Mosasauridae, Tylosaurinae) Sheds New Light on the Tylosaurine Rostrum and Heterochrony

Journal of Vertebrate Paleontology

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The Smallest-Known Neonate Individual of Tylosaurus (Mosasauridae, Tylosaurinae) Sheds New Light on the Tylosaurine Rostrum and Heterochrony

Takuya Konishi, Paulina Jiménez-Huidobro & Michael W. Caldwell

To cite this article: Takuya Konishi, Paulina Jiménez-Huidobro & Michael W. Caldwell (2018): The Smallest-Known Neonate Individual of Tylosaurus (Mosasauridae, Tylosaurinae) Sheds New Light on the Tylosaurine Rostrum and Heterochrony, Journal of Vertebrate Paleontology, DOI: 10.1080/02724634.2018.1510835 To link to this article: https://doi.org/10.1080/02724634.2018.1510835

Published online: 11 Oct 2018.

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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ujvp20 Journal of Vertebrate Paleontology e1510835 (11 pages) # by the Society of Vertebrate Paleontology DOI: 10.1080/02724634.2018.1510835

ARTICLE

THE SMALLEST-KNOWN NEONATE INDIVIDUAL OF TYLOSAURUS (MOSASAURIDAE, TYLOSAURINAE) SHEDS NEW LIGHT ON THE TYLOSAURINE ROSTRUM AND HETEROCHRONY

† TAKUYA KONISHI , , PAULINA JIMENEZ-HUIDOBRO, and MICHAEL W. CALDWELL Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada, [email protected], [email protected], [email protected]

ABSTRACT—We here report on the smallest-known, neonate-sized Tylosaurus specimen, FHSM VP-14845, recovered from the lower Santonian portion of the Niobrara Chalk exposed in Kansas, U.S.A. Lacking any associated adult-sized material, FHSM VP-14845 comprises fragmentary and associated cranial bones, here considered to represent a single neonatal individual with an estimated skull length of 30 cm. Despite its small size, a suite of cranial characters diagnoses FHSM VP-14845 as a species of Tylosaurus, including the elongate basisphenoid morphology. At the same time, FHSM VP-14845 unexpectedly lacks a conical predental rostrum on the premaxilla, generally regarded as diagnostic of this genus. Further, the first and the second premaxillary teeth are closely spaced, with the second set positioned posterolateral to the first, contributing to the overall shortness of the dentigerous premaxilla. Because a conical predental rostrum is already present in ontogenetically young specimens of T. nepaeolicus and T. proriger with respective skull lengths of approximately 40 and 60 cm, formation of such a rostrum must have taken place very early in postnatal ontogeny. Our recognition of a neonate-sized Tylosaurus specimen without an elongate predental rostrum of the premaxilla suggests hypermorphosis as a likely heterochronic process behind the evolution of this iconic tylosaurine feature.

Citation for this article: Konishi, T., P. Jimenez-Huidobro, and M. W. Caldwell. 2018. The smallest-known neonate individual of Tylosaurus (Mosasauridae, Tylosaurinae) sheds new light on the tylosaurine rostrum and heterochrony. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1510835.

INTRODUCTION (<50 cm total skull length) specimens of the genus are rare and not well documented in the literature until recently The fossil record of Tylosaurus (Mosasauridae: (Everhart, 2005a; Konishi and Caldwell, 2007a;Jimenez- Tylosaurinae) begins unequivocally in the late Coniacian (Everhart, 2001, 2005a). Because the first occurrence of a Huidobro et al., 2016; but see Sheldon, 1993). Konishi et al. taxon means that the species evolved before that point in (2010) also suggested that the holotype of Platecarpus angu- time, it is not surprising that possible Tylosaurus are recog- liferus (Cope, 1874) belongs to Tylosaurus sp., hinting at the nized from early Coniacian rocks (Everhart, 2005b) to even possibility that some small Tylosaurus material may have older units of rock and time (i.e., Turonian) (Polcyn et al., been misidentified as various contemporary plioplatecar- 2008; Flores, 2013). Thereafter, the known temporal range pines, a group of small- to medium-sized russellosaurines for the genus extends at least to the early Maastrichtian, sensu Polcyn and Bell (2005). A particularly diminutive making Tylosaurus one of the longest-existing mosasaur gen- mosasaur specimen, FHSM VP-14845, the subject of the cur- era known (Caldwell and Diedrich, 2005;Bullard,2006; rent study, is one such example. Bullard and Caldwell, 2010; Konishi and Caldwell, 2011; Specimen FHSM VP-14845, an exceptionally small-sized mosa- Jimenez-Huidobro and Caldwell, 2016; Konishi et al., 2016). saur specimen consisting of fragmentary jaw and cranial ele- Throughout their known stratigraphic and paleobiogeo- ments, was collected in 1991 from the Smoky Hill Chalk Member graphic range, Tylosaurus attained the largest body size in western Kansas. With the transverse width of the alveolar mar- among sympatric members of the family and, based on their gin barely reaching 1 cm, the specimen likely represents a neo- gastric contents, were apex predators that fed upon various natal individual. The material had been informally assigned to marine vertebrates including other mosasaurs and plesio- Platecarpus, although it has never been formally described. saurs (Martin and Bjork, 1987; Everhart, 2004). Numerous In this contribution, we first describe FHSM VP-14845 as rep- medium- to large-sized specimens presumably pertaining to resenting the smallest-known specimen of Tylosaurus, its gen- subadult and adult Tylosaurus are known, including KU eric assignment augmented through comparison with other, 5033, whose skull and total body length is estimated to have larger North American specimens that are unequivocally assign- been approximately 1.8 m and nearly 13 m, respectively able to the genus. We then discuss its bearings on Tylosaurus (e.g., Everhart, 2005c; pers. observ.). Conversely, small ontogeny. Finally, we present evidence for the developmental stage of this individual to be neonatal and discuss the heterochronic nature of the iconic and conical premaxillary rostrum Corresponding author. †Current address: Department of characteristic of the genus Tylosaurus and the subfamily Biological Sciences, University of Cincinnati, Cincinnati, Ohio 45221- Tylosaurinae. 0006, U.S.A., [email protected]. Color versions of one or more of the figures in the article can be Institutional Abbreviations—AMNH, American Museum of found online at www.tandfonline.com/ujvp. Natural History, New York, New York, U.S.A.; ANSP,

