral margin is uneven, suggesting it may be broken and thus the original lateral extent of the bone could have been greater than preserved (von Huene, 1910:fig. 5). Russell (1967a:26) states, “In a Tylosaurus [proriger] skull in the Yale museum (YPM 4002) the septomaxillary bones are clearly not fused [as described in Plotosaurus bennisoni by Camp, 1942], one appearing on the medial side of each narial opening.” This implies that the septomaxillae do not extend laterally to the maxillae to totally cover the anterior parts of the nares, but the nature and extent of the lateral margins of the septomaxillae are not explicitly described.

Among other Tylosaurus specimens I examined, CMNFV 8162, KUVP 1129, KUVP

28705, and possibly KUVP 65636 and KUVP 66129, have preserved septomaxillae (Fig. 12). In each case, these septomaxillae cover the anterior parts of the nares, and given the observations presented by Merriam (1894) and von Huene (1910), as well as the morphology evident in TMP

2014.011.0001, an anteriorly (within the naris) broad, shield-shaped morphology likely characterizes the Tylosaurus septomaxilla (Fig. 12). CMNFV 8162, KUVP 1129, KUVP 28705,

KUVP 65636, and KUVP 66129 also exhibit what is possibly a posteriorly extending thickened medial edge of the septomaxilla, where it abuts the internarial bar (Fig. 12 B–D, black arrowheads). Such a feature is not readily discernible in TMP 2014.011.0001, but the broken posteromedial fragments of the septomaxilla could conceivably be associated with such a structure (Fig. 12A, black arrowhead).

18 Mandible—A fragment of the right mandible, consisting of posterior portions of the right dentary and splenial in articulation, was prepared separate from the other elements of TMP

2014.011.0001 (Fig. 10), but was preserved lying underneath the muzzle. Some fragments of the dentary and the dentary teeth are still affixed to the matrix ventral to the muzzle. The preserved dentary in TMP 2014.011.0001 exhibits a convex lateral surface and a deep, broad Meckelian groove on the medial face, although part of the splenial ala covers much of the medial surface and is in part depressed into the groove (Fig. 10). The seventh tooth position of the left maxilla is affixed to the posterolateral portion of the dentary fragment.

The Tylosaurus dentary contains 13 teeth, which are largest in the center of the jaw and decrease in size anteriorly and posteriorly, whereas the dentary bone itself becomes increasingly deep posteriorly (Russell, 1967a). The intact piece of the right dentary of TMP 2014.011.0001 spans five tooth positions, although two of the crowns are missing and the fifth position is presumed based only on spacing because its root is not evident either (Fig. 10). A possible elongated foramen is visible on the lateral surface below the third preserved tooth position. The preserved tooth crowns are reduced in both height and diameter compared to those of the maxillae. The height of the preserved dentary fragment and the relatively small teeth, as well as the breadth of the Meckelian groove and splenial ala, suggest the fragment is from the posterior half of the dentary. Other, more posterior fragments of the dentary and possibly other lower jaw elements are present ventrally in the matrix holding the rest of the muzzle (Fig. 5), although no more posterior dentary teeth are discernable. If the intact portion of the dentary contains the posteriormost tooth position, there should be dorsal grooves on the dentary immediately behind that tooth (Russell, 1967a), but no such grooves are evident in the preserved dentary. However, what remains of the dentary posterior to the posteriormost preserved tooth position is highly

19 fragmented, and this could be obscuring any grooves or teeth that may have been present prior to fossilization.

Marginal Dentition—The first seven tooth positions are preserved in the left maxilla and the first nine tooth positions are preserved in the right maxilla of TMP 2014.011.0001, with most of the respective crowns at least partially preserved (Figs. 4, 6, 10). Tylosaurus typically have bicarinate marginal dentition with a labial intercarinal angle of around 120° (Russell, 1967a;

Konishi and Caldwell, 2007a; Konishi et al., 2018). However, in TMP 2014.011.0001, the lingual surfaces of the preserved crowns are embedded in the matrix, and the preservation of the exposed labial surfaces is such that general shape and dimensions can be determined for many of them, but finer details of striae, facets, and carinae are largely not discernable. Several weak facets are visible on an intact portion of the crown of the first right maxillary tooth, characteristic of Tylosaurus teeth (Lindgren and Siverson, 2002), and an anterior carina is visible on one of the preserved dentary teeth (Fig. 13A, B). Portions of the pulp cavities are exposed in left maxillary teeth one, two, and six. All the teeth appear fully erupted and ankylosed, with the exception of left maxillary tooth five and right maxillary teeth five and eight, where a slight gap between the tooth root and the maxillary bone likely indicates the periodontal ligament had yet to fully calcify when the animal died (LeBlanc et al., 2017; Figs. 4, 6).

TMP 2014.011.0001 exhibits homodonty to the extent that the maxillary tooth crowns are similarly conical in shape and slightly posteriorly curved, with some minimal variation in size

(Figs. 4, 6). Some positional variation in tooth size is normal within a single tylosaurine individual (Russell, 1967a), and the general lack of differentiation in marginal dentition is consistent with the literature regarding tylosaurines (e.g., Lindgren and Siverson, 2002; Caldwell et al., 2008; Fernández and Martin, 2009; Jiménez-Huidobro et al., 2018). The lateral and

20 slightly anterior exposure of the first right maxillary tooth crown in TMP 2014.011.0001 exhibits a rounded labial surface. The small preserved fragment of the second right premaxillary tooth crown shows a similarly round lingual face, and a cross section of left maxillary tooth five, while broken, displays an overall round shape (Fig. 13C, D).

Mosasaurs have thecodont tooth attachment, where the tooth is attached to the tooth- bearing element via a socket (Caldwell et al., 2003; LeBlanc et al., 2017; Bertin et al., 2018; but see Zaher and Rieppel, 1999; Luan et al., 2009). There has been disagreement on the orientation of developing replacement teeth in mosasaurs, but a vertical orientation is now established, and seemingly recumbent or other positions are attributed to postmortem taphonomic effects

(Rieppel and Kearney, 2005; Caldwell, 2007). The cross section of the left maxilla of TMP

2014.011.0001 where it is broken immediately posterior to the sixth maxillary tooth reveals a replacement tooth in such a vertical orientation (Fig. 9). This tooth is around Stage III–IV of

Caldwell’s (2007:fig. 6) replacement tooth movement path model, as the crown is nearly full size but has not yet breached the dentigerous margin of the maxilla nor begun developing root cementum. A replacement tooth is similarly exposed where the right maxilla is broken at the ninth tooth position (Fig. 9). Much of its crown has been lost, and the cross section of the tooth is somewhat oblique, but it appears to also be around Stage III–IV of its development.

TMP 2014.011.0001’s relatively large estimated midline skull length (93.6 cm) and body length (at least 6.5 m and possibly as large as 8 m; see Appendix 1) suggest TMP 2014.011.0001 is an adult. Unexpectedly, it exhibits slender, widely spaced maxillary tooth crowns, similar to the typical juvenile Tylosaurus condition (Figs. 4, 6, 14). The anterior four maxillary teeth

(where measurable, i.e., left maxillary tooth two and right maxillary teeth one, two, and four) exhibit crown aspect ratios (height / mesio-distal basal diameter) of 2.26–2.78, meaning the

21 crowns are over twice as tall as they are wide at the base (Appendix 2). The fifth and sixth right maxillary teeth are slightly less slender, with crown aspect ratios of 1.72 and 1.78, respectively

(Appendix 2). More slender anterior teeth are a common morphology in mosasaurs, especially in

Mosasaurinae (e.g., Konishi et al., 2011; LeBlanc et al., 2012; pers. observ.). The better-

preserved right maxillary tooth row of TMP 2014.011.0001 reveals unusually wide spacing

between adjacent crowns (Fig. 6). For all measurable right maxillary tooth crowns (teeth one

through six), the spaces between the teeth are wider than the tooth crowns themselves, reaching

as much as nearly twice as wide as the tooth crown base in the case of right maxillary tooth one

(Appendix 2).

According to Russell (1967a:54), mosasaur teeth “are arranged in a single row in each

mandibular ramus, with the posterior edge of the bony base of one tooth contacting or nearly

contacting the anterior edge of the succeeding tooth base.” This close proximity of adjacent

alveoli remains constant throughout previously reported Tylosaurus ontogeny, although the

crowns themselves grow relatively more robust, and thus closer together, with age (Konishi and

Caldwell, 2007a; Konishi et al., 2018; pers. observ.). In smaller individuals, with relatively

narrower crowns, the tooth root flanges outward from the base of the crown to fill the available

space of the alveolus (Fig. 14A, B). Contrastingly, the root margin is more vertical in larger

individuals, implying that the tooth crown has grown to the point, or near to it, of maximum

capacity for that alveolus (Fig. 14C, D). In TMP 2014.011.0001, despite its relatively large size,

some of the maxillary tooth roots exhibit definite flanging (e.g., right maxillary teeth three and

four). However, some other maxillary tooth roots appear to exhibit a more typical adult

morphology, but with wide gaps between exposed roots (e.g., right maxillary teeth five and six)

(Fig. 6). This morphology could imply the tooth crown spacing is due to adjacent alveoli being

22 widely separated by a span of alveolar bone (sensu Caldwell et al., 2003), a novel condition for either adult or juvenile Tylosaurus, and for mosasaurs in general (Russell, 1967a; Konishi and

Caldwell, 2007a; Konishi et al., 2018; pers. observ.; Fig. 14). However, the preservation of TMP

2014.011.0001 renders the exact morphology of the maxillary tooth root margins somewhat equivocal and obscures the tooth sockets themselves in both maxillae (see Intrageneric

Comparison).

The preserved dentary tooth crowns in TMP 2014.011.0001 are smaller than the preserved maxillary teeth, but generally have a similar lateral shape and aspect ratio, although the anteriormost preserved dentary tooth appears somewhat labiolingually compressed, in contrast with the discernable morphology of the premaxillary and maxillary teeth (Figs. 10,

13B). As with the maxillary teeth, the dentary crowns are too damaged to show fine details of the enamel surface, aside from an anterior carina on the anteriormost preserved dentary tooth (Fig.

13B). The two reliably measurable dentary teeth both have crown aspect ratios of 2.03, falling in range between right maxillary teeth four and five (Appendix 3). The space between the bases of these two adjacent crowns is nearly twice the basal longitudinal diameter of either crown. The exposed roots of the dentary teeth exhibit wide flanging, but the actual margin of the dentary is unclear (Fig. 10).

