New data on cranial anatomy of the ceratopsian dinosaur
Psittacosaurus major
HAIâLU YOU, KYO TANOUE, and PETER DODSON
You, H.âL., Tanoue, K., and Dodson, P. 2008. New data on cranial anatomy of the ceratopsian dinosaur
Psittacosaurus
major
.
Acta Palaeontologica Polonica
53 (2): 183â196.
An exceptionally preserved skull and mandible of ceratopsian dinosaur
Psittacosaurus major
revealed many anatomical
details such as the existence of an elliptical median interpremaxillary foramen, a prominent neurovascular canal on the inâ
ternal wall of the beak, long, slightly divergent basipterygoid processes developed as vertical blades with a deep cleft beâ
tween them, and horizontally oriented vomer. The new specimen shows two autapomorphies of
Psittacosaurus major
, the
transversely narrow dorsal skull roof and very prominent dentary flanges, confirming the presence of two largeâskulled
psittacosaur species in the Lujiatun Bed of the Lower Cretaceous Yixian Formation in Beipiao City, western Liaoning
Province, China, the longâ and narrowâskulled
P. major
, and broadâskulled
P. lujiatunensis
.
K e y w o r d s : Dinosauria, Ceratopsia,
Psittacosaurus
, Cretaceous, Yixian Formation, Liaoning, China.
HaiâLu You [youhailu@gmail.com], Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang
Road, Beijing 100037, P.R. China;
Kyo Tanoue [tanoue@sas.upenn.edu], Department of Earth and Environmental Science, University of Pennsylvania,
240 S. 33rd Street, Philadelphia, PA 19104â6316, USA;
Peter Dodson [dodsonp@vet.upenn.edu], School of Veterinary Medicine and Department of Earth and Environmental
Science, University of Pennsylvania, 3800 Spruce Street, Philadelphia, PA 19104â6045, USA.
Introduction
Psittacosaurus
Osborn, 1923 is a genus of small bipedal
ornithischian dinosaur from the Early Cretaceous of eastern
Asia, especially Mongolia and China. It is one of the less celeâ
brated fruits of the famed American Museum of Natural Hisâ
tory Central Asiatic Expeditions of the 1920s. It was named by
Osborn in 1923 (Osborn 1923), but it was not until the seminal
work of MaryĂĄnska and OsmĂłlska in1975 (MaryaĆska and
OsmĂłlska 1975) that
Psittacosaurus
gained unequivocal
membership as a basal member in the clade Ceratopsia by virâ
tue of possessing a rostral bone. By 1990, more than 115 specâ
imens were recognized in major collections (Sereno 1990a),
and this number has been augmented in the past decade by litâ
erally scores of specimens from Liaoning Province, China.
One specimen includes the skeletons of 34 babies along with
one adult (Meng et al. 2004).
Psittacosaurus
is one of the most
common of all dinosaurs (Dodson 1990). It would seem that
its very commonness has bred contempt of sorts, and only reâ
cently has this genus drawn serious scientific attention, beginâ
ning with the Ph.D. dissertation of Paul Sereno in1987. Until
that time, there was a tendency to synonymize additional Chiâ
nese species of
Psittacosaurus
(i.e.,
Psittacosaurus osborni
Young, 1931;
P. tingi
Young, 1931;
P. sinensis
Young, 1958;
P. youngi
Zhao, 1962; and
P. guyangensis
Cheng, 1982) with
the type species,
P. mongoliensis
Osborn, 1923. Sereno (1987,
1990a, b) carefully reviewed all
Psittacosaurus
material deâ
scribed to date and concluded that of the historical âspeciesâ,
P. mongoliensis
and
P. sinensis
were undoubtedly valid,
P.
youngi
is a synonym of
P. sinensis
, and all other nominal speâ
cies are synonyms of the type species,
P. mongoliensis.
Sereno
himself described two new species,
P. xinjiangensis
(Sereno
and Chao 1988) from northwestern China and
P. meileyinâ
gensis
(Sereno et al. 1988) from northeastern China. A numâ
ber of new species have followed:
P. neimongoliensis
and
P. ordosensis
both from Inner Mongolia (Russell and Zhao
1996);
P. mazongshanensis
from Gansu (Xu 1997);
P. sibiâ
ricus
from West Siberia (Leschinskiy et al. 2000; Averianov et
al. 2006);
P. lujiatunensis
from Liaoning (Zhou et al. 2006);
and
P. major
from Liaoning (Sereno et al. 2007). Thus the
geographic range of
Psittacosaurus
extends for more than
2000 km from east to west, and speciation is no more improbaâ
ble over such a wide span of both geography and climatic conâ
ditions for
Psittacosaurus
than it would be for any vertebrate,
especially a small one. Moreover, Lucas (2006) has posited
that the temporal range of
Psittacosaurus
species is approxiâ
mately 20 million years from the Barremian to the Albian
stages of the Early Cretaceous. This is approximately three
times the average duration of a genus of dinosaur (Dodson
1990), again suggesting the probability of multiple species (if
not genera) of
Psittacosaurus
. Granted that not all of the
named species are necessarily valid, it appears that a stronger
case of multiple congeneric species can be made for
Psittacoâ
saurus
than for almost any other dinosaur.
Another problem that has plagued the study of
Psittacoâ
saurus
is poor preservation compounded by lack of thorough
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Acta Palaeontol. Pol.
53 (2): 183â196, 2008
preparation consistent with the perceived commonness and
lack of value of individual specimens. It is not clear if the enâ
tire rich collection of specimens at the American Museum of
Natural History includes a single completely prepared craâ
nial specimen. Sereno (e.g., 1987, 1990a, b; Sereno and
Chao 1988; Sereno et al. 1988) did not illustrate details of the
palate and braincase but often showed the various apertures
through which these structures might be observed. Likewise,
descriptions allude to incomplete preparation and lost parts
(Russell and Zhao 1996; Xu 1997). Ideally, skulls should
contain no sediment inside, and the jaws should be removed
from their natural articulation, permitting full access to the
palate. Anything less constitutes incomplete preparation.
It is with some trepidation that we describe new specimen
CAGSâIGâVDâ004 of
Psittacosaurus major
Sereno, Zhao,
Brown, and Tan, 2007 from the Lujiatun Bed of the Yixian
Formation in the Lujiatun region of Beipiao City, Liaoning
Province, northeastern China. However, the exceptional preâ
servation and excellent preparation of the specimen seem to
make this project worthwhile inasmuch as certain anatomical
details are revealed for the first time. New discoveries of very
large
Psittacosaurus
skulls (length 190 mm or longer) both
from the Lujiatun region and from Sihetun now number at
least six specimens, none of which were known prior to 2006
(Zhou et al. 2006; LĂŒ et al. 2007; Sereno et al. 2007; H.âL.
