The Smallest-Known Neonate Individual of Tylosaurus (Mosasauridae, Tylosaurinae) Sheds New Light on the Tylosaurine Rostrum and Heterochrony
Received 11 Aug 2017, Accepted 08 Jun 2018, Published online: 11 Oct 2018
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. Jiménez-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.
Citation for this article: Konishi, T., P. Jiménez-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
The fossil record of Tylosaurus (Mosasauridae: Tylosaurinae) begins unequivocally in the late Coniacian (Everhart, 2001, 2005a).
Because the first occurrence of a taxon means that the species evolved
before that point in time, it is not surprising that possible Tylosaurus are recognized from early Coniacian rocks (Everhart, 2005b) to even older units of rock and time (i.e., Turonian) (Polcyn et al., 2008; Flores, 2013). Thereafter, the known temporal range for the genus extends at least to the early Maastrichtian, making Tylosaurus one of the longest-existing mosasaur genera known (Caldwell and Diedrich, 2005; Bullard, 2006; Bullard and Caldwell, 2010; Konishi and Caldwell, 2011; Jiménez-Huidobro and Caldwell, 2016; Konishi et al., 2016). Throughout their known stratigraphic and paleobiogeographic range, Tylosaurus
attained the largest body size among sympatric members of the family
and, based on their gastric contents, were apex predators that fed upon
various marine vertebrates including other mosasaurs and plesiosaurs
(Martin and Bjork, 1987; Everhart, 2004). Numerous medium- to large-sized specimens presumably pertaining to subadult and adult Tylosaurus
are known, including KU 5033, whose skull and total body length is
estimated to have been approximately 1.8 m and nearly 13 m, respectively
(e.g., Everhart, 2005c;
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). Konishi et al. (2010) also suggested that the holotype of Platecarpus anguliferus (Cope, 1874) belongs to Tylosaurus sp., hinting at the possibility that some small Tylosaurus
material may have been misidentified as various contemporary
plioplatecarpines, a group of small- to medium-sized russellosaurines
sensu Polcyn and Bell (2005). A particularly diminutive mosasaur specimen, FHSM VP-14845, the subject of the current study, is one such example.
Specimen
FHSM VP-14845, an exceptionally small-sized mosasaur specimen
consisting of fragmentary jaw and cranial elements, was collected in
1991 from the Smoky Hill Chalk Member in western Kansas. With the
transverse width of the alveolar margin barely reaching 1 cm, the
specimen likely represents a neonatal individual. The material had been
informally assigned to Platecarpus, although it has never been formally described.
In this contribution, we first describe FHSM VP-14845 as representing the smallest-known specimen of Tylosaurus,
its generic assignment augmented through comparison with other, larger
North American specimens that are unequivocally assignable to the genus.
We then discuss its bearings on Tylosaurus 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 characteristic of the genus Tylosaurus and the subfamily Tylosaurinae.Institutional Abbreviations
AMNH, American Museum of Natural History, New York, New York, U.S.A.; ANSP, 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 Institut für Paläontologie, Bonn, Germany; KU, University of Kansas Natural History Museum, Lawrence, Kansas, U.S.A.; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, U.S.A.; RMM, Red Mountain Museum (currently McWane Science Center), Birmingham, Alabama, U.S.A.; TMM, Texas Memorial Museum, University 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
REPTILIA Linnaeus, 1758
SQUAMATA Oppel, 1811
MOSASAURIDAE Gervais, 1852
TYLOSAURINAE Williston, 1897
Liodon (in part) Cope, 1869–1870:200.
Rhinosaurus Marsh, 1872a:17.
Rhamphosaurus Cope, 1872:141.
Tylosaurus Marsh, 1872b:47.
Type species
Macrosaurus proriger Cope (1869).
Holotype
MCZ 4374.Generic Diagnosis
See Russell (1967:171–173) and Jiménez-Huidobro and Caldwell (2016).TYLOSAURUS sp.
