Platecarpus Tympaniticus (Squamata, Mosasauridae): Osteology of an
Exceptionally Preserved Specimen and Its Insights Into the Acquisition of a
Streamlined Body Shape in Mosasaurs
Author(s): Takuya Konishi , Johan Lindgren , Michael W. Caldwell , and Luis Chiappe
Source: Journal of Vertebrate Paleontology, 32(6):1313-1327. 2012.
Published By: The Society of Vertebrate Paleontology
URL: http://www.bioone.org/doi/full/10.1080/02724634.2012.699811
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Journal of Vertebrate Paleontology 32(6):1313–1327, November 2012
© 2012 by the Society of Vertebrate Paleontology
ARTICLE
PLATECARPUS TYMPANITICUS (SQUAMATA, MOSASAURIDAE): OSTEOLOGY OF AN
EXCEPTIONALLY PRESERVED SPECIMEN AND ITS INSIGHTS INTO THE ACQUISITION
OF A STREAMLINED BODY SHAPE IN MOSASAURS
TAKUYA KONISHI,*,1,† JOHAN LINDGREN,2 MICHAEL W. CALDWELL,1,3 and LUIS CHIAPPE4
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9, tkonishi@ualberta.ca;
2
Department of Earth and Ecosystem Sciences, Division of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden,
johan.lindgren@geol.lu.se;
3
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9,
mw.caldwell@ualberta.ca;
4
The Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, California 90007, U.S.A.,
chiappe@nhm.org
1
ABSTRACT—LACM 128319, which was collected in western Kansas, U.S.A., and is assignable to Platecarpus tympaniticus
(Mosasauridae, Plioplatecarpinae), represents arguably one of the most exquisite mosasaur specimens known to date. Measuring 5.67 m from the tip of the snout to the end of the tail, it comprises an exceptionally well-articulated skeleton, accompanied
by soft-tissue remains, such as skin impressions and tracheal cartilage. P. tympaniticus is one of the most numerously collected
mosasaur taxa in North America, but as most specimens are fragmentary or reconstructed to various degrees, LACM 128319
provides a unique opportunity to document the taxon’s osteology from a single skeleton. In this study, we first present a
detailed osteological description of LACM 128319. Following this, we present an analysis of the evolution of a streamlined
body shape in P. tympaniticus, specifically by comparing the length distribution of the dorsal ribs in relevant anguimorphan
taxa. We conclude that both an anterior migration of the rib cage and an increasing regionalization within the dorsal vertebral
series are key features contributing to formation of a streamlined body profile in P. tympaniticus, and probably in many other
hydropedal members of mosasaurs.
INTRODUCTION
In terms of vertebrate evolution, one of the last major
macroevolutionary events that took place during the Mesozoic
Era was a remarkable marine invasion by a group of true lizards,
the mosasaurs. The origin of mosasaurs postdates that of most
other major groups of Mesozoic marine tetrapods, including
ichthyosaurs, plesiosaurs, marine crocodilians (metriorhynchids),
and sea turtles (e.g., Carroll, 1988; Hirayama, 1998; Polcyn et al.,
1999), most of which appear in the Triassic or Early Jurassic.
Mosasaurs quickly reached the top of the trophic hierarchy in
Late Cretaceous marine ecosystems, and successfully maintained
this niche for more than 25 million years, only to go extinct at the
end of the Cretaceous.
One of the most commonly collected species of mosasaur from
anywhere in the world is without doubt Platecarpus tympaniticus
Cope, 1869a, which is known from at least 86 individuals collected
from the Western Interior Basin of North America (Russell,
1967; Kiernan, 2002; Konishi et al., 2010). The density of their
fossils has provided a vast store of morphological information on
mosasaurs in general (e.g., Russell, 1967), the most recent case
constituting a report on an outstandingly well-articulated specimen preserving an array of soft-tissue structures (Lindgren et al.,
2010). The specimen, LACM 128319, was recovered from the upper Santonian–lowermost Campanian portion of the Smoky Hill
Chalk Member of the Niobrara Formation, exposed on the south
side of the Smoky Hill River in Logan County, western Kansas
*Corresponding author. †Current address: P.O. Box 7500, Royal
Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada T0J
0Y0.
(Fig. 1). Lindgren et al. (2010) focused on describing the softtissue anatomy and overall body outline of this specimen (Fig. 2),
comparing it with other highly aquatically adapted mosasaurs. At
the same time, the specimen also provides an excellent opportunity to explore osteological details of P. tympaniticus.
The purpose of this study is twofold: (1) to describe the osteology of LACM 128319 in detail and using these new data to rediagnose the taxon (cf. Konishi et al., 2010); and (2) to compare
the vertebral anatomy of P. tympaniticus with that of progressively more basal anguimorphs (i.e., limbed mosasauroids and
Varanus), to gain insights into the acquisition of a streamlined
body shape in mosasaurs as part of their adaptations to a fully
aquatic existence.
Institutional Abbreviations—AMNH, American Museum of
Natural History, New York, New York, U.S.A.; ANSP, Academy
of Natural Sciences, Philadelphia, Pennsylvania, U.S.A.; FHSM
VP, Sternberg Museum of Natural History, Hays, Kansas,
U.S.A.; FMNH, Field Museum of Natural History, Chicago,
Illinois, U.S.A.; KU, University of Kansas Natural History
Museum, Lawrence, Kansas, U.S.A.; LACM, Natural History
Museum of Los Angeles County, Los Angeles, California,
U.S.A.; M, Canadian Fossil Discovery Centre, Morden, Manitoba, Canada; OMNH, Osaka Museum of Natural History, Osaka, Osaka, Japan; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UALVP, University of
Alberta Laboratory for Vertebrate Paleontology, Edmonton, Alberta, Canada; YPM, Yale Peabody Museum of Natural History,
New Haven, Connecticut, U.S.A.
Anatomical Abbreviations—a, angular; ar, articular; as, astragalus; ax, axis; br, bronchial cartilages; c, cervical vertebra;
cl, clavicle; cm, calcaneum; cr, coronoid; d, dorsal vertebra;
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JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 32, NO. 6, 2012
TABLE 1. Various skeletal measurements of LACM 128319, Platecarpus tympaniticus, in mm.
Skull
Length of skull along midline
Width of frontal between orbits
Length between (base of) first and
sixth maxillary teeth
Height of quadrate
Length of dentary along dental
margin
Length between (base of) first and
sixth dentary teeth
Length of external naris (left)
Length of supratemporal fenestra
Length of orbit
Height of orbit
Axial skeleton
Horizontal length from third to
seventh vertebrae∗
Linear length along tail vertebrae
forming downturned section of tail
Appendicular skeleton
Front paddle (left)
Humerus (left)
Radius (left)
Ulna (left)
Femur (left)
Tibia (left)
Fibula (left)
Ilium (left)
Pubis (left)
∗
517
106
126
99
320
127
146
140
105
84
292
1525
Length
Width
567
127
92
85
129
89
89
170
180
304
∗∗
122
∗∗
101
∗∗
165
∗∗
186
∗∗
171
∗∗
153
From prezygapophysis of third to postzygapophysis of seventh vertebra.
Distal width.