INTRAGENERIC COMPARISON

Marginal Dentition

Maxillary tooth crown aspect ratios (height / mesio-distal basal diameter) for maxillary teeth one through six (where measureable) in eight other Tylosaurus specimens were compared

23 with those of TMP 2014.011.0001 (Fig. 15; Appendix 2). Although I was unable to collect crown aspect ratio measurements for all of maxillary teeth one to six in any of the specimens examined, my data show the teeth of TMP 2014.011.0001 are uniquely slender compared to either adult or juvenile Tylosaurus, as represented by the other specimens. The anterior six maxillary tooth crowns of TMP 2014.011.0001 have aspect ratios ranging from 1.72–2.78, with

right maxillary teeth one (2.78), two (2.32), and four (2.26) exhibiting particularly high crown aspect ratios. Left maxillary tooth two, the only reliably measurable tooth from the left maxilla, also has a high crown aspect ratio of 2.70. The fifth and sixth right maxillary teeth of TMP

2014.011.0001 have crown aspect ratios of 1.72 and 1.78, respectively, falling within the overall range of other Tylosaurus specimens examined (1.35–1.88). Excluding TMP 2014.011.0001,

Tylosaurus crown aspect ratios show no clear ontogenetic trend (Fig. 15), but this could be due to the relatively low sample size in this comparison.

I calculated a crown spacing ratio (crown mesio-distal basal diameter / mesio-distal space between the base of that crown and the base of the crown immediately posterior to it) for each of teeth one to six of the right maxilla of TMP 2014.011.0001. This ratio ranges from 0.52–0.92 for these teeth, lower than expected in comparison with 17 other Tylosaurus specimens (Fig. 16;

Appendix 2). Of the teeth measured in other specimens, only some smaller individuals have any spacing ratios within the range of TMP 2014.011.0001. Additionally, while not included in the present study, Konishi et al. (2018) describe the maxillary tooth crown spacing in the neonate

FHSM VP-14845, the smallest known Tylosaurus (estimated midline skull length 25.6 cm based on maxillary teeth one to six distance of 6.4 cm given by Konishi et al., 2018), as being equivalent to a crown spacing ratio of 0.50–0.59. TMP 2014.011.0001, over 3.5 times larger than

FHSM VP-14845 based on estimated midline skull length, has teeth within or just above this

24 range. Among examined specimens that are similarly sized or larger than TMP 2014.011.0001, all measured crown spacing ratios are greater than 1.00. Thus, their tooth crowns are consistently wider than the spaces between crowns, the opposite of the condition in TMP 2014.011.0001.

Excluding TMP 2014.011.0001, the Tylosaurus specimens examined show a clear increase in crown spacing ratio with increasing midline skull length (Fig. 16B). This relationship is statistically significant for tooth positions one and four.

To further investigate the juvenile-like spacing between maxillary tooth crowns in TMP

2014.011.0001, I directly compared the crown mesio-distal basal diameter for maxillary teeth

one through six in TMP 2014.011.0001 and 18 other Tylosaurus specimens (Fig. 17A; Appendix

2). The maxillary tooth crowns of TMP 2014.011.0001 have unusually narrow bases for the

specimen’s size, indicating that it is likely the tooth crowns, not the sockets or socket spacing,

that are unusual in TMP 2014.011.0001 and result in its widely spaced, juvenile-like dental

morphology. The crown heights of maxillary teeth one through six in TMP 2014.011.0001 are

similar to those of eight other Tylosaurus specimens after accounting for midline skull length

(Fig. 17B). The ratio of crown mesio-distal basal diameter divided by theoretical socket capacity

(crown mesio-distal basal diameter + 0.5 * sum of the mesio-distal spaces between the base of

that crown and the bases of the crowns immediately anterior and posterior to it) for maxillary

teeth two through six of TMP 2014.011.0001 and nine other Tylosaurus specimens increases

with increasing midline skull length, suggesting differential growth rates between tooth crowns

and tooth sockets in Tylosaurus (see Discussion; Fig. 17C). However, values for this ratio in

TMP 2014.011.0001 are lower than expected for a Tylosaurus of its size.

Both of the two measurable dentary teeth of TMP 2014.011.0001 have crown aspect

ratios of 2.03. While the exact tooth positions are unknown, they are from the posterior half of

25 the tooth row, thus I compared these values to those of dentary teeth seven through 13 in four

other Tylosaurus specimens (Fig. 18A; Appendix 3). The dentary crown aspect ratios of 2.03 in

TMP 2014.011.0001 fall just above the 1.36–2.00 range of values I measured in other Tylosaurus

specimens. Among these other Tylosaurus, there may be a slight negative trend in crown aspect

ratio with increasing skull size, but the sample size is too low to be conclusive. The crown spacing ratio for the anteriormost of the two adjacent dentary teeth of TMP 2014.011.0001 is

0.54, which is lower than expected for its size, based on comparison with values for dentary teeth seven through 13 in nine other Tylosaurus specimens (Fig. 18B; Appendix 3). These other

Tylosaurus show an overall increase in crown spacing ratio with increasing skull length. Since the exact tooth positions of the preserved dentary teeth in TMP 2014.011.0001 are not known, direct tooth-by-tooth comparisons of aspect ratio or spacing with other specimens were not possible.

Maxillary Depth

Comparison of relative and absolute maxillary depth in lateral profile of TMP

2014.011.0001 and other specimens of Tylosaurus shows the maxilla of TMP 2014.011.0001 to be relatively shallow (Fig. 19; Appendix 4). In TMP 2014.011.0001, a conservative estimate of the height (combined height of the tooth crown and exposed root in lateral aspect) of maxillary tooth four on either maxilla compared to the anteriorly deepest part of the maxilla, below the posterior terminus of the premaxillomaxillary suture, shows that these teeth are at least as tall as the maxillary lamina dorsal to them. In four other Tylosaurus specimens examined, this ratio of fourth maxillary tooth height to maxilla height ranges from 0.58–0.80, all well below the conservative one-to-one ratio seen in TMP 2014.011.0001 (Fig. 19A). Since the maxillary tooth

26 crowns in TMP 2014.011.0001 are of expected height for a Tylosaurus of comparable size (Fig.

17B; Appendix 2), the high value for this ratio in TMP 2014.011.0001 must be due to a relatively

low-profile maxilla. The ratio of maxilla height to the linear distance from the anterior margin of

the first maxillary tooth root to the posterior margin of the sixth maxillary tooth root is lower

than expected in TMP 2014.011.0001 given its size (Fig. 19B; Appendix 4). This ratio increases

with increasing skull length in the 21 other Tylosaurus specimens measured. Of these other

specimens, only FHSM VP-9350 (estimated midline skull length 38.0 cm) exhibits a ratio less than that of TMP 2014.011.0001. Comparing absolute maxilla depth as a function of estimated midline skull length in TMP 2014.011.0001 and 23 other Tylosaurus specimens shows a clear increase in maxilla height with increasing skull length, with the value for TMP 2014.011.0001 lower than expected based on its size (Fig. 19C; Appendix 4). These three maxillary comparisons show the maxilla of TMP 2014.011.0001 to be unusually shallow at the posterior terminus of the premaxillomaxillary suture. As with the dentition, this condition in TMP 2014.011.0001 is akin to that of congeneric juveniles.

DISCUSSION

Diagnosis

Within Tylosaurinae, there are two well-established genera, Tylosaurus and

Taniwhasaurus. Kaikaifilu, recently described by Otero et al. (2017), is problematic and likely not tylosaurine (Jiménez-Huidobro and Caldwell, 2019; pers. observ.). The status of

Hainosaurus is questionable, as four of five supposed Hainosaurus species (H. bernardi, H. gaudryi, H. pembinensis, and H. neumilleri), including the two from North America, have been

27 reassigned to Tylosaurus (Lindgren, 2005; Bullard and Caldwell, 2010; Jiménez-Huidobro and

Caldwell, 2016; Jiménez-Huidobro and Caldwell, 2019). Hence Jiménez-Huidobro and Caldwell

(2016) propose that Hainosaurus is a junior synonym of Tylosaurus.

Regarding Tylosaurus ‘neumilleri’ (formerly Hainosaurus neumilleri), represented by

only a single, mostly incomplete specimen (SDSM 75705) from the upper Campanian of South

Dakota (Martin, 2007), Jiménez-Huidobro and Caldwell (2019) recognize it as a nomen dubium

and tentatively assign it to Tylosaurus sp. They suggest that the material cannot be distinguished

from either T. pembinensis or T. saskatchewanensis. That may be true for most of the diagnostic features of SDSM 75705 proposed by Martin (2007), but one of SDSM 75705’s apparent

Hainosaurus-like features leading to its original assignment to that genus is its labiolingually compressed, bicarinate maxillary dentition, with the carinae dividing the teeth into seemingly symmetrical halves, similar to the condition seen in the holotype of T. bernardi (IRScNB R23C, formerly the generic type for Hainosaurus) (Dollo, 1885; Lindgren and Siverson, 2002;

Lindgren, 2005; Martin, 2007:figs. 2M, 3E, F). This feature alone may be enough to distinguish it from both T. pembinensis and T. saskatchewanensis, although the literature is contradictory on this point. For instance, Bullard and Caldwell (2010) specifically contrast the teeth of T. pembinensis with those of T. bernardi, reporting instead a similarity to the teeth of other

Tylosaurus, which are typically round or D-shaped in cross section (Russell, 1967a; Lindgren

and Siverson, 2002; pers. observ.). Jiménez-Huidobro et al. (2018) report that the teeth of T.

saskatchewanensis are labiolingually compressed, but liken them to the teeth of T. proriger,

which adhere to the typical Tylosaurus dental morphology described above—i.e., a D-shaped

crown cross section (Russell, 1967a; Lindgren and Siverson, 2002; pers. observ.). By these

descriptions, neither T. pembinensis nor T. saskatchewanensis exhibit the symmetrically

28 labiolingually compressed marginal dentition seen in T. bernardi and SDSM 75705. However,

Jiménez-Huidobro and Caldwell (2019) cite labiolingually compressed teeth as a feature SDSM

75705 shares with both T. pembinensis and T. saskatchewanensis.

In any case, aside from apparent lateral compression in a posterior dentary tooth (Fig.

13B), the marginal dentition of TMP 2014.011.0001 is mostly visible only in lateral view, obscuring any potential labiolingual compression. However, the partial anterior exposure of the first right maxillary tooth, as well as cross sections of the second right premaxillary tooth and the fifth left maxillary tooth, have an overall round shape with no apparent carinae (Fig. 13C, D).

