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Osteology and taxonomy of Mosasaurus conodon Cope 1881 from the
Late Cretaceous of North America
T. Ikejiri and S.G. Lucas
Netherlands Journal of Geosciences / FirstView Article / October 2014, pp 1 - 16
DOI: 10.1017/njg.2014.28, Published online: 29 October 2014
Link to this article: http://journals.cambridge.org/abstract_S0016774614000286
How to cite this article:
T. Ikejiri and S.G. Lucas Osteology and taxonomy of Mosasaurus conodon Cope 1881 from the Late Cretaceous of North
America. Netherlands Journal of Geosciences, Available on CJO 2014 doi:10.1017/njg.2014.28
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Netherlands Journal of Geosciences —– Geologie en Mijnbouw |Page 1 o f 16 . doi: 10.1017/njg.2014.28
Osteology and taxonomy of Mosasaurus conodon Cope 1881
from the Late Cretaceous of North America
T. Ikejiri
1,2,*
&S.G.Lucas
3
1 Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487, USA
2 Alabama Museum of Natural History, Tuscaloosa, Alabama 35487, USA
3 New Mexico Museum of Natural History and Science, 1801 Mountain Road N.W., Albuquerque, New Mexico 87104, USA
* Corresponding author. Email: ikejiri1859@gmail.com
Manuscript received: 1 Februar y 14, accepted: 3 September 14
Abstract
Two well-preserved skeletons of Mosasaurus conodon Cope 1881 (Squamata, Mosasaurinae) from the Pierre Shale (late Campanian) of Colorado and
the Bearpaw Shale (Late Campanian to Early Maastrichtian) of Montana are described. The two specimens are important because they provide new
osteological information, especially on the skull (including jaws with teeth) and forelimbs, whereas those elements are largely missing in the
holotype (AMNH 1380) of M. conodon. Mor phological comparisons of the holotype with the two new specimens allow us to emend the diagnosis
of the species in the genus Mosasaurus, primarily using tooth and forelimb morphologies. Teeth of M. conodon are unique in their combination
of having a slender, gently recurved overall shape (similar to Clidastes) with no serration on the developed carinae (less developed in Clidastes).
The tooth count of M. conodon tends to be low (14–15 in the maxilla, 16 in the dentary and eight in the pterygoid, respectively) when compared
to other species, such as Mosasaurus lemonnieri,Mosasaurus missouriensis and Mosasaurus hoffmanni–Mosasaurus maximus.Theforelimbis
short in the species, characterised by a much lower number of the manual digital formula, 4(+1?)–4(+2?)–4(+1?)–4(+1)–2 than other species
of Mosasaurus. The forelimb bones are generally robust, especially the box-shaped humerus (width-to-length ratio 3/2). A variety of new morpho-
logical data support the conclusions that (1) M. conodon is a nominal species, (2) the European species M. lemonnieri is not a junior synonym and (3) one of the
most complete skeletons of Mosasaurus from South Dakota (SDSM 452) is not assigned to M. conodon (but is likely to be Mosasaurus sp.). To date, M. conodon occurs
only in North America during the late Campanian to early Maastrichtian.
Keywords: Plotosaurini, Mosasaurus, taxonomy, holotype, tooth morphology, palaeobiogeography
Introduction
The genus Mosasaurus (Reptilia, Squamata) is historically one
of the best known mosasaur taxa from upper Campanian to
Maastrichtian marine strata. A number of species of the genus
have been recognised from six continents. These taxa include
Mosasaurus conodon Cope 1881, Mosasaurus missouriensis
Harlan 1834, Mosasaurus maximus Cope 1869, Mosasaurus
ivoensis Persson 1963 (= Tylosaurus ivoensis?; see Lindgren &
Siverson, 2002) and Mosasaurus dekayi Bronn 1838 from North
America, as well as Mosasaurus hoffmanni Mantell 1829 and
Mosasaurus lemonnieri Dollo 1889 from Europe (Russell, 1967;
Machalski et al., 2003). The taxonomic status between the
European and North American species, however, has been
problematic, and a few taxa have been synonymised and
reestablished repeatedly, primarily due to poorly preser ved
and/or largely incomplete skeletons of the holotypes and a
limited number of other associated specimens. For example,
M. maximus and M. dekayi have been suggested to be junior
synonyms of M. hoffmanni (Russell, 1967; Mulder, 1999;
Harrell & Martin, 2014), whereas Lingham-Soliar (1995) deter-
mined M. maximus and M. hoffmanni are different species based
on their quadrates and other bones.
Since it was named by Edward D. Cope in 1881, M. conodon
has been one of the most commonly identified species of
mosasaurs. It should be noted that the holotype, AMNH 1380,
includes only some isolated cranial bones, lower jaw fragments
with a few teeth, 12 cervical and anterior dorsal vertebrae, a
1
© Netherlands Journal of Geosciences Foundation 2014
humerus and a shoulder bone (Fig. 1). This incomplete nature
of the holotypic skeleton provides limited morphological infor-
mation, which has posed challenges to the recognition of the
species. Russell (1967) proposed two main ideas regarding the
taxonomy of M. conodon: (1) the European M. lemonnieri
is a junior synonym of M. conodon and (2) the fairly complete,
articulated skeleton (SDSM 452) from South Dakota, which was
unofficially named as the new species Mosasaurus ‘poultneyi’
in an unpublished MSc thesis (Mar tin, 1953), is assigned to
M. conodon. Notably, the newly established diagnosis for t he
species by Russell (1967) was based on the South Dakota
specimen and a number of specimens assigned to M. lemonnieri,
although some morphological features tend not to apply to the
holotype. Bell (1993, 1997) in fact suggested that SDSM 452 is not
M. conodon, but an indeterminate species of the genus. This identi-
fication allowed him to score only 51 characters out of 142 for that
taxon in his cladistic analysis of the Mosasauridae, which possibly
supported the idea that M. conodon was the basalmost taxon in
the genus and closely related to M. missouriensis.Lingham-Soliar
(2000) later stated that the North American M. conodon and the
European M. lemonnieri are taxonomically distinct. To date,
many small to medium-sized mosasaurs from Campanian and
Maastrichtian strata are often assigned to M. conodon in museum
collections, often based solely on body size and/or stratigraphic
occurrences. Clarification of the taxonomic assignment and
diagnostic features of M. conodon is thus needed.
In 1998, a local landowner, Mr Allen Peterson, in Trinidad,
southern Colorado, discovered a partial mosasaur skeleton,
exhibiting some cranial bones, jaw elements, teeth and many
postcranial bones, in the upper part of the Pierre Shale.
This mosasaur specimen (TSJC 1998.2) displays a nearly identical
size and mor phology to the holotype of M. conodon (AMNH 1380),
especially in its teeth, dentary, coracoid and humerus.
Moreover, we here refer another undescribed skeleton with
an articulated skull, forelimbs and presacral vertebrae
(MOR 006) from the Bearpaw Shale in north-central Montana
to M. conodon. These two new specimens allow us to present
osteological information on the species that is largely missing
in the holotype and thereby provide a better basis for comparing
it with other closely related taxa.
Fig. 1. Hol otype o f M. conodon (AMNH
1380). A, tooth in lateral and anterior
views; B, right coronoid in lateral view;
C, left dentary in medial view; D, cora-
coid; E, right humerus. The arrow in D
indicates the position of the second
coracoid foramen. Scale equals 1 cm
inAand10cminB–E.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
2
This study mainly focuses on the diagnosis of Mosasaurus
conodon based on specimen-based osteological comparisons,
especially between the holotype and the two new specimens.