You, K. Tanoue, and P. Dodson, unpublished data).
Institutional abbreviations
.âCAGSâIG, Chinese Academy
of Geological Sciences, Institute of Geology, Beijing, China;
IVPP, Institute of Vertebrate Paleontology and Paleoanthroâ
pology, Chinese Academy of Sciences, Beijing, China; LH,
Long Hao Institute of Geology and Paleontology, Hohhot,
China.
Description
The skull is large, mature, and very well preserved (Figs. 1,
2). It is complete in nearly every detail, excepted as noted in
the description that follows. The skull is, however, somewhat
sheared laterally, causing it to tilt to the right, as the right
quadrate and cheek are displaced towards the midline. The
left side is somewhat better preserved than the right. The
skull measures 192 mm in basal length from the rostral to the
occipital condyle, and 189 mm from the rostral to the left
quadrate. It is thus about 25% larger than typical skulls of
Psittacosaurus mongoliensis
(Sereno 1987) and similar in
size to the recently described
P. lujiatunensis
(Zhou et al.
2006),
P. sibiricus
(Averianov et al. 2006), and
P. major
(Sereno et al. 2007). Its appearance is typical of every skull
of
Psittacosaurus
, with a high, short face, large orbits and
temporal fenestrae, incipient parietal frill and prominent, latâ
erallyâflaring jugal horn cores. Features that draw attention
are relatively large nares, circular on the left side but elliptiâ
cal on the right, very prominent, laterally flaring palpebrals,
and an infratemporal fenestra that is much narrower ventrally
than dorsally. In some skulls of
Psittacosaurus,
the breadth
of the skull across the jugals is equal to or greater than the
length of skull (e.g.,
P. sinensis
,
P. lujiatunensis
); this is not
true of the present specimen, even allowing for the effect of
crushing (Fig. 1A).
Dermal skull roof
.âThe rostral bone, whose existence was
tentatively acknowledged by Romer (1956) but generally
doubted until MaryaĆska and OsmĂłlskaâs (1975) study, is
seen with exceptional clarity on this specimen. It is a tall eleâ
ment capping the premaxillae and forming almost half of the
cutting edge of the edentulous beak in lateral view (Fig. 2A).
The tip of the rostral is not deflected downward as it is modâ
estly in
P. mongoliensis
and in most species of
Psittacosaurus
,
or strongly pendent as it is in
P. sinensis
, but is nearly straight,
and in any case well above the level of the maxillary teeth. In
great contrast to Neoceratopsia, the front of the rostral is broad
and smoothly rounded. The bone tapers dorsally almost to a
point both in lateral and in rostral views, and likely contacts
the nasals, although there is a gap of several mm now. The exâ
ternal surface of the rostral, particularly in its ventral half,
shows a texture characterized by small vascular foramina, but
the tapering dorsal half is smoothly textured. The prominent
rostralâpremaxillary suture is nearly vertical and widely open;
the ventral last centimeter deflects caudoventrally to meet the
cutting edge. In palatal view, a thin sharp edge is seen defining
a horseshoeâshaped curve reminiscent not of a parrotâs beak
but of a gooseâs. Internally the rostral arches sharply dorsally
to meet the palatine process of the premaxilla.
The premaxilla in lateral view has the form of a broad plate
that fills most of the face. It has a long, caudodorsallyâsloping
contact with the maxilla, broad contact with the lacrimal, and
defines the ventral half of the external naris. As in other speâ
cies of
Psittacosaurus,
a deep horizontal groove in the
premaxillaryâmaxillary suture at the level of the maxillary
ridge marks the beginning of an irregular, meandering groove
in the premaxilla that trends rostrodorsally to the level of the
tip of the rostral, believed by some to represent the caudal edge
of a horny sheath covering the beak (Sereno et al. 1988; Zhou
et al. 2006). On the right side of the specimen, a broad,
smooth, depressed area runs caudodorsally from the meanderâ
ing groove to the external naris that is greater in area than the
naris itself. It is not evident on the left side. Evidence for the
lacrimal canal, as emphasized by (Sereno et al. 1988) is clear.
On the left side, the surface of the premaxilla is somewhat
fractured, and any unossified zones are not clearly demarâ
cated; on the right side an irregular fractured zone on the
premaxilla trends rostroventrally from the level of the lacrimal
foramen for 20 mm. This may be an artifact, but it certainly exâ
poses the nasolacrimal canal, which is filled with red sediâ
ment. Below the rostral end of the canal is a large foramen, and
unrelated to it, is a large 5 mm wide foramen, found only on
the right side. Unreported in other species, we believe it is an
artifact. Extending 40 mm rostrally on a line with the external
maxillary ridge is the cutting edge of the beak, formed in latâ
eral view by equal contributions from the premaxilla and the
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ACTA PALAEONTOLOGICA POLONICA 53 (2), 2008
rostral. The cutting edge of the premaxilla is thicker caudally
and thins somewhat rostrally where it contacts the cutting edge
of the rostral. The premaxilla arches strongly dorsally 12 mm
to form the secondary palate, but fails to meet its fellow on the
midline except right at the rostral suture. A prominent elliptiâ
cal median interpremaxillary foramen is formed that measures
10 mm in length and 3 mm in breadth (Figs. 1C, 3A). Alâ
though also seen in
Hongshanosaurus
(IVPP V 12617) and in
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YOU ET AL.âCRANIAL ANATOMY OF
PSITTACOSAURUS
185
squamosal
parietal
frontal
prefrontal
nasal
palpebral
premaxilla
postorbital
jugal
frontal
squamosal
postorbital
jugal
premaxilla
nasal
palpebral
prefrontal
maxilla
rostral
lacrimal
exoccipital
quadrate
quadratojugal
quadrate
exoccipital
jugal
maxilla
rostral
premaxilla
basioccipital
pterygoid
50 mm
Fig. 1. Ceratopsian dinosaur
Psittacosaurus major
Sereno, Zhao, Brown, and Tan, 2007, new cranial specimen CAGSâIGâVDâ004 from the Early Cretaceous
Lujiatun Bed of Yixian Formation near Beipiao City, Liaoning Province, China, in dorsal (
A
), left lateral (
B
), and ventral (
C
) views. Photographs (A
1
, B
1
, C
1
)
and interpretive outlines (A
2
, B
2
, C
2
).
a juvenile specimen of
P. lujiatunensis
(CAGSâIGâVDâ005),
this foramen seems to have escaped detection until now. Each
premaxilla meets the rostral in a gentle, rostrallyâconvex curve
anchored at the midline.