Published online 11 Oct 2018 Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-2)

FIGURE 2. Dentigerous portion of tylosaurine premaxillae. A–B,FHSM VP-14845, Tylosaurus sp. in A, dorsal and B, ventral views. C,TMM 40092-27, Tylosaurinae, in ventral view. Broken lines in A and B indicate reconstructed outlines of the element. C based on Polcyn et al. (2008:fig. 5C). Abbreviations: inb, internarial bar; rp, resorption pit; rs, predental rostrum; tb1, first tooth base; tb2,secondtoothbase;vp, vomerine process (of premaxilla). Scale bars equal 1 cm (A and B)and5cm(C).

FIGURE 1. Locality and horizon of FHSM VP-14845. A, map of con- tinental U.S.A. with the Santonian (ca. 85 Ma) coastlines of the Western Interior Seaway superimposed, showing the approximate specimen locality (white star) in Gove County, western Kansas; see below for references. B, map of Kansas showing the Niobrara Chalk exposed (in gray), accompanied by a stratigraphic section of the chalk. Within the Niobrara Chalk, FHSM VP-14845 was derived from the lower Smoky Hill Chalk Member just above Marker Unit 7 (MU 7) of Hattin (1982) and is early–middle Santonian (ca. 85 Ma) in age (Ogg et al., 2004). Base map in A from U.S. Geological Survey National Map Viewer (http:// nmviewogc.cr.usgs.gov/viewer.htm); the seaway coastlines after Deep Time Maps and Smith et al. (1994). B modified from Kansas Geological Survey Map M-118, Surficial Geology of Kansas, and the thickness of the chalk after Hodson and Wahl (1960). Abbreviations: FHLM, Fort Hays Limestone Member; KS, Kansas; MU, Marker Unit; SHCM, Smoky Hill Chalk Member; WIS, Western Interior Seaway.

Academy of Natural Sciences Philadelphia, Philadelphia, Pennsylvania, U.S.A.; CMC VP, Cincinnati Museum Center, Cincinnati, Ohio, U.S.A.; FHSM VP, Fort Hays Sternberg Museum, Hays, Kansas, U.S.A.; IPB, Goldfuss-Museum im FIGURE 3. FHSM VP-14845, Tylosaurus sp., incomplete quadrates. Institut fur€ Palaontologie,€ Bonn, Germany; KU, University of Right quadrate in A, lateral, B, medial aspects and left quadrate in C, Kansas Natural History Museum, Lawrence, Kansas, U.S.A.; lateral, and D, medial aspects. Arrow indicates posterior notching along MCZ, Museum of Comparative Zoology, Harvard University, anterior border of cephalic condyle. Note the large stapedial pit whose Cambridge, Massachusetts, U.S.A.; RMM, Red Mountain length equals that of the suprastapedial process and is nearly as wide as Museum (currently McWane Science Center), Birmingham, the quadratic shaft. Quadrate shafts are distally incomplete on both Alabama, U.S.A.; TMM, Texas Memorial Museum, University sides. Abbreviations: ccd, cephalic condyle; spt, stapedial pit; ssp, suprastapedial process. Scale bar equals 1 cm. of Texas, Austin, Texas, U.S.A.; UCMP, University of California Museum of Paleontology, Berkeley, California, U.S.A.; YPM, Yale Peabody Museum of Natural History, New Haven, Connecticut, U.S.A.

SYSTEMATIC PALEONTOLOGY MOSASAURIDAE Gervais, 1852 REPTILIA Linnaeus, 1758 TYLOSAURINAE Williston, 1897 SQUAMATA Oppel, 1811 TYLOSAURUS Marsh, 1872b Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-3)

14840: Gove County, western Kansas, Smoky Hill Chalk Member, Niobrara Chalk. Upper Coniacian–lower Santonian (Everhart, 2001). FHSM VP-14843: Gove County, western Kansas, Smoky Hill Chalk Member, Niobrara Chalk. Upper Coniacian–lower Santonian (Everhart, 2001). More precise provenances for the latter two specimens are unknown.