∗∗
FIGURE 1. Locality of LACM 128319, Platecarpus tympaniticus, indicated by a star in southern part of Logan County, Kansas, U.S.A. Coordinates are provided in main text. Abbreviation: KS, Kansas.
dt, dentary; en, external naris; f, frontal; fi, fibula; fm, femur;
h, humerus; has, hemal arch–spine complexes; hyd, hyoid; i,
intermediate caudal vertebra; icl, interclavical; il, ilium; im, intermedium; inb, internarial bar; is, ischium; j, jugal; m, maxilla; mc1, first metacarpal; mdk, median dorsal keel; mt5, fifth
metatarsal; od, odontoid; of, obturator foramen; os, orbitosphenoid; p, pygal vertebra; par, prearticular; pb, parietal table; pf,
parietal foramen; ph, phalanges; pm, premaxilla; pof, postorbitofrontal; prf, prefrontal; pu, pubis; q, quadrate; r, radius; s,
surangular; sa, sclerotic aperture; sc, scapula; sm, septomaxilla;
sp, splenial; sq, squamosal; ss, suspensorial ramus; str, sternal ribs;
st-qp, quadrate process of supratemporal; t, terminal caudal vertebra; t4, fourth tarsal; tb, tibia; tp, transverse process; tr, tracheal
cartilages; u, ulna; ul, ulnare. Where two sides are labeled, each
abbreviation is preceded by r (right) or l (left). Numerals following hyphens indicate vertebral counts.
MATERIALS AND METHODS
Throughout this paper, we use the term ‘streamlined’ in the
sense that it is “a teardrop contour offering the least possible resistance to a current of air, water, etc.” (Stein, 1980:1299). In this
definition, streamlined aquatic vertebrates are particularly efficient swimmers, because inertial drag becomes minimized “when
their maximum width is about one-fourth of their length and
is placed about one-third of the length from the leading tip”
(Pough et al., 1999:218). Consequently, we do not apply the term
‘streamlined’ to animals with other body forms, including terrestrial lizards such as the Komodo dragon, whose body is widest in
the middle.
A detailed skeletal line drawing was produced by tracing
a high-resolution photographic image of LACM 128319 using
Adobe Photoshop CS3. Skeletal elements were measured using a
steel measuring tape for rib lengths, and digital calipers for other
measurements. All rib lengths were measured in a straight line
as a distance between the preserved proximal and distal ends of
each rib. Among the anterior dorsal ribs, only the last one is complete. The distinction between the last anterior and the first posterior dorsal rib was based on the pointed distal morphology of
the latter. Various other skeletal measurements of LACM 128319
are listed in Table 1.
SYSTEMATIC PALEONTOLOGY
REPTILIA Linnaeus, 1758
SQUAMATA Oppel, 1811
MOSASAURIDAE Gervais, 1852
RUSSELLOSAURINA Polcyn and Bell, 2005
PLIOPLATECARPINAE Dollo, 1884
PLATECARPUS Cope, 1869a
Type Species—Platecarpus tympaniticus, by monotypy (Konishi and Caldwell, 2011).
Diagnosis—As for type and only species.
PLATECARPUS TYMPANITICUS Cope, 1869a
(Figs. 2–6, 8F, 9A)
KONISHI ET AL.—PLATECARPUS AND STREAMLINING IN MOSASAURS
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FIGURE 2. Platecarpus tympaniticus, LACM 128319, entire skeleton arranged in what is considered as showing natural articulation. A, interpretative
line drawing; B, photograph. Asterisk indicates provisional identification.
Holotype—ANSP 8484, 8487–88, 8491, 8558–59, 8562, individually numbered elements belonging to a single individual. For details about the type material, and the occurrence of P. tympaniticus, see Konishi et al. (2010).
Referred Material, Locality, and Horizon—LACM 128319,
discovered approximately 20 km SSE of Russell Springs, Logan
County, western Kansas, U.S.A. (NW 14 Sec. 15; T15S; R34W)
(101◦ 5 W, 38◦ 45 N) (Fig. 1). Upper part of the Smoky Hill Chalk
Member (Niobrara Formation), late Santonian–earliest Campanian (84–83 Ma) in age (Hattin, 1982; Ogg et al., 2004).
Emended Diagnosis—Readers are referred to Konishi et al.
(2010) for a thorough diagnosis of both the genus and species.
Only new or revised characters are listed below. A mediumsized russellosaurine mosasaur with an adult body length of about
5.5 m, mandible length typically 0.6 m; only last maxillary tooth
suborbital; septomaxilla elongate, occurring inside narial chamber and posterior to expanded portion of external naris; prefrontal bordering only posterior 10% of lateral border of external
naris; posterolateral frontal flanges singularly surrounding anterior parietal table; edentulous portion posterior to last dentary
tooth two alveoli long; angular articulation surface with splenial
ornamented with diagonally oriented grooves and ridges; coronoid process moderately developed with posterior border posteroventrally inclined; zygosphenes and zygantra rudimentary on
cervicals; 20–23 dorsal vertebrae, of which 9 are anterior dorsals;
last anterior (ninth) dorsal rib nearly twice as long as first posterior (10th) dorsal rib; scapular neck present; humerus as long as
distally wide; four to five carpals; phalangeal formula 4-5-5-4-3 or
one additional phalanx each for any given digit. For dentition, see
diagnosis for Plioplatecarpinae in Konishi and Caldwell (2011).
Taxonomic Remarks—Because a complete and exhaustive
synonymy list of Platecarpus tympaniticus was provided by
Konishi et al. (2010), and because we are unaware of any new
junior synonyms of the taxon, this list will not be repeated here.
DESCRIPTION AND COMPARISONS
The anatomy of Platecarpus tympaniticus is relatively well understood from a large number of partial specimens curated in
a number of museums and institutions worldwide. However, a
largely intact and articulated skeleton, as well as the preservation
of soft-tissue structures, makes LACM 128319 uniquely informative among all the specimens assigned to P. tympaniticus (Konishi
et al., 2010). Consequently, the balance of the following description focuses on the articulated osteology of the specimen, and
the new data on body shape as derived from that anatomy. The
anatomical description is followed by a brief taphonomic description of LACM 128319.
Skull
Premaxilla—The premaxillary-maxillary suture extends posteriorly to the level of the third maxillary tooth, and rises posteriorly at about 45◦ . The dentigerous portion is trapezoid in dorsal
view and similar to that of Latoplatecarpus willistoni Konishi and
Caldwell, 2011, a more derived plioplatecarpine; this is in contrast
to the semicircular outline in Plesioplatecarpus planifrons (Cope,
1874) (see Konishi and Caldwell, 2007:fig. 2). The internarial bar
reaches the anterior processes of the frontal, and it is laterally expanded along its posterior two-thirds (Fig. 3). The anterior onethird of this element forms the medial border of the expanded
bony naris. Although the first premaxillary teeth are broken, a
predental rostrum is absent based on the remaining tooth bases.
Maxilla—The posterior end of the premaxillary-maxillary
suture coincides with the anteriorly deepest portion of the
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JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 32, NO. 6, 2012
FIGURE 3. Platecarpus tympaniticus, LACM 128319, first slab containing skull and first nine vertebrae. A, interpretative line drawing; B, photograph
(Color figure available online).
maxilla (similar to the condition in Plesioplatecarpus, but differing from that of Latoplatecarpus, where the maxilla continues to
rise posteriorly beyond the suture). Of the 12 maxillary teeth,
only the last tooth is suborbital, whereas at least two tooth positions are situated underneath the orbit in Latoplatecarpus (Konishi and Caldwell, 2011). In LACM 128319, the depth of the maxilla where the narial embayment is most pronounced is approximately equal to the longest preserved tooth (the second left)
(Fig. 3). Konishi and Caldwell (2011) reported that the corresponding maxillary depth is more than 20% smaller than a fully
erupted maxillary tooth in Latoplatecarpus willistoni Konishi and
Caldwell, 2011, and conversely it is 15% greater than such a tooth
in Plesioplatecarpus planifrons. Thus, the condition in Platecar-
pus tympaniticus falls between these two taxa, in accordance
with the plioplatecarpine phylogeny presented by Konishi and
Caldwell (2011). Posteriorly, a tongue-like dorsal flange of the
maxilla forms a typically mosasaurid sigmoidal suture with the
prefrontal.