The maxilla is a triangular bone whose base constitutes the
toothrow, and whose apex over the middle of the toothrow
reaches a near fourâway junction with the premaxilla, lacrimal
and jugal (Fig. 1B). The apex of the maxilla appears to taper to
a point rather than having a trapezoidal form as in
P. lujiatuâ
nensis
,
P. neimongoliensis
, and
P. meileyingensis
(Zhou et al.
2006). It is not, however, low, as the apex is slightly above the
ventral border of the orbit. The maxilla has a prominent deâ
pression dorsal to the horizontal external maxillary ridge that
defines the dorsal edge of the deep buccal emargination. The
depression, also prominent in
P. mongoliensis
and
P. lujiatuâ
nensis
, does not have the triangular form characteristic of that
of
P. mongoliensis,
(resembling instead the impression of a
finger pressed into heavy clay), and is emphatically not to be
considered as an antorbital fossa, which has been lost in
psittacosaurs (Sereno 2000).
186
ACTA PALAEONTOLOGICA POLONICA 53 (2), 2008
frontal
squamosal
jugal
premaxilla
nasal
palpebral
prefrontal
maxilla
rostral
exoccipital
-opisthotic
quadrate
quadratojugal
rostral
maxilla
premaxilla
nasal
prefrontal
frontal
laterosphenoid
squamosal
parietal
postorbital
parasphenoid
postorbital
basioccipital
pterygoid
basisphenoid
quadrate
pterygoid
quadrate
squamosal
jugal
maxilla
rostral
quadratojugal
parietal
exoccipital
basioccipital
pterygoid
basisphenoid
supraoccipital
50 mm
palpebral
Fig. 2. Ceratopsian dinosaur
Psittacosaurus major
Sereno, Zhao, Brown, and Tan, 2007, new cranial specimen CAGSâIGâVDâ004 from the Early Cretaceous
Lujiatun Bed of Yixian Formation near Beipiao City, Liaoning Province, China, in right lateral (
A
), rostral (
B
), and caudal (
C
) views. Photographs (A
1
, B
1
, C
1
)
and interpretive outlines (A
2
, B
2
, C
2
).
The maxillaryâjugal suture runs directly ventrally from
the apex of the maxilla until it reaches the external maxillary
ridge, then it bends abruptly to run caudoventrally, as the
jugal overlaps the maxilla for several centimeters until the
latter terminates. At the bend, there is a slight but not espeâ
cially noteworthy blunt prominence; the same site is highly
prominent in
P. mongoliensis
and other species, but it lackâ
ing in
P. sinensis
and
P. neimongoliensis
(Zhou et al. 2006).
Our specimen appears to agree with the latter two species in
this character. There is a series of prominent ventrallyâdiâ
rected neurovascular foramina beneath the external maxilâ
lary ridge; these number four on the left side and three on the
right side. The caudal foramen is the largest of these, measurâ
ing 5 or 6 mm in long axis. In palatal view, strong preâ
maxillary processes of the maxillae join the secondary palate
in fused symphysis behind the premaxillae. The curved preâ
maxillaryâmaxillary suture is visible, but the flat intermaxilâ
lary suture, extending for 19 mm caudal to the premaxilla, is
not. On the internal wall of the beak, rostrodorsal to the first
maxillary tooth, a prominent neurovascular canal, probably
carrying terminal branches of the maxillary branch of the
trigeminal nerve and maxillary branches of the carotid artery,
passes between the premaxillary process of the maxilla and
the premaxilla itself (Fig. 3A). The size of the foramen sugâ
gests that the oral mucosa of the beak was well supplied and
possibly fleshy. The medial surface of the maxilla is flat and
high. Caudally between the pterygoid symphysis and a cauâ
dally convex suture between the maxilla and mandibular
ramus of the pterygoid, there is a small cleft in the maxilla
measuring about 6 mm in length for a neurovascular foraâ
men. The position is close to that of the palatine foramen,
identified in basal neoceratopsians as an opening for palatine
branches of the maxillary artery and nerve (Brown and
Schlaikjer 1940). Ventral to the palatine foramen the maxilla
overlaps the pterygoid with a shallow convex tab. For an alâ
ternate description, see the palatine below.
The lacrimal is typical of that of psittacosaurs, and forms
the rostral border of the orbit and prominently displays the
orbital opening of the nasolacrimal duct. It is not especially
well preserved in the present specimen, although it is better
on the left than on the right side. It is overlapped on its rostral
edge by the premaxilla.
The nasal forms the rostromedial part of the skull roof,
and then flexes rostroventrally above the caudal end of the
external naris to form the internarial bridge. The nasal forms
the dorsal border of the external naris and continues rostroâ
ventrally past the naris to meet or almost meet the rostral as a
slender, tapering rostral process. The rostral process of the
nasal appears to sandwich the premaxillae, uniquely barely
visible on the midline from the external naris down to the
rostral. The external nares are large, nearly circular on the
left side and measure 23 mm by 20 mm in horizontal and verâ
tical axes respectively, and are elliptical on the right side,
measuring 26 mm by 15 mm in long and short axes, respecâ
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YOU ET AL.âCRANIAL ANATOMY OF
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187
maxilla
premaxilla
rostral
parietal
postorbital
squamosal
frontal
palpebral
laterosphenoid
exoccipital-
opisthotic
basioccipital
parietal
basisphenoid
jugal
pterygoid
pterygoid
parasphenoid
20 mm
20 mm
Fig. 3. Ceratopsian dinosaur
Psittacosaurus major
Sereno, Zhao, Brown, and Tan, 2007, new cranial specimen CAGSâIGâVDâ004 from the Early Cretaâ
ceous Lujiatun Bed of Yixian Formation near Beipiao City, Liaoning Province, China, in palatal view (
A
) (with arrows indicating the neurovascular openâ
ings in the secondary palate formed by the rostralâpremaxillaâmaxilla complex), and right lateral and ventral view (
B
) of the caudodorsal portion of the skull,
showing the internal (ventral) surface of the frontal and the quadrate cotyla of the squamosal. Photographs (A
1
, B
1
) and interpretive outlines (A
2
, B
2
).
tively. Due to the maturity of the specimen, the internasal and
nasalâfrontal sutures are hard to discern. In many specimens
of
Psittacosaurus
(e.g., Sereno et al. 1988; Russell and Zhao
1996; Zhou et al. 2006), there is a swelling or tumescence of
either the nasals or prefrontals on the skull roof between the
orbits and the external naris, and the abrupt narrowing of the
snout at the level of the caudal end of the external nares coinâ
cides with the nasal flexure. Consequently, psittacosaurs
have a bluntânosed appearance, perhaps most extreme in
P.
lujiatunensis
(Zhou et al. 2006) and
P. sibiricus
(Averianov
et al. 2006). In the present specimen there is little if any such
swelling, and the skull roof tapers smoothly from the rostroâ
dorsal region of the nares to the internarial bridge, as in
P.
majo
r (Sereno et al. 2007). The nasals are comparatively narâ
row, and are squeezed caudally between the prefrontals. The
combined width of the nasals at the narrowest (10 mm) is less
than that of either prefrontal measured at the same point (14
mm left, 15 mm right). This is in great difference from
P.
lujiatunensis
, in which the nasals are twice as wide at the preâ
frontals (Zhou et al. 2006).
