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A PARTIAL SKELETON OF THE BASAL MOSASAUR HALISAURUS
PLATYSPONDYLUS FROM THE SEVERN FORMATION (UPPER
CRETACEOUS: MAASTRICHTIAN) OF MARYLAND
Author(s): ROBERT B. HOLMES and HANS-DIETER SUES
Source: Journal of Paleontology, 74(2):309-316. 2000.
Published By: The Paleontological Society
DOI: http://dx.doi.org/10.1666/0022-3360(2000)074<0309:APSOTB>2.0.CO;2
URL: http://www.bioone.org/doi/full/10.1666/0022-3360%282000%29074%3C0309%3AAPSOTB
%3E2.0.CO%3B2
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309
J. Paleont., 74(2), 2000, pp. 309–316
Copyright q2000, The Paleontological Society
0022-3360/00/0074-0309$03.00
A PARTIAL SKELETON OF THE BASAL MOSASAUR
HALISAURUS PLATYSPONDYLUS FROM THE SEVERN FORMATION
(UPPER CRETACEOUS: MAASTRICHTIAN) OF MARYLAND
ROBERT B. HOLMES
AND
HANS-DIETER SUES
Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada, ,rholmes@mus-nature.ca.and
Department of Palaeobiology, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario M5S 2C6, Canada, ,hdsues@rom.on.ca.
A
BSTRACT
—A new specimen of the basal mosasaur Halisaurus platyspondylus from the Severn Formation of Prince Georges County,
Maryland (Upper Cretaceous: middle Maastrichtian) represents the most complete partial skeleton of this uncommon taxon to be
described to date. The characteristic dorsoventral compression of the vertebral centrum is most pronounced in the anterior trunk
vertebrae, and the centra of the posterior trunk vertebrae exhibit proportions more similar to those in other mosasaurs such as Pliopla-
tecarpus. The postorbitofrontal forms its primary contact with the frontal rather than the parietal, and the supraoccipital is firmly
attached to the ventral side of the parietal. The plane of articulation between the parietals and supratemporal is neither vertical (as in
Varanidae) nor horizontal (as in Mosasauridae), but forms an angle of about 55 degrees with the horizontal. The quadrate bears a long,
ventrally-flared suprastapedial process but appears to lack an infrastapedial process. Close similarities in the structure of the frontal and
parietal with ‘‘Clidastes’’ sternbergii support referral of the latter to Halisaurus, but reference of Phosphorosaurus ortliebi to Halisaurus
is questionable. Halisaurus has been recorded from the Santonian to the late Maastrichtian.
INTRODUCTION
T
HE HOLOTYPE
of Halisaurus platyspondylus Marsh, 1869,
(YPM 444) was collected by John G. Meirs from late
Maastrichtian strata of the upper New Egypt Formation in the
pit-mine of the Cream Ridge Marl Company near Hornerstown,
Upper Freehold Township, in Monmouth County, New Jersey
(Baird, 1986a, 1986b). Marsh (1869, p. 396) described it as com-
prising a posterior cervical vertebra, an anterior thoracic verte-
bra, a right splenial (which is actually a left angular; Baird,
1986a), and a portion of the ‘‘base of the skull’’ (basioccipital
and basisphenoid). Marsh did not provide any figures in his orig-
inal account, but Baird (1986a, fig. 1) has since published pho-
tographs of this material. In his brief description, Marsh (1869,
p. 395) particularly noted the dorsoventral flattening of the cen-
tra that gives the intercentral articular surfaces a ‘‘very trans-
versely elliptical outline.’’
Marsh (1870) substituted the generic nomen Baptosaurus for
Halisaurus to avoid confusion with Halosaurus Johnson, 1864,
a genus of teleostean fish. However, this substitution is not valid
under the ICZN rules concerning homonymy. Subsequently,two
additional species of Baptosaurus, B. fraternus Cope, 1869–
1870, and B. onchognathus Merriam, 1894, were named. The
former is probably a subjective junior synonym of Plioplatecar-
pus depressus, and the holotype of the latter was destroyed dur-
ing World War II (Russell, 1967). The invalid generic nomen
Baptosaurus remained in general use until the 1950s (McDowell
and Bogert, 1954), but Romer (1956) reintroduced the correct
name Halisaurus.