The diagnoses for the species presented by Cope (1881) and
Russell (1967) are reviewed and applied to a re-examination
of M. lemonnieri and SDSM 452. Implications for biostrati-
graphic and palaeobiogeographic distributions of the species
are then discussed.
The institutional abbreviations used in this report are as
follows:AMNH –American Museum of Natural History, New York,
USA; FHSM –Sternberg Museum of Natural History, Fort Hays
State University, Hays, Kansas, USA; FMNH –Field Museum of
Natural History, Chicago, USA; Goldfuss –Goldfuss-Museum,
Institut f¨
ur Pal¨
aontologie, Der Universit¨
at Bonn, Bonn, Germany;
IRSNB –Royal Belgian Institute of Natural Sciences, Brussels,
Belgium; KUVP –University of Kansas, Museum of Natural
History, Lawrence, Kansas, USA; MNHN –Mus´
ee National
d’Histoire Naturelle, Laboratoire de Pal´
eontologie, Paris, France;
MOR –Museum of the Rockies, Bozeman, Montana, USA;
MSC –McWane Science Center, Birmingham, Alabama, USA;
NHMM –Natuurhistorisch Museum Maastricht, Maastricht,
USA; NHMUK –Natural Histor y Museum, London, UK (formerly
the British Museum of Natural History); NJSM –New Jersey
State Museum, Trenton, New Jersey, USA; RMM –former Red
Mountain Museum (paleontological collection now stored at
MSC), Birmingham, Alabama, USA; SDSM –South Dakota School
of Mines and Technology, Rapid City, South Dakota, USA;
TMM –Texas Memorial Museum, University of Texas, Austin,
USA; TSJC –Louden-Henritze Archaeology Museum, Trinidad
State Junior College, Trinidad, Colorado, USA; UAM –University
of Alabama Museums, Tuscaloosa, Alabama, USA; UNSM –University
of Nebraska State Museum, Lincoln, Nebraska, USA;
USNM –National Museum of Natural History, Washington,
D.C.,USA;YPM–Yale Peabody Museum, New Haven,
Connecticut, USA.
The osteological abbreviations are as follows:
Cranial skeleton: ar, articular; ba, basioccipital condyle; d,
dentary; f, frontal; j, jugal; m, maxilla; oc, occipital segment;
p,parietal;paf, parietal foramen; pm, premaxilla; pof, postor-
bitofrontal; pr, prootic; prf, prefrontal; pt,pterygoid;q,quad-
rate; sa,surangular;sp,splenial;sq, squamosal.
Quadrate: aa, anterodorsal ridge of ala; isp, infrastapedial process;
ssp, suprastapedial process.
Vertebrae: CV, cervical vertebra; DS, dorsal vertebra; P, pygal
vertebra including a sacral vertebra; CdC, anterior caudal
vertebra with chevron.
Appendicular skeleton: cg, glenoid condyle; ect, ectepicondyle; hu,
humerus; i, intermedium; mc, metacarpal; pc, pectoral crest; pf,
pisiform; pgp, postglenoid process; r, radius; ra, radiare; ul,ulna;
ula, ulnare; 2-4, 2-4 distal carpals.
Age and geological context
TSJC 1998.2 was collected from a construction site near the
downtown of Trinidad, in south-central Colorado (NW1/4,
NE1/4 Sec. 24, T33 S, R64 W) (Fig. 2). The layer yielding the
bones is in the upper part of the Pierre Shale, about 45–60 m
below the base of the overlying Trinidad Sandstone. Another
medium-sized mosasaur, TSJC 1966.P.2, was collected from
about 2 km east of the site. The beds containing the bones
are characterised by greyish shale with indurated limestone
concretions. The bone layer of TSJC 1996.P2 is about the same
stratigraphic level as TSJC 1998.2. Following Lee and Knowlton
(1917), the two skeletons are estimated to belong to either the
Baculites cuneatus or B. compressus ammonite zones.
MOR 006 was found on US Fish and Wildlife Lands in Phillips
County, north-central Montana (MOR locality number BS-136)
(Fig. 2). Although precise locality and stratigraphic data are
not available, the specimen was most likely collected from an
upper Campanian horizon (P. Leiggi, written commun., 2002).
Fig. 2. Representative fossil sites of M. conodon in North
America. Light grey areas indicate the Late Cretaceous
(Campanian) landmass. Dark grey colour indicates distri-
bution of Upper Cretaceous marine surface rock/strata.
Letters for locality: A, the holotype (AMNH 1380) from
New Jersey B, TSJC 1998.2 from Colorado; C, MOR 006
from Montana. A list of other specimens (dots) is
explainedinthetext.ThemapismodifiedfromIkejiri
et al. (2 013).
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
3
Systematic palaeontology
MOSASAURIDAE Gervais, 1853
MOSASAURUS Conybeare, 1822
MOSASAURUS CONODON Cope 1881
Clidastes conodon, Cope 1881
?Mosasaurus lemonnieri, Dollo, 1889
Mosasaurus conodon, Baird & Case, 1966
Mosasaurus conodon, Russell, 1967
Mosasaurus lemonnieri, Russell, 1967
Mosasaurus conodon, Lingham-Soliar, 2000
Distribution
M. conodon is largely known from Upper Cretaceous strata of
North America, which are associated with the Western Interior
Seaway, the Gulf of Mexico and southern Atlantic Seaboard
(Fig. 2). Fossils have been collected from various upper
Campanian and lower Maastrichtian strata, such as the
Navesink Formation (New Jersey), Severn Formation (Maryland),
upper Pierre Shale (Colorado and South Dakota), Bearpaw Shale
(Montana), Marlbrook Marl (Arkansas) and Demopolis Chalk
(Alabama).
Emended diagnosis
Medium-sized Mosasaurus exhibiting the following combination
of features: relatively narrow snout; slender maxilla and dentary;
greatly expanded splenial ventroposteriorly; 13–14 maxillary
teeth, 15–16 dentary teeth and eight pterygoid teeth; slender,
transversely compressed teeth on premaxilla, maxilla and
dentary; oval-shaped cross-section of teeth; no serration on
well-developed carina(e); single anterior carina on teeth of
anterior jaw (tooth pm1-maxillary tooth m4 and dentary tooth
d1–d4 or d5), anterior and posterior carinae in m5 and m6 and
d6 and d7, single posterior carina in m7–m14 and d8–d16;
narrow posteroventral angle of jugal (70–80°); relatively large
infrastapedial process placed low on poster ior quadrate (c. 2/5
of total height from the mandibular articulation); large
box-shaped humerus (ratio of total height-to-transverse
width = 3:2); well-developed, hooked entepicondyle of
humerus; strongly constricted medial surface of humeral
mid-shaft; radius much larger than ulna; strongly concave
proximal articular surface of intermedium; reduced digital
formula in manus [4(+1?)–4(+2?)–4(+1?)–4(+1)–2] (much less
than M. hoffmanni–M. maximus and SDSM 452).
Holotype
AMNH 1380, consisting of incomplete right dentary with one
tooth, a few tooth fragments, coronoid, splenial, angular, articular,
squamosal, 12 vertebrae (at least six cervical and four dorsal
vertebrae), partial lef t (?) scapula, left coracoid, r ight humerus,
right ulna and rib fragments (Fig. 1). It was most likely collected
from the Navesink Formation (upper Campanian to lower
Maastrichtian) in Freehold, Monmouth County, New Jersey
(Baird & Case, 1966; Gallagher, 1993).