The prefrontal is a somewhat Lâshaped bone, forming the
rostrodorsal rim of the orbit on the skull roof, and then bendâ
ing rostroventrally onto the side of the face to meet the lacriâ
mal and premaxilla. The prefrontals taper caudolaterally and
terminate in the middle of the dorsal rim of the orbit.
The triangular palpebrals are very prominent in this speciâ
men, articulating broadly with the prefrontal at the rostroâ
dorsal corner of the orbit. In dorsal view, the palpebral projects
directly caudolaterally; for the better preserved left palpebral,
the length of the base is 23 mm and the height of the apex of
the triangle is 26 mm. The rostral edge of the palpebral is
rolled ventrally into a flange at right angles to the body of the
bone, so that viewed rostrally, the bone is also triangular, meaâ
suring 15 mm in thickness at its base (on the right side), taperâ
ing to 5 mm in thickness at the apex. The triangular flange eviâ
dently necessitates lateral flaring of the palpebral, causing efâ
fective shielding of the corner of the eyeball at least.
The frontal is a large central element of the skull roof that
forms the caudodorsal part of the rim of the orbit. It extends
from the nasals and prefrontals rostrally to the parietal and
postorbital caudally. The frontal measures 33 mm in breadth at
its narrowest between the orbits. Its length is less certain due to
the problem of determining its rostral and caudal limits, but
appears to be about 55 mm. The caudolateral part of the frontal
forms part of the shallow rim that marks the rostral end of the
supratemporal fenestrae. The interfrontal suture cannot be deâ
tected, but a low median ridge plainly reveals its position. The
central region of the frontal is shallowly depressed relative to
the lateral edge, and the frontal reaches its highest relief (of 3
or 4 mm) caudolaterally at the thickened caudodorsal edge of
the orbit, where it contacts the postorbital.
The internal (ventral) surface of the frontal clearly shows
the hourglassâshaped trace of the rostral regions of the brain,
defined by scars that must trace the attachment of a cartilagiâ
nous orbitosphenoid (Fig. 3B). The scar is 9.5 mm wide at its
narrowest, nearly twice the width reported by Russell and
Zhao (1996) for the same measurement in
P. neimongoâ
liensis
(4.7 mm), the skull of which measures 132 mm in
length. The frontal is thickest over the brain (ca. 9.6 mm),
and tapers to a thin edge at the orbital rim (3 mm), the bevel
conforming somewhat to the globe of the eyeball.
The bony orbit is very large, measuring 55 mm in length
and 61 mm in height. The eyeball itself must have been only
a modest fraction of that size due to the narrowness of the
interorbital space. The distance between the dorsal orbital
rim and the orbitosphenoid scar on the frontal bone is only 13
mm, and the distance between the ventral rim of the orbit and
the palatine is about 22 mm. Thus, the maximum conceivable
diameter of the eyeball would be 44 mm, assuming a ventral
position in the orbit; the minimum 26 mm; and the reasonâ
able intermediate estimate, placing the eyeball more or less
in the middle of the orbit, is 35 mm, leaving a generous space
of roughly 10 mm all around the eyeball for the adnexa. Put
another way, the eyeball fills only about 35% of the area of
the orbit. If allowance be made for crushing of the skull, the
dorsal measurement is unaffected but the ventral measureâ
ment might be increased to 30 mm, allowing for an eyeball
60 mm in diameter, which is too great for the horizontal meaâ
sure of the orbit. The intermediate value is 43 mm. In this
case, the eyeball occupies 55% of the surface area of the orâ
bit, and leaves a rim of 6 to 9 mm around the socket. Such an
eyeball would have been somewhat exophthalmic, especially
dorsally, giving its bearer a slightly bugâeyed appearance.
The parietal is the most caudal element of the central skull
roof. It bounds the supratemporal fenestrae both medially and
caudally, and along with the frontal and postorbital, particiâ
pates in the formation of the rostral boundary of the fenestra.
The orientation of the parietal is predominantly horizontal, alâ
though the sagittal crest is convex dorsally and in lateral view
stands above the temporal bar formed by the postorbital and
squamosal. The parietal fans out caudally into a frill that overâ
hangs the occiput. Although all psittacosaurs show this frill, it
is particularly strongly developed in the present specimen,
with a length (measured parasagittally because of the sagittal
emargination) of 30 to 33 mm, and a breadth of 69 mm. The
gently biconvex caudal margin of the frill has a cardioid shape
and is strongly emarginated on the midline. Moreover, and apâ
parently uniquely among psittacosaurs, the caudal edge of the
parietal has the form of a thickened bar, contrasting with the
extremely thin web of the parietal fan. The bar is thicker on the
right side than on the left, and measures up to 9 mm in thickâ
ness. These precocious characters are reminiscent of the frills
of certain Canadian Late Cretaceous centrosaurine (Dodson
and Currie 1990) and chasmosaurine (Godfrey and Holmes
1995) ceratopsids, albeit in miniature. The caudal edge of the
frill is horizontal in caudal view. The supratemporal fenestrae
are large and elliptical, and their long axes are parallel to the
long axis of the skull. The left and right supratemporal feneâ
strae measure 61 mm and 57 mm in length, and 34 mm and
34 mm in width, on the left and right sides, respectively.