Russell (1970) referred mosasaurid material from the Moore-
ville Chalk (formerly Mooreville Member of the Selma For-
mation) in Alabama to Halisaurus on the basis of its distinctive
vertebral structure. He also noted the close similarity between
the frontals from the Mooreville Chalk and those of ‘‘Clidastes’’
sternbergii Wiman, 1920, from the Smoky Hill Member of the
Niobrara Chalk (Santonian) in Kansas. On this basis, Russell
argued that the latter taxon should be referred to Halisaurus. He
proposed a new combination, Halisaurus sternbergi to include
both the holotype of ‘‘Clidastes’’ sternbergii and the material
from the Mooreville Chalk. Additional undescribed specimens,
similar to but not necessarily conspecific with H. sternbergi,
were reported by Bell (1997).
Lingham-Soliar (1996) referred Phosphorosaurus ortliebi
Dollo, 1889, from the upper Maastrichtian ‘‘Craie phosphate´e’’
of Ciply (Belgium), to Halisaurus. P. ortliebi had previously
been synonymized with Plioplatecarpus by Russell (1967), pre-
sumably on the basis of the very large, anteriorly placed parietal
foramen. However, the quadrates ofthis form differ considerably
from those of Plioplatecarpus (Lingham-Soliar, 1996, fig. 5).
Lingham-Soliar (1996) reassigned Phosphorosaurus to Halisau-
rus based on the structure of the frontal, which closelyresembles
that of Halisaurus sternbergi (e.g., Russell, 1970, fig. 165).
However, Phosphorosaurus differs from Halisaurus in several
cranial features, and its referral of that taxon is problematical
(see below).
Although various isolated bones from Maastrichtian-age strata
in the eastern United States have been referred to Halisaurus
platyspondylus over the years, this taxon ‘‘remains one of the
rarest and least well-known of the mosasaurs’’ (Baird, 1986a, p.
72). The new specimen reported here, USNM 442450, compris-
es, for the first time, numerous associated cranial and postcranial
bones of a single individual. It represents a remarkable find be-
cause partial skeletons of mosasaurs from the Late Cretaceous
marine strata exposed along the Atlantic coast from New Jersey
to the Carolinas are rare (Baird, 1986a). USNM 442450 was
collected by Dr. Peter M. Kranz (Washington, DC) from mid-
Maastrichtian strata of the Severn Formation in a temporary ex-
posure south of Oxon Hill, just north of Kerby Hill Farm along
Indian Head Highway (MD-210), in Prince Georges County
(Maryland) in 1989. The bones were preserved in a poorly con-
solidated, glauconitic sandy matrix that was easily removed us-
ing a dental pick and soft brush after initial consolidation of the
bones with acrylic resin (Acryloid B 72).
Institutional abbreviations.—FMNH, The Field Museum,
Chicago; NJSM, New Jersey State Museum, Trenton; NMC, Ca-
nadian Museum of Nature, Ottawa; USNM, National Museum
of Natural History, Washington, DC; YPM, Peabody Museum
of Natural History, Yale University; YPM-PU, former Princeton
University collection, now housed in the Peabody Museum of
Natural History at Yale University.
SYSTEMATIC PALEONTOLOGY
Family M
OSASAURIDAE
Gervais, 1853
Genus H
ALISAURUS
Marsh, 1869
Baptosaurus M
ARSH
, 1870 (invalid substitute nomen)
Type species.—Halisaurus platyspondylus Marsh, 1869.
310 JOURNAL OF PALEONTOLOGY, V. 74, NO. 2, 2000
Referred species.—Halisaurus sternbergi (Wiman, 1920).