DESCRIPTION AND COMPARISON OF FHSM VP-14845 Premaxilla—The rostrum is mostly incomplete except for its distal end, which indicates that the predental rostrum was only about 3 mm long, barely equivalent to the basal diameter of the premaxillary tooth crowns (Fig. 2). Inferred from the remaining bone surfaces, and also from the posterolateral posi- tioning of the second premaxillary tooth relative to the first one (see below), the reconstructed outline of the dentigerous portion of the premaxilla in dorsal aspect describes a gently pointed arc that, compared with adult Tylosaurus specimens, is proportionally much shorter. As shown in Table 1, FHSM VP- 14845 is the only Tylosaurus specimen we examined that showed the distance across the widest part of the premaxilla exceeding the snout length anterior to that part of the bone (length:width ratio ¼ 0.86; Fig. 2, Table 1). The outline also dif- fers sufficiently from that of adult forms of Plesioplatecarpus and Platecarpus known from the Niobrara Chalk in exhibiting a pointed anterior end instead of a transversely straight one (e.g., Russell, 1967:fig. 83). In adult forms of Ectenosaurus clidastoides FIGURE 4. FHSM VP-14845, Tylosaurus sp., incomplete pterygoids in and the mosasaurine Clidastes, the two sides of the snout con- ventral aspect. Dentigerous portion of A,rightandB, left pterygoids. verge at a more acute angle than in FHSM VP-14845 (compare Quadratic ramus of C, left pterygoid. Functional tooth crowns have been Russell, 1967:figs. 72 and 86; ANSP 10193 [C. propython holo- dissolved. Note the anteroposteriorly linear tooth alignment and also these type, pers. observ.]). Polcyn et al. (2008) reported on a tylosaur- teeth originating from a flat bony base, not along an elevated ridge. ine premaxilla TMM 40092-27 from the Turonian portion Abbreviations: ecpp, ectopterygoidal process; qr, quadrate ramus. Scale bar equals 1 cm. (Arcadia Park Shale) of the Eagle Ford Shale exposed in north- eastern Texas. The seemingly complete dentigerous portion of this geologically older material is essentially equidimensional (length:width ratio ¼ 0.98; Fig. 2C), and is characterized by read- Liodon (in part) Cope, 1869–1870:200. ily converging sides forming an abbreviated predental rostrum, Rhinosaurus Marsh, 1872a:17. the length of which is comparable to the basal diameter of the Rhamphosaurus Cope, 1872:141. premaxillary teeth (see also Polcyn et al., 2008:fig. 5C). Tylosaurus Marsh, 1872b:47. Consequently, the overall dorsal outline of the dentigerous portion of FHSM VP-14845 bears the closest similarity to that of the Type species—Macrosaurus proriger Cope (1869). Turonian-aged TMM 40092-27, particularly concerning its overall Holotype—MCZ 4374. shortness and abbreviated predental rostrum. Generic Diagnosis—See Russell (1967:171–173) and Somewhat unexpectedly, both pairs of premaxillary teeth Jimenez-Huidobro and Caldwell (2016). project anteriorly and laterally at the base (Fig. 2B), implying a procumbent nature atypical of tylosaurines (e.g., Bell, 1997:fig. TYLOSAURUS sp. 5C). Also unusual are closely spaced first and second premaxil- – lary teeth, where the second pair is also located posterolateral (Figs. 2 5, 7 9, 12) to the first pair (Fig. 2B). In larger individuals, the anterior — and posterior pairs are well separated anteroposteriorly, where Referred Specimens FHSM VP-14845: dentigerous portion the posterior pair occurs more or less posterior to the anterior of premaxilla with broken base of internarial bar, other tooth- set (see below). Nevertheless, the premaxillary tooth crowns bearing jaw fragments, right and left splenial fragments, right are smooth and bicarinate, exhibiting a ‘D’-shaped cross-sec- and left partial coronoids, incomplete quadrates on both sides, tion (Fig. 2B), the latter being consistent with the intercarinal right and left partial pterygoids, and partial braincase (basi- angle reported by Konishi and Caldwell (2007a)inTylosaurus sphenoid). FHSM VP-14840: anterior portion of premaxilla, of about 120 on the labial side. The same angle would be close broken at the second tooth row. FHSM VP-14843: premaxilla, to 180 in a given subadult and adult plioplatecarpine tooth, with broken posterior portion of internarial bar. including Ectenosaurus (e.g., Konishi and Caldwell, 2011), and Locality and Horizon—FHSM VP-14845: approximately marginal teeth of adult Clidastes spp. are longitudinally elong- 2.4 km (1.5 miles) south of Castle Rock, southeastern Gove ate ovals in cross-section, with predominantly fore-and-aft car- County (coordinates: SW1/4, Sec. 14, T14S, R26W [approxi- inal orientation (e.g., CMC VP 7554 [C. liodontus] and YPM 0 00 0 00 mately 38 49 51.65 N, 100 11 14.25 W]) in western Kansas, 40350 [Clidastes sp.]; T.K., pers. observ.). from above Hattin’s(1982) Marker Unit 7 (M. J. Everhart, Quadrate—Whereas the lateral side of the preserved quad- pers. comm., Oct. 15, 2013) in the Smoky Hill Chalk Member rates show signs of cortical bone loss postmortem, the more of the Niobrara Chalk (Fig. 1; Everhart, 2001). Lower to mid- intact medial surface clearly exhibits a very large stapedial pit, dle Santonian, Upper Cretaceous (Everhart, 2001). FHSM VP- whose vertical length is comparable to that of the Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-4)

FIGURE 6. RMM 5610, Tylosaurus proriger, base of braincase (basioccipital and basisphenoid) in A, dorsal and B, ventral views. Note that basisphenoid morphology is virtually identical to that of FHSM VP-14845. Abbreviations: ala, alar process; ba, exit for basilar artery branch; bpp, basipterygoid process; ds, dorsum sellae; ftrc, facet for tra- FIGURE 5. FHSM VP-14845, Tylosaurus sp., basisphenoid in A, dor- becular cartilage; icb, exit for internal carotid branch; plp, posterolat- sal and B, ventral views. Abbreviations: ala, alar process; ba, exit for eral process (of basisphenoid); ppr, parasphenoid process; sel, sella basilar artery branch; bpp, basipterygoid process; ds, dorsum sellae; turcica. Scale bar equals 1 cm. ftrc, facet for trabecular cartilage; icb, exit for internal carotid branch; plp, posterolateral process; sel, sella turcica. Scale bar equals 1 cm.

suprastapedial process and its width is approximately 50% of the anteroposterior width of the shaft (Fig. 3A). Departing from a typical abbreviated morphology of the suprastapedial process in Tylosaurus, the immature tylosaur exhibits a suprastapedial process that is slender but substantially long relative to the rest of the quadrate, its length appearing to be almost half the estimated quadrate height based on obser- vation of FHSM VP-15632 (T. kansasensis sensu Everhart, 2005a; T. nepaeolicus sensu Jimenez-Huidobro et al., 2016) and RMM 5610 (T. proriger), in which the ventral limit of the stapedial pit corresponds to the midheight of the quadrate (e.g., Everhart, 2005a:fig. 3d). In FHSM VP-14845, the suprastapedial process curves down gently and projects posteriorly as well as ventrally from the long axis of the shaft and also that of the stapedial pit, forming a large meatus FIGURE 7. FHSM VP-14845, Tylosaurus sp., right splenial in A, medial, B, lateral, and C, posterior views. Scale bar equals 1 cm. (i.e., stapedial notch). In larger specimens attributed to various Tylosaurus species, the suprastapedial process projects more ventrally and the corresponding meatus is proportionally narrow (e.g., Russell, 1967:fig. 94A [T. proriger]; Bell, 1997:fig. 7B [T. nepaeolicus]; and Everhart, 2005a:fig. 3 [T. known from the lower Smoky Hill Chalk Member in Kansas ‘kansasensis’]). As a shared Tylosaurus feature, the anterior (e.g., Konishi and Caldwell, 2007b:fig. 3), the pterygoid tooth border of the cephalic condyle is shallowly excavated (Fig. row follows a gentle sinusoid curvature, closely following the 3A, arrow). Unfortunately, at least the ventral half of the lateral edge of the pterygoid anterior to the level of the ectop- element was lost postmortem. terygoidal process (the tooth row runs more or less along a Pterygoids—The main dentigerous portion is preserved on straight midline of the pterygoid in Tylosaurus) (Konishi and both sides, and a distal portion of the left quadrate ramus is Caldwell, 2011). In Clidastes, numerous, closely packed ptery- also preserved (Fig. 4). As is typical in tylosaurine pterygoids, goid teeth occur along a pronounced vertical ridge, which the pterygoid tooth row is straight and wider anterior to the Tylosaurus lacks (e.g., Bell, 1997:character 42[1]; KU 1022). As ectopterygoidal process (e.g., KU 28705, Tylosaurus sp.). In expected, resorption pits occur posterolaterally to the func- adult plioplatecarpines, such as Plesioplatecarpus planifrons tional tooth positions. Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-5)