Prefrontal—A small posterolateral portion of the narial opening is bordered by this element (to a much smaller degree than
in Plesioplatecarpus planifrons; see Konishi and Caldwell, 2007).
The supraorbital process is minimally developed. The dorsal and
lateral faces of the element meet along an obtuse corner lateral
to the frontal. The prefrontal and postorbitofrontal are in contact, thereby excluding the frontal from the supraorbital border
(Fig. 3).
KONISHI ET AL.—PLATECARPUS AND STREAMLINING IN MOSASAURS
Frontal—The distance between the anterolateral processes of
the frontal appears to be about 30% of its interorbital width, a
low value that is comparable to that of Plesioplatecarpus planifrons (UALVP 24240; 36%) but not with that of Latoplatecarpus (TMP 1984.162.0001; at least 50%) (Konishi and Caldwell,
2011). A median dorsal keel extends for more than 75% of the
frontal table length. The broadly triangular frontal table is indented along the supraorbital border, a feature usually associated
with P. planifrons (Konishi and Caldwell, 2007). The posterolateral flanges singularly surround the parietal foramen. In dorsal
aspect, the posterior border of each flange makes a direct contact
with the postorbitofrontal. The frontal ala is rounded in outline.
Parietal—The roughly pentagonal parietal table is somewhat
wider than it is long. The anterior tip of the almond-shaped
parietal foramen extends slightly beyond the anterior edge of the
parietal table; a condition that is fundamentally different from
that of Latoplatecarpus willistoni, in which a pair of posteromedian flanges of the frontal surrounds or borders the anterior half
of the foramen (Konishi and Caldwell, 2011). From the table, a
short postorbital process extends laterally to form the anteromedial corner of the upper temporal fenestra, but it does not
form the dorsal surface of the skull table posterior to the frontal.
Posteriorly, lateral borders of the table meet to form an obtuse
parietal crest. The preserved left suspensorial ramus extends
towards the posterolateral corner of the upper temporal fenestra, where it contacts the parietal process of the supratemporal.
There is no indication that the ramus contacts the squamosal.
Postorbitofrontal—At least the posterior half of the supraorbital border is formed by the anterior ramus of this element.
There is a potential sutural boundary between the anterior ramus (i.e., postfrontal proper) and the rest of the element (i.e.,
postorbital proper), although such a feature is rare and taphonomic alterations cannot be ruled out. The jugal process bears an
anteroventral projection, and it descends to articulate with the
dorsal end of the ascending process of the jugal. The squamosal
process is extremely elongate in LACM 128319; it reaches the
posterior edge of the squamosal-supratemporal complex posteriorly, a feature previously known only in Plioplatecarpus houzeaui
Dollo, 1889, among plioplatecarpine mosasaurs.
Sclerotic Ring—A well-preserved sclerotic ring consists of at
least 10 sclerotic ossicles of three different types (Marsh, 1880),
arranged seemingly in no particular order. The sclerotic aperture
and the ring are both somewhat compressed dorsoventrally, thus
agreeing with the shape of the orbit that is itself somewhat longer
than deep. Although the eyeball of Platecarpus tympaniticus may
have been ellipsoidal rather than spherical, its lens was almost
certainly spherical as a result of a high degree of adaptation for
life in water (Kardong, 2002).
Orbitosphenoid—A putative orbitosphenoid is identified, being partially exposed between the posterior edge of the sclerotic
ring and the jugal process of the postorbitofrontal (Fig. 3). It is a
laterally flattened element with only a subtle curvature. Along
with the relative size of the element and its proximity to the
frontal-parietal suture, these features agree with the orbitosphenoid identified in Latoplatecarpus willistoni (Konishi and Caldwell, 2011).
Septomaxilla—All the palatal elements are embedded in the
matrix except for the putative right septomaxilla. Lying medial
to the maxilla and with its anterior end reaching the posterior
edge of the expanded portion of the external naris, the position
of the element is identical to the paired septomaxillae reported
in Plesioplatecarpus planifrons by Konishi and Caldwell (2007).
The element is expanded anteriorly, followed by a slender and
straight portion that extends posteriorly under the prefrontal
past the maxilla-prefrontal contact. An identical anterior portion
is present in the septomaxilla of P. planifrons, followed by a
dorsoventrally flattened, gently concave midportion that extends
posteriorly to contact the palatine underneath the prefrontal
1317
(Konishi and Caldwell, 2007:figs. 2, 3). Given the topological
identity with the septomaxilla in P. planifrons, and as no other
known cranial elements match the preceding morphological
description of this element, our identification of this element in
LACM 128319 as the first-known septomaxilla in Platecarpus
tympaniticus is made with confidence. Whether the aforementioned morphological differences between the two taxa are
interspecific or arising from preservation cannot be ascertained
at this point.
Squamosal—The elongate postorbital process reaches the anterodorsal corner of the lower temporal fenestra. The lateral
wall of the process gradually tapers to a point along the anterior half of its length. Posteroventrally, the nearly horizontal
quadrate process of the squamosal articulates with the quadrate
along a long suprastapedial process rather than at the cephalic
condyle (see Konishi and Caldwell, 2011). Medially, the body of
the squamosal articulates with the supratemporal that also fills
the gap between the parietal process of the squamosal and the
suspensorial ramus of the parietal.
Supratemporal—Dorsally and anteromedially, the supratemporal extends as a tongue-like parietal process to articulate with
the suspensorial ramus of the parietal from underneath. The
body of the element abuts that of the squamosal from the medial side, and it is concealed in lateral aspect. The bulbous ventral projection, the quadrate process, of the supratemporal caps
the suprastapedial process of the quadrate distal to the squamosal
(Konishi and Caldwell, 2011; Fig. 3A).
Quadrate—Shaped like a question mark, the quadrate forms
a suspensorial articulation at the distal portion of its long
suprastapedial process. The quadrate shaft is tilted anteriorly at
about 45◦ to the horizontal, and the distal end of the suprastapedial process and the mandibular condyle are aligned vertically.
As a result, approximately the posterior half of the lower temporal fenestra is occupied by the quadrate (Fig. 3). Although a separate contribution detailing this new interpretation of the quadrate
orientation in plioplatecarpine mosasaurs is underway (T. Konishi, in prep.), all of these conditions in LACM 128319 are in accordance with those observed in Latoplatecarpus willistoni (TMP
1984.162.0001, DMNH 8769; Konishi and Caldwell, 2011), Plesioplatecarpus planifrons (FHSM VP-2296 [pers. observ.]; Konishi,
2010) and Platecarpus sp., cf. P. tympaniticus (KU 4862, YPM
1286, 40585 [pers. observ.]; Konishi, 2010), and are here considered to represent the original condition. In P. tympaniticus,
the quadrate shaft has previously been reconstructed in an upright orientation with the entire suprastapedial process projecting posterior to the mandibular condyle (e.g., Williston, 1898:pls.
13, 27; Russell, 1967:figs. 20, 38), an interpretation that is not
supported by the aforementioned specimens, including LACM
128319, where at least one quadrate is preserved in articulation
with the squamosal and supratemporal. Laterally adjacent to the
squamosal, the dorsal surface of the suprastapedial process is longitudinally sulcate, and laterally adjacent to the supratemporal,
the same surface bears a deeper, circular excavation (Fig. 3).