Referred specimens
MOR 006, nearly complete skull and jaws except for coronoids
(reconstructed), 41 articulated presacral to pygal vertebrae,
left and right pectoral girdle bones, nearly complete articulated
forelimbs except a few distal phalanges, ischia, many ribs and
chevrons (from the Bearpaw Shale of north-central Montana);
MOR 5051, partial left maxilla with four teeth (from the Pierre
Shale); RMM 2204 (now stored at MSC), three isolated teeth,
two pterygoid teeth, skull fragments, seven trunk and six
caudal vertebrae (from the Demopolis Chalk of Lowndes County,
Alabama); RMM 3037 (now stored at MSC), partial lef t dentary
and lower jaw, pterygoids, 18 isolated teeth, coracoid, humerus
(from lower Demopolis Chalk of Sumter County, Alabama); TSJC
1998.2, fairly large disarticulated skeleton, including
incomplete skull with squamosal, postorbitofrontal and left
paroccipital process, several partial jaw elements with teeth,
six cervical vertebrae, 25 dorsal vertebrae, nine pygal vertebrae
(including sacral?), nine intermediate caudal vertebrae, partial
left coracoid, right coracoid, right humerus, ulna, radius,
tibiae, three metacarpals, nine disarticulated phalanges and
rib fragments; TSJC 1966.P.2, occipital condyle, posterior
portion of lower jaw, most of the cervical and dorsal vertebrae,
humerus, ulna, radius, three metacarpals, f ive phalanges, rib
fragments, chevron (both TSJC specimens from the Pierre Shale
of southern Colorado); UAM 1994.0008.0004, teeth and some
cranial bones (from the Demopolis Chalk of Marengo County,
Alabama); USNM 18255, partial right maxilla with tooth; USNM
11396, one cervical vertebra, 15 articulated trunk to pygal
vertebrae, some caudal vertebrae, chevrons, partial scapula, ilia,
pubes, ischia, ulna, phalanges, rib fragments (from the Marlbrook
Marl of Hempstead County, Arkansas); USNM 18255, partial premax-
illa and right maxilla, tooth, left humerus, partial coracoid and
scapula, radius (from the Pierre Shale of Hughes County, South
Dakota); USNM 336480, tooth (from the Severn Formation of
Prince George’s County, Maryland); YPM 1573, teeth, jaw fragments,
three cervical vertebrae including atlas, ulna, radius (from the
McLean Pits of Middletown, New Jersey).
Remarks on excluded specimens
Russell (1967, p. 135) referred 17 specimens to M. conodon.
Some of these specimens are, however, too incomplete and/or
fragmentary to identify to species level with certainty, such as
AMNH 1387, 1395, 1397, ANSP 8469, 8480, 8501, 8502, 8504,
8509, and YPM 279, 1500 and 1510. AMNH 1395 consists of
an isolated tooth, jaw fragments and a coracoid. The tooth,
likely from the posterior portion of a jaw, does not resemble
that of M. conodon because of its transversely compressed
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
4
structure and car inae with serrations. Some isolated teeth of
YPM 1573 are morphologically similar to TSJC 1998.2. USNM
11904 includes 14 trunk vertebrae, one sacral and some pygal
vertebrae, a phalanx, a radius and two ribs. USNM 11396 con-
sists of a partial scapula, all pelvic bones, two phalanges, and
15 dorsal, some pygal, and many caudal vertebrae, but these
portions of the skeleton are not useful to identify M. conodon.
USNM 18255 is a relatively small individual of Mosasaurus,and
the two isolated teeth are greatly flattened transversely, similar
to AMNH 1395. This tooth morphology indicates the two speci-
mens do not assign to Mosasaurus. SDSM 452 is not assignable
to M. conodon, a view we base on a number of mor phological
features as presented below.
Description
Skull
The nearly complete articulated skull of MOR 006 (Figs 3 and 4)
indicates that M. conodon has a much more slender skull in
overall shape than M. maximus (TMM 313, NJSM 11053),
M. hoffmanni (NMHNP AC. 9648), M. missouriensis (KUVP 1034)
and Mosasaurus sp. (UNSM 77040). A small but deep concavity
is located in the lateral margin near the mid-section of the
frontal in MOR 006 (Fig. 4). This margin is weakly concave in
M. missouriensis (Bell, 1997, p. 305; Williston, 1898, pl. 20;
personal observation of KUVP 1034) and M. maximus (NJSM
11053, TMM 313), but absent in M. hoffmanni (see Lingham-Soliar,
1995, f igs 4 and 6) and M. lemonnieri (Lingham-Soliar, 2000).
The parietal of MOR 006 is relatively shorter anteroposteriorly
and wider transversely than that of M. lemonnieri. The parietal-
frontal suture is not clearly visible in the specimen, but a weak
line is visible along the anterior margin of the parietal f lares.
According to Bell (1997), a relatively small parietal foramen,
defined as smaller than or equal to the area of the stapedial pit,
is commonly found in Mosasaurus missouriensis,M. maximus
and UNSM 77040 (Mosasaurus sp.), but the foramen is relatively
large in M. conodon (MOR 006 and TSJC 1998.2). The two
specimens exhibit an oval-shaped parietal foramen that is
slightly elongated anteroposteriorly. A similar oval-shaped
parietal foramen also appears in M. maximus (NJSM 11052; see
Mulder, 1999; Lingham-Soliar, 1995, fig. 7) and M. hoffmanni
(IRSNB R26; see Lingham-Soliar, 1995, fig. 6e), but the outline
tends to be wider transversely, forming a nearly circular shape,
as in M. missouriensis (KUVP 1034) and M. lemonnieri (IRSNB
3127 and 3211).
In MOR 006, the postorbitofrontal-squamosal ramus
reaches the end of the supratemporal fenestra in M. maximus,
M. missouriensis (Bell, 1997) and M. lemonnieri (Lingham-Soliar,
2000), as well as in MOR 006.
The posteroventral angle of the jugal is about 70–80
o
in
MOR 006 (Fig. 3). This angle is smaller than in M. lemonnieri
(85–95
°
: IRSNB 3127, 3189), M. hoffmanni (90–95
°
:NHMUK
PV OR 11589, IRSNB R26), M. maximus (c. 90
°
: NJSM 11053)
Fig. 3. Skull of M. conodon (MOR 006) in lateral view. Scale equals 10 cm.
Fig. 4. Skull of M . conodon (MOR 006) in (A) dorsal and
(B) lateral views. Light grey colour indicates missing or
reconstructed portion in the original skull. Scale equals
10 cm. Osteological abbreviations are listed in the text.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
5
and the holotype of M. missouriensis (c. 90
°
:Goldfuss1327).In
M. conodon (MOR 006), the posteroventral process of the jugal
is greatly expanded posteriorly, which can be distinguished
from M. lemonnieri,M. hoffmanni and M. maximus.
Thisprocessisalsopositioned much higher in M. conodon
(MOR 006) than it is in M. lemonnieri and M. hoffmanni.
The well-preserved squamosal has a robust overall structure
withacircularcross-sectioninTSJC1998.2.Itbearsavery
shallow trench on the dorsolateral surface of the anterior
wing, which differs from a much deeper trench in M. lemonnieri
(Lingham-Soliar, 2000).
The well-preserved braincase of MOR 006 (Fig. 5) displays
tightly sutured occipital elements. The basisphenoid is
wide transversely and expanded to the anterior margin.