TABLE 1. Length: width ratio of dentigerous portion of premaxilla among small Tylosaurus specimens from North America. Note that TMM 40092-27, whose corresponding ratio is 0.98 (not shown here), is not only larger but is also stratigraphically much older (Turonian, Eagle Ford Fm., Texas; Polcyn et al., 2008) than any of the specimens listed herein.

FHSM VP-14845 FHSM VP-14843 FHSM VP-14840 RMM 5610 Dentigerous premaxilla length : width ratio 0.86 1.22 1.71 1.88

Basisphenoid—Overall, the basisphenoid is more elongate 7B). Such a cotylar morphology would have allowed the sple- than those in plioplatecarpines (cf. Russell, 1967:33) or in nial to pivot medially with a greater angle than laterally with Clidastes (CMC VP 7554 and KU 1022; pers. observ.). respect to the angular, consistent with the hypothesis that the Anteriorly, the sella turcica terminates as a pair of smooth, ver- mandible bent outward at the intramandibular joint in mosa- tical surfaces for articulation with the trabecular cartilage (e.g., saurs (e.g., Lee et al., 1999:fig. 1). As mentioned already, much Oelrich, 1956:fig. 14; Fig. 5A). In adult plioplatecarpines, these of the outer surface of the lateral wing is eroded postmortem. surfaces are posteriorly inclined and less well defined (Russell, Tooth-Bearing Elements—There are more than a dozen 1967:fig. 10). In RMM 5610, a juvenile specimen of Tylosaurus fragments of tooth-bearing elements that are neither fragments proriger with a skull length of about 50 cm, a pair of elongate of the premaxilla nor the pterygoid (Fig. 8). None of these jaw ovoid foramina for the internal carotid branches pierce the fragments preserve functional tooth crowns, and the only sella turcica just in front of the dorsum sellae (Fig. 6A). An crown that is preserved intact occurs in a single replacement identical condition is discernible on FHSM VP-14845 (Fig. pit (Fig. 8B). Two long sections of jaw rami, here identified as 5A). In the adults of the plioplatecarpine Platecarpus, these those pertaining to the right dentary and the left maxilla, can foramina occur more anteriorly, around the midlength of the be assembled out of some of these jaw fragments. In stark con- sella turcica (Russell, 1967:fig. 10). Unlike Platecarpus, no dis- trast to the premaxillary dentition, adjacent functional tooth tinct foramina for the basilar artery are present on the floor of crowns in jaw rami are separated from each other by a gap at the sella turcica in FHSM VP-14845 or RMM 5610. least 50% greater than a single basal crown diameter (Konishi As is the case with Tylosaurus generally, the alar process is and Caldwell, 2007a; Fig. 8A, B, double-headed arrows). On markedly longer than the sella turcica (Figs. 5A, 6A), whereas both the maxilla and the dentary, the medial dental margin is the opposite condition characterizes Platecarpus (Russell, approximately at the same level as the lateral margin and 1967:fig. 10). In another plioplatecarpine, Selmasaurus john- forms a complete ovoid alveolus. In contrast, alveoli in contem- soni, and in a mosasaurine, Clidastes, these two features are porary plioplatecarpines are circular (e.g., Konishi and about equal in length (FHSM VP-13910 [S. johnsoni holotype], Caldwell, 2007b:fig. 3). ANSP 10193 [C. propython holotype], and CMC VP 7554 [C. Coronoids—On both right and left coronoids (Fig. 9), the liodontus]; pers. observ.). Near the posterior edge of the dor- coronoid processes are tall and acutely angled in profile. On sum sellae on the dorsal surface is a single, irregularly shaped the left coronoid, the posterior border is apparently complete median foramen that may have served as an exit for the basilar and is vertically straight, lacking a distinct posteroventral pro- artery. This region is also pierced by a circular foramen in cess that would merge with the coronoid buttress of the suran- RMM 5610. The base of the posterolateral process is preserved gular. In RMM 5610 (Tylosaurus proriger), such a process is on the left side of FHSM VP-14845, indicating that its main well developed and projects posteriorly beyond the posterior axis trended more posteriorly than laterally, a typical edge of the coronoid process (Fig. 10). The lack of this process Tylosaurus character (cf. Fig. 5B). On the ventral surface, the in FHSM VP-14845 is not preservational, however; both the base of the left basipterygoid process is preserved, showing lateral and medial borders forming the base of the element that the process projected ventrally at its base (Fig. 5B), con- meet posteriorly to form the posteroventral corner of the cor- sistent among Tylosaurus specimens, including RMM 5610 onoid, much as in RMM 5610 (Figs. 9D [arrow], 10B [arrow]). (Fig. 6B) (see also Jimenez-Huidobro and Caldwell, 2016:fig. The medial wing is largely incomplete on both sides, whereas 7A). The same process projects anterolaterally in contempor- the short lateral wing is complete on the left element. In dorso- ary plioplatecarpines (e.g., FHSM VP-401, Ectenosaurus clidas- ventral aspect, the coronoid exhibits a gentle medial curvature, toides; FHSM VP-13910, Selmasaurus johnsoni; AMNH 1820, even though both coronoids are incomplete anteriorly. Platecarpus tympaniticus; also see Rieppel and Zaher, 2000:fig. 2C) and more anteriorly than laterally, but barely ventrally, in mosasaurines (e.g., Russell, 1967:fig. 12; Clidastes sp., cf. C. lio- DISCUSSION dontus [KU 1022, pers. observ.]; Plotosaurus bennisoni [holo- Tylosaurus type, UCMP 32778, pers. observ.]; Prognathodon kianda Biostratigraphy of in Western Kansas [Schulp et al., 2008:fig. 6H, I]). These vertically oriented bases Collected from above Marker Unit (MU) 7 (Hattin, 1982;M. of the paired basipterygoid processes occur close to the midsa- J. Everhart, pers. comm.) of the Smoky Hill Chalk Member gittal plane, forming a distinct median cleft between them exposed in western Kansas, U.S.A., FHSM VP-14845 not only (Figs. 5B, 6B). represents the smallest Tylosaurus specimen known to date, Splenials—Both splenials are preserved near their articular but it also bridges a significant biostratigraphic gap that existed cotyle, and the better-preserved right element is described between MU 5 and MU 9 for the genus (Everhart, 2001). here. On the medial aspect, the base of the medial wing is bro- Among the two nominal and the one then unnamed ken postmortem and has been pushed up against the lateral Tylosaurus species known from upper Coniacian–lower wing (Fig. 7A). On the articular cotyle, the surface medial to Campanian strata of the member, Everhart (2001) indicated the vertical keel is wider and more strongly excavated than the for the respective taxa the following stratigraphic ranges: surface lateral to it, exposing the medial surface of the keel Tylosaurus ‘kansasensis’ sensu Everhart, 2005a (MUs 1–5; (Fig. 7A, C). In lateral aspect, although the cortical layer has upper Coniacian–lowermost Santonian), T. nepaeolicus (MUs been eroded postmortem, it is nonetheless discernible that the 1–5; upper Coniacian–lowermost Santonian), and T. proriger lateral border of the splenial cotyle is vertically straight (Fig. (MUs 9–23; middle Santonian–lower Campanian) (e.g., Fig. 1). Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-6)