Jugal—The ‘L’-shaped jugal bears an acute posterior process
at its posteroventral corner. The distal end of the horizontal
(= suborbital) ramus turns upward, whereas that of the vertical
(= postorbital) ramus is expanded and laterally excavated.
When measured from its posteroventral process, the horizontal
ramus is approximately 2.7 times longer than the vertical ramus.
Because the jaws are closed in LACM 128319, the jugal corner
overlaps the coronoid process (Fig. 3).
Braincase—Unfortunately, all braincase elements are embedded in matrix in LACM 128319.
Mandible
Dentary—The slender dentary bears 12 teeth. The first dentary tooth occurs between the first and second premaxillary teeth,
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and the last between the 10th and 11th maxillary teeth. Thus, it
is likely that the marginal teeth interdigitated in Platecarpus tympaniticus, a condition seldom preserved in plioplatecarpine fossils because the teeth are often disoriented, as in LACM 128319.
There is an approximately two-alveolus-long portion without
teeth posterior to the 12th dentary tooth. Under this portion,
the lateral wall of the bone appears horizontally sulcate. The
largest concentration of foramina on the same wall occurs anteriorly beneath the first four teeth. A slight predental projection is
present.
Splenial—As preserved, the splenial tapers to a point under
the ninth dentary tooth in LACM 128319, whereas it reaches to
the 11th tooth in another specimen (FMNH UC-600) of Platecarpus tympaniticus. The intramandibular joint is in alignment with
the posterior end of the anterior surangular foramen, and thus
a significant posterior portion of the splenial occurs under the
surangular. A similar condition is observed in FMNH UC-600.
Angular—The anterior articular surface with the splenial is ornamented with medioventrally directed grooves and ridges, a feature previously considered to be characteristic of more derived
plioplatecarpines (Konishi and Caldwell, 2011).
Surangular—The dorsal border of the surangular that is located posterior to the coronoid is horizontally straight, and the
former element deepens anteriorly. A little over 30% of the
coronoid-surangular suture protrudes beyond the intramandibular joint, a condition that is nearly identical to that in FMNH UC
600 (see Konishi and Caldwell, 2011). The anterior surangular
foramen and its associated sulcus occur underneath the anterior
one-third (34%) of the coronoid suture, similar to the condition
in L. willistoni (Konishi and Caldwell, 2011). The precise outline of the posterior coronoid-surangular suture cannot be ascertained due to poor preservation.
Coronoid—In LACM 128319, the coronoid is largely hidden
underneath the jugal with the coronoid process occurring medial
to the jugal corner. Because of this and the incomplete preservation of the suture with the surangular, little can be said about
the element. The posterior border of the coronoid process is inclined at about 60◦ from the horizontal, consistent with what was
reported by Konishi and Caldwell (2011) for this taxon.
Articular-Prearticular—Even though the mandible experienced mediolateral compression during the process of fossilization, the retroarticular process remains rotated medially from the
parasagittal plane. As described by Konishi and Caldwell (2011),
the straight medial border and rounded lateral border of this
process meet at the posteromedial corner (Fig. 3, left side). As
preserved, the glenoid surface formed by the articular appears
to be semicircular rather than crescent-shaped. The prearticular
is only visible through the gap above the intramandibular joint,
bridging the dentary and postdentary complex.
Hyoid—A rod-like element with a slight anterior curvature
rests along the posteroventral border of the postdentary complex
(Fig. 3A), and is here identified as a hyoid bone. The element
is approximately 50% as long as the postdentary complex, and
appears to be complete. No segmentation is present, and the element is interpreted to represent the first ceratobranchial, which
is the only ossified portion of the hyoid apparatus in most lizards
(Romer, 1956). The anterior end of the element is slightly expanded, whereas the bone tapers uniformly along the posterior
50% of its length, ending in a blunt terminus. A short, cartilaginous epibranchial likely articulated with this end (e.g., Romer,
1956:fig. 198).
Tracheal and Bronchial Cartilages—Several strings of rings of
various completeness appear in the lower temporal fenestra, between the retroarticular processes, underneath the fourth and
fifth, and the seventh and eighth, vertebrae (Fig. 3). Of those
four groups, the anterior three segments comprise tracheal rings,
whereas the posterior-most segment is interpreted to comprise
bronchial rings. As noted by Lindgren et al. (2010), the tracheal
rings exhibit a wide range of diameters that are likely the result
of preservational artifacts. Nevertheless, the bronchial rings are
consistently smaller than the tracheal rings in diameter, and the
former are observed as arranged in two subparallel rows (Lindgren et al., 2010) (Fig. 3A). The bifurcation of the trachea most
likely occurred underneath the sixth cervical vertebra (Lindgren
et al., 2010).
Postcranial Axial Skeleton
The vertebral formula is as follows: seven cervicals, 20 dorsals,
?five pygals, ?28 intermediate caudal, and 56 terminal caudal vertebrae, amounting to a total of 116 vertebrae, of which 89 are
caudals. Based on “a remarkably complete specimen” (Williston, 1910:537), most likely referable to P. tympaniticus, Williston
(1910) reported seven cervicals, 23 dorsals, and six pygals, with
an estimated total caudal count of 85 to 86. Based on other specimens, Williston (1910) also provided ranges of 22 to 23 dorsal
and five to six pygal vertebrae for Platecarpus, and subsequently
suggested presence of individual variations among mosasaurs
with respect to the vertebral number. Twenty dorsal vertebrae in
LACM 128319 thus represents the lowest known count for these
vertebrae in P. tympaniticus, and the number of the caudal vertebrae constitutes about 77% of the total vertebral count (Fig. 2).
In the following section, the axial skeleton of LACM 128319 is
described according to those regions.
Cervical Vertebrae—Of the four atlantal elements, only the
odontoid is preserved above the gap between the axis and the
third cervical, and it is dislocated. The axis and the third cervical
are disarticulated, but the position of the former with respect to
the skull is considered to be close to the original, and it is located
at the level of the quadratic suspensorium. Apparently, the skull
of LACM 128319 became detached from the rest of the skeleton
at the junction of these two vertebrae, and migrated slightly forward before or during burial. The rest of the cervicals are articulated, and together follow a dorsally concave course. This portion
of the axial skeleton is only about 5% of the total body length and
is shorter than the skull by about 45% (Table 1). Along those five
posterior cervical vertebrae, the horizontal length of the neural
spines increases posteriorly. The anterior-most preserved rib is
located on the fifth cervical vertebra, whereas two more anteriorly adjacent ribs are present in various plioplatecarpine taxa,
including Platecarpus tympaniticus (e.g., FMNH VP-322). The
synapophyseal facet on the fifth cervical is approximately twice
as deep as the anterior ones (Fig. 3). The cervical ribs become
increasingly curved and longer posteriorly (Figs. 3, 7).
Dorsal Vertebrae—Overall, the dorsal section of the skeleton
is gently arched as preserved, particularly along the ‘thoracic’
region (Lindgren et al., 2010) (Figs. 2, 4). The neural spines
are poorly preserved along this anterior dorsal region, but they
are shown to be broadly rectangular along the posterior dorsal
region, and slightly taller than the corresponding vertebrae. The
first nine dorsal vertebrae, seven of which are preserved, would
have each born anterior dorsal ribs at least 350 mm from tip to tip
in linear measurement, and are markedly longer compared with
the last cervical rib and the 10th (= the first posterior) dorsal rib
(Fig. 7). Both the 9th and 10th dorsal ribs are complete, and the
former is nearly twice as long as the latter (Fig. 7, stars). Posterior
to the ninth dorsal vertebra, the ribs become gradually shorter
posteriorly, the last one being shorter than the fifth cervical rib.