Diagnosis (modified from Bell [1993, 1997]).—Prefrontals
extend far anteriorly, forming about one-half of lateral narial
margin; maxilla lacking recurved posterolateral wing; ventral
rim of parietal foramen supported by raised boss attenuated pos-
teriorly into triangular ridge; plane of parietal-supratemporal
contact oblique, forming an angle of about 55 degrees with the
horizontal; quadrate exhibiting ‘‘question mark’’ lateral profile;
expanded ventral termination of suprastapedial process articu-
lating with, but not fusing to, a horizontal facet immediately
above articular condyle; infrastapedial process apparently ab-
sent; quadrate ventral articulation gently domed, convex in any
view; coronoid with greatly expanded posterior wing; surangular
reaches posteriorly only to midpoint of the glenoid on the dorsal
edge of the ramus, leaving the articular to form the bulk of
glenoid surface; dorsal margin of articular contribution to glen-
oid strongly convex; vertebral centra dorsoventrally conpressed,
cotyles/condyles subrectangular in outline; synapophyses of pos-
terior cervical and anterior trunk vertebrae flare distally and pro-
ject well below the ventral rim of centrum.
H
ALISAURUS PLATYSPONDYLUS
Marsh, 1869
Diagnosis (modified from Bell, 1993).—Premaxillary rostrum
with large foramina; median keel on dorsal surface of frontals
well-developed anteriorly; replacement teeth form in subdental
crypts; external nares broadly rounded and wide posteriorly; sur-
angular-articular suture passes posteroventrally from the dorsal
rim of the jaw, tracing a broad arc across the lateral surface of
the ramus.
DESCRIPTION OF
H.
PLATYSPONDYLUS
Skull.—USNM 442450 comprises both frontals, parietals, and
prefrontals, a partial braincase, left quadrate, both pterygoids,
partial mandibular rami, 26 vertebrae, ribs, and other indeter-
minate fragments of bone.
The coalesced frontals were broken into several pieces during
collection, and are incomplete anteriorly. Nevertheless, enough
is preserved to establish that they closely resemble the frontals
in other specimens referred to Halisaurus, such as FMNH PR
195 (Russell, 1970) and YPM-PU 18818 (Baird and Case,
1966). The supraorbital margins of the frontals extend more or
less parallel to each other. Ventrally, each frontal is extensively
excavated anterolaterally for the reception the prefrontal and
bears a much more shallow, less extensive depression for artic-
ulation with the postorbitofrontal posterolaterally. Both surfaces
are fluted, indicating firm sutural attachments in life. They are
not confluent, indicating that the postorbitofrontal and prefrontal
were not in contact with each other, and that each frontal formed
a very short segment of the supraorbital border. Although a piece
of the posteromedian portion of the frontal ‘‘shield’’ is missing,
the edges of a prominent triangular boss are visible on its dorsal
surface. Anterior to this boss, the median ridge is represented
only by a slight rise on the dorsal surface, which becomes much
more pronounced more anteriorly, where it forms a distinct,
rounded keel. The dorsal surface of each frontal bears fine radial
striations. The transverse frontoparietal suture, which lacks only
a short section at the midline, is essentially indistinguishable
from that of YPM-PU 18818 (Baird and Case, 1966, fig. 1A).
The right prefrontal is nearly complete, and the left is repre-
sented only by its central portion. The horizontal posterior ramus
of the prefrontal bears a modest supraorbital process, compara-
ble in size and shape to that in Platecarpus ictericus (Russell,
1967, fig. 83). The prefrontal projects far anteriorly to about the
midpoint of the external naris. The preserved, broadly rounded
rim indicates that the naris was unusually wide posteriorly (Fig.
1). The lateral surface of the vertical lamina of the prefrontal
bears a distinct facet for the reception of the maxilla. The shape
of its edge indicates that the maxilla lacked the posterodorsally
reflected lappet present in most other mosasaurs (Bell, 1997).
The central portion of the co-ossified parietals forms a raised
‘‘table’’ with an almost flat dorsal surface. The larger, anterior
triangular portion of this table is set off from a smaller, posterior
subrectangular portion by a constriction. Anteriorly, it bears a
large pineal foramen that is separated from the frontoparietal
suture by a distance about equal to its diameter. The foramen
passes posteroventrally into the cranial cavity at an angle of
about 15 degrees to the vertical plane. Ventrally, the rim of the
foramen is supported by a raised boss that tapers posteriorly into
a long, prominent triangular tongue of bone separated from the
remainder of the ventral surface by deep lateral furrows (Fig.