The postorbital is a threeâpronged bone that contributes to
the skull roof, to the temporal bar, and to the postorbital
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ACTA PALAEONTOLOGICA POLONICA 53 (2), 2008
barâjugal bar that separates the orbit from the infratemporal
fenestra. The postorbital achieves little presence on the skull
roof, as the caudolateral corner of the frontal apparently occuâ
pies a more lateral position than in the other species of
Psitâ
tacosaurus
(Sereno et al. 1988; Russell and Zhao 1996; Zhou
et al. 2006). However, there is a modest prominence above the
caudodorsal corner of the eye where the postorbital meets the
frontal, and the orbital rim is 12 mm thick here. The postâ
orbitalâjugal bar is surprisingly thin, only 8 mm wide in lateral
view opposite the most rostral extent of the infratemporal
fenestra, but twice that figure ventrally. The jugal process of
the postorbital is long and slender, overlapping the jugal and
forming most of the caudal border of the orbit. There is a modâ
est smooth eminence on the postorbital near its lower end, no
doubt marking the position where the elaborate jugal âhornâ
(more reasonably, jugal eminence) of
P. sinensis
begins (Seâ
reno et al. 1988). There is also a laterallyâdirected modest
prominence in the middle of the threeâway junction of the
three prongs of the postorbital, rostrodorsal to the adjacent
corner of the infratemporal fenestra, but no ridges run either
caudally or ventrally from this eminence as in other species
(Sereno et al. 1988; Zhou et al. 2006). The squamosal process
of the postorbital overlaps the postorbital process of the squaâ
mosal and forms the entire dorsal border of the infratemporal
fenestra in lateral view, as expected.
The squamosal provides the cotyla for the dorsal end of the
quadrate, contributes to the temporal bar separating the tempoâ
ral fenestrae, forms much of the lateral border of the supraâ
temporal fenestra and also the caudolateral corner, and anâ
chors the caudolateral edge of the parietal frill. A stout mediâ
allyâdirected process marks the squamosal contribution to the
parietal frill, although the parietalâsquamosal suture cannot be
determined. The entire frill is situated rostral to the dorsal end
of the quadrate, and a slight dorsal orientation of the medial
process of the squamosal elevates the caudal margin of the frill
above the level of the temporal bar, making it visible in lateral
view. The rostrodorsal end of the quadrate is well supported
by the squamosal, which sends a tapering process ventrally
down the rostral edge of the quadrate. The process is fanâ
shaped proximally, and is supported by the pterygoid wing of
the quadrate. The quadrate process extends ventrally for 22
mm and then is apparently truncated by a fracture. There is a
25 mm hiatus between the ventral process of the squamosal
and the dorsal process of the quadratojugal. It is possible that
they contacted each other as in
P. lujiatunensis
, but this cannot
be determined. The squamosal terminates caudally as a broad,
thin tab applied to the rostrolateral surface of the paroccipital
process of the exoccipital. On the right side of the specimen,
the cheek unit, consisting of the quadrate, quadratojugal and
jugal have been removed, exposing the quadrate cotyla of the
squamosal with exceptional clarity (Fig. 3B). It is clear that the
caudolateral orientation of the planar surface so formed proâ
vides little restriction or provides little caudal bracing for the
head of the quadrate.
The jugal is one of the most distinctive bones of the entire
psittacosaur skull. It runs from the maxilla and lacrimal
rostrally to the quadratojugal caudally, it forms the ventral
borders of the orbit and the infratemporal fenestra, and it
bears the ventrolaterallyâflaring jugal horn that is the hallâ
mark of ceratopsians generally and of
Psittacosaurus
specifiâ
cally. The lateral flare of this specimen is comparatively
modest. The width between jugal horns is estimated at 160
mm, only about 85% of basal skull length; in
P. sinensis
,
P.
lujiatunensis
, and
Hongshanosaurus
the width of the skull
exceeds skull length (Sereno 1990a; Zhou et al. 2006; You
and Xu 2005). The jugal overlaps the maxilla in lateral view
along the external maxillary ridge as far caudally as the distal
end of the toothrow, and then it forms a lateral flange that
overhangs the mandible, terminating in the jugal horn. The
jugal is modestly thickened at the suborbital margin (5.5
mm), at the rostral base of the lateral flange (7 to 8 mm), and
along the rostroventral edge of the infratemporal fenestra.
The central suborbital portion of the jugal is somewhat thin.
There appears to be a low diagonal ridge running caudoâ
ventrally from the region of the jugalâlacrimalâmaxillary
junction to the rostral edge of the jugal flange. Another ridge
runs from the rostroventral edge of the infratemporal fenestra
ventrally to the apex of the jugal horn; this is one of the definâ
ing features of the genus (Sereno 1987). The jugal horn core
itself measures about 25 mm in length and 27 mm across its
base. The right horn is the more robust, measuring 19 mm in
thickness at its base, compared to only 12 mm on the left side.
The lateral surface shows coarse vascular grooves cited as
evidence for the keratinous sheath, but the ventral surface of
the horn core is smooth. Caudoventrally the jugal divides
into two short prongs into which the quadratojugal slots. The
larger dorsal prong forms a small part of the caudal border of
the infratemporal fenestra, and lies as a tab on the surface of
the quadratojugal. The ventral process underlies that bone.
The quadratojugal lies between the lower end of the
quadrate and the jugal, and is wedgeâshaped in caudal view,
broadest ventrally and tapering dorsally. It is partially covâ
ered in lateral view by the dorsal prong of the jugal which diâ
vides the bone into two parts, as in
P. lujiatunensis
(Zhou et
al. 2006) but unlike the other species of
Psittacosaurus
(Sereno et al. 1988; Russell and Zhao 1996). The quadratoâ
jugal wraps rostrodorsally around the shaft of the quadrate,
and forms the caudal border of the infratemporal fenestra for
20 mm or so. It tapers dorsally and may be broken, so its full
dorsal extent cannot be determined. There is a noticeable
prominence on the prong situated near the caudal base of the
jugal horn, but it is not as pronounced as in
P. sinensis
and
P. meileyingensis
(Sereno et al. 1988; Zhou et al. 2006).
The infratemporal fenestra measures 77 mm in height. As
in
P. lujiatunensis
and
P. major
, it narrows ventrally, from a
dorsal width of 52 mm to a ventral width of 20 mm. In other
species of
Psittacosaurus
it is rectangular and unreduced
ventrally.
Palatoquadrate complex
.âThe components of the palate,
from rostral to caudal, consist of the premaxilla, the maxilla,
the vomer, the palatine, the ectopterygoid and the pterygoid.
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189
The vomer is preserved only as a single fragment. Similarly,
the palatines are mostly missing except for caudal fragments.
These have already been well described elsewhere (Sereno et
al. 1988; Zhou et al. 2006).