Fragments of calcified sternal ribs are preserved loosely associated with the distal ends of the anterior dorsal ribs (costal segments), with their long axes being roughly perpendicular to those
of the corresponding, ossified ones. The partial interclavicle is
located below the last two cervicals, and using its position as a
proxy, the anterior portion of the sternum would have occurred
below the first dorsal vertebra.
Caudal Vertebrae—Five pygal vertebrae are confirmed in
LACM 128319, whereas it constitutes the minimum count. Russell (1967) reported five pygal vertebrae in Platecarpus, most
KONISHI ET AL.—PLATECARPUS AND STREAMLINING IN MOSASAURS
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FIGURE 4. Platecarpus tympaniticus, LACM 128319, second slab containing much of trunk area inclusive of front paddle. A, interpretative line
drawing; B, photograph (Color figure available online).
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FIGURE 5. Platecarpus tympaniticus, LACM 128319, third slab containing pelvic area inclusive of hind paddles, and pre-bend section of tail. A,
interpretative line drawing; B, photograph. Asterisk indicates provisional identification (Color figure available online).
likely based on two specimens of P. tympaniticus sensu Konishi
et al. (2010) (FMNH UC-600 and YPM 1350A). Assuming that
our identification of the first intermediate caudal vertebra is correct (Figs. 2, 5), 28 such vertebrae are present in LACM 128319.
It is clear that both the pygals and nearly all intermediate caudal vertebrae bear neural spines that are posteriorly inclined at
a similar angle. In the posterior-most section of the intermedi-
ate caudal series, the neural spines begin to change their angle
of inclination with respect to the long axis of the corresponding
centra, so that on the last intermediate caudal, the spine becomes
nearly perpendicular to the axis. At the 3rd terminal caudal, the
long axis of the neural spine becomes perpendicular to that of
the centrum, and from the 5th to the 14th, the neural spines
are inclined anteriorly (Fig. 6). The spine resumes its vertical
KONISHI ET AL.—PLATECARPUS AND STREAMLINING IN MOSASAURS
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FIGURE 6. Platecarpus tympaniticus, LACM 128319, fourth slab containing downturned portion of tail. A, interpretative line drawing; B, photograph
(Color figure available online).
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FIGURE 7. Distribution of rib lengths in Platecarpus tympaniticus,
LACM 128319. Note that cervical ribs 3 and 4 are most likely lost post
mortem. Black star indicates last anterior dorsal rib and white star, first
posterior dorsal rib. The former is nearly twice as long as the latter.
Length of the 8th rib is a close estimate, and those of the 9th to 15th
ribs constitute minimal values because these two ribs are incomplete. All
the other ribs are complete
orientation on the 15th terminal caudal and, posterior to the 16th,
the neural spines become posteriorly inclined again.
Based on examination of FHSM VP-322, assignable to P. tympaniticus, Caldwell and Konishi (2007) reported that the 31st to
48th caudal neural spines are vertical, whereas all the others are
inclined posteriorly. Reexamination of this specimen reveals that
the situation is more complex and comparable to that of LACM
128319 (Table 2). In particular, most (35th to 45th) of the caudal vertebrae, whose neural spines were interpreted as vertical
by Caldwell and Konishi (2007), possess anteriorly inclined neural spines, and they are located between vertically oriented neural
spines (33rd to 34th and 46th to 47th). The corresponding vertebrae are the 36th to 37th (= 3rd to 4th terminal) and 48th to 49th
(= 15th to 16th terminal) caudals in LACM 128319. The height
of the neural spines also undergoes changes that appear to correspond closely to the changes in their orientation. In the pre-bend
section, the spines show a constant, gradual decrease from the
base of the tail to the 24th intermediate caudal vertebra. The neural spines regain the height posterior to this vertebra, and achieve
the peak height from 39th to 44th (6th to 11th terminal) caudals
where the main tail bend occurs (Fig. 6). Williston (1910:538) observed “a distinct elevation of the spines” in the distal caudal series of Platecarpus, similar to the condition in Tylosaurus proriger reported by Osborn (1899) (Russell, 1967). The height of the
spines decreases steadily beyond the bend except for a slight increase near the end, where the tail curves gently upward.
By comparing the ratio of the dorsal centrum length to the
ventral centrum length around the tail bend of the specimen,
Lindgren et al. (2010:fig. 7A) demonstrated that the vertebrae
occurring near or at the tail bend consistently showed a distinctly
greater dorsal centrum length, which cumulatively contributed
to the downturned tail morphology in LACM 128319. The
TABLE 2.
128319.
authenticity of the tail bend can also be demonstrated by the
evenly spaced neural spines and vestigial prezygapophyses
throughout the caudal skeleton, inclusive of those in the bent
portion (Figs. 5, 6). In FHSM VP-322, a virtually complete
P. tympaniticus skeleton, the tail is preserved and/or reconstructed to be straight when extended. Close examination of the
specimen indicates that the stretched tail shows an uneven spacing of the neural spines: they are more closely spaced in areas
of the column where dorsal flexure occurs. Such flexures are best
interpreted to be secondary, given the corresponding crowding of
the neural spines, and this interpretation provides another line of
support for the downturned nature of the tail in Platecarpus and
other mosasaurs, such as Plotosaurus (Lindgren et al., 2007, 2010;
see also Lindgren et al., 2011, for a discussion on the evolution
of a dorsoventrally expanded tail fin in mosasaurine mosasaurs).
It is also noted that anterior to the tail bend, the last six pairs
of transverse processes align nearly horizontally, because their
position in the corresponding centra becomes increasingly higher
at the same time the latter begin to follow a gradually descending course immediately anterior to the main tail bend (Fig. 2).
An identical condition was reported by Osborn (1899) from the
30th to 38th caudal vertebrae of Tylosaurus proriger (Russell,
1967). As observed by Schumacher and Varner (1996, 2007), the
main downturned portion of the tail consists exclusively of terminal caudal vertebrae lacking transverse processes. Contrary
to Schumacher and Varner’s (2007:41) observation in Platecarpus, however, the tail of LACM 128319 does not exhibit “a gentle sinuous curve . . . with a slight upward flexure anterior to”
the major downturned bend. Such a condition appears prevalent among the known remains of Tylosaurus (e.g., AMNH 221,
FHSM VP-3), and is potentially specific to this taxon (Osborn,
1899).
The hemal arch–spine complexes are oriented nearly horizontal anterior to the bend (see the articulated ones from the 17th to
28th intermediate caudals), whereas they gain an angle with the
long axis of the corresponding vertebrae constituting the downturned portion of the tail (Figs. 5, 6). The authenticity of the
near-horizontal orientation of the hemal arch–spine complexes
anterior to the tail bend is not unequivocally demonstrated, because the hemal arches articulate with the corresponding centra,
and hence they become mobile once the soft tissues degrade. A
specimen of Mosasaurus sp. (TMP 1983.064.0001) exhibits an unambiguously downturned tail with evenly spaced neural spines,
whereas the hemal arch–spine complexes of the caudal vertebrae
preceding the bend are inclined at about 45◦ from the main axis of
the tail. Because the hemal arch–spine complexes are fused to respective caudal centra in Mosasaurus, it may be that these structures in LACM 128319 were originally inclined at a higher angle
and became more horizontal in the process of fossilization. Still,
LACM 128319 and TMP 1983.064.0001 share the conspicuously
downturned tail, regardless of the preserved hemal arch–spine
complex orientation. Therefore, the unequivocal nature of the
tail bend in the former specimen is readily demonstrated. At the
very posterior end, the tail of the specimen is subtly upturned.