2). In other mosasaurs in which the structure of this area is
known (such as Plioplatecarpus and Platecarpus), the ventral
margin of the foramen is not conspicuously raised, and its pos-
terior margin turns posteriorly into a triangular trough on the
ventral surface of the parietal. The bladelike supratemporal rami
are oriented approximately vertically as they diverge from the
central plate of the parietal. However, in contrast to other known
mosasaurs, in which the rami have assumed an approximately
horizontal orientation posteriorly, the rami of USNM 442450
form an angle of about 55 degrees with the horizontal plane at
their posterior ends. The thickened posterior end of each supra-
temporal ramus bears a ventral facet for contact with the par-
occipital process. Laterally, it bears a long, horizontal, triangular
facet for articulation with the supratemporal. Postmortem com-
pression has displaced the anterior process of the supratemporal
to expose a relatively smooth surface that hints at the possibility
of some anteroposterior movement between the suspensorial ra-
mus and supratemporal.
The right pterygoid (Fig. 3), which lacks only the distal end
of its quadratic process, and fragments of the left pterygoid are
preserved. In overall form, the pterygoid resembles that of Pla-
tecarpus. The quadratic process expands posteriorly to form a
vertical blade. A raised ridge on the palatal ramus bears nine
teeth, the anterior and posterior of which are smaller than the
more central teeth, which are similar in size to the marginal
teeth.
The complete left quadrate (Fig. 4) closely resembles that of
Halisaurus sternbergi (Wiman, 1920, fig. 5) and is similar to
that of Phosphorosaurus ortliebi (Dollo, 1889; Lingham-Soliar,
1996), although neither quadrate is complete in the holotype of
the latter. In contrast with other mosasaurs, where the circular
or oval tympanic rim is uniformly curved in lateral view, the
rim in USNM 442450 is slightly concave anteriorly,but abruptly
turns posteriorly in a tight arc to join the nearly straight dorsal
margin. This results in a profile resembling a question mark
(Bell, 1997), similar to that in Halisaurus sternbergi. The ex-
panded ventral termination of the large suprastapedial process
articulates with, but apparently is not fused to, a transversely
oval, horizontal shelf immediately above the distal articular con-
dyle. A thick flange bearing coarse fluting projects posterome-
dially from the condylar base posterior to this shelf; it may be
homologous to the infrastapedial process in other mosasaurs, but
its orientation relative to the suprastapedial process makes this
doubtful.
Only the posterior portion of the braincase (Fig. 5) is pre-
served. The occipital condyle resembles those of Platecarpus
and Plioplatecarpus in outline, and is not compressed dorsoven-
trally. The prootic bears a well-developed otosphenoidal crest.
In contrast to many mosasaurs, but as in derived Mosasaurinae
(Mosasaurus, Plesiotylosaurus, Plotosaurus, and Prognathodon;
Russell, 1967), the supraoccipital is firmly attached to the pari-
etals.
311HOLMES AND SUES—HALISAURUS FROM MARYLAND
F
IGURE
1—Halisaurus platyspondylus, USNM 442450, reconstruction of
skull in dorsal view. Unshaded portions based on the holotype of Hal-
isaurus sternbergii (Wiman, 1920). Abbreviations used in the figures:
an, angular; ar-pra, articular-prearticular; bo, basioccipital; bp.p, bas-
ipterygoid process; ec.p, ectopterygoid process; eo, exoccipital; f, fron-
tal; fa.d, facet for dentary on lateral surface of splenial; f-p.su, fron-
toparietal suture; gl.f, glenoid facet; op, opisthotic; ot.cr, otosphenoidal
crest; p, parietal; prf, prefrontal; q, quadrate; q.p, quadrate process; sa,
surangular; so, supraoccipital; sp, splenial; s.p, suprastapedial process;
st, supratemporal; t.c, tympanic crest. Scale bar equals 5 cm.