Since the anterior maxillary teeth of SDSM 75705 are clearly labiolingually compressed and bicarinate, and given the questionable specific diagnosis of that specimen, the possibility that

TMP 2014.011.0001 is conspecific with SDSM 75705 is at best equivocal. Similarly, TMP

2014.011.0001 can reasonably be excluded from Hainosaurus due to lacking this Hainosaurus- type labiolingually compressed dentition, and due to the likely synonymy of that genus with

Tylosaurus as suggested by recent works (Lindgren, 2005; Bullard and Caldwell, 2010; Jiménez-

Huidobro and Caldwell, 2016; Jiménez-Huidobro and Caldwell, 2019). Furthermore, a generic assignment of TMP 2014.011.0001 to Taniwhasaurus can be ruled out because the premaxilla lacks a conspicuous dorsal sagittal crest (Figs. 3, 4). Such a crest is present on the anterior portion of the premaxilla in Taniwhasaurus, while the premaxilla is dorsally smooth in

Tylosaurus (Caldwell et al., 2005; Caldwell et al., 2008; Fernández and Martin, 2009).

Therefore, TMP 2014.011.0001 is confidently assigned to Tylosaurus within Tylosaurinae.

In North America, four unequivocal Tylosaurus species are known: T. proriger, T. nepaeolicus (with T. ‘kansasensis’ as a junior synonym), T. pembinensis, and T. saskatchewanensis (Bullard and Caldwell, 2010; Jiménez-Huidobro, 2016; Jiménez-Huidobro et

29 al., 2016; Jiménez-Huidobro et al., 2018). Based on the cranial morphology alone, TMP

2014.011.0001 is sufficiently distinct from all of these nominal Tylosaurus species by exhibiting

the following suite of characters: (1) marginal teeth with consistently high crown aspect ratios;

and (2) same with consistently wide spacing between crowns, both of these characters arising

from narrow crown basal diameters; and (3) maxilla anteriorly low-profile (Figs. 15–19;

Appendices 2–4). These features in combination are so exclusive to TMP 2014.011.0001 that

they warrant the erection of a new species within Tylosaurus. I note that SDSM 39966, a very

large tylosaurine from the middle Campanian Pierre Shale Formation of South Dakota, appears

to have relatively narrow, widely spaced marginal tooth crowns based on published photographs,

but I was unable to obtain reliable data from this specimen to include in my quantitative

comparisons. Meredith et al. (2007) diagnosed this specimen as cf. Tylosaurus sp., but it is

confidently assignable to Tylosaurus given the likely synonymy of Hainosaurus and Tylosaurus.

Given that TMP 2014.011.0001 is the northernmost known specimen of Tylosaurinae

from North America, coming from the northern reaches of the Late Cretaceous Western Interior

Seaway (Fig. 2), it is possible that its unique dental morphology might represent some kind of

seasonal variation associated with migration. This scenario presumes a significant difference

between southern and northern prey communities, possibly driving seasonal change in

Tylosaurus dentition as they migrated between the two regions. Seasonal migration of mosasaurs

within the Western Interior Seaway is conceivable (e.g., Grigoriev and Grabovskiy, 2019), and

seasonal change in dentition is known in a number of extant vertebrates, including some sharks,

rays, teleosts, and salamanders (Stewart, 1958; Witten et al., 2005; Berkovitz and Shellis, 2017).

However, the literature lacks reports of such dental plasticity in squamates, and I consider it

highly unlikely that TMP 2014.011.0001 is merely a seasonal variant of a known Tylosaurus

30 species based on the following reasoning. First, the hundreds of Tylosaurus specimens known from further south in the seaway (Russell, 1967a, 1988; Nicholls, 1988; Kiernan, 2002; Jiménez-

Huidobro, 2016) display a conserved dental morphology (e.g., Russell, 1967a; Everhart, 2005;

Bullard and Caldwell, 2010; Jiménez-Huidobro et al., 2016, 2018; pers. observ.). If TMP

2014.011.0001 were a seasonal migrant from these southern locales, with a seasonal dentition, it would be expected that the known Tylosaurus fossil assemblage from lower latitudes would include some individuals, either about to embark on a northward migration or freshly returned from one, showing mixed dental characteristics (i.e., in the midst of transitioning between seasonal phenotypes). Given the lack of overlap in maxillary crown aspect ratio and spacing between TMP 2014.011.0001 and other Tylosaurus specimens of comparable size as I demonstrated (Figs. 15, 16; Appendix 2), this scenario seems highly improbable. Moreover, the gracile construction of the maxilla seen in TMP 2014.011.0001, seemingly independent of ontogeny, is outside of the morphological spectrum exhibited by contemporary congeners of comparable size in more southerly North American locales, and is likely too drastic a structural change to vary on a seasonal basis (Fig. 19; Appendix 4). I hence maintain the designation of

TMP 2014.011.0001 as a new species within Tylosaurus.

Osteology

Septomaxilla—TMP 2014.011.0001 has a septomaxilla that is transversely broad and slightly dorsally concave, forming the floor of the anterior naris between the internarial bar and the maxilla (Figs. 3, 12A). Phylogenetic studies of Squamata generally place mosasauroids within Toxicofera, sometimes within Pythonomorpha and sometimes within Varanoidea or elsewhere within Anguimorpha (e.g., Caldwell et al., 1995; Lee, 1997, 2005; Caldwell, 1999b;

31 Lee and Caldwell, 2000; Conrad, 2008; Reeder et al., 2015; Simões et al., 2017, 2018; Paparella

et al., 2018; but see Gauthier et al., 2012; Fig. 1A). A broad septomaxilla is common in these

clades (Lapage, 1928; Bellairs, 1949; Lee and Scanlon, 2002; Evans, 2008). A broad, triangular

septomaxilla is also seen in the fossil taxa Coniasaurus gracilodens and Primitivus

manduriensis, both dolichosaurs, a hypothesized sister group to Mosasauroidea (Caldwell,

1999a, 1999b; Simões et al., 2017; Paparella et al., 2018). Given this morphological consistency

and its similarity to the broad septomaxillae of the basal mosasaurid described by Bell and

Polcyn (2005b) and the basal russellosaurine mosasaurs Tethysaurus nopcsai and Yaguarasaurus

columbianus (Páramo, 1994; Páramo-Fonseca, 2000; Bardet et al., 2003), and herein also confirmed in the large tylosaurine TMP 2014.011.0001, perhaps a broad septomaxilla that

completely bridges the naris anteriorly between the internarial bar and the maxilla is a

plesiomorphic trait within Mosasauridae.

This characterization of the basal mosasaur septomaxilla contrasts notably with that of

Caldwell et al. (1995), which was later reiterated and expanded to represent the condition for

mosasaurs in general by Caldwell (1999b, 2012). Caldwell et al. (1995:524) state that one of the

features associated with apparent retraction of the nares in more basal mosasaurs is “extreme

elongation and narrowing of the septomaxilla,” citing only Camp (1942) as a reference for this

characteristic. I consider this problematic for two reasons: (1) Camp (1942) describes the

septomaxilla of only Plotosaurus bennisoni; and (2) P. bennisoni is a relatively derived, rather

than basal, member of Mosasauridae (Russell, 1967a; Bell, 1997; Simões et al., 2017). As

outlined above, the elongate septomaxillary morphology of P. bennisoni is representative of

neither mosasaurs as a whole nor basal mosasaurs in particular.

32 Downward Maxillary Extensions—The downward extension of the maxillary cortical bone over the roots of the maxillary teeth in the anterior teeth of TMP 2014.011.0001 (Fig. 11), is known in the non-tylosaurine mosasaurs Goronyosaurus nigeriensis (Lingham-Soliar, 2002) and the holotype of ‘Platecarpus’ somenensis (MNHN 1895-7, pers. observ.). Lingham-Soliar

(2002) argues that the downward extension of the maxilla in Goronyosaurus nigeriensis, only appearing on the third left maxillary tooth, serves to increase the length of the corresponding tooth relative to the neighboring ones without increasing the height of the crown itself, and analogizes the effect to the dentition of the Nile crocodile. The condition in TMP 2014.011.0001, however, is different, in that downward extension of the maxilla over the tooth root is visible in left maxillary teeth one, two, and three, as well as right maxillary tooth one (unable to determine for right maxillary tooth two). While the maxillary extensions in TMP 2014.011.0001 simply cover these tooth roots rather than acting as extended pedestals for the teeth as suggested by

Lingham-Soliar (2002), in general, the anterior four maxillary tooth crowns of TMP

2014.011.0001 are taller and more slender than the other preserved teeth (Appendix 2). In comparison to other Tylosaurus specimens, all these anterior maxillary teeth of TMP

2014.011.0001 have relatively high crown aspect ratios, although the heights of the crowns are not unusual for the genus (Figs. 15, 17B; Appendix 2). The maxillary extensions in TMP

2014.011.0001 could serve to better anchor the roots of the anterior teeth, allowing for the growth of relatively gracile crowns, as opposed to lengthening a singular ‘fang’ as postulated by

Lingham-Soliar (2002) in Goronyosaurus nigeriensis,

Ontogenetic Changes in Dentition—Little information exists in the literature regarding

Tylosaurus dental ontogeny. Jiménez-Huidobro et al. (2016) recharacterize T. ‘kansasensis’ as a juvenile ontogimorph of T. nepaeolicus, but report little difference between the teeth of T.

33 ‘kansasensis’ and those of T. proriger and adult T. nepaeolicus, similar to the original description of T. ‘kansasensis’ (Everhart, 2005). However, dentition is not the focus of those papers, and hence their assessment of marginal dentition speaks more to a gross morphological consistency in dentition among Tylosaurus, both juvenile and adult, rather than a lack of informative ontogenetic variation. Konishi and Caldwell (2007a) propose that, over the course of

Tylosaurus ontogeny, the jaw bones scale isometrically with tooth alveolus size but allometrically with tooth crown spacing, such that the alveoli maintain a consistent relative spacing while the tooth crowns themselves become relatively larger and thus closer together.

Konishi and Caldwell (2007a) base their argument on the observation that juvenile Tylosaurus teeth are more slender than those of adults, and Konishi et al. (2018) and Stewart and Mallon

(2018) also remark on the slenderness of the tooth crowns in small Tylosaurus individuals (Fig.

14).

Tooth slenderness can be most directly quantified by measuring crown aspect ratio, but the tooth crowns are often missing or incomplete in Tylosaurus specimens (Russell, 1967a; pers. observ.). Thus, while a number of juvenile Tylosaurus specimens are known, it is difficult to assess ontogenetic changes in tooth size and shape. Tooth crown spacing, on the other hand, can often be observed in the absence of the crown itself and, consequently, my crown spacing dataset is more extensive than my crown aspect ratio dataset (Figs. 15, 16, 18; Appendices 2, 3). Given the proposed isometry between Tylosaurus alveoli and jaws (Russell, 1967a; Konishi and

Caldwell, 2007a; pers. observ.), ontogenetic change in tooth crown spacing is then related to ontogenetic variation in tooth crown aspect ratio. Konishi et al. (2018) use crown spacing comparisons to support Konishi and Caldwell’s (2007a) Tylosaurus ontogenetic arguments, and elements of my limited crown aspect ratio data, corroborated by my more-extensive crown

34 spacing data, as well as observations of teeth that were representative but unsuitable for

measurement, further support the conclusions of Konishi and Caldwell (2007a) (Figs. 15, 16, 18;

Appendices 2, 3). My data lend additional strong support to these conclusions, in that the ratio of

basal crown diameter to theoretical socket diameter increases with skull size in Tylosaurus,

indicating that crown diameter increases at a faster rate than socket size as the animal grows

(Fig. 17C).