This feature is also confirmed in another conspecific specimen
(FHSM VP-322) without crowding of the neural spines. Although
Huene (1911) described a mounted skeleton of a fairly complete
Comparison of changes in caudal neural spine orientation between two specimens of Platecarpus tympaniticus, FHSM VP-322 and LACM
Specimen
FHSM VP-322
LACM 128319
Posteriorly oriented
Vertically oriented
Anteriorly oriented
Vertically oriented
Posteriorly oriented
C1–C32
C1–C35
C33–C34
C36–C37
C35–C45
C38–C47
C46–C47
C48–C49
C48–C89
C50–C89
Abbreviation: C, caudal vertebra.
KONISHI ET AL.—PLATECARPUS AND STREAMLINING IN MOSASAURS
Platecarpus sp., the gently upturned tail shown in his figure is best
attributed to a taphonomic artifact. Nearly all the vertebrae that
constitute the upturned portion of this tail are separate from each
other by a small gap, an indication that the integrity of the original tail morphology of the specimen has now been lost (pers.
observ.).
Appendicular Skeleton
Forelimb—Except for the unpaired interclavicle, only the left
forelimb elements are identified with confidence in this specimen.
The scapula shows a condition typical of Platecarpus tympaniticus. The condylar neck is distinctly present, and the anteroventral
border of the blade forms an obtuse angle of approximately 120◦
to the long axis of the neck. Posteriorly, the embayed portion of
the ventral border of the blade exceeds the rest of the border in
length. The blade is incomplete, and the condyle is partially embedded in matrix.
The coracoid is only partially exposed. The coracoid emargination as well as the coracoid foramen are present, and the element appears slightly larger than the scapula. Part of the articular
condyle and a large portion of the coracoid fan posterior to the
emargination are still covered by matrix.
Adjacent to the coracoid, a slender, straight element is identified as an interclavicle. It is bilaterally asymmetrical, and is narrower on one side than the other. The exposed surface is subtly
convex, suggesting that the ventral side is exposed.
Overall, and as preserved, the long axis of the forelimb extends posteroventrally at slightly over 30◦ from the horizontal
(Fig. 2). The humerus is nearly as large as the scapula (e.g.,
AMNH 2006) (Fig. 4). Well-developed ect- and entepicondyles
give the humeral shaft a highly constricted profile. In extensor aspect, the length of the element equals that across the two
condyles. Whereas the proximal border of the element is nearly
straight, the distal border is strongly convex, a condition shared
with Latoplatecarpus nichollsae (Cuthbertson, Mallon, Campione, and Holmes, 2007) (Konishi and Caldwell, 2009:fig. 15A;
Konishi and Caldwell, 2011). No features on the flexor side of
the element are observable.
The radius in LACM 128319 shows a striking similarity to
that in Plioplatecarpus primaevus Russell, 1967, in exhibiting a
strongly developed anterior flange along the anterodistal border
(Fig. 4; Holmes, 1996:fig. 15). The flange is broadly convex, and
occupies more than half of the length of the anterior radial border in both LACM 128319 and P. primaevus. At the same time,
the morphology and development of the flange in specimens of
Platecarpus tympaniticus vary, because some are angular (e.g.,
YPM 1426) and some occupy less than half of the anterior border distally (e.g., YPM 884, 1426) (Russell, 1967:fig. 53). In these
specimens, the radial shaft is distinctly constricted in the middle,
whereas the radius has an overall blocky appearance in LACM
128319 or Plioplatecarpus primaevus. In all specimens examined
of Platecarpus tympaniticus, the radius is distinctly longer distally
than proximally, whereas the distinction is much less obvious in
Plioplatecarpus primaevus (Holmes, 1996:fig. 15).
The ulna of LACM 128319 shows the hourglass shape typical
of Platecarpus tympaniticus. The proximal and distal borders are
similar in length, and the shaft is distinctly constricted. The articulation facet for the ulnare faces distoanteriorly. Together with
the radius, the ulna borders a large portion of the antebrachial
foramen that is slightly longer than wide.
Four well-ossified carpals are preserved, consisting of the intermedium, ulnare, and the third and fourth distal carpals. This
constitutes a typical carpal count in Platecarpus tympaniticus, but
an additional carpal, the second distal one, is occasionally noted
to be ossified in this taxon (e.g., KU 1001; Caldwell, 1996), in
the less derived form Plesioplatecarpus planifrons (FHSM VP2116), and in the more derived Plioplatecarpus sp. (Holmes et al.,
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1999). The intermedium and ulnare form the distal border of
the antebrachial foramen, and the ulnare is the largest of the
four carpals. The two distal carpals are almost equal in size
and rounded. When compared with mosasaurines (e.g., Russell,
1967:figs. 50–52), in which the space between the zeugopodium
and the metacarpals is filled with tightly spaced and well-ossified
carpals, the same gap in Platecarpus was largely cartilaginous
except for the central region, and is now closed postmortem in
LACM 128319 (Fig. 4).
All five digits are present, though with varying degree of
completeness. As already indicated, the digits have been pushed
proximally into the space that was not filled by ossified carpals.
Despite this, it is clear in LACM 128319 that the fifth digit was
divaricate from, and shorter than, the rest. The first four digits are
each tightly curved posteriorly, a feature potentially unique to
plioplatecarpines (see Russell, 1967:figs. 50–55). As preserved,
the phalangeal formula of LACM 128319 is 3-4-4-4-3, whereas
that of FHSM VP-322 is 4-5-5-4-3. It is thus likely that the last
two digits include all the phalanges in the former specimen.
Hind Limb—The pelvic region is the most disturbed portion of
the specimen, although the cause of this cannot be conclusively
determined (see Taphonomy below).
The ilium possesses a long iliac blade that curves gently anteriorly and tapers distally to a blunt end. The pubic blade is also
long, except that it expands distally. The pubic tubercle is obtusely angled, and the obturator foramen pierces its base. Only
the proximal portion of the left ischium is exposed.
The femur is about 1.5 times longer than it is wide at its distal
end, and is constricted in the middle. At the distal end, which
is wider than the proximal one, the tibial facet is larger than
the fibular facet at an approximate ratio of 2:1. This ratio is not
clearly indicated in Russell (1967:fig. 62), but the same ratio is
present in Plioplatecarpus primaevus (Holmes, 1996:fig. 17).
The tibia has a tighter curvature along the anterior border than
the posterior one. This condition is consistent with other plioplatecarpines, such as Latoplatecarpus nichollsae (Cuthbertson
et al., 2007) and Plioplatecarpus primaevus (Holmes, 1996:fig. 17;
Cuthbertson et al., 2007:602 and fig. 3). When compared with
these two taxa, the midshaft constriction is more pronounced in
Platecarpus tympaniticus (Cuthbertson et al., 2007; Fig. 5). When
compared with the femur and the fibula, the tibia is proportionately much more robust.
The fibula resembles the femur in outline but is approximately
60% of the size of the latter. As in the former two elements,
the fibula is wider distally. The anterior border is shallowly
emarginated, a condition that is similar to the posterior border
of the tibia, and thus the crural foramen is assumed to have been
lenticular in outline.