F
IGURE
2—Halisaurus platyspondylus, USNM 442450, ventral view of
parietals (slightly restored) showing the raised boss surrounding and
tongue-like thickening behind the pineal foramen. Scale bar equals 2
cm.
Neither lower jaw is complete, but there is enough comple-
mentary information from both mandibular rami to permit re-
construction of most constituent bones (Fig. 6). Only the dentary
and coronoid are not preserved on either side. Although the
glenoid surface is formed by both the articular and the suran-
gular, Halisaurus is unusual among mosasauroids in that the
surangular reaches only to the midpoint of the glenoid on the
dorsal edge of the ramus (see Bell 1997, character 79); hence
the contribution of the surangular is considerably smaller than
that of the articular. This may represent a plesiomorphic condi-
tion. However, unlike in other halisaurines, where the suture
drops straight ventrally from this point and then curves anteri-
orly (Bell, 1993) the surangular-articular suture of USNM
442450 passes ventroposteriorly, tracing a broad arc as it turns
ventrally and finally anteriorly. At the posterolateral end of the
glenoid facet, the dorsal edge of the articular rises to form a
stout conical process. The thickened retroarticular process is
squared off posteriorly and is strongly inflected, assuming a
nearly horizontal orientation The form of the joint between the
angular and splenial is virtually identical to that figured for Pla-
tecarpus (Russell, 1967, fig. 28).
Vertebrae.—Although the atlas and axis are missing, the re-
maining cervical vertebrae are present and well preserved (Fig.
7). Two anterior thoracic vertebrae, one bearing a prominent
midventral tubercle, probably represent T1 and T2 or T3. Based
on comparisons with a complete presacral vertebral column of
Plioplatecarpus (NMC 11035; Holmes, 1996), the remaining 14
dorsal vertebrae form a more or less continuous series from ap-
proximately T10 to T23, the latter probably at or close to the
sacrum (Table 1), although a shift anterior or posterior by a few
segments would not be incompatable with the morphology ob-
served. The anterior seven vertebrae of this series are generally
well preserved and complete, but the rest lack the neural arches
and transverse processes. Only a few caudal vertebrae are pre-
served, comprising two pygals, two anterior caudals, and one
posterior caudal.
Zygosphenes and zygantra are absent. The synapophyses are
unusually large in the anterior portion of the vertebral column,
312 JOURNAL OF PALEONTOLOGY, V. 74, NO. 2, 2000
F
IGURE
3—Halisaurus platyspondylus, USNM 442450, right pterygoid in ventral view. Scale bar equals 2 cm.
F
IGURE
4—Halisaurus platyspondylus, USNM 442450, left quadrate in 1, lateral, 2, posterior, 3, medial, and 4, anterior views. Scale bar equals 1 cm.
and flare distally to project well below the ventral rim of the
centrum in the posterior cervical and anterior thoracic region.
The diagnostic dorsoventrally compressed, subrectangular out-
line of the condyle is apparent throughout the preserved column,
although it is most pronounced anteriorly (see Table 1). For ex-
ample, the ratio of condylar width to height averages 0.54 in the
preserved cervical vertebrae (compared with 0.77 for the equiv-
alent series of vertebrae in Plioplatecarpus, NMC 11035),
whereas this ratio is 0.62 (0.79 in Plioplatecarpus) in the mid-
trunk vertebrae (T10–T13). In the posterior trunk vertebrae
(T18–T21), it increases to 0.74 (0.88 in Plioplatecarpus). Dor-
soventral compression of the vertebral centrum, rather than a
circular condylar outline, is a common condition among vara-
noid lizards, and probably represents the plesiomorphic condi-
tion for this group. However, the width/height ratio for the con-
dyle generally does not exceed 4:3 in the latter (Bell, 1997), and
the ratio of 2:1 in Halisaurus may represent a derived condition.
On the thoracic vertebrae (Fig. 8), the prezygapophyses are
connected to the synapophyses by a thick web of bone that be-
comes more pronounced posteriorly, and, on mid-thoracic ver-
tebrae, bears a sharp ridge along its curved anterolateral border.