(Figs. 2–5, 7−9, 12)
Referred Specimens
FHSM VP-14845: dentigerous portion of premaxilla with broken base of internarial bar, other tooth-bearing jaw fragments, right and left splenial fragments, right and left partial coronoids, incomplete quadrates on both sides, right and left partial pterygoids, and partial braincase (basisphenoid). FHSM VP-14840: anterior portion of premaxilla, broken at the second tooth row. FHSM VP-14843: premaxilla, with broken posterior portion of internarial bar.Locality and Horizon
FHSM
VP-14845: approximately 2.4 km (1.5 miles) south of Castle Rock,
southeastern Gove County (coordinates: SW1/4, Sec. 14, T14S, R26W
[approximately 38°49′51.65″N, 100°11′14.25″W]) in western Kansas, from
above Hattin’s (1982) Marker Unit 7 (M. J. Everhart, pers. comm., Oct. 15, 2013) in the Smoky Hill Chalk Member of the Niobrara Chalk (Fig. 1; Everhart, 2001). Lower to middle Santonian, Upper Cretaceous (Everhart, 2001).
FHSM VP-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.
FIGURE 1. Locality and horizon of FHSM VP-14845. A,
map of continental 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.
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 positioning 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 = 0.86; Fig. 2, Table 1). The outline also differs 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 and the mosasaurine Clidastes, the two sides of the snout converge at a more acute angle than in FHSM VP-14845 (compare Russell, 1967
:figs. 72 and 86; ANSP 10193 [C. propython holotype, pers. observ.]). Polcyn et al. (2008)
reported on a tylosaurine premaxilla TMM 40092-27 from the Turonian
portion (Arcadia Park Shale) of the Eagle Ford Shale exposed in
northeastern Texas. The seemingly complete dentigerous portion of this
geologically older material is essentially equidimensional (length:width
ratio = 0.98; Fig. 2C),
and is characterized by readily converging sides forming an abbreviated
predental rostrum, the length of which is comparable to the basal
diameter of the premaxillary teeth (see also Polcyn et al., 2008:fig.
5C). Consequently, the overall dorsal outline of the dentigerous
portion of FHSM VP-14845 bears the closest similarity to that of the
Turonian-aged TMM 40092-27, particularly concerning its overall
shortness and abbreviated predental rostrum.
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, second tooth base; vp, vomerine process (of premaxilla). Scale bars equal 1 cm (A and B) and 5 cm (C).
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.
Somewhat unexpectedly, both pairs of premaxillary teeth project anteriorly and laterally at the base (Fig. 2B), implying a procumbent nature atypical of tylosaurines (e.g., Bell, 1997:fig.
5C).
Also unusual are closely spaced first and second premaxillary
teeth, where the second pair is also located posterolateral to the first
pair (Fig. 2B).
In larger individuals, the anterior and posterior pairs are well
separated anteroposteriorly, where the posterior pair occurs more or
less posterior to the anterior set (see below). Nevertheless, the
premaxillary tooth crowns are smooth and bicarinate, exhibiting a
‘D’-shaped cross-section (Fig. 2B), the latter being consistent with the intercarinal angle reported by Konishi and Caldwell (2007a) in Tylosaurus
of about 120° on the labial side. The same angle would be close to 180°
in a given subadult and adult plioplatecarpine tooth, including Ectenosaurus (e.g., Konishi and Caldwell, 2011), and marginal teeth of adult Clidastes
spp. are longitudinally elongate ovals in cross-section, with
predominantly fore-and-aft carinal orientation (e.g., CMC VP 7554 [C. liodontus] and YPM 40350 [Clidastes sp.]; T.K., pers. observ.).
Quadrate
Whereas
the lateral side of the preserved quadrates show signs of cortical bone
loss postmortem, the more intact medial surface clearly exhibits a very
large stapedial pit, whose vertical length is comparable to that of the
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
observation of FHSM VP-15632 (T. kansasensis sensu Everhart, 2005a; T. nepaeolicus sensu Jiménez-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 (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. ‘kansasensis’]). As a shared Tylosaurus feature, the anterior border of the cephalic condyle is shallowly excavated (Fig. 3A, arrow). Unfortunately, at least the ventral half of the element was lost postmortem.