TheoccipitalcondyleofMOR006hasanearlycircularshape
in posterior view.
The quadrate of MOR 006 has a rectangular-shaped dorsal
end in anterior view and a relatively small suprastapedial
process (Fig. 6). The notch of the suprastapedial process is
placed slightly above two-thirds of the total height of the quad-
rate. A relatively large infrastapedial process is located slightly
below the mid-point of the overall quadrate height, which is
about the same position as in M. lemonnieri (IRSNB 3189;
see Lingham-Soliar, 2000, fig. 2) and M. maximus (NJSM
11052, 11053) but lower than in M. hoffmanni (ISRNB R26,
NHMUK PV OR 11589: half to three-fif ths of the overall height)
and M. missouriensis (about half of the overall height).
When compared to M. maximus (NJSM 11053), MOR 006 has
a less-developed external ridge of the suprastapedial process.
The stapedial pit of MOR 006 is large, with a nearly circular
outline, as in other species of Mosasaurus.
The quadrates of MOR 006 are smaller than those of
M. missouriensis and M. maximus relative to overall skull size
(Table 1; Appendix 1). Based on the ratio of the quadrate
height-to-dentary length, M. conodon (MOR 006) has a lower
ratio (0.19) than M. missouriensis (0.22inKUVP1034)and
M. maximus (0.23 in NJSM 11053), indicating that the
former species has a relatively slender skull among species in
the genus.
Jaws
MOR 006 has nearly complete upper and lower jaws, except
for largely reconstructed coronoids (Figs 3 and 4). The entire
structure of the lower and upper jaws is slender. The premaxilla
of MOR 006 is narrow transversely when compared to other
species of Mosasaurus (Fig. 7). The anterior end of the premaxilla
is slightly pointed in MOR 006, which is similar to M. lemonnieri
(Lingham-Soliar, 2000). The coronal cross-section of the premax-
illa is nearly sub-rectangular, with nearly straight ventral and
gently curved dorsal margins in rostral view. A well-developed
median ridge runs along the anteroposterior axis on the ventral
surface of the premaxilla, which reaches about half the height
Fig. 5. Braincase of M. conodon (MOR 006) in poster ior view. Scale equals
10 cm .
Fig. 6. Left quadrate of M. conodon (MOR 006) in (A)
posterior and (B) lateral views. Scale equals 10 cm. Oste-
ological abbreviations are listed in the text.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
6
of the tooth crown of pm1. The ridge does not directly contact
the anterior-most tip of the premaxilla but stops near the
anterior base of pm1 (Fig. 7C). The posterior end of the
maxillo-premaxillary suture occurs above m6 and m7.
TheslendermaxillaeofMOR006(Appendix1)canbedistin-
guished from fairly robust maxillae of M. lemonnieri and
strongly broad bones of M. hoffmanni (Lingham-Soliar, 1995,
2000). In lateral view, MOR 006 shows that the dorsal and
ventral outlines are relatively low and nearly parallel from near
m4 to m14 or m15 (Figs 3 and 4). The lateral surface of the
mid-maxilla is inclined about 80° from the horizontal in coronal
cross-sectional view, although the medial surface is nearly
perpendicular. A posterodorsal process is absent (or possibly
damaged) in MOR 006, whereas it is repor ted in M. missouriensis
(KUVP 1034) and Mosasaurus sp. (UNSM 77040) (Bell, 1997).
The very slender dentary in the holotype of Mosasaurus
conodon (Fig. 1), which has been suggested to be diagnostic
of the species by Cope (1881) and Russell (1967), is also seen
in MOR 006 (Figs 3 and4; Appendix 1). The slenderness is
morphologically similar to Clidastes, rather than to other
species of Mosasaurus. In MOR 006 and AMNH 1380, the ventral
margin of the posterior end of the dentar y is greatly expanded
ventrally (Fig. 8). In the cross-section of the mid-dentary,
the lateral surface is convex, but the medial surface is slightly
concave. A narrow, trench-like mandibular canal extends from
nearly the anterior-most tip to the mid-portion of the dentary
on the medial surface, being gradually expanded posteriorly.
On the medial surface a small, oval-shaped concavity (10 320 mm
in diameter) is placed below d7 and d8 (the arrow in Fig. 8).
This feature is probably not pathological based on the smooth
surface morphology and nearly identical size and relative
position on both of the dentaries.
Tooth counts
MOR 006 has 14 teeth in the left maxilla and 15 in the right
maxilla, including a dental alveolus. This discrepancy between
left and right upper jaws indicates a small degree of intraspecific
variation in M. conodon. Other species of Mosasaurus tend to have
ahigher toothcount(e.g.16inM. maximus), but M. hoffmanni
and M. missouriensis (KUVP 1031) display a lesser number
(14 teeth) (Table 3). Russell (1967) reported that Clidastes
liodontus has 14–15 teeth, whereas Clidastes propython has
16–18 teeth.
Russell (1967, p. 133) repor ted t hat 17 teeth in the dent ary,
based on SDSM 452, is diagnostic of M. conodon,althoughMartin
(1953) repor ted the jaw bones were largely reconstructed based
on Clidastes. Both of the dentaries of MOR 006, however, have
a total of 16 teeth (Tables 2 and 3). The same tooth count
appears in C. liodontus,butC. propython tends to have a higher
number (17 teet h) (Russell, 1967). Within Mosasaurus,theden-
tary holds a total of 16 teeth in M. lemonnieri (IRSNB specimens;
Lingham-Soliar, 2000) and 15 teeth in M. missouriensis
Table 1. Comparisons of skull measurements in Mosasaurus.
M. conodon
MOR 006
M. maximus
NJSM 11053
M. missouriensis
KUVP 1034
Skull length (pm–ba) 977 mm 1,208 mm 1,110 mm
Dentary length
(ventral margin)
620 mm 818 mm 609 mm
Skull height (q–pa) 200 mm 317 mm 224 mm
Skull width (q–q) 321 mm 329 mm 307 mm (sq–sq)
Quadrate height 115 mm 190 mm 135 mm
Frontal width 203 mm 260 mm 175 mm (pa–pa)
Length ration q:de 0.19 0.23 0.22
Osteological abbreviations are listed in Fig. 2.
Fig. 7. Premaxilla of M. conodon (MOR 006) in (A) dorsal,
(B) anterior, (C) ventral and (D) left lateral views. Scale
equals 10 cm.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
7
(KUVP 1034). The more derived species, M. hoffmanni–M. max-
imus exhibits 14 teeth, which is the lowest number in the genus.
Russell (1967) stated that M. conodon has a total of
10 pterygoid teeth, but the specimen he based this on is not
specified. Notably, MOR 006 has only eight in both pterygoids.
M. lemonnieri (multiple IRSNB specimens?) has 11–12 pterygoid
teeth (Lingham-Soliar, 2000), although eight teeth appear in
M. hoffmanni (IRSNB R26; Lingham-Soliar, 1995) and
M. missouriensis (KUVP 1032; Williston, 1898). C. propython
(ANSP10193;KUVP1022)displays13–14 pterygoid teeth.
Tooth morphology
In general, mosasaur teeth morphologically vary in (1) overall
shape (curvature and robustness), (2) surface texture (smooth
or faceted), (3) overall size, (4) position of a carina (when
present), (5) serrations (if present) and (6) cross-sectional
shape. The holotype of M. conodon (AMNH 1380) includes
two well-preserved marginal teeth: one located in the anterior
portion of the right dentary and an isolated tooth crown
(Fig. 1). The two teeth are slender and slightly recurved in
overall shape, which is suggested to be one of the diagnostic
features in the species (Cope, 1881; Russell,1967). This overall
tooth morphology in AMNH 1380 is nearly identical to several
well-preserved teeth in TSJC 1998.2 (Fig. 9). In contrast, overall
tooth shape is much more robust in M. hoffmanni–M. maximus.