A similar feature is present in Plioplatecarpus primaevus (R. B.
H., unpublished data), but the ridge is not as pronounced.
Neural and hemal arches are not preserved on any of the
caudal vertebrae. However, enough of the bases of the hemal
arches is visible to indicate that, unlike in russellosaurines (Ty-
losaurus
1
plioplatecarpines; Bell, 1997) and varanoids gener-
ally, the arches were fused to the centrum.
DISCUSSION
Skull structure and phylogenetic relationships.—Various au-
thors (Russell, 1967; Carroll and deBraga, 1992; deBraga and
Carroll, 1993; Bell, 1993) have argued that the skull of Hali-
saurus is more primitive than that of any other known mosa-
saurid, and, in several important features, is more similar to that
of varanoid lizards. According to deBraga and Carroll (1993),
the postorbitofrontal of Halisaurus, as in varanoid lizards, is
firmly attached to the parietal and only loosely connected to the
frontal. The vertical supratemporal ramus of the parietal is firmly
sutured to the supratemporal laterally and to the paroccipital
process medially. The supratemporal arcade, comprising the su-
pratemporal, squamosal, and postorbitofrontal, forms a rigid
structural link between the suspensorium and parietal. Because
the braincase is firmly attached to the ventral surface of the
parietal, the parietal-braincase-temporal arcade can be consid-
ered a single structural unit. deBraga and Carroll (1993) argued
that movement at the frontoparietal suture was possible when
forces transferred from the pterygoid to the palatine and maxilla
resulted in elevation or depression of the snout relative to the
posterior part of the skull. The quadrate, which is firmly attached
313HOLMES AND SUES—HALISAURUS FROM MARYLAND
F
IGURE
5—Halisaurus platyspondylus, USNM 442450. Reconstructed
braincase in 1, occipital and 2, left lateral views. Scale bar equals 2 cm.
F
IGURE
6—Halisaurus platyspondylus, USNM 442450. Partially reconstructed right mandibular ramus in 1, lateral and 2, medial views. Arrow
denotes intramandibular joint. Scale bar equals 5 cm.
to the pterygoid, could have moved anteroposteriorly about its
dorsal point of articulation, producing protraction and retraction
of the jaws as in extant Varanus (Frazzetta, 1962). In mosasaurs
other than Halisaurus, the postorbitofrontal primarily attached
to the frontal rather than the parietal. The short supratemporal
ramus of the parietal has a horizontal orientation and forms an
unfused, overlapping contact with the supratemporal. Unlike the
condition in varanoid lizards and Halisaurus, the occipital unit
of the braincase could have rotated about an anterior transverse
axis as a consequence of its loose dorsal contacts with the ven-
tral surface of the conjoined parietals and lateral contacts with
the supratemporal processes of the parietal through their unfused
contacts with the supratemporals. This movement could have
been transmitted through the temporal arcade to the frontal, pro-
ducing elevation or depression of the snout. A significant con-
sequence of this unique mechanical linkage is the effective de-
coupling of the quadrate, and therefore jaw protraction/retrac-
tion, from the mesokinetic system (Russell, 1964).
The new specimen demonstrates that, contrary to previous
claims, the skull of Halisaurus does not exhibit a structure sug-
gestive of kinesis like that hypothesized for varanoids, but rather
shows a mosaic of varanoid and mosasauroid features of uncer-
tain biomechanical significance. Halisaurus resembles more de-
rived mosasaurs in the postorbitofrontal forming its primary con-
tact with the frontal rather than with the parietal, and in the
presence of a short supratemporal ramus of the parietal that
forms a loose articulation with the supratemporal. However, the
orientation of the supratemporal process is neither horizontal (as
in more derived mosasaurs) nor vertical (as in varanoids), but is
oblique, with the plane of its dorsolateral surface forming an
angle of about 55 degrees with the horizontal. Unlike the con-
dition in most mosasaurs, the occipital unit of the braincase is
firmly attached to the ventral surface of the parietal, precluding
any dorsoventral movement. Some of these features appear to
be plesiomorphic, and, if considered together, the character com-
plex could be interpreted as representing a stage transitional be-
tween varanoids and more derived mosasaurids, although it is
incompatible with either hypothesized kinetic system.