FIGURE 3. FHSM VP-14845, Tylosaurus sp., incomplete quadrates. Right quadrate in A, lateral, B, medial aspects and left quadrate in C, lateral, and D,
medial aspects. Arrow indicates posterior notching along anterior
border of cephalic condyle. Note the large stapedial pit whose length
equals that of the suprastapedial process and is nearly as wide as the
quadratic shaft. Quadrate shafts are distally incomplete on both sides. Abbreviations: ccd, cephalic condyle; spt, stapedial pit; ssp, suprastapedial process. Scale bar equals 1 cm.
Pterygoids
The main dentigerous portion is preserved on both sides, and a distal portion of the left quadrate ramus is also preserved (Fig. 4).
As is typical in tylosaurine pterygoids, the pterygoid tooth row is
straight and wider anterior to the ectopterygoidal process (e.g., KU
28705, Tylosaurus sp.). In adult plioplatecarpines, such as Plesioplatecarpus planifrons known from the lower Smoky Hill Chalk Member in Kansas (e.g., Konishi and Caldwell, 2007b:fig.
3), the pterygoid tooth row follows a gentle sinusoid curvature,
closely following the lateral edge of the pterygoid anterior to the
level of the ectopterygoidal process (the tooth row runs more or less
along a straight midline of the pterygoid in Tylosaurus) (Konishi and Caldwell, 2011). In Clidastes, numerous, closely packed pterygoid teeth occur along a pronounced vertical ridge, which Tylosaurus lacks (e.g., Bell, 1997:character 42[1]; KU 1022). As expected, resorption pits occur posterolaterally to the functional tooth positions.
FIGURE 4. FHSM VP-14845, Tylosaurus sp., incomplete pterygoids in ventral aspect. Dentigerous portion of A, right and B, left pterygoids. Quadratic ramus of C,
left pterygoid. Functional tooth crowns have been dissolved. Note the
anteroposteriorly linear tooth alignment and also these teeth
originating from a flat bony base, not along an elevated ridge. Abbreviations: ecpp, ectopterygoidal process; qr, quadrate ramus. Scale bar equals 1 cm.
Basisphenoid
Overall, the basisphenoid is more elongate than those in plioplatecarpines (cf. Russell, 1967:33) or in Clidastes (CMC VP 7554 and KU 1022; pers. observ.). Anteriorly, the sella turcica terminates as a pair of smooth, vertical surfaces for articulation with the trabecular cartilage (e.g., Oelrich, 1956:fig. 14; Fig. 5A). In adult plioplatecarpines, these surfaces are posteriorly inclined and less well defined (Russell, 1967:fig. 10). In RMM 5610, a juvenile specimen of Tylosaurus proriger with a skull length of about 50 cm, a pair of elongate ovoid foramina for the internal carotid branches pierce the sella turcica just in front of the dorsum sellae (Fig. 6A). An identical condition is discernible on FHSM VP-14845 (Fig. 5A). In the adults of the plioplatecarpine Platecarpus, these foramina occur more anteriorly, around the midlength of the sella turcica (Russell, 1967:fig. 10). Unlike Platecarpus, no distinct foramina for the basilar artery are present on the floor of the sella turcica in FHSM VP-14845 or RMM 5610.
FIGURE 5. FHSM VP-14845, Tylosaurus sp., basisphenoid in A, dorsal and B, ventral views. Abbreviations: ala, alar process; ba, exit for basilar artery branch; bpp, basipterygoid process; ds, dorsum sellae; ftrc, facet for trabecular cartilage; icb, exit for internal carotid branch; plp, posterolateral process; sel, sella turcica. Scale bar equals 1 cm.
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 trabecular cartilage; icb, exit for internal carotid branch; plp, posterolateral process (of basisphenoid); ppr, parasphenoid process; sel, sella turcica. Scale bar equals 1 cm.