Paedomorphosis—The high-aspect-ratio, widely spaced maxillary crowns in TMP

2014.011.0001 resemble the condition in smaller, presumably juvenile, congeneric individuals

(Figs. 14–18). Both the overall slenderness and spacing of the crowns in TMP 2014.011.0001 are

due to unusually restricted crown basal diameters in this presumed adult specimen (Fig. 17), a

condition that may represent paedomorphosis. While all russellosaurine mosasaurs present

slender tooth crowns as juveniles, only tylosaurines, and particularly Tylosaurus, show a

consistent ontogenetic increase in tooth crown robustness (e.g., Russell, 1967a; Konishi and

Caldwell, 2007a; Palci et al., 2013; Stewart and Mallon, 2018; pers. observ.), and it is assumed

this was also the case in the last common ancestor between TMP 2014.011.0001 and its

congeneric sister taxon.

Additionally, several metrics in this study indicate that the maxilla of TMP

2014.011.0001 is relatively shallow in lateral aspect compared to other Tylosaurus (see above;

Fig. 19; Appendix 4). A reduced maxillary lamina could be another paedomorphic character in

TMP 2014.011.0001, as its maxilla height is more typical of smaller individuals of Tylosaurus in

both relative and absolute senses (Fig. 19). With my data showing that maxillary tooth crown

height in TMP 2014.011.0001 is consistent with Tylosaurus ontogenetic trends (Fig. 17B;

Appendix 2), and lacking any clear evidence for extra space between its tooth sockets, there is no

35 indication that the apparent low profile of the maxilla in TMP 2014.011.0001 is due to

anomalous dentition or to elongation of the maxilla rather than simply reduced height of the

lamina. In summary, the skull of TMP 2014.011.0001 is paedomorphic in exhibiting slender

marginal tooth crowns and shallow maxillae anchoring these teeth.

Paleobiogeography

Northernmost Tylosaurus—TMP 2014.011.0001, recovered from approximately 62°N

paleolatitude in northwest Alberta, Canada, represents the first unequivocal tylosaurine mosasaur

found north of 60°N paleolatitude in North America, extending the known range of tylosaurines

on that continent northward by about 550 km, and is the northernmost known occurrence of

Tylosaurus worldwide (Fig. 20). Globally, only a single specimen of Tylosaurinae is known from

a more northerly locale, that being Tylosaurinae indet. material from the Turonian of Chukotka

in Russia’s Far East region (Grigoriev and Grabovskiy, 2019). Some Swedish tylosaurine

specimens come from localities that are currently north of TMP 2014.011.0001’s discovery site

(Persson, 1959; Lindgren, 2005), but during the Late Cretaceous these locales were farther south

(Smith et al., 1994). TMP 2014.011.0001 is one of only a few dozen tylosaurine specimens

known from Canada (Nicholls, 1988; Bullard and Caldwell, 2010; Jiménez-Huidobro et al.,

2018), compared to hundreds from Kansas (Russell, 1967a, 1988; Jiménez-Huidobro, 2016).

Previously, according to the literature, the northernmost North American tylosaurine specimen was the holotype of T. saskatchewanensis, RSM P2588.1, from the upper Campanian of the

Bearpaw Formation near Herbert Ferry in Saskatchewan, with a paleolatitude of about 56°N

(Jiménez-Huidobro et al., 2018). However, another tylosaurine specimen, TMP 1983.126.0001, undescribed but mentioned briefly by Caldwell (2005), from the upper Campanian of the

36 Bearpaw Formation just east of Dinosaur Provincial Park in Alberta, has a provenance slightly north of that of RSM P2588.1, in terms of both current latitude and paleolatitude (about 57°N paleolatitude). Jiménez-Huidobro et al. (2018) do mention TMP 1983.126.0001, but they mistakenly maintain it originated south of RSM P2588.1. The confusion likely arises from the vague locality information provided by Caldwell (2005), but precise locality coordinates obtained from the respective institutions reveal TMP 1983.126.0001 to be the more northerly of the two specimens.

Coming from the Santonian–lower middle Campanian Puskwaskau Formation, TMP

2014.011.0001 is potentially the oldest known tylosaurine from Canada, as no Canadian tylosaurine material has yet been reported from the Santonian (Nicholls, 1988; Russell, 1988;

Jiménez-Huidobro et al., 2018; Jiménez-Huidobro and Caldwell, 2019; Fig. 2). The Puskwaskau

Formation partially overlaps temporally with other well-known mosasaur-bearing localities such as the Smoky Hill Chalk Member of the Niobrara Formation in Kansas, the Mooreville Chalk

Formation in Alabama, and the Pembina and Sharon Springs Members of the Pierre Shale

Formation in South Dakota, Wyoming, and Manitoba. However, compared to these formations, far less is known about the vertebrate faunal assemblage of the Puskwaskau Formation, where only a handful of marine reptile and teleost fossils have been reported (Collom, 2001; Bell et al.,

2014; Vavrek et al., 2016). The mosasaur material known from the Puskwasakau Formation amounts to an isolated tooth crown and a dorsal vertebra both attributed to Plioplatecarpinae, a caudal centrum assigned to Russellosaurina, and undescribed skull material from Kakut Creek, where TMP 2014.011.0001 was also found (Collom, 2001; Bell et al., 2014). The discrepancy in known vertebrate diversity between the Puskwaskau Formation and other formations farther south is likely due, at least partially, to the relative remoteness and inaccessibility of its

37 outcroppings, which has resulted in less effort toward finding and collecting fossils from the

formation (Driscoll et al., 2018). This reflects an overall collection bias in the North American

mosasaur fossil record. Among nominal North American Tylosaurus species, T. nepaeolicus and

T. proriger are known collectively from over 200 specimens, almost entirely from Alabama,

Kansas, and Texas, while species coming exclusively from localities north of Kansas are known

from far fewer specimens (Russell, 1967a; Kiernan, 2002; Driscoll et al., 2018; Fig. 20).

Tylosaurus pembinensis is known from about a dozen specimens from southern Manitoba, while

T. saskatchewanensis and T. ‘neumilleri’ are each based on a single specimen, from southern

Saskatchewan and South Dakota, respectively (Martin, 2007; Bullard and Caldwell, 2010;

Jiménez-Huidobro et al., 2018; Jiménez-Huidobro and Caldwell, 2019). I herein establish T. borealis, sp. nov., based on TMP 2014.011.0001 from northwest Alberta.

Pasch and May (2001) describe bite marks on the bones of a hadrosaur (UAMES 12275) from the middle Turonian of Alaska, comparing the marks to the teeth of a Tylosaurus proriger specimen. They speculate that the bites were made by a marine scavenger with similar teeth, possibly a mosasaur. While a mosasaur could feasibly be the culprit, Pasch and May (2001) offer no conclusive evidence of such, let alone that the scavenger was a T. proriger specifically.

Unequivocally, the middle Turonian age and the Pacific Rim provenance of UAMES 12275 rule out T. proriger, which do not appear in the fossil record until the late Santonian and are known exclusively from the Western Interior Seaway (Pasch and May, 2001; Jiménez-Huidobro and

Caldwell, 2019). Only a few tylosaurine specimens are known globally from the Turonian, none of them from the Pacific Basin (Polcyn et al., 2008; Bell et al., 2013; Loera Flores, 2013;

Grigoriev and Grabovskiy, 2019). Aside from a fragmentary specimen from the Campanian of

Japan diagnosed as Tylosaurus? sp. (Sato et al., 2012), the only known Pacific tylosaurines are

38 Taniwhasaurus, which do not appear in the fossil record until the Santonian (Jiménez-Huidobro and Caldwell, 2019).

Both the marine and terrestrial fossil records indicate that the Bering Strait closed around the Albian–Cenomanian transition and remained closed well into the Cenozoic, aside from an apparent opening and reclosing during the Coniacian (Briggs, 1987; Marincovich et al., 1990;

Iba et al., 2011; Zakharov et al., 2011), precluding the movement of aquatic fauna from the northern Western Interior Seaway into the northern Pacific Ocean. In its north-south pathway across the North American continent, the Western Interior Seaway was bounded on the west by the orogen of the North American Cordillera (Miall et al., 2008), thus keeping the seaway separated from the Pacific Ocean for its entire length. As a result, the mosasaur community of the Western Interior Seaway, so far as the available fossil record is concerned, was distinct from that of the Pacific coast of North America (Camp, 1942; Russell, 1988; Nicholls and Meckert,

2002).

The distribution of Tylosaurinae bears this out. All North American tylosaurines have come from the Western Interior Seaway and the Atlantic coast (Russell, 1988); none have ever been found on the west coast of the continent. The only unequivocal tylosaurine mosasaurs known from the entire Pacific Basin are Taniwhasaurus, and thus far these have come from the

Santonian–Campanian of Antarctica, Japan, and New Zealand, but not North America (Caldwell et al., 2005; Caldwell et al., 2008; Fernández and Martin, 2009; Jiménez-Huidobro, 2016).

Taniwhasaurus occur at relatively high latitudes in both the Northern and Southern Hemispheres, comparable to the high northern latitude of TMP 2014.011.0001. However, as described above,

TMP 2014.011.0001’s conspicuous lack of a sagittal ridge on the dorsal surface of the premaxilla

(Caldwell et al., 2005; Caldwell et al., 2008; Fernández and Martin, 2009), combined with the

39 Bering land bridge and the North American Cordillera acting as physical barriers between the

Western Interior Seaway and Pacific Ocean, rules out Taniwhasaurus as a reasonable diagnosis for the specimen.