Three tarsals are confirmed (e.g., Russell, 1967:fig. 62). The
proximal border of the astragalus, the largest of the three, is
notched to border the distal end of the crural foramen. One calcaneum is located next to the astragalus, and is proximodistally
compressed and rectangular in outline. The fourth distal tarsal
is rounded and equidimensional in outline, and the left one lies
adjacent to the distal end of the fibula from the same side. It is assumed that all the tarsals are represented in the specimen, as the
same set of bones is reported in the holotype of Latoplatecarpus
nichollsae (Cuthbertson et al., 2007).
Albeit being disarticulated, the pedal digits from both the left
and right sides of the animal are preserved. The fifth metatarsal is
vaguely keyhole-shaped, and is proximally nearly as wide as the
distal end of the fibula (Fig. 5). The first to the third metatarsals
are preserved in association immediately distal to the tibia on the
left side. All the other metatarsals are difficult to assign any digit
numbers because they are scattered over a small area and are
similar in form to one another. On this slab, there are two distinct
areas preserving pedal phalanges, and they are readily identifiable as left and right pedal ones (Fig. 5). A gently curved outline
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of the digits is faintly preserved on the left side, whereas all
phalanges are clumped together on the right side. As preserved
on the left side, the phalangeal formula is 2-5-4-3-?, whereas it is
3-6-5-3-2 in FHSM VP-322 (Platecarpus tympaniticus), 5-5-5-4-3
according to Russell (1967:fig. 6, citing Capps, 1907) (Platecarpus
sp.), and 4-5-5-5-3 in the holotype of Latoplatecarpus nichollsae
(Cuthbertson et al., 2007). Whereas Russell (1967) reported five
phalanges for the first pedal digit of Platecarpus, a general lack of
well-preserved hind limbs in mosasaurs hampers its confirmation.
Taphonomy
Whatever the cause(s), the skeletal disturbance in the anterior
dorsal and pelvic regions of LACM 128319 must have occurred
after the animal settled on the seafloor on its left side, taking the
high overall articulation and completeness of the specimen into
consideration (e.g., Reisdorf et al., 2012). Under oxygen-depleted
conditions, disarticulation of a marine reptile skeleton is caused
predominantly by bottom currents, as low as 0.2–0.4 m/s, as scavengers are lacking in general (Reisdorf et al., 2012).
The right side of LACM 128319 lacks the front paddle and
most of the anterior ribs. This is most likely because the
mosasaur’s thorax on this side was exposed furthest from
the sediment-water interface, and was most strongly affected by
the currents (Reisdorf et al., 2012). An alternative explanation
is that this side of the animal was actively scavenged, although
an apparent lack of disarticulation along the vertebral column in
this region and of any preserved bite marks by scavengers (e.g.,
Squalicorax) renders an intense scavenging of this part of the
mosasaur body less likely. Because the area above the anterior
dorsal vertebrae and the third to fifth dorsal vertebrae are
heavily weathered, the third possibility is that the right anterior
dorsal ribs, along with the seventh and eighth dorsal vertebrae,
were originally preserved but lost to erosion prior to collecting
of the specimen. The last possibility is also in accordance with
the overall integrity of the specimen.
The highly localized and high degree of disturbance in the hip
region of LACM 128319 seems to lack a reasonable explanation.
Because most elements constituting the hind limbs are preserved
in pairs inclusive of phalanges, it is unlikely that scavenging was
the major cause of the disturbance. At the same time, for bottom
currents weak enough not to disturb articulation of the distal end
of the tail to dislocate some of the largest vertebrae (= pygals)
of the animal seems equally unlikely. An explosive rupture of the
abdomen during decomposition of the animal is another possibility, but this view faces the following difficulties: (1) there is
no evidence that such a rupture dislocate skeletal elements; and
(2) high hydrostatic pressures at the depth of the sea render it
physically impossible to explode a carcass at the water depth
greater than 1 m (Reisdorf et al., 2012). It may be that localized
scavenging by low-oxygen-tolerant benthic invertebrates took
place (Beardmore et al., 2012).
DISCUSSION
Osteology of Platecarpus tympaniticus
It is rare to find a mosasaur skeleton in near-perfect articulation and with soft-tissue structures so well preserved. Indeed,
LACM 128319 represents arguably the best mosasaur specimen
in this regard (Lindgren et al., 2010), such that the rich store of osteological and soft-tissue details greatly adds to our understanding of the anatomy of Platecarpus tympaniticus specifically and
that of mosasaurs in general.
In the skull, a right septomaxilla is identified (Fig. 3). Its topological relationships with the surrounding bones are identical to
those for the element identified as a septomaxilla in UALVP
24240, Plesioplatecarpus planifrons, which, according to Konishi
and Caldwell (2011), is the sister taxon of the clade containing
Platecarpus tympaniticus as its basal-most taxon. On the intramandibular joint surface, the angular exhibits medioventrally directed grooves and ridges. In the global phylogenetic analysis of
plioplatecarpines by Konishi and Caldwell (2011), this feature
constituted a single unequivocal character that defined the clade
that is sister to Latoplatecarpus willistoni. The presence of this
feature in Platecarpus tympaniticus, considered to be less derived
than the former taxon, implies that the intramandibular joint surface among the derived plioplatecarpines exhibits a greater morphological range than was considered by Konishi and Caldwell
(2011). At the same time, this observation reinforces Konishi and
Caldwell’s (2011) suggestion that “Plioplatecarpus” nichollsae,
the basal-most taxon of the clade that Latoplatecarpus willistoni
is sister to, is congeneric with the latter taxon.
Postcranially, LACM 128319 exhibits a wide array of important anatomical features resulting from the fact that it is an in situ
articulated skeleton. Overall, the neck column is concave upward,
followed by a hunched anterior dorsal region continuing posteriorly as a more or less horizontal string of vertebrae up to the
last intermediate caudal vertebra. Posterior to the last intermediate caudal, the tail bends gently downwards (Fig. 2). The neck
curvature likely contributed to a shortening of the neck in Platecarpus tympaniticus, presumably for a reason functionally similar
to that present in extant cetaceans, in which the cervical vertebrae are so abbreviated in length that there is a secondary loss of
the neck (e.g., Romer and Parsons, 1977). Partially accentuated
by the curvature of the neck, the anterior dorsal vertebral column in LACM 128319 exhibits a distinctly hunched outline. An
arched dorsal region in mosasaurs was first reconstructed as a line
drawing in Tylosaurus proriger Cope, 1869b, by Osborn (1899:fig.
13), although no rationale for this interpretation was provided.
Camp (1942:pl. 5) subsequently indicated a slightly curved dorsal contour in the anterior dorsal region of Plotosaurus bennisoni
(Camp, 1942), which was recently reconstructed with a more distinct curvature by Lindgren et al. (2007:fig. 3). Two more recent
studies, Holmes (1996:fig. 1) and Dortangs et al. (2002:fig. 7),
reconstructed skeletons of Plioplatecarpus primaevus and Prognathodon saturator Dortangs, Schulp, Mulder, Jagt, Peeters, and
de Graaf, 2002, respectively, with a dorsal curvature to the dorsal
column. Otherwise, researchers have mainly referred to Williston
(1898:pl. 72) for full skeletal restorations of mosasaurs, in which
the vertebral column is, though schematically, shown as horizontally straight from the atlas to the last caudal vertebra. LACM
128319 provides little support for such reconstructions.