Splenials
Both splenials are preserved near their articular cotyle, and the better-preserved right element is described here. On the medial aspect, the base of the medial wing is broken postmortem and has been pushed up against the lateral wing (Fig. 7A). On the articular cotyle, the surface medial to the vertical keel is wider and more strongly excavated than the surface lateral to it, exposing the medial surface of the keel (Fig. 7A, C). In lateral aspect, although the cortical layer has been eroded postmortem, it is nonetheless discernible that the lateral border of the splenial cotyle is vertically straight (Fig. 7B). Such a cotylar morphology would have allowed the splenial to pivot medially with a greater angle than laterally with respect to the angular, consistent with the hypothesis that the mandible bent outward at the intramandibular joint in mosasaurs (e.g., Lee et al., 1999:fig. 1). As mentioned already, much of the outer surface of the lateral wing is eroded postmortem.
FIGURE 7. FHSM VP-14845, Tylosaurus sp., right splenial in A, medial, B, lateral, and C, posterior views. Scale bar equals 1 cm.
Tooth-Bearing Elements
There are more than a dozen fragments of tooth-bearing elements that are neither fragments of the premaxilla nor the pterygoid (Fig. 8). None of these jaw fragments preserve functional tooth crowns, and the only crown that is preserved intact occurs in a single replacement pit (Fig. 8B). Two long sections of jaw rami, here identified as those pertaining to the right dentary and the left maxilla, can be assembled out of some of these jaw fragments. In stark contrast to the premaxillary dentition, adjacent functional tooth crowns in jaw rami are separated from each other by a gap at least 50% greater than a single basal crown diameter (Konishi and Caldwell, 2007a; Fig. 8A, B, double-headed arrows). On both the maxilla and the dentary, the medial dental margin is approximately at the same level as the lateral margin and forms a complete ovoid alveolus. In contrast, alveoli in contemporary plioplatecarpines are circular (e.g., Konishi and Caldwell, 2007b:fig. 3).
FIGURE 8. FHSM VP-14845, Tylosaurus sp., tooth-bearing elements (TBEs) compared with those of RMM 5610, Tylosaurus proriger. A, partial right dentary of FHSM VP-14845, anterior to the right; B, partial left maxilla of the same in occlusal view, anterior to the left; C, premaxilla and left maxilla of RMM 5610 in lateral view showing eighth and ninth maxillary teeth; D,
close-up view of the same teeth, showing the interdental distance that
is approximately 35% greater than the basal crown length of adjacent
teeth. Abbreviations: c, basal crown length; d, interdental distance; rp, resorption pit; t, tooth. Arrows indicate interdental gap. Scale bars equal 2 cm.
Coronoids
On both right and left coronoids (Fig. 9), the coronoid processes are tall and acutely angled in profile. On the left coronoid, the posterior border is apparently complete and is vertically straight, lacking a distinct posteroventral process that would merge with the coronoid buttress of the surangular. In RMM 5610 (Tylosaurus proriger), such a process is well developed and projects posteriorly beyond the posterior edge of the coronoid process (Fig. 10). The lack of this process in FHSM VP-14845 is not preservational, however; both the lateral and medial borders forming the base of the element meet posteriorly to form the posteroventral corner of the coronoid, much as in RMM 5610 (Figs. 9D [arrow], 10B [arrow]). The medial wing is largely incomplete on both sides, whereas the short lateral wing is complete on the left element. In dorsoventral aspect, the coronoid exhibits a gentle medial curvature, even though both coronoids are incomplete anteriorly.
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 10. RMM 5610, Tylosaurus proriger, right coronoid in A, lateral and B, ventral views. Arrow indicates posteroventral corner. Scale bar equals 2 cm.