While there is some evidence for glaciation at times during the Late Cretaceous

(Bornemann et al., 2008; Ladant and Donnadieu, 2016), global temperatures were warmer during that time period. Isotopic data, floral assemblages, and the presence of large terrestrial reptiles, including champsosaurs and various dinosaurs, in the North American Arctic during the Late

Cretaceous indicate the Arctic climate, while variable, was at least cool-temperate even during cooler periods (Marincovich et al., 1990; Tarduno et al., 1998; Fiorillo and Gangloff, 2000;

Zakharov et al., 2011). Marine temperatures reflected this warmer climate as well. There was less of a latitudinal water temperature gradient than there is today (Lowenstam, 1964;

Marincovich et al., 1990; Nicholls and Russell, 1990; Thiede et al., 1990) and, while estimates vary, average sea surface temperatures from the Arctic of the Late Cretaceous are consistently found to have been well above current temperature averages (Lowenstam, 1964; Herman and

Spicer, 1996; Bice and Norris, 2002; Payne et al., 2012). This reduced latitudinal temperature gradient during the Late Cretaceous has been hypothesized to be linked to latitudinally wider- ranging faunal provinces (Marincovich et al., 1990), including in the Western Interior Seaway, which Nicholls and Russell (1990) and others (see Nicholls and Russell, 1990) have divided into only two faunal provinces within its entire extent. More recently, Bernard et al. (2010) and

Harrell et al. (2016) have suggested mosasaur endothermy, which, combined with the reduced

Late Cretaceous water temperature gradient, suggests less heterogeneity in mosasaur distributions. In this light, finding the new Tylosaurus specimen TMP 2014.011.0001 from northwest Alberta to be taxonomically distinct from more southerly specimens is unexpected,

40 hinting at a more complex, hitherto unsuspected heterogeneous provinciality for the marine fauna in the Late Cretaceous Western Interior Seaway.

Tylosaurus Distribution in Western Interior Seaway—Nicholls and Russell (1990) used relative abundance data for marine vertebrates in their biogeographical analysis of the

Western Interior Seaway, comparing vertebrate fossil assemblages from five localities along the seaway’s path. From north to south these localities are (1) the Anderson River Plain on the northern coast of the Northwest Territories; (2) the Pembina Member of the Pierre Shale

Formation in southern Manitoba; (3) the Sharon Springs Member of the Pierre Shale Formation, extending from South Dakota to Kansas; (4) the Smoky Hill Chalk Member of the Niobrara

Formation in western Kansas; and (5) the Mooreville Chalk Formation of central Alabama (Fig.

20). By Nicholls and Russell’s (1990) accounting, these assemblages all date to the early

Campanian, although the Smoky Hill Chalk assemblage from Kansas also includes older material from the Santonian and Coniacian. In the decades since Nicholls and Russell’s (1990) study, new findings have shown this age correlation to be largely nonexistent. What Russell

(1967b) identified as Platecarpus from the Anderson River is assignable to Plioplatecarpus

based on a distinct bulge on the quadrate shaft (T. Konishi, pers. comm., 2019). Given that

Pliopatecarpus do not appear in the fossil record until around 74 Ma, the Anderson River assemblage cannot be older than late Campanian (Konishi and Caldwell, 2011). Additionally, the

Pembina and Sharon Springs Members of the Pierre Shale Formation are now placed in the early middle Campanian and the Mooreville Chalk Formation straddles the Santonian–Campanian boundary (Kiernan, 2002; Martin et al., 2007).

Correlation issues aside, according to Nicholls and Russell (1990), vertebrate

distributions in these assemblages reveal two faunal provinces in the Western Interior Seaway,

41 the Northern and Southern Interior Subprovinces, with the boundary between the two located south of the Niobrara assemblage. They suggest Platecarpus as the most abundant mosasaurs in

the Northern Interior Subprovince and Clidastes as the most abundant mosasaurs in the Southern

Interior Subprovince. This characterization of the mosasaur fauna is problematic because, as

mentioned above, Plioplatecarpus are now recognized as the only definitively known mosasaur

genus from the Anderson River assemblage. Similarly, based on current alpha taxonomy, the

supposed Platecarpus sensu Nicholls and Russell (1990) from the Pembina and Sharon Springs

Members of the Pierre Shale Formation are likely Latoplatecarpus, with Platecarpus now

recognized only from Kansas and farther south (Konishi and Caldwell, 2009, 2011; Konishi et

al., 2010). Furthermore, even beyond these taxonomic issues, sampling bias renders the

Anderson River assemblage unsuitable for Nicholls and Russell’s (1990) relative abundance

comparison.

Firstly, according to Nicholls and Russell (1990), the vertebrate record for the Anderson

River assemblage comes from a single field expedition and includes only 21 specimens, a

striking contrast to the extensive fieldwork and 600+ to 7000+ specimens associated with each of

the other assemblages. A single sample of only 21 specimens only provides a very coarse image

of the vertebrate fauna of the Anderson River. Tylosaurus are relatively uncommon among the

mosasaurs of the Western Interior Seaway (Nicholls and Russell, 1990), and hence a lack of

Tylosaurus in the limited fossil record of the Anderson River does not necessarily denote their

absence there. While exposed farther south, the Pembina Member of the Pierre Shale Formation

in southern Manitoba, Canada, has produced at least 18 tylosaurine specimens (Nicholls, 1988).

Additionally, the southern four assemblages are within a latitudinal range that is less than

the distance between the Anderson River assemblage and the Pembina assemblage (Fig. 20).

42 This is particularly problematic, as the Anderson River assemblage not only has a drastically lower sample size than the other study areas, but it was also considered by Nicholls and Russell

(1990) to be informative for a latitudinal range larger than that for the other four assemblages combined. Thus, in my view, the Anderson River assemblage is hardly representative of the

Anderson River locality, let alone the huge swath of seaway that existed between the Anderson

River and the Pembina outcropping in southern Manitoba—essentially the entire Canadian

portion of the Western Interior Seaway. Although tylosaurines have yet to be reported from the

Anderson River, TMP 2014.011.0001 strongly indicates that they were likely permanent

inhabitants of the northern reaches of the seaway. Corroborating this biogeographic assessment

within the seaway, the Pacific Basin tylosaurines Taniwhasaurus oweni and Ta. antarcticus are known from comparable paleolatitudes in the Southern Hemisphere, around 60°S and 64°S paleolatitude, respectively (Caldwell et al., 2005; Fernández and Martin, 2009), and Grigoriev and Grabovskiy (2019) report on Russian Tylosaurinae indet. material from well north of TMP

2014.011.0001’s locality.

Furthermore, while the majority of North American Tylosaurus material comes from the

southern half of the Western Interior Seaway, particularly Kansas, Alabama, and Texas, TMP

2014.011.0001 adds to existing evidence of persistent Tylosaurus diversity in the middle third of

the seaway, north of Kansas, from the Santonian (likely age of TMP 2014.011.0001) through the

latest Campanian (T. saskatchewanensis), with T. proriger, T. pembinensis, and T. ‘neumilleri’

also known from this geographic and temporal range (Russell, 1967a, 1988; Thurmond, 1969;

Echols, 1972; Martin and Bjork, 1987; Martin, 2007; Meredith et al., 2007; Polcyn et al., 2008;

Bullard and Caldwell, 2010; Loera Flores, 2013; Driscoll et al., 2018; Jiménez-Huidobro et al.,

2018; Jiménez-Huidobro and Caldwell, 2019; Fig. 20). Interestingly, in Alabama, where the Late

43 Cretaceous mosasaur fossil record is essentially continuous from the Santonian through the

Maastrichtian, Tylosaurus become conspicuously absent by the middle Campanian (Kiernan,

2002), although Tylosaurus persist at least into the middle Campanian of the Ozan Formation of

Texas (Thurmond, 1969; Echols, 1972; Russell, 1988; Driscoll et al., 2018). While this apparent faunal change in the Gulf could be an artifact of preservation and/or collection, it coincides with at least two possibly related phenomena. One is the increasing presence of large mosasaurines in the Gulf mosasaur fossil record in the latter half of the Campanian and the Maastrichtian

(Russell, 1967a; Kiernan, 2002), and the other is the separation of the Western Interior Seaway from the Gulf of Mexico during the Campanian, a process that was completed during the

Maastrichtian, if not the late Campanian (Sohl et al., 1991; Kauffman and Caldwell, 1993; Berry,

2017; Blakey and Ranney, 2018). Perhaps due to increased competition with large Gulf mosasaurines, Tylosaurus in North America may have become restricted to the seaway as it separated from the Gulf of Mexico during the middle and late Campanian and receded northward during the Maastrichtian. Further, given the temporally stable Tylosaurus diversity north of

Kansas from the Santonian through the uppermost Campanian, there is likely more undiscovered

Tylosaurus diversity in the northern latitudes of the Western Interior Seaway, possibly into the

Maastrichtian and into the Arctic Circle (Fig. 20).

Food Habits and Paleoecology

With its slender, widely spaced dentition and relatively low-profile maxilla, TMP

2014.011.0001 shows possible dietary specialization unique among its congeners. Compared to most other mosasaurs, Tylosaurus exhibit a long muzzle relative to total skull size (Russell,

1967a). Reducing the height of the maxilla, as seen in TMP 2014.011.0001, results in a lower

44 profile muzzle and could be a way to enhance the maneuverability of this long snout,

approaching the longirostrine morphology of some extant piscivorous, secondarily aquatic

tetrapod taxa (Cleuren and De Vree, 2000; Werth, 2000; McCurry et al., 2017). Particularly

high-aspect-ratio anterior teeth in TMP 2014.011.0001 invites comparison to extant taxa with a

similar dental condition, such as gharials and South Asian river dolphins. These taxa also exhibit

long, low-profile snouts with the highest-aspect-ratio teeth borne anteriorly, and are known to feed primarily on fish (Williston, 1914; Cleuren and De Vree, 2000; Werth, 2000; McCurry et al., 2017). It then follows that the analogous dental features in TMP 2014.011.0001 might also be a specialization for piscivory.

Massare’s (1987, 1997) feeding guilds—‘Cut,’ ‘Pierce I,’ ‘Pierce II/General,’ ‘Smash,’

‘Crunch,’ and ‘Crush’—have long represented the standard model for assessing tooth function in

Mesozoic marine reptiles, including mosasaurs. This model is largely qualitative, based on observations about the shape and wear of a given tooth, but does include two quantitative measures, relative tooth size and aspect ratio, though Massare (1987) only loosely associates different guilds with these measures. More recent quantitative treatments of Massare’s (1987) feeding guild model have either included non-reptile marine taxa, thus reducing the resolution

among marine reptiles, or have focused on Jurassic taxa, thus excluding mosasaurs (Ciampaglio

et al., 2005; Foffa et al., 2018). According to Massare (1987:table 3), tooth crown aspect ratios

are 1.5–2.5 for the ‘Cut’ guild and usually 2.0–3.0 for the ‘Pierce II/General’ guild. While there

is some clear overlap between the guilds, typical Tylosaurus fall within her model’s ‘Cut’ guild, best adapted for bony prey such as reptiles and large fish (Massare 1987; Ross, 2009), an assessment supported by fossilized adult Tylosaurus gut contents (Massare, 1987:table 1;

Everhart, 2004). However, TMP 2014.011.0001’s teeth, at least the anterior ones, are

45 representative of the ‘Pierce II/General’ guild, pointed but more slender than teeth in the ‘Cut’ guild, and defined by the model as more particularly suited for piscivory, corroborating the above comparison to other piscivorous taxa (Massare, 1987; Ross, 2009; Fig. 21).