Interestingly, the horizontal section of the dorsal column
in LACM 128319 largely corresponds to the dorsal vertebrae
with short ribs (i.e., 10th to 20th dorsals). Thus, the anteriorly
sloping portion (= hunched portion) of Platecarpus tympaniticus
corresponds to the deepest part of the torso supported by long
ribs, where such a skeletal arrangement likely contributed to
forming a tapering anterior portion of the overall streamlined
body outline in this mosasaur (Fig. 8). In most modern cetaceans,
the portion of the thoracic column that is analogous to the anterior dorsal column of mosasaurs also demonstrates a consistent
posterior rise (De Panafieu, 2007). As in other streamlined
marine vertebrates including whales, but unlike in Varanus
including the water monitor Varanus salvator (Laurenti, 1768),
the deepest part of the torso in Platecarpus tympaniticus occurs
about one-third of its body length from the leading tip (Pough
et al., 1999; Fig. 8F, solid vertical line).
Although most pygal and anterior-most intermediate caudal
vertebrae have experienced postmortem disturbance, the posterior dorsal section and articulated intermediate caudal section
align along a straight horizontal line; hence, the disturbed section of the vertebrae is also assumed to have been horizontally
straight. This interpretation deviates slightly from that presented
in Lindgren et al. (2010:fig. 8), in which the posterior dorsal and
KONISHI ET AL.—PLATECARPUS AND STREAMLINING IN MOSASAURS
1325
Although the poor preservation of the hind paddle in LACM
128319 precludes this interpretation, other plioplatecarpine specimens with well-articulated hind-limb elements, such as FHSM
VP-322 (Platecarpus tympaniticus) and M750306 (cf. Latoplatecarpus), show the fifth digit to be equally as divergent as that
of the front paddle in LACM 128319. Recently, the close morphological correspondence between the front and hind paddles in mosasaurs was demonstrated in the derived mosasaurine
mosasaur Plotosaurus bennisoni as well (Lindgren et al., 2008).
Evolution of Streamlined Body Shape in Mosasaurs
FIGURE 8. Comparison of body profile in cetaceans, scombroid fish,
a pelagic shark, a mosasaur, and a water monitor. A, dolphin; B, blue
whale; C, swordfish; D, tuna; E, Greenland shark; F, Platecarpus tympaniticus; and G, Varanus salvator (water monitor). A–E modified from
Pough et al. (1999:fig. 8-7b), F from Lindgren et al. (2010), and G is based
on various online resources and Koch et al. (2007). Solid vertical bars indicate deepest part in each animal. Note that in swordfish, this bar still
occurs at about one-third of the length from the front of the animal when
the long prong is not considered. In V. salvator, the deepest part occurs
much more anteriorly than the rest, and the depth-to-length ratio of the
body is smaller. Not to scale.
pygal vertebrae of the specimen successively form a gently sloping column posteriorly. This difference in the skeletal reconstruction, however, little affects the overall fusiform body outline of
the mosasaur indicated in Figure 8F. As previously stated, the tail
bends gently ventrally posterior to the last intermediate vertebra;
thus, the downturned portion of the caudal skeleton lacks transverse processes and was laterally flattened (e.g., Schumacher and
Varner, 2007; Lindgren et al., 2010).
Lindgren et al. (2010:fig. 8) indicated that the fifth digit in both
the front and hind paddles was highly divaricate from the rest.
In LACM 128319, the nine anterior dorsal ribs are readily
distinguishable from the posterior dorsal ribs by being at least
1.75 times longer than the latter ones (Fig. 7). As stated earlier,
the 9th and 10th ribs at the junction of these rib series are both
complete, and the former is nearly twice as long as the latter.
In the body cavity of LACM 128319, Lindgren et al. (2010)
identified a possible trace of a kidney between the first and the
third posterior dorsal ribs and against the vertebral column. In
extant terrestrial lizards (e.g., Varanus), however, kidneys are
placed much more posteriorly near the base of the hind limb
(Lindgren et al., 2010:fig. 9A). Using cetaceans as analogues,
Lindgren et al. (2010) argued that the anterior locations of internal organs such as kidneys and intestines in Platecarpus tympaniticus would indicate an anterior migration of the rib cage to
achieve a more streamlined body profile. Here, we bolster this
inference based on the osteological comparison of the specimen
with Varanus, a more basal anguimorph than mosasaurs sensu
Palci and Caldwell (2007), and with Carsosaurus and Komensaurus, limbed mosasauroids occurring at the base of the clade
that also contains Platecarpus tympaniticus (Caldwell and Palci,
2007).
The rib cage in extant monitor lizards such as the water
monitor Varanus salvator is wider than deep, and the ribs
become longest towards the midpoint between the pectoral and
pelvic girdles (De Panafieu, 2007:35; Fig. 9D). The ribs become
gradually shorter away from this midpoint in both anterior and
posterior directions, and there is no abrupt change in length
between adjacent ribs except immediately anterior to the pelvic
girdle (De Panafieu, 2007:35; Fig. 9D). In two primitive, limbed
mosasauroids (Fig. 9B, C), the long ribs occupy approximately
the anterior two-thirds of the dorsal series. It is also noticeable
that the posteriad reduction in rib length appears gradual in these
two basal mosasauroids. In Platecarpus tympaniticus (Fig. 9A),
on the other hand, the long ribs are restricted to the anterior 50%
of the dorsal series, and this section of the column is followed
immediately by an abrupt reduction in the rib length by nearly
50%.
It is hence plausible that in the course of postcranial adaptation to a fully aquatic life in derived mosasauroids, not only do
their limbs became modified into flippers (i.e., hydropedality),
but their rib cage also became more anteriorly positioned, and
the length difference between the anterior dorsal and posterior
dorsal rib series became increasingly distinct. Because the rib
cage outline in Varanus salvator is well reflected in their body
outline (Koch et al., 2007; Fig. 8G), the anteriorly concentrated
long ribs in Platecarpus tympaniticus must have contributed
to forming an anteriorly deep, streamlined body outline as in
modern cetaceans, scombroids, and pelagic sharks (Fig. 8A–F).
By inference, such a distinct modification to an overall body
shape almost certainly influenced the arrangement of the internal organs, and an anterior migration of internal organs
analogous to that of whales likely took place (Lindgren et al.,
2010).
Unfortunately, ribs are not well preserved in other members of
the Plioplatecarpinae, neither in more basal (e.g., Ectenosaurus,
Plesioplatecarpus) nor more derived (e.g., Latoplatecarpus,
1326
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 32, NO. 6, 2012
ACKNOWLEDGMENTS
We would like to thank first the staff at the Natural History
Museum of Los Angeles County for their extensive support at
the museum in the course of this study: P. Johnson, K. Urhausen,
and D. Goodreau. We are especially grateful for S. Abramowicz’s photographic work, which formed a basis for our description of the beautiful mosasaur specimen. Comments provided
by A. Schulp and M. Everhart substantially improved the original manuscript of this paper. This research was fully or partially
funded by the following grants: Alberta Ingenuity Fund Ph.D.
Student Scholarship to T.K.; the Swedish Research Council, the
Crafoord Foundation, and the Royal Swedish Academy of Sciences to J.L.; and an NSERC Discovery Grant (no. 238458-01)
and Chair’s Research Allowance to M.W.C.
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FIGURE 9. Changes in length distribution of dorsal ribs among anguimorphans including Platecarpus tympaniticus. A, Platecarpus tympaniticus; B, Komensaurus carrolli; C, Carsosaurus marchesetti; D, Varanus
salvator (water monitor). Not to scale. According to Palci and Caldwell
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Submitted December 23, 2011; revisions received April 4, 2012;
accepted May 26, 2012.
Handling editor: Johannes Müller.