The vomer is not well preserved in this specimen, but sevâ
eral observations are offered anyway. It forms a median bar
separating the choanae and runs between the maxillary symâ
physis and the pterygoids. It is universally described as
arched (Sereno 1990; Sereno et al. 1988; You and Xu 2005;
Zhou et al. 2006), although this term is unexplained. It is
rarely figured, but when figured it appears in palatal view
(You and Xu 2005; Zhou et al. 2006) in which neither a lonâ
gitudinal nor a transverse arch could in principle be seen. In
some specimens, including CAGSâIGâVDâ005 and
Hongâ
shanosaurus houi
(You and Xu 2005), the vomer is primarily
horizontal in orientation and is not at all arched. The vomer
attaches to the dorsal surface of the maxillary symphysis, at a
level well dorsal to the external maxillary ridge, in the vicinâ
ity of the ventral rim of the circumnarial depression. It does
not form part of the secondary palate as it does in basal
neoceratopsians. This is similar to the level of the vomerine
prominence of the palatine process of the pterygoids. Thus
the vomer is horizontal in orientation in psittacosaurs; it does
not arch caudodorsally as in basal neoceratopsians (OsmĂłlâ
ska 1986). The choanae of psittacosaurs are large and open
ventrally onto the roof of the oral cavity in a more or less horâ
izontal plane.
The palatine is discerned with difficulty, and does not apâ
pear to be very prominent in the present specimen. Possibly
lateral crushing has destroyed important parts. In broadâ
snouted specimens such as CAGSâIGâVDâ005, the palatines
are more prominent, both rostrocaudally and mediolaterally.
In CAGSâIGâVDâ005, they are broadly applied to the side of
the pterygoid, but do not rise as far dorsally as the vomerine
prominence. The rostral edge is directed transversely in a sadâ
dleâlike lateral slope that ends as a thickened buttress on the
medial surface of the jugal at a four oâclock position in the
right orbit or eight oâclock position in the left orbit. This correâ
sponds to the transverse palatine wing of OsmĂłlska (1986).
The transverse saddle defines the caudal border of the choana,
which appears to be very capacious. The choana is bordered
rostrally by the maxillary symphysis, medially by the vomer,
and caudally by the pterygoid. In CAGSâIGâVDâ004, the orâ
bital buttress of the palatine can be discerned on the left side,
slightly more ventral (closer to the 7 oâclock position) than in
CAGSâIGâVDâ005, on a level with the seventh maxillary
tooth. The palatine is applied to the medial surface of the
maxilla caudally. Due to uncertainty, two alternate descripâ
tions are presented. One interpretation of the palatine is that it
forms the caudoventral edge applied to the maxilla as far venâ
trally as the dental foramina, covers the maxilla opposite the
last two teeth, forms a rounded tabâlike process that overlaps
the rostromedial medial base of the mandibular process of the
pterygoid, and is cleft dorsal to this to form a palatine foramen.
The rostral border would ascend as a thin irregular edge dorsal
to the penultimate maxillary tooth. Dorsal to the maxilla the
palatine contacts the palatine process of the pterygoid, but the
thin lamina cannot be followed. The alternate interpretation is
that the palatine is entirely dorsal to the palatine foramen (see
maxilla above). In any case, the choana appears to extend from
the level of the first maxillary tooth to the level of the seventh
maxillary tooth.
Both ectopterygoids are poorly preserved, and can be
identified as only as fragments on the caudodorsal bases of
the left and right mandibular processes of the pterygoids.
They are directed rostrolaterally across the caudal edge of the
maxilla dorsal to the last tooth, and meet the medial surface
of the maxillaryâjugal ridge. The ectopterygoid contacts the
jugal as in other specimens (Sereno 1987; Sereno et al. 1988;
Zhou et al. 2006).
The complex, threeâdimensional pterygoids are well preâ
served, and consist of quadrate, mandibular and palatine
rami and a central plate, all well exposed. The pterygoids are
separated along the caudal midline by a cleft that continues
rostrally for 19 mm (combined length of cleft including both
basisphenoid plus pterygoid components is 37 mm) and then
forms a symphysis resulting in a short central plate at the
level of the caudal end of the maxilla. The quadrate ramus of
the pterygoid is very thin and overlaps the pterygoid wing of
the quadrate medially. In ventromedial view it can be seen as
far caudoventrally as about threeâquarters of the distance beâ
tween the central plate of the pterygoid and the quadrate
condyle, and from there the caudal margin meanders rostroâ
dorsally to the vicinity of the rostral end of the cranioâ
quadrate passage. The irregularity of the course suggests loss
of thin bone by breakage or flaking. A thickened ridge curves
caudoventrally from the basipterygoid process to the ventral
edge of the quadrate ramus. In so doing, a distinctive pocket
is formed on the medial surface of the quadrate ramus, the
thickness of which reflects the breadth of the strong basiâ
pterygoid processes. The mandibular ramus of the pterygoid
lightly overlaps the caudolateral aspect of the maxilla immeâ
diately caudal to the toothrow and the adjacent palatine, and
forms a strong pendant process that defines the ventral meâ
dial edge of the mandibular adductor fossa. This fossa is
bounded laterally by the medial surface of the jugal, and
rostrally by the ectopterygoid. The process measures 26 mm
in length (measured medially from the maxillary suture to the
broad rounded tip), 17 mm in breadth at its base, and only
4 mm in thickness. The palatine rami of the pterygoids asâ
cend steeply rostrodorsally from the central plate at an angle
of about 60
°
, and remain closely appressed to each other on
their rostral aspect. They form a rounded process expanded
in lateral view on the midline almost at the level of the lacriâ
mal canal in the rostral edge of the orbit. Here they would be
expected to embrace the vomers, which are not preserved.
From the caudal edge of the vomerine prominence, thin
laminae of the pterygoid descend caudoventrally towards the
basipterygoid processes; the profile of the dorsal edge correâ
sponds more or less with the ventral rim of the orbit. The
space between the laminae, open dorsally, forms an interâ
pterygoid vacuity, which is floored rostroventrally by the
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ACTA PALAEONTOLOGICA POLONICA 53 (2), 2008
pterygoid symphysis (central plate plus palatine rami) and is
continuous caudally with the basisphenoidâpterygoid cleft.
In some specimens, including CAGSâIGâVDâ005 and
Hongâ
shanosaurus houi
(You and Xu 2005), a median bar connects
the parasphenoid to the vomerine prominence of the pteryâ
goid, but no such connection is preserved in the present specâ
imen.
The quadrate is a tall in lateral view, gently convex rosâ
trally and concave caudally. Only the condyles and a small
caudal edge are visible ventrally, but in
P. mongoliensis
none
of the quadrate is visible ventrally (Sereno et al. 1988). Dorsal
to the quadratojugal, the quadrate is visible for the remainder
of its course. The left quadrate measures 88 mm in height, the
right quadrate 100 mm. The left mandibular condyle is well
preserved, and measures 31 mm in width and 14 mm in length.