DISCUSSION
Biostratigraphy of Tylosaurus in Western Kansas
Collected from above Marker Unit (MU) 7 (Hattin, 1982; M. J. Everhart, pers. comm.) of the Smoky Hill Chalk Member exposed in western Kansas, U.S.A., FHSM VP-14845 not only represents the smallest Tylosaurus specimen known to date, but it also bridges a significant biostratigraphic gap that existed between MU 5 and MU 9 for the genus (Everhart, 2001). Among the two nominal and the one then unnamed Tylosaurus species known from upper Coniacian–lower Campanian strata of the member, Everhart (2001) indicated for the respective taxa the following stratigraphic ranges: Tylosaurus ‘kansasensis’ sensu Everhart, 2005a (MUs 1–5; upper Coniacian–lowermost Santonian), T. nepaeolicus (MUs 1–5; upper Coniacian–lowermost Santonian), and T. proriger (MUs 9–23; middle Santonian–lower Campanian) (e.g., Fig. 1). Everhart (2005a:233) later confined the age of T. kansasensis to the late Coniacian. Recently, Jiménez-Huidobro et al. (2016) synonymized Tylosaurus kansasensis Everhart, 2005, with T. nepaeolicus (Cope, 1874) based on the sympatry of the two species, and on their significant morphological overlap on supposed key diagnostic characters of T. kansasensis. Jiménez-Huidobro et al. (2016) also suggested that specimens assigned to T. kansasensis were likely juveniles of T. nepaeolicus (Cope, 1874).The fact that FHSM VP-14845 is from the MU 7 (lower– middle Santonian) renders it possible that it could represent any one of these Tylosaurus species known from Kansas and increases the possibility that stratigraphically older ‘Lower Chalk’ T. nepaeolicus (sensu Jiménez-Huidobro et al., 2016) and younger ‘Upper Chalk’ T. proriger briefly coexisted in the Western Interior Seaway (cf. Everhart, 2001). In fact, Everhart (2001) hypothesized that the taxon range zone for T. proriger may have extended to include MU 5 (upper Coniacian), based on the occurrences of two large tylosaurine specimens (FHSM VP-13742 and 13908) from between MU 4 and MU 5. Although taxonomic assignment of FHSM VP-14845 to one or another species of Tylosaurus remains equivocal at the moment, its fine-scale morphological similarities to the basisphenoid of RMM 5610, a T. proriger juvenile, supports the possibility that FHSM VP-14845 could be assigned to T. proriger. It is noteworthy that the coronoid process in at least one adult and two juvenile specimens of Tylosaurus nepaeolicus sensu Jiménez-Huidobro et al. (2016)—AMNH 124, FHSM VP-2295 [T. kansasensis holotype], and VP-2495, respectively—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.
FIGURE 11. FHSM VP-2495, Tylosaurus nepaeolicus sensu Jiménez-Huidobro et al., 2016, left postdentary complex showing overhanging posterior coronoid process (arrow). Scale bar equals 5 cm.
Cranial Ontogeny in Tylosaurus
Premaxilla and Predental Rostrum
Everhart (2005) reported that the relative length of the predental rostrum on the premaxilla in Tylosaurus kansasensis ranged from 2.5% to 3.0% of mandibular length, smaller than the same ratio in T. nepaeolicus (4.2%) and in T. proriger (4.8%). Considering T. nepaeolicus to be a senior synonym of T. kansasensis, where specimens assigned to the latter are generally smaller in size than those assigned to the former, Jiménez-Huidobro et al. (2016) hypothesized that the predental rostrum grew longer ontogenetically in Tylosaurus, although they did not characterize rostral lengthening in much detail. In FHSM VP-14845, the edentulous rostrum beyond the first premaxillary tooth pair is present but much smaller (≪10 mm in length) than is observed in other specimens of Tylosaurus of Santonian–Campanian age (Figs. 2, 12; Thurmond, 1969; Sheldon, 1993). In addition, the outline of the projection describes a gentle parabolic arc in dorsoventral aspect, which is in stark contrast to the more triangular and more elongate morphology that is typical of a Tylosaurus rostrum. Notwithstanding, the presence of a small but distinct predental projection in FHSM VP-14845 precludes it from being Platecarpus, a ‘short-snouted’ plioplatecarpine, as it was originally considered to be.
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.
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 interdental gaps become stout and conical in adults, closing such gaps. At the level of the crown base, the juvenile maxilla (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 augment the ontogenetic argument of Jiménez-Huidobro et al. (2016) for this element in Tylosaurus: namely, the smaller or younger the animal, the proportionately more slender the suprastapedial process and the greater the size of the stapedial notch (Jiménez-Huidobro et al., 2016:78). Specimen FHSM VP-14845 exhibits a substantial gap between the suprastapedial process and the shaft, the former being elongate and deflected medially (Fig. 3). The stapedial pit is enormous, as long as the suprastapedial process itself, which is noteworthy given its small relative size in a typical adult Tylosaurus quadrate (Russell, 1967; Bell, 1997:fig. 7B). Also of note is that, at least in medial aspect, the quadrate shaft of FHSM VP-14845 is only slightly wider than that of the stapedial pit. It thus seems that the entire quadrate developed with positive allometry relative to the suprastapedial pit in both vertical and horizontal dimensions. In sum, a tylosaurine quadrate early in ontogeny is characterized as possessing a long, slender suprastapedial process, a wide stapedial notch, and a very large stapedial pit.Ontogenetic Status of FHSM VP-14845: Prenatal or a Neonate?