314 JOURNAL OF PALEONTOLOGY, V. 74, NO. 2, 2000
F
IGURE
7—Halisaurus platyspondylus, USNM 442450. Cervical vertebrae (C3-7) in left lateral view (top) and posterior view (bottom). Scale bar
equals 1 cm.
315HOLMES AND SUES—HALISAURUS FROM MARYLAND
T
ABLE
1—Measurements (in mm) of vertebrae of Halisaurus platyspondylus,
USNM 442450. Centrum height and width measured at condyle.
Vertebra Centrum
length Centrum
width Centrum
height Centrum
height/width
C3 52 27 13.5 0.50
C4 55 29.5 13.5 0.46
C5 55.5 27 15 0.56
C6 54 28 16 0.57
C7 52 29 17 0.59
T1 51.5 29.5 17.5 0.59
T10 48 30 18 0.60
T11 47.5 31 19.5 0.63
T12 48 30 19.5 0.65
T13 47 33 20 0.61
T14 49 32 21 0.66
T15 50 32.5 22 0.68
T16 51 33 23 0.70
T17 53 33.5 26 0.78
T18 50.5134.5 26 0.75
T19 48133.5 26 0.78
T20 51 34 24 0.71
T21 50 34 24 0.71
T22 54 — — —
T23 53 33126.5 —
ant. pygal 42131 27 0.87
post. pygal 36 33 27 0.82
ant. caudal 33 30 29 0.97
ant. caudal 29 28 25 0.89
post. caudal 20.5 20 16 0.80
F
IGURE
8—Halisaurus platyspondylus, USNM 442450. Dorsal vertebrae
in left lateral view (left) and posterior view (right). 1, Anterior dorsal
vertebra (T1 or T2). 2, mid-dorsal vertebra (probably T12). Scale bar
equals 1 cm.
Classification of Halisaurus.—Bell (1993), accepting the iden-
tification of H. D. Sues, referred USNM 442450 to Halisaurus,
along with specimens NJSM 12146 and 12259. Vertebral struc-
ture alone is suffucient to confirm this identification. Since no
differences between USNM 442450 and the fragmentary type
of H. platyspondylus are apparent, the specimen is assigned to
this species. Other isolated halisaurine bones from the Upper
Cretaceous of Maryland and New Jersey have been assigned to
H. platyspondylous (Baird, 1986b; Baird and Case, 1966; Bell,
1993). Specimens identified from other localities (Bell, 1993;
Caldwell and Bell, 1995; Russell, 1970; Wiman, 1920) are ex-
cluded here from consideration pending further study.
Since the new find is more complete than the holotype of
Halisaurus platyspondylus, it permits evaluation of other mo-
sasauroid taxa that have been referred to Halisaurus. Numerous
similarities between USNM 442450 and ‘‘Clidastes’’ sternbergii
Wiman, 1920, particularly in the structure of the parietal, the
position of the parietal foramen, and the structure of the quadrate
support referral of the latter to Halisaurus (Russell, 1967, 1970).
The reassignment of Phosphorosaurus ortliebi Dollo, 1889 to
Halisaurus by Lingham-Soliar (1996) is questionable. Although
the frontals are similar in the two taxa, especially in the absence
of supraorbital emarginations (Lingham-Soliar, 1996), Halisau-
rus bears a median frontal keel only anteriorly, rather than a
median crest that runs prominently the full length of the frontal
as in Phosphorosaurus. Unlike USNM 442450 and Halisaurus
sternbergi, Phosphorosaurus has a much larger pineal foramen
located at the frontoparietal suture. However, in one specimen
of Halisaurus cf. H. sternbergi (USNM 3777; Bell, 1993), the
foramen is situated very close to this suture, but the same char-
acter-state occurs convergently in several other mosasauroidtaxa
(see Bell, 1993). Variation in size and relative position of the
pineal foramen is not unexpected because the foramen represents
a remnant of the large median fontanelle between the frontals
and parietals in early developmental stages of extant lepidosaurs
(Rieppel, 1992) and would not, in its self, preclude close rela-
tionship between Halisaurus and Phosphorosaurus. The incom-
plete quadrates of the holotype of P. ortliebi (Dollo, 1889, pl.