The transverse axis of the mandibular condyle is approxiâ
mately at 90
°
to the long axis of the skull. The broad, eggâ
shellâthin pterygoid wing of the quadrate is typical in its develâ
opment, and completes the medial wall of the infratemporal
fossa as far rostrally as the postorbital bar. The pterygoid wing
is bowed slightly laterally, creating a space between it and the
prooticâopisthotic, called the cranioquadrate passage by Seâ
reno (1987). It contacts the former on a small thickening or
prominence at the rostral end of the horizontal crista prootica.
The pterygoid wing of the quadrate and its complement on the
pterygoid form a curtain of bone that obscures the ventral porâ
tions of the braincase in lateral view, including most of the craâ
nial nerve foramina. Fortuitously, these bones are broken
away on the right side of the skull, providing lateral access to
some of these structures. Otherwise, deep braincase structures
are typically visible only through the orbit (e.g., Sereno 1987;
Zhou et al. 2006), or in palatal view.
Braincase
.âOn the occipital surface, the visible structures
include the basioccipital, exoccipitals, and supraoccipital, as
well as the parietal and squamosals. The fused occipital
condyle, which measures 18 mm in diameter, reveals no suâ
tures by which the basioccipital and exoccipital components
can be discerned, although other specimens of less maturity
show it to be composed primary of the basioccipital. The
condyle is about 25% smaller in diameter than the 25 mm reâ
ported for the skull of
Psittacosaurus lujiatunensis
, which is
of similar size (Zhou et al. 2006). The condyle forms a good
smooth hemisphere, mounted on a very short pedestal, which
is angled somewhat caudoventrally. The pendent basiocciâ
pital tubera hang beneath the condyle, and are heavy, with a
broad median cleft between them, unlike the solid plate of
bone in basal neoceratopsians (e.g., You and Dodson 2003;
You et al. 2005). They span a width of 33 mm, almost twice
that of the condyle itself, and hang 26 (left) to 29 (right) mm
below the level of the exoccipitals.
The exoccipitals form wingâlike, caudolaterallyâdirected
paroccipital processes that contribute indirectly to the support
of the dorsal end of the quadrate by supporting the caudal exâ
tremity of the squamosal. The distal ends of the paroccipital
processes are dorsoventrally expanded and project caudally
beyond the quadrate (forming the extremity of the skull in latâ
eral view) and also hang well below the head of the quadrate,
forming an ample tympanic recess between it and the concave
caudal edge of the quadrate. The paroccipital process supports
the squamosal dorsally and, at midâlength, the parietal frill.
The betterâpreserved left paroccipital process measures 58
mm in length from the foramen magnum to the tip. The
dorsoventrally expanded tip measures 32 mm in width.
The supraoccipital forms a sharp sagittal ridge or septum
that runs from the foramen magnum to the ventral surface of
the parietal. Dorsally the septum expands to form a shelf or
pedicle that supports a ventral sagittal process of the parietal.
The septum divides the occipital surface into left and right
compartments for the insertion of cervical epaxial muscles.
The supraoccipitalâexoccipital suture cannot be determined.
The basisphenoid shows two major features in ventral
view. The first are paired plateâlike processes that conform in
general size and shape to the basal tubera of the basioccipital,
from which they are separated by a shallow transverse groove.
The second feature consists of long, rostrallyâprojecting basiâ
pterygoid processes, which are received in pockets on the meâ
dial surfaces of the pterygoids near the level of the caudal end
of the orbit in lateral view. The basipterygoid processes are
strongly separated from each other, continuing rostrally the
channel that divides the basal tubera. On the right side of the
skull, the wings of the quadrate and pterygoid are broken
away, exposing this region with exceptional clarity. In ventral
view the basipterygoid processes seem to be rods, but in lateral
or oblique view, it is seen that they are vertical blades measurâ
ing 12 mm in height. The basipterygoid processes diverge
only slightly in the present specimen, but in a small specimen
of
P. lujiatunensis
(CAGSâIGâVDâ005) that measures 77 mm
in length, the processes diverge from each other at an angle of
almost 60
°
. The basipterygoid processes measure 30 mm in
length from the basioccipitalâbasisphenoid suture to the pteryâ
goids, and appear to be much longer than those in
P.
lujiatunensis
(Zhao et al. 2006: figs. 2F, 3D). Between the proâ
cesses is a deep cleft, which is continuous caudally with the
cleft between the basal tubera and rostrally with a cleft beâ
tween the pterygoids. Nothing like this is seen in basal neoâ
ceratopsians such as
Archaeoceratops
(You and Dodson 2003)
or
Auroraceratops
(You et al. 2005). In CAGSâIGâVDâ005,
which is relatively low, the basipterygoid processes are oriâ
ented horizontally, but in CAGSâIGâVDâ004 the processes are
shallowly declined, and the basisphenoidâpterygoid articulaâ
tion is located slightly below the level of the occipital condyle.
Situated dorsally on midline above and between the basiâ
pterygoid processes is a median rostrum, the parasphenoid, a
vertical blade with an expanded rostral tip, which is readily
seen through the orbit projecting rostral to the pterygoid proâ
cesses of the quadrate. The dorsal edge of the parasphenoid is
concave, leaving an important recess between it and the
laterosphenoids above.
The rostrodorsallyâossified element of the braincase is the
laterosphenoid. It sits on the basisphenoid, and forms a strong
pillar that curves rostrodorsolaterally underneath the rostroâ
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YOU ET AL.âCRANIAL ANATOMY OF
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191
medial corner of the supratemporal fenestra to contact the
skull roof at the caudodorsal edge of the orbit. The lateroâ
sphenoid is supported dorsally by the parietal, and fits into a
cotyla on the ventral surfaces of the frontal and postorbital. As
is generally the case in archosaurs, the joint is a synovial not a
fibrous one. The rostroventral edge of the laterosphenoid that
supported the mid and rostral portions of the brain due to
breakage is very thin, friable, and incomplete. Some of it has
clearly been lost due to breakage, but almost certainly this reâ
gion was continued rostrally by an unossified orbitosphenoid.
The laterosphenoid forms the rostral and a short part of the
rostromedial wall of the mandibular adductor chamber ventral
to the supratemporal fenestra. A prominent crack in the rostral
part of the adductor chamber lines up with the trigeminal foraâ
men ventrally, and this is tentatively accepted as the lateroâ
sphenoidâprootic junction.
The prootic and opisthotic are situated caudal to the
laterosphenoid, and form the medial wall of the adductor
chamber ventral to the parietal, but are not suturally distinct
from each other. The crista prootica is a long horizontal ridge
that more or less marks the dorsal extent of the pterygoid
wing of the quadrate. Dorsal to this, the surface of the prootic
is smooth, providing a surface for origin of the jaw adductor
musculature. The crista prootica may be seen in ventral view
as a sharp, curled ridge directed caudolaterally more or less
parallel to the ventral edge of the paroccipital process. It thus
defines the medial edge of the cranioquadrate passage and
the rostrolateral edge of the tympanic canal, which measures
nearly 7 mm in width. The stapes would be situated here if it
was present, but none was preserved.