An interesting question is the functional significance, if any, of the increased interdental spacing in TMP 2014.011.0001. Churchill and Clementz (2015) propose that, in pinnipeds, wider spacing between teeth is associated with a reduced role of dentition in feeding, but the piercing tooth shape of TMP 2014.011.0001 suggests that is likely not the case here. Perhaps the dental morphology of TMP 2014.011.0001 is something of an evolutionary compromise, converging toward the fish-specialist model exemplified by such extant taxa as gharials and South Asian river dolphins, but within the phylogenetic constraints of its tylosaurine origins, i.e., conserving the standard Tylosaurus dental count. Thus the increased spacing between crowns in TMP

2014.011.0001 may not serve an explicit functional purpose, but is merely a necessary evolutionary consequence of the reduced crown diameters.

As already discussed, the narrow marginal tooth crown diameters and reduced maxillary lamina in TMP 2014.011.0001 likely represent paedomorphosis (Fig. 14), suggesting a similar diet to smaller tylosaurs known from more southern waters. Ontogenetic change in dentition in mosasaurs is presumably related to a change in diet, especially considering the accompanying increase in body size and the known ontogenetic changes in dentition and diet of extant monitor lizards (Mertens, 1942; Lenz, 2004; Polcyn and Bell, 2005b; Konishi and Caldwell, 2007a;

Polcyn et al., 2014; Fig. 22). Although very few exist, studies of stable isotopes in mosasaur fossils support such an inference, with juvenile mosasaurs exhibiting stable isotope values indicating nearer-to-shore environments and shallower foraging habits than larger ontogimorphs

(Clementz and Koch, 2001; Robbins et al., 2008; Schulp et al., 2013; Polcyn et al., 2014). This

46 may suggest that juvenile mosasaurs inhabited nearshore nurseries that perhaps offered abundant

small prey as well as protection from larger predators, an idea with a long history among marine

reptile researchers (e.g., Williston, 1914; Russell, 1967a; Street et al., 2019; but see Everhart,

2002a, 2007; Houssaye and Tafforeau, 2012; Field et al., 2015). The Puskwaskau Formation,

where TMP 2014.011.0001 was found (Fig. 2), represents a relatively shallow marine setting

(less than 50 m deep) (Hu, 1997; Hu and Plint, 2009; Bell et al., 2014), perhaps similar to such

nursery environments. The dentition of TMP 2014.011.0001 may therefore have been adapted

for feeding on the smaller fish that likely inhabited such shallow settings, comparable to the

presumed diet of juvenile tylosaurs in similar environments.

Nicholls and Russell (1990) note that fish were abundant throughout the Late Cretaceous

Western Interior Seaway, including in the northern latitudes, as reported by Russell (1967b),

with much isolated fish material remaining uncatalogued in museum collections. With the

ubiquity of fish, both large and small, in the seaway, it is intriguing that TMP 2014.011.0001 is

the only known example of divergence from the standard Tylosaurus dental model toward a

possible specialization in ichthyophagy. TMP 2014.011.0001 also being from a locality well

north of any other known Tylosaurus implies that there was a significantly different faunal assemblage at higher latitudes in the Western Interior Seaway.

CONCLUSIONS

TMP 2014.011.0001, recovered from approximately 62°N paleolatitude near present-day

Grande Prairie, northwest Alberta, Canada, is confidently assignable to Tylosaurus within

Tylosaurinae based on the following suite of characters: broad, parallel-sided, and dorsally

47 smooth premaxilla between the maxillae; round and robust cross section of the dentigerous portion of the premaxilla; and overall homodont dentition. Furthermore, based on the specimen’s unusually slender and widely spaced tooth crowns, low-profile maxilla, and downward extension of the maxillary cortical bone to cover the anterior tooth roots, I establish T. borealis, sp. nov., with TMP 2014.011.0001 as the holotype. The likely Santonian age of TMP 2014.011.0001 and its high-latitude locale add to existing evidence for continuous and high Tylosaurus diversity in the middle and northern stretches of the Western Interior Seaway, beginning at latest in the

Santonian, and continuing throughout the Campanian and possibly even into the Maastrichtian.

Comparison with other Tylosaurus specimens shows its narrow maxillary crown diameters and reduced maxillary lamina to likely be paedomorphic characters, perhaps reflecting a specialization for piscivory. Since TMP 2014.011.0001 is the northernmost known Tylosaurus to date, this dietary specialization could indicate a difference in both predator and prey communities in the northern waters of the Late Cretaceous Western Interior Seaway.

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70 FIGURES

FIGURE 1. Phylogenetic hypotheses of A, Squamata, showing the placement of Mosasauridae; and B, Mosasauridae, showing the placement of Tylosaurinae. Phylogenetic tree in A based on

Reeder et al. (2015), Paparella et al. (2018), and Simões et al. (2018). Phylogenetic tree in B based on Bell and Polcyn (2005a). In A, black and white arrowheads indicate Toxicofera and

Pythonomorpha, respectively.

71 72 FIGURE 2. Locality and horizon of TMP 2014.011.0001, holotype of Tylosaurus borealis, sp.

nov. A, map of Canada with inset showing the specimen locality (indicated by the white

arrowhead) in Kakut Creek near Grand Prairie, Alberta, Canada; B, stratigraphic chart of the

Puskwaskau Formation with accompanying time scale. Double-headed black arrow indicates

most likely stratigraphic position of specimen. A modified from Vavrek et al. (2016) with map of

Canada modified from Bell et al. (2014) and Vavrek et al. (2016). B modified from Bell et al.

(2014).

73 74 FIGURE 3. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Incomplete skull in dorsal view. A, photograph; B, line drawing. Abbreviations: see Anatomical Abbreviations section in text. Scale bar equals 10 cm.

75 76 FIGURE 4. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Incomplete skull in left dorsolateral view. A, photograph; B, line drawing. Note the slender second left maxillary tooth crown, as well as the low-profile maxillary lamina. Abbreviations: see Anatomical

Abbreviations section in text. Scale bar equals 10 cm.

77 78 FIGURE 5. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Incomplete skull in left lateral view. Abbreviations: see Anatomical Abbreviations section in text. Scale bar equals

10 cm.

79 80 FIGURE 6. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Incomplete skull in right lateral view. Note the slenderness and wide spacing of the maxillary tooth crowns, as well as the low-profile maxillary lamina. A, photograph; B, line drawing. Abbreviations: see

Anatomical Abbreviations section in text. Scale bar equals 10 cm.

81 82 FIGURE 7. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Incomplete skull in anterior view. A, photograph.; B, line drawing. Hatched area with dotted border in B indicates

the broken face of the premaxilla, anterior to which the remainder of the bone is missing.

Abbreviations: see Anatomical Abbreviations section in text. Scale bar equals 10 cm.

83 84 FIGURE 8. Tylosaurus vomerine processes of the premaxilla. A, premaxilla of TMP

2014.011.0001, holotype of T. borealis, sp. nov., in anterior view; B, premaxilla of FHSM VP-

14845, Tylosaurus sp., in anterior view; C, snout of KUVP 115002, Tylosaurus sp., in palatal view; D, snout of KUVP 65636, Tylosaurus sp., in palatal view. Abbreviations: see Anatomical

Abbreviations section in text. Scale bars equal 2 cm.

85 86 FIGURE 9. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Incomplete skull in posterior view. Abbreviations: see Anatomical Abbreviations section in text. Scale bar equals

10 cm.

87 88 FIGURE 10. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Incomplete right lower jaw. A, lateral view; B, medial view. Abbreviations: see Anatomical Abbreviations section in text. Scale bar equals 10 cm.

89 90 FIGURE 11. Downward extension of the maxilla in mosasaurs. A, right maxilla of TMP

2014.011.0001, holotype of Tylosaurus borealis, sp. nov., in lateral view; B, left maxilla of the same in lateral view; C, premaxilla and right maxilla of MNHN 1895-7, holotype of

‘Platecarpus’ somenensis, in lateral view; D, left maxilla of IGF 14750, neotype of

Goronyosaurus nigeriensis, in lateral view. White arrowheads indicate downward extension of maxillary cortical bone over tooth roots, black arrowheads indicate where this maxillary cortical bone has been lost, exposing the surface of the tooth root. Photo in D modified from Lingham-

Soliar (1991). Abbreviations: see Anatomical Abbreviations section in text. Scale bars equal 5 cm, scale bar in A applies to A–C.

91 92 FIGURE 12. Tylosaurus snouts in dorsal view showing septomaxillae, anterior to the right. A,

TMP 2014.011.0001, holotype of T. borealis, sp. nov.; B, KUVP 1129, T. nepaeolicus; C,

KUVP 28705, T. proriger; D, CMNFV 8162, T. proriger. Black arrowheads indicate possible thickened medial edge of septomaxilla. White arrowheads in A indicate outline of lateral edge of right vomer visible through plastically depressed right septomaxilla. Abbreviations: see

Anatomical Abbreviations section in text. Scale bar equals 5 cm.

93 94 FIGURE 13. TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov. Features of the

marginal dentition. A, weak facets visible laterally on right maxillary tooth one. Apex toward the

bottom; B, anterior carina (indicated by white arrowheads) and apparent lateral compression of

the anteriormost preserved right dentary tooth, from the posterior half of the tooth row. Apex

toward the top; C, cross section of right premaxillary tooth two in occlusal view, showing

rounded lingual surface of crown; D, cross section of left maxillary tooth five in occlusal view,

showing overall round shape; E, line drawing of the same. For C, D, and E, arrow indicates anterior. Scale bars equal 1 cm, upper scale bar applies to A and B, lower scale bar applies to C,

D, and E.

95 96 FIGURE 14. Juvenile and adult Tylosaurus marginal dentition. A, RMM 5610, T. proriger, estimated midline skull length of 51.6 cm; B, KUVP 66129, T. proriger, estimated midline skull length of 50.6 cm; C, FFHM 1997-10, T. proriger, estimated midline skull length of 101.6 cm;

D, CMNFV 40320, T. pembinensis, estimated midline skull length of ca. 120 cm. Notice the more robust crowns and reduced skirting of exposed roots, and hence reduced space between crowns, in teeth of larger Tylosaurus specimens relative to smaller ones. Abbreviations: see

Anatomical Abbreviations section in text. Scale bars equal 5 cm, upper scale bar applies to A and

B, lower scale bar applies to C and D.