10, figs. 10, 11; Lingham-Soliar, 1996, fig. 5) closely resemble,
as far as preserved, those of USNM 442450 and Halisaurus
sternbergi (Wiman, 1920, fig. 5). However, as reconstructed by
Lingham-Soliar (1996, fig. 1), the quadrate more closely resem-
bles that of Prognathodon (e.g., Dollo, 1889, pl. 10, figs. 8, 9).
The parietals of Phosphorosaurus differ from those of USNM
442450 (see above) in the presence of a deep, elliptical fossa on
the ventral surface. Additional material, especially vertebrae, is
needed to resolve the status of Phosphorosaurus ortliebi, and
we retain this taxon as distinct at the present time.
The skull of USNM 442450 shows some arguably plesio-
morphic characters, but its overall structure, comprising a mo-
saic of mosasaurid and varanoid features, represents an inter-
mediate pattern of uncertain phylogenetic significance. Although
the pronounced dorsoventral compression of the vertebral centra
is possibly plesiomorphic, it is more pronounced than in any of
the hypothesized outgroups (Bell, 1993, 1997), and may well
represent an autapomorphic condition.
Type.—Holotype, YPM 444. New Egypt Formation (Upper
Cretaceous: upper Maastrichtian), pit-mine of the Cream Ridge
Marl Company near Hornerstown, Monmouth Co., New Jersey.
Occurrence.—As noted by Caldwell and Bell (1995), Hali-
saurus had a wide stratigraphic and geographic distribution. The
holotype of H. platyspondylus (YPM 444) was collected from
the Upper Egypt Formation (upper Maastrichtian) of New Jer-
sey, but specimens referable to Halisaurus have also been re-
ported from strata of roughly equivalent age in Belgium (Ling-
ham-Soliar, 1994), Niger (Lingham-Soliar, 1991), the Severn
Formation of Maryland (Baird and Case, 1966 [as ‘‘Navesink’’];
Baird, 1986b), the Navesink Formation (lower Maastrichtian) of
New Jersey (Bukowski, 1983), the Mount Laurel Formation
(lower Maastrichtian) of Delaware (Baird, 1986b), the Mer-
chantville Formation (lower Campanian) of Delaware (Baird,
1986b), the Mooreville Chalk (lower Campanian) of Alabama
(Russell, 1970), the lower to middle Campanian of Texas (Cald-
well and Bell, 1995), the Niobrara Formation (Santonian) of
316 JOURNAL OF PALEONTOLOGY, V. 74, NO. 2, 2000
Kansas (Baird, 1986b; Merriam, 1894; Russell, 1970; Wiman,
1920), and the Santonian of east-central Peru (Caldwell and Bell,
1995). Although some of the these identifications are based only
on isolated vertebrae (e.g., Baird, 1986a, 1986b; Caldwell and
Bell, 1995), more extensive skeletal remains are known from a
number of localities ranging in age from the Santonian (Russell,
1970) to the late Maastrichtian. If correctly identified in each
instance, these records indicate an unusually long stratigraphic
range for Halisaurus, possibly appearing soon after the first re-
ported occurrence of the Mosasauridae in the Turonianof Britain
(Woodward, 1905) and persisting to the late Maastrichtian. Other
known mosasaurid genera generally exhibit more restricted
stratigraphic ranges (Bell, 1993, 1997).
ACKNOWLEDGMENTS
We thank P. M. Kranz (Washington, D.C.) for collecting the
new specimen, donating it to the National Museum of Natural
History, and inviting us to study it. N. Hotton III and R. W.
Purdy (USNM) facilitated the extended loan of the material. D.
M. Scott assisted with the layout and scanning of the illustra-
tions.
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