MOR 006 preserves most teeth in the nearly complete lef t and
right upper and lower jaws, although the tooth surfaces are
damaged by a high degree of pyrite mineralisation. In MOR
006, the two-thirds apicalmost portion of the teeth from the
mid-portions of the jaws are more curved distally and also slightly
curved lingually in M. conodon compared to M. hoffmanni–
M. maximus. The teeth of M. lemonnieri in the mid-portion of the
jaws are much straighter than those of M. conodon. Additionally
the pm1, pm2, d1 and d2 teeth have a much stronger curvature than
other teeth in the jaws of MOR 006.
The smooth tooth surface in AMNH 1380 (Fig. 1) has been
suggested to be another diagnostic feature of M. conodon
(Cope, 1881). This tooth morphology is also found in all
preserved teeth in TSJC 1998.2 (Fig. 9) and Clidastes (Russell,
1967), as well as in M. hoffmanni (MNHN AC9648). In contrast,
well-developed facets or striae occur in the holotypes of
M. maximus (AMNH 1389) and M. missouriensis (Goldfuss 1327;
illustrated in Harlan, 1834). Lingham-Soliar (2000) suggested
that in M. lemonnieri the tooth facets are better developed in
more mature than in immature individuals. However, to our
knowledge, all known specimens of M. conodon,includingthe
two largest, TSJC 1998.2 and AMNH 1380, commonly have a ver y
smooth dental surface. This evidence indicates that the facet
surface is absent throughout the postnatal ontogeny of
M. conodon.
Tooth size is variable depending on its relative jaw position
in MOR 006. The two premaxillary teeth are much slenderer
than other teeth in the maxilla in MOR 006 (Table 2). M3 is
Fig. 8. Dentar y of M. conodon (MOR 006) in medial view. Arrow indicates
a depression (see explanation in text). Scale equals 5 cm.
Table 2. Measurements (in millimetres) of teeth and variation in positions of carina in M. conodon (MOR 006).
Left upper jaw: premaxillary (1–2) and maxillary (3–16) teeth
Tooth number 1234567 8910111213141516
Crown height 32 28 36 35 30 28 34 29 32 31 ? 27 ? ? ? ?
Crown transverse width 16 18 22 22 21 24 28 26 29 28 ? 26 ? ? ? ?
Position of carina ? AAAAAPAP? P P? P? ? ? ?
Left lower jaw: dentar y (1–16) teeth
Tooth number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Crown height 22 29 31 45 47 42 46 41 47 ? 44 45 45 39 37 ?
Crown transverse width 15 15 15 26 29 27 29 29 28 ? 28 21 23 19 18 ?
Position of carina A A A A ? A P A P A(?) P P ? P P P P P ?
A, anterior; P, posterior carina present.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
8
the longest tooth, whereas more posterior teeth (m6–m12) are
wider transversely, in the upper jaw, although a few posterior
teeth are missing. Based on the sizes of the aveoli, tooth size
gradually decreases from m6 to the distal teeth. In the dentary,
d1 and d2 are relatively small and have much weaker recurva-
ture than pm1 and pm2, whereas d5 is the longest tooth in
the dentar y of MOR 006. The tooth length from d4 to d13 is
nearly sub-equal in MOR 006, which appears slightly different
from M. lemonnieri (IRSNB 3132), which has a more anterior po-
sition of sub-equal-sized teeth (from d3 to d11; Lingham-Soliar,
2000).
A mix of single and double carinae in jaws is suggested to
be another diagnostic feature of M. conodon (Cope, 1881).
The well-preserved maxilla and dentary of TSJC 1998.2 also
display this morphology. In the partial left dentary of TSJC
1998.2, three teeth and two aveoli are exhibited, and the rostral-
most tooth has a single anterior carina, the middle tooth has both
anterior and posterior carinae, and the caudal-most tooth has
a single posterior carina. Interestingly, a similar type of varia-
tion occurs in the premaxilla-maxilla and dentar y of MOR 006
(Table 2). In the upper jaw, m4 and m5 have double carinae,
but an extensively weathered tooth surface does not allow us
to examine this feature in m6. Only an anterior carina is present
between pm1 and m4. Only the posterior carina is present from
m6 to m10; the rest of the posterior teeth are missing in
the maxilla. These features are unique to M. conodon or, at
least, sharply different from M. maximus,M. lemonnieri and
M. missouriensis.
Well-preserved teeth in TSJC 1998.2 and AMNH 1380 show
the absence of serrations under light microscopic examination
(Fig. 10). Well-developed thin carinae in the Colorado specimen
indicate that serrations are not worn or physically damaged.
Such unserrated carinae are also known in Clidastes (Russell,
1967), but M. conodon tends to have better-developed edges.
Highly developed serrations are, in contrast, commonly found
in M. lemonnieri,M. hoffmanni and M. maximus, including small
(juvenile) individuals.
A transversely compressed cross-section of the teeth,
characterized by nearly symmetrical lingual and labial circum-
ferences, is also an autapomorphic feature in Mosasaurus
conodon. This oval-shaped cross-section is found in all teeth on
the premaxilla, maxilla and dentary of MOR 006. This feature is
morphologically dif ferentiated easily from a U-shaped cross-
section in M. hoffmanni,M. maximus and M. dekayi.Inthose
species of Mosasaurus the distal carina is placed strictly labially,
and the angle between the anterior and posterior carinae is less
than 90° in the premaxillary teeth and gradually spread to
the caudal teeth but never meets 180° nor an oval-shaped
cross-section.
Based on a series of tooth morphologies, we suggest teeth,
even isolated ones, are the most useful elements to identify
M. conodon.M. conodon and Clidastes share morphologically
very similar teeth in the jaws, but the former taxon displays
Table 3. Tooth count in selected taxa of Mosasaurus and Clidastes.
M. conodon MOR 006 C. liodontus C. propython M. lemonnieri
IRSNB 3127
M. maximus
NJSM 11053
M. hoffmanni IRSNB R12,
MNHN AC9648
Maxilla 14–15 14–15* 16–18* 15** 16 14***
Dentary 16 16* 17* 16** 14 14***
* Data from Russell (1967).
** Data from Lingham-Soliar (2000).
*** Data from Lingham-Soliar (1995).
Fig. 9. Teeth of M. conodon (TSJC 1998.2) in (A) lateral and (B) dorsal
views. Scale equals 1 cm.
Fig. 10. An terior carina of tooth of M. conodon (TSJC 1998.2) in side view
under light microscopy. Scale equals 1 mm. Note about 0.2 mm of the edge
is a carina with no serration.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
9
10–20% larger teeth due to larger overall body size and
better-developed carinae than the latter taxon.
In M. conodon (MOR 006), the pterygoid teeth can be
distinguished from all other teeth in the jaws based on:
(1) the transversely wider base of the tooth crown than the
crown length and (2) the absence of carinae. The latter charac-
teristic is dif ferent from the holotype of M. hoffmanni (MNHN
AC9648), which displays carinae. The apical one-fifth to one-
fourth of the tooth crown is strongly curved and occasionally
hooked in MOR 006.