97 98 FIGURE 15. Anterior six (where measureable) maxillary tooth crown aspect ratios in lateral view (crown height / crown mesio-distal basal diameter) for TMP 2014.011.0001, holotype of

Tylosaurus borealis, sp. nov., and eight other Tylosaurus specimens, plotted as a function of midline skull length. A, summary plot showing crown aspect ratio data for all six anterior tooth positions; B, crown aspect ratios plotted separately for each tooth position. Data points in A with the same estimated midline skull length represent multiple teeth from a single specimen. A shows no clear trend in the data, and regression analysis, excluding TMP 2014.011.0001, reveals no significant trend for any individual tooth position. Note that the values for maxillary teeth one, two, and four of TMP 2014.011.0001 are greater than any teeth measured in either smaller or larger specimens (see B). Specimen pictured in the legend is the right maxilla of FHSM VP-

7262, T. nepaeolicus.

99 100 FIGURE 16. Anterior six (where measureable) maxillary tooth crown spacing ratios (crown mesio-distal basal diameter / mesio-distal space between the base of that crown and the base of the crown immediately posterior to it) for TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., and 17 other Tylosaurus specimens. A, summary plot showing crown spacing ratio data for all six anterior tooth positions; B, crown spacing ratios plotted separately for each tooth position. Data points in A with the same estimated midline skull length represent multiple teeth from a single specimen. Note that the values for TMP 2014.011.0001 are all less than 1.0, meaning the spaces between crowns are wider than the crowns themselves, and the lowest values among all the data are from teeth belonging to TMP 2014.011.0001. Excepting TMP

2014.011.0001, A and B show a clear trend of increasing crown spacing ratio with increasing skull length among Tylosaurus specimens, and regression analysis, excluding TMP

2014.011.0001, shows this relationship to be significant for maxillary tooth positions one (R2 =

0.73, F(1, 4) = 10.85, p = 0.030) and four (R2 = 0.74, F(1, 6) = 17.06, p = 0.006). For these two tooth positions, the dashed line is the regression line and the gray dotted curves indicate the 95% confidence interval for the regression. Note that the values for TMP 2014.011.0001 are more typical of a smaller individual. Specimen pictured in the legend is the right maxilla of FHSM

VP-7262, T. nepaeolicus.

101 102 FIGURE 17. Comparison of maxillary tooth crown dimensions (where measureable) as functions of estimated midline skull length in TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., and other Tylosaurus specimens. A, mesio-distal basal diameter of maxillary tooth crowns one through six in TMP 2014.011.0001 and 18 other Tylosaurus specimens; B, height in lateral aspect of maxillary tooth crowns one through six in TMP 2014.011.0001 and eight other

Tylosaurus specimens; C, the ratio of crown mesio-distal basal diameter to theoretical socket capacity (crown mesio-distal basal diameter + 0.5 * sum of the mesio-distal spaces between the base of that crown and the bases of the crowns immediately anterior and posterior to it; see c in the legend) for maxillary teeth two through six in TMP 2014.011.0001 and nine other Tylosaurus specimens. All metrics plotted in A–C show an overall increase with increasing skull length, with the values for TMP 2014.011.0001 being unusually low in A and C. Specimen pictured in the legend is the right maxilla of FHSM VP-7262, T. nepaeolicus.

103 104 FIGURE 18. Comparison of posterior dentary tooth crown metrics as functions of estimated

midline skull length for TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., and other Tylosaurus specimens. Data points with the same estimated midline skull length represent multiple teeth from a single specimen. A, tooth crown aspect ratios (crown height / crown mesio- distal basal diameter) for dentary teeth seven through 13 for TMP 2014.011.0001 and four other

Tylosaurus specimens; B, tooth crown spacing ratios (crown mesio-distal basal diameter / mesio- distal space between the base of that crown and the base of the crown immediately posterior to it) for dentary teeth seven through 12 for TMP 2014.011.0001 and nine other Tylosaurus specimens. A potentially shows an overall slight negative trend while B shows an overall positive trend, with the values for TMP 2014.011.0001 being the highest among examined specimens in A and lower than expected in B. Specimen pictured in the legend is the right maxilla of FHSM VP-7262, T. nepaeolicus.

105 106 FIGURE 19. Relative height of the maxilla as a function of estimated midline skull length in

TMP 2014.011.0001, holotype of Tylosaurus borealis, sp. nov., and other Tylosaurus specimens.

A, the ratio of the height (crown plus root) of maxillary tooth four in lateral aspect to the height of the maxilla in TMP 2014.011.0001 and four other Tylosaurus specimens; B, the ratio of the height of the maxilla in lateral aspect at the posterior terminus of the premaxillomaxillary suture to the distance occupied by the first six maxillary teeth in TMP 2014.011.0001 and 21 other

Tylosaurus specimens; C, the height of the maxilla in TMP 2014.011.0001 and 23 other

Tylosaurus specimens. Regression analysis, excluding TMP 2014.011.0001, shows a significant positive correlation in B (R2 = 0.42, F(1, 19) = 13.81, p = 0.001) and C (R2 = 0.90, F(1, 21) =

189.45, p < 0.001). In B and C, the dashed line is the regression line and the gray dotted curves

indicate the 95% confidence interval for the regression. Note that the values for TMP

2014.011.0001 are unusually high in A and lower than expected in B and C. Specimen pictured

in the legend is the right maxilla of FHSM VP-7262, T. nepaeolicus.

107 108 FIGURE 20. North American Tylosaurus paleobiogeography. Known localities for each species are marked by specific icons, with some icons representing multiple specimens of the same species from the same general locality. The boundaries of the Western Interior Seaway and other seas (shaded gray) approximate their Santonian extent and are based on Kauffman (1984),

Scotese (2014), and Blakey and Ranney (2018). Paleolatitudes also represent the Santonian and are based on Scotese (2014) and Blakey and Ranney (2018). Note the new species is the only

Tylosaurus known to date from north of 60oN paleolatitude. Numbered circles show the localities of the marine vertebrate assemblages from Nicholls and Russell’s (1990) relative abundance study of the Western Interior Seaway, these being 1, the Anderson River Plain on the northern coast of the Northwest Territories; 2, the Pembina Member of the Pierre Shale Formation in southern Manitoba; 3, the Sharon Springs Member of the Pierre Shale Formation, extending from South Dakota to Kansas; 4, the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas; and 5, the Mooreville Chalk Formation of central Alabama. Tylosaurus species shown within these circles are not necessarily included in these assemblages, as they may be from different geological formations or members. Base map modified from Bell et al. (2014) and

Vavrek et al. (2016). Tylosaurus locality data compiled from Russell (1967a, 1988), Thurmond

(1969), Echols (1972), Martin and Bjork (1987), Martin (2007), Meredith et al. (2007), Polcyn et al. (2008), Bullard and Caldwell (2010), Loera Flores (2013), Driscoll et al. (2018), Jiménez-

Huidobro et al. (2018), and Jiménez-Huidobro and Caldwell (2019).

109 110 FIGURE 21. Range of crown aspect ratios (crown height / crown mesio-distal basal diameter) for maxillary teeth one through six in different species of Tylosaurus, with corresponding feeding guilds as per Massare (1987). The species are T. nepaeolicus, including T. ‘kansasensis’

(eight teeth; two specimens); T. proriger (nine teeth; four specimens); T. saskatchewanensis (two teeth from the holotype, RSM P2588.1); and T. borealis, sp. nov. (five teeth from the holotype,

TMP 2014.011.0001). Note that the range of aspect ratios for T. borealis, sp. nov., is twice that

of the combined range of the other Tylosaurus species examined, and extends well into the

Pierce II/General feeding guild, uniquely beyond the range of the Cut guild. Multiple age classes are represented in the data for T. nepaeolicus and T. proriger, while the T. saskatchewanensis and T. borealis, sp. nov., individuals are adults. Abbreviations: T. b., sp. nov., Tylosaurus borealis, sp. nov.; T. n., Tylosaurus nepaeolicus; T. p., Tylosaurus proriger; T. s., Tylosaurus saskatchewanensis.

111 112 FIGURE 22. Ontogenetic series of Varanus niloticus (sensu Dowell et al., 2016) skulls in left lateral view. Numerical order 1–4 corresponds to progressive ontogenetic stages. Note the ontogenetic change in shape and size of marginal dentition and increase in bowing of the jaw bones. Skulls to scale with each other. Modified from Mertens (1942:plate 27).

113 114 APPENDICES

APPENDIX 1. Methodology for determining Tylosaurus size metrics. I estimated midline skull length (anterior tip of the rostrum to posterior margin of the occipital condyle) as a proxy for body size. This is one of several Tylosaurus size metrics presented by Everhart (2002b) and

Stewart and Mallon (2018), in turn based on metrics put forth by Russell (1967a). For examined specimens whose midline skull length is provided in Everhart (2002b:table 1) or Stewart and

Mallon (2018:appendix 1), I used the provided measurement. For other specimens, I estimated midline skull length based on the distance from the anterior margin of the first maxillary tooth root to the posterior margin of the sixth maxillary tooth root. When this distance was not provided by Everhart (2002b:table 1) or Stewart and Mallon (2018:appendix 1) and I could not measure it myself, I used the corresponding distance in the dentary, which I measured myself when not provided by Everhart (2002b:table 1) or Stewart and Mallon (2018:appendix 1). I then used the following conversion factors:

Maxillary Tooth 1–6 Length = 0.2500 * Midline Skull Length; and

Dentary Tooth 1–6 Length = 0.2504 * Midline Skull Length.

I calculated these conversion factors based on measurements provided in Everhart

(2002b:table 1). I divided one to six maxillary/dentary tooth length by midline skull length for each specimen where both measurements were provided (excluding measurements noted as approximate by Everhart, 2002b), and then averaged these ratios to calculate my conversion factors.

115 Using the same methodology, I also arrived at two conversion factors to estimate total body length:

Maxillary Tooth 1–6 Length = 0.0290 * Total Body Length; and

Dentary Tooth 1–6 Length = 0.0301 * Total Body Length.

The body lengths given by Everhart (2002b:table 1) are estimates and are rounded to the nearest decimeter, whereas other measurements are recorded to the nearest millimeter. Thus I consider these body size conversion factors to be less reliable than the midline skull length conversion factors, and have used estimated midline skull length as a proxy for body size in my analysis throughout.