FIGURE 9. FHSM VP-14845, Tylosaurus sp., coronoids. Right coronoid in A, lateral, B, medial, and C, dorsal views. Left coronoid in D, lateral, E, medial, F, dorsal, and G, ventral views. Arrow indicates posteroventral corner. Scale bar equals 1 cm.

FIGURE 8. FHSM VP-14845, Tylosaurus sp., tooth-bearing elements another species of Tylosaurus remains equivocal at the (TBEs) compared with those of RMM 5610, Tylosaurus proriger. A, moment, its fine-scale morphological similarities to the basi- partial right dentary of FHSM VP-14845, anterior to the right; B, par- sphenoid of RMM 5610, a T. proriger juvenile, supports the tial left maxilla of the same in occlusal view, anterior to the left; C, pre- possibility that FHSM VP-14845 could be assigned to T. pro- maxilla and left maxilla of RMM 5610 in lateral view showing eighth riger. It is noteworthy that the coronoid process in at least one and ninth maxillary teeth; D, close-up view of the same teeth, showing adult and two juvenile specimens of Tylosaurus nepaeolicus the interdental distance that is approximately 35% greater than the — basal crown length of adjacent teeth. Abbreviations: c, basal crown sensu Jimenez-Huidobro et al. (2016) AMNH 124, FHSM length; d, interdental distance; rp, resorption pit; t, tooth. Arrows indi- VP-2295 [T. kansasensis holotype], and VP-2495, respect- cate interdental gap. Scale bars equal 2 cm. ively—overhangs the posterior border of the element (Fig. 11, arrow), whereas it does not extend beyond the posterior border of the element in FHSM VP-14845, a condition it shares with both small/juvenile (RMM 5610; Fig. 10) and large/adult (FHSM VP-3) T. proriger specimens. Everhart (2005a:233) later confined the age of T. kansasensis to the late Coniacian. Recently, Jimenez-Huidobro et al. Tylosaurus (2016) synonymized Tylosaurus kansasensis Everhart, 2005, Cranial Ontogeny in with T. nepaeolicus (Cope, 1874) based on the sympatry of the Premaxilla and Predental Rostrum—Everhart (2005) two species, and on their significant morphological overlap on reported that the relative length of the predental rostrum on supposed key diagnostic characters of T. kansasensis. Jimenez- the premaxilla in Tylosaurus kansasensis ranged from 2.5% to Huidobro et al. (2016) also suggested that specimens assigned 3.0% of mandibular length, smaller than the same ratio in T. to T. kansasensis were likely juveniles of T. nepaeolicus nepaeolicus (4.2%) and in T. proriger (4.8%). Considering T. (Cope, 1874). nepaeolicus to be a senior synonym of T. kansasensis, where The fact that FHSM VP-14845 is from the MU 7 (lower– specimens assigned to the latter are generally smaller in size middle Santonian) renders it possible that it could represent than those assigned to the former, Jimenez-Huidobro et al. any one of these Tylosaurus species known from Kansas and (2016) hypothesized that the predental rostrum grew longer increases the possibility that stratigraphically older ‘Lower ontogenetically in Tylosaurus, although they did not character- Chalk’ T. nepaeolicus (sensu Jimenez-Huidobro et al., 2016) ize rostral lengthening in much detail. In FHSM VP-14845, the and younger ‘Upper Chalk’ T. proriger briefly coexisted in the edentulous rostrum beyond the first premaxillary tooth pair is Western Interior Seaway (cf. Everhart, 2001). In fact, Everhart present but much smaller (10 mm in length) than is observed (2001) hypothesized that the taxon range zone for T. proriger in other specimens of Tylosaurus of Santonian–Campanian age may have extended to include MU 5 (upper Coniacian), based (Figs. 2, 12; Thurmond, 1969; Sheldon, 1993). In addition, the on the occurrences of two large tylosaurine specimens (FHSM outline of the projection describes a gentle parabolic arc in VP-13742 and 13908) from between MU 4 and MU 5. dorsoventral aspect, which is in stark contrast to the more tri- Although taxonomic assignment of FHSM VP-14845 to one or angular and more elongate morphology that is typical of a Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-7)