Based on the dentary tooth crown diameter, and under the assumption that tooth crowns grow isometrically to overall body size, Field et al. (2015) estimated the size of the smallest Clidastes sp. specimen they reported (YPM 058126) to be 0.66 m, or about 22% the length of a 3-m-long adult. As Konishi and Caldwell (2007a) reported in Tylosaurus proriger, however, tooth crowns in Tylosaurus exhibit positive allometry relative to the respective tooth-bearing elements, except for those on the premaxilla at a very early ontogenetic stage (i.e., FHSM VP-14845; this study). Jaws of a juvenile possessing slender tooth crowns and large interdental gaps (see above) are later filled by enlarged, and not by additional, adult tooth crowns. Comparison between the smaller and larger specimens Field et al. (2015) identified as Clidastes reveals that the interdental gaps are indeed proportionately larger in smaller specimens (e.g., YPM 058126, the gap larger than the basal crown diameter) than in larger specimens (YPM 1314, the gap smaller than basal crown diameter). Hence, it is possible that Field et al. (2015) underestimated the total body length of YPM 058126, which renders the neonate status of this specimen that Field et al. (2015) suggested less unequivocal. Also of note, YPM 1253, the third smallest specimen of Clidastes identified by Field et al. (2015), indeed pertains to a Platecarpus-like plioplatecarpine: a short premaxillomaxillary suture (about two and a half alveoli long), a round basal tooth crown cross-section, and clear presence of hemapophyses (i.e., a hemal arch–spine complex not fused to the caudal vertebra) are all characteristic of this short-snouted plioplatecarpine common in the Smoky Hill Chalk Member (T.K., pers. observ., 2005).By estimating both the skull length (SL) and the total body length (TBL), we further evaluated the possible developmental stage of FHSM VP-14845 at the time of its death. To do this, we used data from two large Tylosaurus skeletons collected from the Kansas Chalk: AMNH FR-221, an 8.83-m-long, nearly complete and articulated Tylosaurus proriger skeleton (Osborn, 1899a, 1899b), and FHSM VP-3, another articulated, partially reconstructed skeleton of T. proriger that is slightly smaller than AMNH FR-221 (Russell, 1967). By comparing six-alveolus lengths between AMNH FR-221 and FHSM VP-14845, the skull length (SL) of the latter was estimated to be about 30 cm. Comparison between SLs of FHSM VP-14845 and AMNH FR-221 was made subsequently, yielding the estimated total body length (TBL) of 2.23 m for FHSM VP-14845 (Appendix 1). This estimated TBL is 25.3% that of AMNH FR-221 and 17.2% that of KU 5033 (the ‘Bunker tylosaur’), the largest-known T. proriger specimen collected from Kansas at an estimated TBL of 13 m (Everhart, 2002, 2005c; pers. observ.), and 24.8–27.9% of an estimated maximum TBL for T. nepaeolicus at 8–9 m (Everhart, 2002). Based on Caldwell and Lee (2001:fig. 2, and references therein), the estimated TBL for FHSM VP-14845 falls well within the neonate TBL range expected for extant varanoid lizards, at approximately 10–40% maternal TBL.