In addition, I derived an alternative estimate for Tylosaurus body size based on the proportions of T. dyspelor AMNH FR 221 as reported by Osborn (1899). According to Osborn

(1899), the skull of AMNH FR 221 is 1.19 m long (tip of rostrum to back of supratemporal arch, slightly longer than midline skull length) and its total body length is 8.83 m, and thus:

Midline Skull Length ≈ 0.1348 * Total Body Length.

This conversion results in estimated tylosaur body lengths consistently shorter than those calculated using the above Everhart (2002b)-based conversion factor. The Osborn (1899) conversion and the Everhart (2002b) conversion likely serve as reasonable lower and upper bounds, respectively, for estimating overall Tylosaurus body size.

116 APPENDIX 2. Tylosaurus tooth crown measurements and ratios for maxillary teeth one through six (where measureable). All teeth are from the right maxilla, except for those tooth positions in parentheses, which are from the left maxilla. Midline skull length is the distance (cm) from the tip of the rostrum to the back of the occipital condyle. Abbreviations: CAR, crown aspect ratio

(crown height / crown mesio-distal basal diameter); CD, crown mesio-distal basal diameter; CH, crown height; CSR, crown spacing ratio (crown mesio-distal basal diameter / mesio-distal distance between the base of that crown and the crown immediately posterior to it); HT, holotype specimen; MSL, estimated midline skull length (cm); SC, theoretical socket capacity

(crown mesio-distal basal diameter + 0.5 * sum of the mesio-distal spaces between the base of that crown and the bases of the crowns immediately anterior and posterior to it).

117 CH CD SC MSL Source for Specimen Species Tooth (cm) (cm) (cm) CAR CSR CD / SC (cm) MSL BMNH R3626/R3627 Tylosaurus sp. (4) 2.98 1.71 1.74 65.2 pers. observ. CMNFV 8162 T. proriger 1 1.33 57.4 Stewart and 2 1.06 1.98 1.20 0.53 Mallon (2018) 3 2.08 1.13 2.13 1.84 1.04 0.53 5 1.86 0.99 1.88 CMNFV Tylosaurus sp. 1 1.06 76.4 pers. observ. 40930/40937/40943 2 1.69 3.19 1.06 0.53 3 1.86 3.28 1.34 0.57 4 1.82 3.23 1.26 0.56 (6) 1.29 FFHM 1997-10 T. proriger (1) 3.85 2.86 1.35 2.24 101.6 Everhart (2) 4.09 2.68 4.02 1.53 1.84 0.67 (2002b) (3) 2.51 4.18 1.34 0.60 118 (5) 4.00 2.63 1.52 1.45 FHSM VP-3 T. proriger (2) 4.94 2.81 1.76 1.34 108.0 Everhart (3) 5.61 3.26 5.02 1.72 2.34 0.65 (2002b) (4) 2.87 4.53 1.41 0.63 (5) 4.80 3.08 1.56 FHSM VP-78 T. 'kansasensis' 1 0.78 38.4 pers. observ. 2 0.63 1.62 0.64 0.39 3 0.71 1.56 0.88 0.45 FHSM VP-2209 T. nepaeolicus 1 1.70 82.0 Everhart 3 1.79 0.97 (2002b) 4 1.84 6 1.91 0.96 FHSM VP-2295 T. 'kansasensis' HT 1 2.56 1.58 1.62 63.2 pers. observ. 3 2.62 1.41 1.86 5 2.86 1.61 1.77 1.35 6 3.00 1.66 1.80 FHSM VP-3366 T. 'kansasensis' 2 1.64 1.68 70.0 pers. observ. 3 1.73 5 1.62 FHSM VP-6907 T. proriger 1 1.44 67.5 pers. observ. 3 1.52 1.00 (4) 1.52 1.04 (5) 1.63 (6) 1.61 FHSM VP-7262 T. nepaeolicus 1 1.61 1.25 72.0 Everhart 2 2.99 1.87 3.15 1.60 1.48 0.59 (2002b) 3 2.81 2.03 3.30 1.39 1.57 0.61 4 2.82 1.97 1.43 6 2.54 1.84 1.38 1.12 FHSM VP-15632 T. 'kansasensis' (2) 0.77 36.8 pers. observ. (4) 0.76 0.92 119 (6) 0.79 1.57 0.81 0.50 FHSM VP-18520 T. 'kansasensis' 1 1.52 1.29 71.2 pers. observ. 2 1.54 4 1.63 6 1.63 0.85 FMNH UR 820 T. proriger (2) 1.75 3.28 1.34 0.53 92.8 pers. observ. (3) 2.24 3.81 1.22 0.59 (4) 2.07 3.99 1.08 0.52 (5) 2.17 4.03 1.24 0.54 (6) 2.46 KUVP 1168 Tylosaurus sp. 3 0.81 81.2 pers. observ. 6 0.86 KUVP 65636 Tylosaurus sp. (2) 2.49 99.2 pers. observ. (4) 2.48 1.29 (5) 2.68 KUVP 66129 T. proriger 2 0.96 50.6 Stewart and 5 1.72 1.11 1.55 Mallon (2018) RMM 5610 T. proriger 2 1.40 51.6 pers. observ. 3 1.25 2.25 1.15 0.56 4 1.05 2.13 0.88 0.49 5 1.24 2.49 0.99 0.50 6 1.07 2.06 1.34 0.52 RSM P2588.1 T. saskatchewanensis 4 5.60 3.49 1.60 1.63 107.2 pers. observ. HT 5 4.65 3.02 1.54 TMP 2014.011.0001 T. borealis, sp. nov. 1 3.27 1.18 2.78 0.52 93.6 pers. observ. HT 2 3.79 1.64 3.96 2.32 0.69 0.41 3 1.66 4.21 0.61 0.39 4 4.45 1.97 4.46 2.26 0.88 0.44 5 3.64 2.11 4.78 1.72 0.68 0.44 6 3.72 2.10 4.79 1.78 0.92 0.44

120 APPENDIX 3. Tylosaurus tooth crown ratios for dentary teeth seven through 13 (where measureable). All teeth are from the right dentary, except for those tooth positions in parentheses, which are from the left dentary. Midline skull length is the distance (cm) from the tip of the rostrum to the back of the occipital condyle. Abbreviations: see Appendix 2.

121 Specimen Species Tooth CAR CSR MSL (cm) Source for MSL BMNH R3626/R3627 Tylosaurus sp. 9 1.94 65.2 pers. observ. 11 1.59 0.82 12 2.00 0.54 13 1.86 CMNFV 40930/40937/40943 Tylosaurus sp. (8) 0.98 76.4 pers. observ. (9) 1.31 FFHM 1997-10 T. proriger (9) 1.36 101.6 Everhart (2002b) (11) 1.53 (13) 1.46 FHSM VP-2495 T. 'kansasensis' 9 0.73 41.9 pers. observ. (10) 1.55 (11) 0.57 12 0.54

122 FHSM VP-3366 T. 'kansasensis' (9) 0.82 70.0 pers. observ. (12) 0.64 FHSM VP-7262 T. nepaeolicus (11) 0.76 72.0 Everhart (2002b) FHSM VP-15632 T. 'kansasensis' (10) 0.43 36.8 pers. observ. (11) 0.39 KUVP 1033 T. proriger (7) 1.81 78.4 pers. observ. KUVP 1168 Tylosaurus sp. 9 0.78 81.2 pers. observ. (12) 0.71 KUVP 66129 T. proriger (9) 0.56 50.6 Stewart and Mallon (2018) RSM P2588.1 T. saskatchewanensis HT 7 1.69 1.82 107.2 pers. observ. 8 1.73 1.06 9 1.78 0.96 10 1.57 1.95 11 1.89 0.84 12 1.57 0.79 13 1.46 TMP 2014.011.0001 T. borealis, sp. nov. HT ? 2.03 0.54 93.6 pers. observ. ? + 1 2.03

123 APPENDIX 4. Tylosaurus relative maxillary depth measurements and ratios. All measurements taken from the right maxilla, except for those values in parentheses, which are from the left maxilla. Midline skull length is the distance (cm) from the tip of the rostrum to the back of the occipital condyle. Abbreviations: m, height of the maxilla in lateral view at the posterior end of the premaxillomaxillary suture; M1–6, distance from the anterior margin of the first maxillary tooth root to the posterior margin of the sixth maxillary tooth root; M4, height (crown + exposed root) of maxillary tooth four. For other abbreviations, see Appendix 2.

124 m / MSL Specimen Species m (cm) M4 / m M1–6 (cm) Source for MSL BMNH R3626/R3627 Tylosaurus sp. (4.92) (0.74) (0.30) 65.2 pers. observ. CMNFV 8162 T. proriger 3.34 0.26 57.4 Stewart and Mallon (2018) CMNFV 40930/40937/40943 Tylosaurus sp. 6.47 0.34 76.4 pers. observ. FFHM 1997-10 T. proriger (9.65) (0.64) (0.34) 101.6 Everhart (2002b) FHSM VP-3 T. proriger (9.04) (0.36) 108.0 Everhart (2002b) FHSM VP-78 T. 'kansasensis' 2.83 0.29 38.4 pers. observ. FHSM VP-2209 T. nepaeolicus (6.84) (0.33) 82.0 pers. observ. FHSM VP-2295 T. 'kansasensis' HT 4.38 0.28 63.2 pers. observ. FHSM VP-3366 T. 'kansasensis' (5.12) (0.29) 70.0 pers. observ. FHSM VP-7262 T. nepaeolicus 6.02 0.58 0.31 72.0 Everhart (2002b) FHSM VP-9350 T. 'kansasensis' (1.96) (0.21) 38.0 pers. observ. FHSM VP-15632 T. 'kansasensis' (2.73) (0.29) 37.6 pers. observ. FHSM VP-18520 T. 'kansasensis' 5.50 0.32 71.2 pers. observ. 125 KUVP 1032 T. proriger 8.14 119.5 Everhart (2002b) KUVP 1033 T. proriger (6.27) (0.32) 78.4 pers. observ. KUVP 1129 T. nepaeolicus 4.28 0.26 66.8 pers. observ. KUVP 1168 Tylosaurus sp. 5.37 0.26 81.2 pers. observ. KUVP 50090 Tylosaurus sp. 8.13 0.31 104.4 pers. observ. KUVP 65636 Tylosaurus sp. 7.98 (0.31) 99.2 pers. observ. KUVP 66129 T. proriger 3.91 0.29 50.6 Stewart and Mallon (2018) KUVP 115002 Tylosaurus sp. 4.68 76.7 pers. observ. RMM 5610 T. proriger (3.23) (0.25) 51.6 pers. observ. RSM P2588.1 T. saskatchewanensis HT 8.12 0.80 0.30 107.2 pers. observ. TMP 2014.011.0001 T. borealis, sp. nov. HT 5.48 1.00 0.23 93.6 pers. observ.