of the predental rostrum continues, so that the length of the rostrum and that of the first premaxillary alveolus become nearly equal (RMM 5610; Fig. 12). In FFHM 1997-10, a 1.2-m- long skull referable to T. proriger (Everhart, 2001), the predental rostrum is even longer than the first premaxillary alveolus, suggesting that in Tylosaurus the rostrum continued lengthening at a greater rate than the rest of the premaxilla, and likely the rest of the skull, beyond the ontogenetic stage represented by RMM 5610 (e.g., FFHM 1997-10). This growth pattern of the rostrum suggests a relatively late offset (cessation) of rostral development through Tylosaurus life history relative to its hypothetical, russellosaurine ancestor, in which the rostrum was either short or absent. In terms of a heterochronic pattern of evolution, we therefore recognize that rostral development in the Tylosaurus premaxilla exhibited hypermorphosis. Dentition—In Tylosaurus proriger, Konishi and Caldwell (2007a) suggested that there was positive allometry in the basal crown diameter of marginal teeth relative to the jawbones, where slender juvenile tooth crowns with substantial inter- FIGURE 10. RMM 5610, Tylosaurus proriger, right coronoid in A, lat- dental gaps become stout and conical in adults, closing such eral and B, ventral views. Arrow indicates posteroventral corner. Scale gaps. At the level of the crown base, the juvenile maxilla bar equals 2 cm. (RMM 5610) exhibits an interdental gap that is nearly 1.5 times greater than the anteroposterior basal length of an adjacent tooth crown (Fig. 8C, D). The gaps exhibited on the maxilla and dentary of FHSM VP-14845 are even more substantial, becoming 1.7–2.0 times as long as the anteroposterior basal length of adjacent crowns (Fig. 8A, B, double-headed arrows), lending further support to Konishi and Caldwell’s (2007a) hypothesis. In contrast, the interdental gap between the first and the second teeth on the premaxilla of FHSM VP-14845 is distinctly smaller than the basal crown diameter of the adjacent teeth (Fig. 2B), here considered associated with its very early ontogenetic stage preceding alveolar elongation (Fig. 12). Morphologically, tooth crowns in both the premaxilla and other jawbones of FHSM VP-14845 are slender, as in RMM 5610. Quadrates—The preserved portions of the quadrates aug- FIGURE 11. FHSM VP-2495, Tylosaurus nepaeolicus sensu Jimenez- ment the ontogenetic argument of Jimenez-Huidobro et al. Huidobro et al., 2016, left postdentary complex showing overhanging (2016) for this element in Tylosaurus: namely, the smaller or posterior coronoid process (arrow). Scale bar equals 5 cm. younger the animal, the proportionately more slender the suprastapedial process and the greater the size of the stapedial notch (Jimenez-Huidobro et al., 2016:78). Specimen FHSM VP-14845 exhibits a substantial gap between the Tylosaurus rostrum. Notwithstanding, the presence of a small suprastapedial process and the shaft, the former being elong- but distinct predental projection in FHSM VP-14845 precludes ate and deflected medially (Fig. 3). The stapedial pit is enor- it from being Platecarpus,a‘short-snouted’ plioplatecarpine, as mous, as long as the suprastapedial process itself, which is it was originally considered to be. noteworthy given its small relative size in a typical adult Fortunately, there are two Tylosaurus specimens in the Fort Tylosaurus quadrate (Russell, 1967;Bell,1997:fig.7B).Also Hays Sternberg Museum collections that, like FHSM VP- of note is that, at least in medial aspect, the quadrate shaft 14845, exhibit a very small predental rostrum that is well below of FHSM VP-14845 is only slightly wider than that of the stapedial pit. It thus seems that the entire quadrate devel- 10 mm in length (Fig. 12). Also found in Gove County, FHSM oped with positive allometry relative to the suprastapedial VP-14843 and 14840 not only show a progressively longer pre- pit in both vertical and horizontal dimensions. In sum, a dental rostrum compared with FHSM VP-14845, but they also tylosaurine quadrate early in ontogeny is characterized as show a corresponding increase in the longitudinal dimension of possessing a long, slender suprastapedial process, a wide sta- the first premaxillary tooth alveolus (Fig. 12). Accompanying pedial notch, and a very large stapedial pit. this seemingly ontogenetic alveolar elongation, it is also appar- ent that the second tooth pair migrates progressively with respect to the first tooth pair, from a more posterolateral pos- Ontogenetic Status of FHSM VP-14845: Prenatal or ition in FHSM VP-14845 to a more posterior position in a Neonate? FHSM VP-14840 (t2 in Fig. 12). This in turn results in an Based on the dentary tooth crown diameter, and under the increase in the length:width ratio of the dentigerous premaxilla, assumption that tooth crowns grow isometrically to overall where the entire dentigerous portion including the predental body size, Field et al. (2015) estimated the size of the smallest portion becomes elongate. As Tylosaurus ontogeny progresses Clidastes sp. specimen they reported (YPM 058126) to be 0.66 beyond the size class represented by FHSM VP-14840, the m, or about 22% the length of a 3-m-long adult. As Konishi alveolar elongation seems to slow down while the elongation and Caldwell (2007a) reported in Tylosaurus proriger, Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-8)

FIGURE 12. Rostral and alveolar elongation in early Tylosaurus ontogeny. All the FHSM specimens are referable to Tylosaurus sp., whereas RMM 5610 pertains to T. proriger. Note progressively more anteroposterior alignment of two pairs of premaxillary teeth in association with alveolar elongation, which shows slowing down (here between FHSM VP-14840 and RMM 5610) while the rostrum maintains positive allometric growth. Abbreviation: t2, second premaxillary tooth. Scale bar equals 2 cm.