Still, given the exceptionally large adult size of Tylosaurus compared with extant Varanus, it may be possible that FHSM VP-14845 was a prenatal individual close to parturition, a possibility that Field et al. (2015) did not consider for any of the small Clidastes specimens they analyzed. Although direct evidence supporting or countering such a possibility is lacking in FHSM VP-14845, we present here an argument favoring the likelihood that FHSM VP-14845 was a neonate. First, FHSM VP-14845, consisting of associated fragmentary bones pertaining to a single individual, was discovered without any associated adult or juvenile bones (M. J. Everhart, pers. comm., 2018), indicating that it was preserved by itself ex utero (e.g., O’Keefe and Chiappe, 2011; Field et al., 2015). Second, even if FHSM VP-14845 was an offspring from an exceptionally large Tylosaurus proriger individual such as KU 5033, the TBL ratio of 17.2% between the two specimens still exceeds the equivalent ratio of 15% estimated in Carsosaurus marchesetti, a basal mosasauroid (Caldwell and Lee, 2001). Among extant cetaceans, another clade of secondarily aquatic tetrapods that are universally viviparous, the TBL ratio between the neonate and the mother is negatively correlated with the maternal TBL across a variety of whale species, both in mysticetes (R2 = 0.417) and in odontocetes (R2 = 0.315) and when both clades are combined (R2 = 0.620; Fig. 13). In extant viviparous/ovoviviparous chondrichthyan taxa without embryonic cannibalism, some of the smaller species (e.g., Squalus acanthias) exhibit the longest gestation period of up to 24 months, indicating that smaller taxa give birth to a small number (2–14 in S. acanthias) of large offspring relative to maternal size (Pough et al., 2013:fig. 5-11). Indeed, the whale shark (Rhincodon typus), the largest extant shark species growing up to at least 12 m in TBL, is known with the litter size of 300, the largest recorded among extant sharks (Stevens, 2007, and references therein). Perinatal embryos of a 10.6-m gravid whale shark ranged from 58 to 64 cm in TBL, which amounts to 5.5–6.0% of the maternal TBL (Joung et al., 1996). The same ratio in Squalus acanthias becomes fivefold, where we recorded 26.5–31.7% based on 10 perinatal embryos from three females (T.K., pers. observ.). Although circumstantial, these lines of evidence lend more support to FHSM VP-14845 having been a precocial neonate ex utero.
FIGURE 13. Extant cetacean neonate size distribution against maternal body length for A, mysticetes, B, odontocetes, and C,
mysticetes and odontocetes combined. Note the general decline in
neonate size relative to maternal body size. Data compiled from Boyd
et al. (1999) and Jefferson et al. (2008).
In 2016, Jiménez-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 certain 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; Simões 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).
Handling editor: Patrick Druckenmiller.
ACKNOWLEDGMENTS
We sincerely thank M. J. Everhart for his kind hospitality in accommodating our collections visits at the FHSM and elsewhere in Kansas, and for providing us with additional information 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.APPENDIX 1.
Skull and body length estimates for FHSM VP-14845. To estimate the skull length (SL) and the total body length (TBL) of FHSM VP-14845, we used selected measurements of FHSM VP-3 and AMNH FR-221 from Russell (1967:appendix A) and Osborn (1899a), respectively, as follows.- Skull length (SL) FHSM VP-3 = 1058 mm; length between first and sixth dentary teeth (D1–D6) measured across tooth bases = 225 mm
- (SL) FHSM VP-14845 = (D1–D6) FHSM VP-14845 × 1058 mm/225 mm — A
(D1–D6) FHSM VP-14845 = 64 mm — B
Inserting B into A, we derive:
(SL) FHSM VP-14845 = 300.9 mm — C
- 3. Total body length (TBL) AMNH FR-221 = 8830 mm, whereas (SL) AMNH FR-221 = 1190 mm. From C, then, (TBL) FHSM VP-14845 can be obtained as follows:
(TBL) FHSM VP-14845 = (TBL) AMNH FR-221 × C/(SL) AMNH FR-221 = 8830 mm × 300.9 mm/1190 mm = 2232.7 mm
Thus, approximately, (TBL) FHSM VP-14845 = 2.23 m
Similarly, using the M1–M6 length as the best proxy for D1–D6, we attained the following SL estimate for juvenile T. proriger (RRM 5610):- 4. (SL) RMM 5610 = (M1–M6) RMM 56 × 1058 mm/225 mm
= 130 mm × 1058 mm/225 mm
= 611 mm
≈ 60 cm
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