Vertebrae
MOR 006 has 41 articulated presacral–pygal vertebrae.
The cervical vertebrae indicate MOR 006 is 15–20% smaller
than AMNH 1380. The two specimens show that cervical
vertebrae have a more slender overall structure in M. conodon
than in M. hoffmanni and M. maximus. In TSJC 1998.2 (M. conodon)
the fourth cervical vertebra has the largest hypophyseal peduncle,
although the seventh cervical vertebra has only a small pinched
convexity, but lacks an articular surface. The posterior cervical
vertebrae of TSJC 1998.2 and AMNH 1380 display a heart-shaped
posterior face of the centrum with slightly concave dorsal
and rounded ventral margins. The synapophyses are slightly
elongated dorsoventrally in the cervical series in the genus
Mosasaurus, but the expansion is weaker in M. conodon (AMNH
1380, MOR 006, TSJC 1998.2) than it is in M. hoffmanni and
M. maximus.Cope(1881)listedthepresenceof thezygantrum
as a diagnostic character of M. conodon. This accessor y vertebral
articulation is found in the mid-cervical to anterior dorsal
vertebrae of MOR 006, TSJC 1998.2 and AMNH 1380, and is also
reported in M. lemonnieri (Lingham-Soliar, 2000).
Table 4. Comparisons of counts of vertebrae in mosasaurs (modified from
Nicholls, 1988).
Taxon CV DS P CdC
Tylosaurus proriger
1
723 7 ?
Hainosaurus bernardi
2
10 >19 20 49
Platecarpus
1
730 5 59
Clidastes
1
735 7 70
Plotosaurus tuckeri
3
930 30 59
Mosasaurus sp. (SDSM 452) 7 38 8 21
Mosasaurus sp (FMNH P26956) 7 32 9 ?
M. conodon (TSJC 1998.2) 7 >25? 8 or 9 >10?
M. conodon (MOR 006) 7 35 (or 36) ? ?
M. maximus (TMM 313 and
NJSM 11053)
4
7 24 (+8?) 9 or 10 ?
M. lemonnieri
5
731–38 13–22 ?15–40
Data sources:
1
Williston (1898, p. 143);
2
Dollo (1882, p. 153);
3
Camp (1942);
4
Langston (1966, fig. 2);
5
Lingham-Soliar (2000).
Fig. 11. Pectoral girdles of Mosasaurus. A, right coracoid (top) and scapula
(bottom) of M. conodon (MOR 006); B, right coracoid of M. conodon
(TSJC 1998.2); C, left coracoid of Mosasaurus sp. (FMNH P26956). Arrow
indicates the second coracoid foramen. Grey colour indicates a missing
portion. Scale equals 10 cm.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
10
The total number of dorsal (trunk) vertebrae varies in various
genera of mosasaurs (Nicholls, 1988). In the genus Mosasaurus,
M. conodon tends to have a higher number (35 in MOR 006)
(Table 4). SDSM 452 has one of the highest dorsal vertebral counts
among specimens assigned to Mosasaurus. Lingham-Soliar (2000)
reported 31–38 dorsal vertebrae in M. lemonnieri,butthe
specimen(s) were not specified. The mounted skeleton of
M. maximus (TMM 313) exhibits the smallest number (24 in total)
in the genus, but, possibly, several vertebrae are missing
(Langston, 1966, fig. 2).
Eight disarticulated pygal vertebrae of M. conodon (TSJC
1998.2) have a relatively large, elongate centrum, as do the
posterior dorsal vertebrae. According to Osborn (1899), the
transverse processes of the first pygal vertebra (aka. sacral)
are more than twice as long as the synapophyses of the last dorsal
vertebra in Mosasaurus, but the difference is 120–130% in TSJC
1998.2.
TSJC 1998.2 has several centra of the intermediate caudal
vertebrae with fused chevrons. The shape of the centra exhibits
a typical triangle shape in posterior view, which is evidently
taller dorsoventrally than the transverse width.
Pectoral girdle
Nearly complete scapulae and coracoids are articulated in
MOR 006 (Fig. 11A), although they are slightly flattened due
to taphonomic processes. The scapula has about the same
surfaceareaasthecoracoid.The long and straight medial
margin of the scapula is in stark contrast to the short, strongly
constricted lateral edge. The scapula and coracoid have
about the same width in MOR 006 and AMNH 1380, but in SDSM
452 the scapula is slightly more emarginated than the coracoid.
The scapula (MOR 006) has a rectangular-shaped coracoid
articular head, which exhibits a weak convexity and many small
pits. The anteromedial corner of the fan is greatly expanded,
which contrasts with the reduced anterolateral corner. The
medial margin is slightly damaged, but is nearly straight in
MOR 006. The entire medial edge from the scapular head to
the corner of the fan is thicker than the lateral edge, as in
the coracoid.
The right coracoid of TSJC 1998.2 exhibits excellent three-
dimensional preservation (Fig. 11B) and shares a number of
morphological similarities with AMNH 1380, such as a relatively
expanded medial border, a thicker medial margin of the fan
(i.e. the thickest portion in the fan) and a well-developed
ridge-like anterior margin on the proximal head.
Russell (1967) discussed variation in relative size between
the lengths of the medial border of the coracoid fan and the
circumference of an outer line of the fan among mosasaurs
(e.g. ratio of the length-to-circumference: c. 0.33 in Platecarpus
and Tylosaurus;largerthan0.33inM. conodon and Clidastes).
Three specimens of M. conodon showvariationintheratio
ranging from c. 0.24 (AMNH 1380) to 0.28 (TSJC 1998.2) to
0.30 (MOR 006). Two other specimens of Mosasaurus sp. have a
very similar ratio: 0.29 in FMNH P26956 and 0.31 in SDSM 452.
Based on these data, the ratio is not useful for species-level
taxonomic assignment among Mosasaurus. TSJC 1998.2 (Fig.11B)
has a relatively large coracoid foramen, as commonly seen in
Mosasaurus and Clidastes in Mosasaurinae, but sharply different
to its relatively small size in plioplatecarpine mosasaurs.
A single coracoid foramen is usually found in most individuals
among various mosasaur taxa, but a few specimens assigned to
the genus Mosasaurus display two foramina. Notably, two cor-
acoid foramina occur in the large individual of Mosasaurus sp.
(FMNH P26956). The double coracoid foramina appear in only
the left coracoid (Fig. 11C), but the right coracoid exhibits
one regular foramen in the typical spot. The accessor y second
foramen is slightly larger in diameter than the typical anterior
foramen. The coracoid of the holotype of M. conodon (AMNH
1380) is only about four-fif ths complete, but a smooth natural
edge, forming a part of a foramen, indicates the presence of
this second coracoid foramen (arrow in Fig. 1D), which is at a nearly
identical position, as seen in the Field Museum material.
All preserved coracoids of MOR 006 and TSJC 1998.2 show only
a single foramen, indicating intraspecific variation in M. conodon.
Limb bones
The right humerus of the holotype of Mosasaurus conodon
(AMNH 1380) (Fig. 1E), is large and overall very robust.