however, tooth crowns in Tylosaurus exhibit positive allometry Lee (2001:fig. 2, and references therein), the estimated TBL relative to the respective tooth-bearing elements, except for for FHSM VP-14845 falls well within the neonate TBL range those on the premaxilla at a very early ontogenetic stage (i.e., expected for extant varanoid lizards, at approximately 10–40% FHSM VP-14845; this study). Jaws of a juvenile possessing maternal TBL. slender tooth crowns and large interdental gaps (see above) Still, given the exceptionally large adult size of Tylosaurus are later filled by enlarged, and not by additional, adult tooth compared with extant Varanus, it may be possible that FHSM crowns. Comparison between the smaller and larger specimens VP-14845 was a prenatal individual close to parturition, a pos- Field et al. (2015) identified as Clidastes reveals that the inter- sibility that Field et al. (2015) did not consider for any of the dental gaps are indeed proportionately larger in smaller speci- small Clidastes specimens they analyzed. Although direct evi- mens (e.g., YPM 058126, the gap larger than the basal crown dence supporting or countering such a possibility is lacking in diameter) than in larger specimens (YPM 1314, the gap smaller FHSM VP-14845, we present here an argument favoring the than basal crown diameter). Hence, it is possible that Field likelihood that FHSM VP-14845 was a neonate. First, FHSM et al. (2015) underestimated the total body length of YPM VP-14845, consisting of associated fragmentary bones pertain- 058126, which renders the neonate status of this specimen that ing to a single individual, was discovered without any associ- Field et al. (2015) suggested less unequivocal. Also of note, ated adult or juvenile bones (M. J. Everhart, pers. comm., YPM 1253, the third smallest specimen of Clidastes identified 2018), indicating that it was preserved by itself ex utero (e.g., by Field et al. (2015), indeed pertains to a Platecarpus-like plio- O’Keefe and Chiappe, 2011; Field et al., 2015). Second, even if platecarpine: a short premaxillomaxillary suture (about two FHSM VP-14845 was an offspring from an exceptionally large and a half alveoli long), a round basal tooth crown cross-sec- Tylosaurus proriger individual such as KU 5033, the TBL ratio tion, and clear presence of hemapophyses (i.e., a hemal arch–- of 17.2% between the two specimens still exceeds the equiva- spine complex not fused to the caudal vertebra) are all lent ratio of 15% estimated in Carsosaurus marchesetti, a basal characteristic of this short-snouted plioplatecarpine common in mosasauroid (Caldwell and Lee, 2001). Among extant ceta- the Smoky Hill Chalk Member (T.K., pers. observ., 2005). ceans, another clade of secondarily aquatic tetrapods that are By estimating both the skull length (SL) and the total body universally viviparous, the TBL ratio between the neonate and length (TBL), we further evaluated the possible developmental the mother is negatively correlated with the maternal TBL stage of FHSM VP-14845 at the time of its death. To do this, across a variety of whale species, both in mysticetes we used data from two large Tylosaurus skeletons collected (R2 ¼ 0.417) and in odontocetes (R2 ¼ 0.315) and when both from the Kansas Chalk: AMNH FR-221, an 8.83-m-long, nearly clades are combined (R2 ¼ 0.620; Fig. 13). In extant viviparous/ complete and articulated Tylosaurus proriger skeleton ovoviviparous chondrichthyan taxa without embryonic canni- (Osborn, 1899a, 1899b), and FHSM VP-3, another articulated, balism, some of the smaller species (e.g., Squalus acanthias) partially reconstructed skeleton of T. proriger that is slightly exhibit the longest gestation period of up to 24 months, indicat- smaller than AMNH FR-221 (Russell, 1967). By comparing ing that smaller taxa give birth to a small number (2–14 in S. six-alveolus lengths between AMNH FR-221 and FHSM VP- acanthias) of large offspring relative to maternal size (Pough 14845, the skull length (SL) of the latter was estimated to be et al., 2013:fig. 5-11). Indeed, the whale shark (Rhincodon about 30 cm. Comparison between SLs of FHSM VP-14845 typus), the largest extant shark species growing up to at least and AMNH FR-221 was made subsequently, yielding the esti- 12 m in TBL, is known with the litter size of 300, the largest mated total body length (TBL) of 2.23 m for FHSM VP-14845 recorded among extant sharks (Stevens, 2007, and references (Appendix 1). This estimated TBL is 25.3% that of AMNH therein). Perinatal embryos of a 10.6-m gravid whale shark FR-221 and 17.2% that of KU 5033 (the ‘Bunker tylosaur’), ranged from 58 to 64 cm in TBL, which amounts to 5.5–6.0% the largest-known T. proriger specimen collected from Kansas of the maternal TBL (Joung et al., 1996). The same ratio in at an estimated TBL of 13 m (Everhart, 2002, 2005c; pers. Squalus acanthias becomes fivefold, where we recorded observ.), and 24.8–27.9% of an estimated maximum TBL for T. 26.5–31.7% based on 10 perinatal embryos from three females nepaeolicus at 8–9 m (Everhart, 2002). Based on Caldwell and (T.K., pers. observ.). Although circumstantial, these lines of Konishi et al.—Earliest-known ontogeny in Tylosaurus (e1510835-9)