Fig. 12. Right humerus of M. conodon
(TSJC 1998.2) in (A) anterior,(B) dor-
sal and (C) lateral views. Scale equals
10 cm. Osteological abbreviations are
listed in the text.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
11
The overall size and shape are nearly identical to the humer us of
TSJC 1998.2 (Fig. 12). The two humeri have nearly cubic three-
dimensional shape, with the ratio of 3:2.4:2 of the greatest
height-to-anteroposterior breadth-to-transverse width at the
ectepicondyle. The humerus of TSJC 1998.2 has a well-developed
pectoral crest, as seen in AMNH 1380. The pectoral crest stops at
nearly a quarter of the total length from the proximal end in TSJC
1998.2,whichismuchshorterthanM. hoffmanni (IRSN R12;
Lingham-Soliar, 1995, fig. 20). The anterior and posterior surfa-
ces of the mid-shaf t are strongly constricted, whic h is also similar
to IRSN R12. The entepicondylar process is well-developed in
TSJC 1998.2, which exhibits a hook-like medial end that is
strongly curved, but M. hoffmanni (IRSN R12) has a much-less
developed process.
MOR 006 has nearly complete, articulated forelimbs, which
are relatively short and robust for the genus (Fig. 13). The ulna
is smaller than the radius in Mosasaurus,butthesizediscrep-
ancy in MOR 006 is greater than in other species, such as
M. hoffmanni (NHMM 1993024; Lingham-Soliar, 2000, fig. 21)
and SDSM 452. The radius of MOR 006 has a greatly expanded
distal end. The mid-shaft is strongly constricted, and the least
circumference is at about one-sixth of the total distance distal
from the proximal end, which differs from it being located about
the middle of the shaft as in SDSM 452 and M. missouriensis
(KUVP 1032).
Based on the complete set of tarsals of MOR 006 (M. conodon)
(Fig. 13), the manual formula is the same as in SDSM 452
(Mosasaurus sp.) and NHMM 1993024 (M. hoffmanni). The
proximal end of the intermedium is strongly concave, as found
in SDSM 452, but this expansion is much gentler in M. hoffmanni.
The very elongate pisiform of MOR 006 also morphologically dif-
fers from that of M. hoffmanni (NHMM 1993024; Lingham-Soliar,
2000).
The manual digital formula can distinguish Mosasaurus con-
odon from at least a few other species of Mosasaurus. Although
articulated manus are generally not common in any mosasaurs,
MOR 006 has a nearly complete set except for perhaps four or
five distal phalanges, based on relative size and articulation
of preserved bones. The specimen allows an estimation of
4(+1?)–4(+2?)–4(+1?)–4(+1)–2 in the left manus (Fig. 13;
Appendix 1). Notably, the formula of M. conodon is much
less than that of SDSM 452, which has 9–10(?)–10–10(?)–4
(?) (Martin, 1953). One specimen of M. hoffmanni (NHMM
1993024) exhibits 9–10–10–10–3 (Lingham-Soliar, 1995,
fig. 21). Each of those metacarpals and phalanges in SDSM
452 and M. hoffmanni is relatively shorter than those of
M. conodon.
The eight well-preserved, isolated phalanges of TSJC 1998.2
are relatively short for a mosasaur. The proximal and distal ends
are greatly expanded, as is typically seen in Mosasaurus.Artic-
ular surfaces in the proximal and distal ends are smooth. In
MOR 006, the proximal end is slightly convex, especially on
metacarpals II–IV, although the distal ends are slightly concave
or flat (Fig. 13). Metacarpal I has a large, hook-like corner
on the ventral margin of the proximal end, which appears in
various taxa of Mosasaurini, such as Mosasaurus,Plotosaurus
and Clidastes.Mosasaurus, however, has a much more strongly
constricted mid-shaft than the two other genera. The ventral
margin of the mid-shaft is also more constricted in all elements
of digit I than is seen in digits II–IV in M. conodon,whichalso
occurs in SDSM 452.
The well-preserved tibia has a greatly expanded mid-shaft in
TSJC 1998.2. Both proximal and distal articular surfaces display
a rectangular shape with a shallow concavity, which is in con-
trast to the f lattened surface in Clidastes. The isolated fibula
of TSJC 1998.2 also exhibits heavily built overall structure, with
Fig. 13. Left forelimb of M. conodon (MOR 006). Light grey colour indi-
cates missing portion. Scale equals 10 cm. Osteological abbreviations
are listed in the text.
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
12
a prominent proximal end, which is thought to be a diagnostic
character of M. conodon (Russell, 1967). Some disarticulated
metapodials of TSJC can be separated into either elongate or
short morphotypes. Based on an articulated manus and pes in
SDSM 452, the metatarsals are slightly more elongate than
the metacarpals.
Discussion and conclusions
Despite the incomplete and fragmentary nature of the holotypic
skeleton (AMNH 1380), M. conodon is,wesuggest,anominal
species for two main reasons. First, the elements preserved in
the holotype are enough to allow referral of some other
Table 5. Comparisons of diagnostic morphological features for M. conodon with previous studies.
Cope (1881) Key specimen(s):
AMNH 1380
Russell (1967) SDSM 452, AMNH
1380 and M. lemonnieri
This study AMNH 1380, MOR 006
and TSJC 1998.2
Large body size (about size of
‘Liodon validus’)
Not used Small–medium size Mosasaurus
NA Tuberosity present(?) below stapedial
pit on lower medial body of quadrate
Not included
NA Suprastapedial and infrastapedial
process small in lateral profile
Revised (placed above a third
of the quadrate height)
Narrow splenial angular joint Not included Not included
Slender dentaries Dentary very slender, as in Clidastes,
dorsal and ventral margins converge
gradually anteriorly, element comes
to rounded tip anterior to f irst
dentary tooth
Adapted from Russell
16 (plus two or three more)
teeth on dentary
17 teeth on dentary Revised (16 teeth on dentary)
NA 15 teeth on maxilla Revised (14–15 teeth on maxilla)
NA 10 teeth on pterygoid Revised (8 teeth on pterygoid)
NA Ventral wing of coronoid well
developed on medial surface of lower
jaw
Not included
Subcircular teeth (in cross-section) Same as Cope Adapted from Cope
Bicarinate middle teeth Not included Uncertain if autapomor phy of the
species
Teeth not faceted and smooth enamel External f acets narrow and more
numerous than in M. maximus
Adapted from Cope & Russell
Anterior teet h have only anterior
carina
Not included Adapted from Cope (uncertain if
autapomorphy of the species)
NA Marginal teeth slenderer than in M.
missouriensis, M. maximus and M.
ivoensis, tips posteriorly recurved
Adapted from Russell
Coracoid has ‘deep fissure’extending
toward foramen
Not included Not included
Humerus wider than long
(height:width = c. 1.0)
Same as Cope Height:width = 3:2
Eight cervicals? Not included Excluded
The presence of zygosphenes
zygantra
Not included Excluded (synapoporphy in
Mosasaurini?)
Nearly circular vertebral articulations Not included Excluded
Chevron co-ossified with caudal
vertebra
Not included Excluded (synapomorphy in
Mosasaurinae)
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13
specimens to the same species. Second, the holotype and the
newly described specimens we here refer to M. conodon,espe-
cially MOR 006 and TSJC 1998.2, allow a review of previous di-
agnostic characters (Table 5) and thereby reinforce the
diagnosis. These emended diagnostic characters can distin-
guish M. conodon from other species of Mosasaurus,suchas
M. lemonnieri,M. hoffmanni–M. maximus and M. missouriensis,
as well as SDSM 452, as discussed below.
M. conodon differs from M. lemonnieri,assuggestedby
Lingham-Soliar (2000). Considerable differences appear in
the tooth morphology, the tooth count in the maxilla, dentary
and pterygoid (Table 3), and the position of the infrastapedial
process in the quadrate. M. conodon displays ver y smooth
tooth surfaces (no facets) and developed carinae without any
serrations, as are also suggested to be diagnostic characters of
Clidastes (Russell, 1967), which indicate plesiomorphic features
in the lineage of Mosasaurini. In contrast, M. lemonnieri pos-
sesses well-developed facets and developed serrate carinae.
Lingham-Soliar (2000) suggested these developed facets and
serrations might occur during ontogeny in M. lemonnieri,
without presenting specific data on juvenile specimens.
Notably, the two specimens described above, AMNH 1380 and
TSJC 1998.2, are fairly large for M. conodon (Table 1), and
thepresenceofsomefusedcranialbonesandrugosearticular
surfaces in appendicular bones indicates fully-grown individuals.
Thus, comparisons of tooth morphology with other species of
Mosasaurus should be relevant for taxonomic assignment.
M. hoffmanni–M. maximus can be differentiated from
M. conodon by tooth morphology, especially the presence of
well-developed serrations on the carinae. Their teeth are also
morphologically different from those of M. conodon in cross-
section. The two large species show a U-shaped cross-section
instead of the transversely compressed, oval-shaped outline
in M. conodon.TheNorthAmericanM. maximus tends to possess
well-developed facets on the entire tooth surface. Some very
large specimens assigned to M. hoffmanni,includingthe
holotype, exhibit smooth tooth surfaces, as seen in M. conodon
(personal observation in MNHNAC9648). Besides tooth mor-
phology, there is an apparent difference body size. M. conodon
is at least 20% smaller than one of the largest known specimens
of M. maximus and M. missouriensis (based on skull length;
Table 1). In M. conodon, the posteroventral process of the jugal
is located higher on the vertical ramus, and the quadrate is small
relative to the overall skull and jaw size; the ratio of quadrate
(dorsoventoral height)-to-dentary (anteroposterior length) is
0.19 in M. conodon and 0.23 in M. maximus (Table 1). M. conodon
also exhibits a smaller manual phalangeal formula (Table 3) and
a box-shaped humerus with a well-developed entepicondyle crest
that is more heavily built than in M. hoffmanni–M. maximus.
The new Colorado and Montana specimens indicate that
the nearly complete mounted skeleton, SDSM 452, should not
be referable to M. conodon based on the following features
found in the South Dakota specimen: a higher tooth count in
the pterygoid, a higher position of the infrastapedial process
in the quadrate, gradually expanded splenial and ventral
margin of the posterior dentary and a higher number of the
manual digital formula (Fig. 13). The species-level taxonomic
identification of SDSM 452 is diff icult primarily due to the large
amount of plaster reconstruction in the skull region (Martin,
1953). For now, we suggest that Mosasaurus sp. is a reasonable
option for identification of SDSM 452, following Bell (1997).
The diagnosis of Mosasaurus missouriensis is currently not
well understood, mainly due to the incomplete nature of the
holotype. If KUVP 1034 (a fairly complete, well-preserved skull)
is assigned to this species as suggested by Bell (1997), this species
can be separated from M. conodon by fewer teeth in the pterygoid,
a smaller number of the manual digital formula, a smooth tooth
surface and a robust humerus.
Stratigraphically, M. conodon has one of the oldest records
within the clade Plotosaurini. A number of specimens listed
above demonstrate that M. conodon occurs in upper Campanian
to lower Maastrichtian strata. TSJC 1998.2 is one of the oldest
known specimens in the species, which is estimated to be strati-
graphically in the late Campanian Baculites cuneatus/compressus
Biozone (c. 74 Ma; Ogg et al., 2012). There are difficulties in
determining the precise stratigraphic levels of AMNH 1380
and MOR 006, although they must occur in late Campanian
to early Maastrichtian strata (Gallagher, 1993; P. Leiggi, written
commun., 2002), more precisely ranging through the Exiteloceras
jenneyi (late Campanian) and Baculites eliasi (early Maastrichtian)
ammonite zones (Gill & Cobban, 1973; Rice & Shurr, 1983). In the
Gulf Coastal Plain, a number of specimens assigned to M. conodon
are known only from the Demopolis Chalk, which is late Campanian
to early Maastrichtian in age (Raymond et al., 1988; Ikejiri et al.,
2013). This leads to the conclusion that M. conodon is from a dif-
ferent time interval to M. maximus in North America (Russell,
1967; Gallagher, 1993), as, to date, the two species have not
yet been found in the same stratigraphic unit.
The European M. lemonnieri is mainly known from the late
Maastrichtian age (e.g. the upper Maastrichtian Opoka of
central Poland and upper Maastrichtian formations of the
Netherlands; Lingham-Soliar, 2000; Machalski et al., 2003).
If M. conodon and M. lemonnieri are phylogenetially closely
related, as suggested by Lingham-Soliar (2000), the divergence
of this clade to Europe might have happened before the
Maastrichtian. To test this hypothetical scenario, a cladistic
analysis including various taxa of Mosasaurini and detailed
data of their stratigraphic occurrences will be needed.
To date, populations of M. conodon are palaeogeographically
restricted to North America, including the Western Interior
Seaway (the most nor therly record in Phillips County, north-
eastern Montana), the Gulf Coastal Plain and the Mississippi
Embayment areas (Russell, 1967; Kiernan, 2002; Ikejiri et al.,
2013), and the Atlantic Seaboard area in New Jersey (Gallagher,
1993, 2002). However, some relatively small species of
Mosasaurus from outside of North America, which are
Netherlands Journal of Geosciences —– Geologie en Mijnbouw
14
established by mostly incomplete, fragmentary skeletons, are
known, such as M. hobetsuensis and M. prismaticus from Japan
(Suzuki, 1985a,b; Sakurai et al., 1999; Tanimoto, 2005),
M. beaugei from Morocco and Syria (Bardet et al., 2004) and
M. mokoroa from New Zealand (Welles & Gregg, 1971). Hopefully,
the specimen-based study presented here will increase our
knowledge of osteological information and help clarify some
taxonomic problems and the phylogenetic relationships of various
species of Mosasaurus.
Acknowledgments
We thank Allen Peterson, who allowed one of us (TI) to excavate
TSJC 1998.2 and who donated the specimen. TI also thanks Loretta
Martin and Roy Rankin at TSJC for their support in the excavation
of TSJC 1998.2. John R. Horner (MOR) gave us the opportunity to
describe MOR 006. We also appreciate valuable discussion and
comments from Gorden Bell Jr., Michael Everhart, T. Lynn Harrell
Jr., Johan Lindgren and Mike Polcyn. Patrick Leiggi (MOR)
checked information of the fossil site of MOR 006 for us. Brooks
Britt and Rodney Sheets allowed TI to prepare the specimen
(TSJC1998.2) at the fossil laboratory of the Museum of Western
Colorado. Kenneth Carpenter and Jeffrey Martz helped to identify
some elements of TSJC 1998.2. The original map used for Fig. 2 was
made by Sandy Ebersole. Special thanks are due to the curators
and collection managers at the following institutions (abbrevia-
tions listed in the text) that granted access to specimens: AMNH,
Institut f¨
ur Pal¨
aontologie (Der Universit¨
at Bonn), FMNH, FHSM,
KUVP, MOR, MSC, NHMM, NJSM, SDSM, TMM, TSJC, UAM, UNNM,
USNM and YPM. Richard Zakrzewski, Anne Schulp and anonymous
reviewers provided constructive comments on an earlier draft of the
manuscript. We thank Mike Polcyn for the invitation to contribute to
this symposium volume.
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