of the subfamily. In T. proriger, such a robust conical rostrum was already present, although still growing, even as the individual’s skull reached 60 cm in length (RMM 5610; Fig. 12; Appendix 1), and in T. nepaeolicus sensu Jimenez-Huidobro et al. (2016), a conical rostrum is evident on a specimen with a skull length as short as 40 cm (IPB R322; Jimenez-Huidobro et al., 2016:fig. 3B). In 2016, Jimenez-Huidobro et al. (p. 78) suggested that the premaxillary rostrum “seems to be shorter” in small specimens of T. nepaeolicus that Everhart (2005) regarded as T. kansasensis. Nevertheless, the exact onset of the conical premaxillary rostrum development in Tylosaurus ontogeny remained unclear and its universal presence has been assumed for T. nepaeolicus and T. proriger. Recognizing here that FHSM VP-14845 can be assigned to Tylosaurus despite the lack of the conical rostrum allows formulation of the following hypotheses: (1) at least cer- tain Tylosaurus species were born without a conical premaxillary rostrum; (2) alveolar, as well as predental, elongation continued and contributed to development of a prow-like dentigerous premaxilla in Tylosaurus; and (3) the onset of rostrum morphogenesis began exceptionally early in Tylosaurus postnatal ontogeny. Finally, from an evolutionary perspective, we further conclude that hypermorphosis is a major heterochronic driver behind the evolution of a conical tylosaurine rostrum, given the lack of such a feature in plioplatecarpines, a generally well-supported sister clade of tylosaurines (e.g., Konishi and Caldwell, 2011; Simoes~ et al., 2017). At the same time, we reject the possibility of sexual selection as a driver of tylosaurine rostrum evolution, given its presence exceptionally early in their postnatal ontogeny. It is a possibility that the bony rostrum was selected for a sex-independent function in tylosaurines, such as for ramming, which killer whales today employ when hunting cetaceans of various sizes (Ford et al., 1998, 2005; Visser et al., 2010).

ACKNOWLEDGMENTS We sincerely thank M. J. Everhart for his kind hospitality in accommodating our collections visits at the FHSM and else- where in Kansas, and for providing us with additional informa- tion on and photographs of FHSM VP-14845. We also thank incisive comments provided by our reviewers, A. Schulp and M. Polcyn. S. Garvey assisted T.K. with measurements of Squalus acanthius specimens. This research was in part funded by NSERC (Natural Sciences and Engineering Research Council of Canada) Discovery Grant no. 238458, NSERC Accelerator Grant no. 412275, and a Faculty of Science Chairs Research Allowance to M.W.C.

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Schulp, A. S., M. J. Polcyn, O. Mateus, L. L. Jacobs, and M. L. Morais. 1. Skull length (SL) FHSM VP-3 ¼ 1058 mm; length between 2008. A new species of Prognathodon (Squamata, Mosasauridae) first and sixth dentary teeth (D1–D6) measured across from the Maastrichtian of Angola, and the affinities of the mosa- ¼ – tooth bases 225 mm saur genus Liodon; pp. 1 12 in M. J. Everhart (ed.), Proceedings ¼ – of the Second Mosasaur Meeting, Hays, Kansas. Fort Hays 2. (SL) FHSM VP-14845 (D1 D6) FHSM VP-14845 — Studies Special Issue 3. May 3–6, 2007. Fort Hays State 1058 mm/225 mm A University, Hays, Kansas. Sheldon, M. A. 1993. Ontogenetic study of selected mosasaurs of North We calculated (D1–D6) FHSM VP-14845 using the more com- America. M.S. thesis, University of Texas, Austin, Texas, 184 pp. plete left maxilla because tooth size does not vary greatly ~ Simoes, T. R., O. Vernygora, I. Paparella, P. Jimenez-Huidobro, and between upper and lower jaws (but see Russell’s[1967] very M. W. Caldwell. 2017. Mosasauroid phylogeny under multiple likely measuring error for FHSM VP-4 (¼ VP-3), where the phylogenetic methods provides new insights on the evolution of aquatic adaptation in the group. PLoS ONE 12:e0176773. doi: 10. distance between the first and sixth teeth for a maxilla is more 1371/journal.pone.0176773. than twice as long as that for a dentary. These values are simi- Smith, A. G., D. G. Smith, and B. M. Funnell. 1994. Atlas of Mesozoic lar in other specimens). Based on this we obtained: and Cenozoic Coastlines. Cambridge University Press, Cambridge, U.K., 99 pp. (D1–D6) FHSM VP-14845 ¼ 64 mm — B Stevens, J. D. 2007. Whale shark (Rhincodon typus) biology and ecol- Inserting B into A, we derive: ogy: a review of the primary literature. Fisheries Research 84:4–9. (SL) FHSM VP-14845 ¼ 300.9 mm — C Thurmond, J. T. 1969. Notes on mosasaurs from Texas. Texas Journal of Science 21:69–80. 3. Total body length (TBL) AMNH FR-221 ¼ 8830 mm, Visser, I. N., J. Azeschmar, J. Halliday, A. Abraham, P. Ball, R. ¼ Bradley, S. Daly, T. Hatwell, T. Johnson, W. Johnson, L. Kay, T. whereas (SL) AMNH FR-221 1190 mm. From C, then, Maessen, V. McKay, T. Peters, N. Turner, B. Umuroa, and D. S. (TBL) FHSM VP-14845 can be obtained as follows: Pace. 2010. First record of predation on false killer whales (TBL) FHSM VP-14845 ¼ (TBL) AMNH FR-221 C/(SL) (Pseudorca crassidens) by killer whales (Orcinus orca). Aquatic ¼ ¼ Mammals 36:195–204. AMNH FR-221 8830 mm 300.9 mm/1190 mm 2232.7 mm Williston, S. W. 1897. Range and distribution of the mosasaurs. Kansas Thus, approximately, (TBL) FHSM VP-14845 ¼ 2.23 m University Quarterly 6:177–189. Similarly, using the M1–M6 length as the best proxy for Submitted on July 20, 2017; revisions received May 18, 2018; accepted D1–D6, we attained the following SL estimate for juvenile T. June 8, 2018. proriger (RRM 5610): Handling editor: Patrick Druckenmiller. 4. (SL) RMM 5610 ¼ (M1–M6) RMM 56 1058 mm/225 mm

APPENDIX 1. ¼ 130 mm 1058 mm/225 mm Skull and body length estimates for FHSM VP-14845. To esti- ¼ 611 mm mate the skull length (SL) and the total body length (TBL) of 60 cm FHSM VP-14845, we used selected measurements of FHSM VP-3 and AMNH FR-221 from Russell (1967:appendix A) According to the above calculations, the skull of RMM 5610 and Osborn (1899a), respectively, as follows. is approximately twice as long as that of FHSM VP-14845.