The cranial nerve foramina are not well preserved. A maâ
jor landmark is the trigeminal foramen, situated ventrally beâ
tween the laterosphenoid and the prootic. On the right side of
the present specimen there is a fractured area just dorsal to
the basisphenoid that measures 10 mm in width. The triâ
geminal foramen is visible on the left side situated on the
level of the rostrodorsal corner of the infratemporal fenestra.
It measures about 4 mm in diameter, is situated in a pit, and
directs the course of the trigeminal nerve ventrolaterally. On
the right side, the trigeminal foramen is less distinct, but sevâ
eral grooves course rostrodorsally along the ventral edge of
the laterosphenoid. At least one of these seems to correspond
to the groove reported for cranial nerve III, the oculomotor
nerve, in
P. lujiatunensis
(Zhou et al. 2006). A pit ventral and
slightly caudal to trigeminal foramen on the left side may
correspond to the foramen for cranial nerve VII, the facial
nerve. On the ventrolateral aspect of the paroccipital process
there is a triangular depression defined by two ridges, one
running dorsolaterally from the occipital condyle, and one
caudodorsally from the basioccipital. In this depression are
three foramina, representing the exits for cranial nerves IX,
X, XI, and XII, the glossopharyngeal, vagus, accessory and
hypoglossal nerves, respectively.
Mandible
.âBoth mandibles are preserved. The mandible is
long and deep in lateral view and slender in dorsal view. The
left mandible measures 188 mm in length and 68 mm in
height. The existence of an external mandibular foramen is unâ
certain since the portion including the dentaryâsurangularâ
angular junction is missing. The mandibular ramus is nearly
straight in dorsal view. A distinctive flange is developed
ventrolaterally from the middle one third of the mandible. The
jaw articulation is approximately 10 mm lower than the base
of the tooth row. The following description of the mandibular
elements is based mainly on the better preserved left mandible
(Fig. 4).
The predentary caps the dentaries rostrally. Its length is
36 mm, width 39 mm and height 41 mm. Possibly due to the
compression of the rostral bone, it is wider than the rostral. In
lateral view, it is short and deep as in
P. lujiatunensis
,
P.
meileyingensis
,
P. neimongoliensis
, and
P. sinensis
, and it difâ
fers from relatively long and shallow predentary of
P. monâ
goliensis
and
P. sibiricus
(Young 1958; Sereno 1987; Sereno
et al. 1988; Russell and Zhao 1996; Averianov et al. 2006;
Zhou et al. 2006). It is triangular in lateral view and semicircuâ
lar in rostral view. It forms most, if not all, of the cutting edge
of the edentulous lower beak. In dorsal view, the cutting edge
is horseshoeâshaped as in the opposing rostral. The right dorâ
sal process, which is approximately 10 mm long articulates
dorsally with the rostrodorsal margin of right dentary. The
ventral process extends caudoventrally in lateral view tapering
distally. The dorsal oneâthird of its external surface and most
of the dorsal surface except for the small area immediately
rostral to mandibular symphysis are pitted. In contrast, the
ventral twoâthirds of the rostral surface is mostly characterized
by a smooth texture, but there also are shallow grooves. In dorâ
sal view, the caudal margin of its dorsal surface is composed
of caudally concave two arches, which receive the rostrally
convex ends of dentaries, anchored at the midline.
The dentary fills nearly half of the mandible in lateral view.
Its rostral end is covered by the caudal margin of predentary,
with the predentaryâdentary suture running caudoventrally in
lateral view. The dentary measures 87 mm in length and 68
mm in height. Medially, the rostral ends of the dentaries conâ
tact with each other at the symphysis. The caudodorsal portion
of the dentary rises to form the rostral half of the coronoid proâ
cess including its apex. On both lateral and medial sides of the
apex, striations for pseudotemporalis muscle attachment run
caudodorsally (Ostrom 1964). Slightly rostrally from the midâ
point of the ventral margin stretches the rostral end of the unâ
usually strong ventrolateral flange, which is present to some
degree in most psittacosaurids.
P. sinensis
and
P. neimongoâ
liensis
do not possess this flange (Sereno et al. 1988; Russell
and Zhao 1996). The flange reaches the middle of the ventral
margin of angular. In medial view, the ventrolateral flange exâ
tends ventral to splenial. The dentary is widest opposite the
rostral end of the flange in dorsal view, reaching the width of
22 mm including the flange. A prominence extending across
the lateral surface from the coronoid process to the rostral corâ
ner of the flange noted in
P. meileyingensis
,
P. mongoliensis
,
and
P. lujiatunensis
(Sereno 1987; Sereno et al. 1988; Zhou et
al. 2006) is poorly developed.
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ACTA PALAEONTOLOGICA POLONICA 53 (2), 2008
The surangular is a large bone forming most of the upper
half of the caudal third of the mandible. It is bounded by the
dorsal half of the dentary rostrally, the angular ventrally and
the articular caudally. In lateral view, it is subtriangular, taâ
pering caudoventrally. It is 65 mm long and 38 mm high. The
rostrodorsal corner of the surangular contributes the caudal
half of the coronoid process. In dorsal view, it is mostly slenâ
der. However, at its caudal end, it bears a lateral projection
slightly tilting laterally. Together with the medially projectâ
ing articular, the left and right mandibles have their greatest
widths of 30 mm and 34 mm, respectively, at the point of the
projections. The projection is lateral to and coâplanar with
the glenoid surface of the articular, thus participating in jaw
articulation.
The angular forms most of the ventral half of the caudal
third of the mandible. It contacts the ventral half of dentary
rostrally, the surangular dorsally and the articular caudally. It
is slightly larger than surangular. In lateral view, it is a subâ
triangular element tapering caudally and roughly symmetrical
to the surangular across the surangularâangular suture. Its
length measures 76 mm and height 34 mm. In medial view, it
is overlapped by the prearticular and partially by caudal end of
the splenial. The suture between the surangular and the anguâ
lar is ambiguous in medial view. The ventrolateral flange of
the mandible ends at the middle of the angular.
The splenial covers the midsection of the medial side of
the mandible. It is bounded by the dentary rostrodorsally, the
coronoid dorsally, the prearticular caudally and the angular
caudoventrally. Its length is 78 mm and height 29 mm. The
rostral end along the ventral margin is located immediately
caudal to the predentary. From the rostral end, the rostroâ
dorsal margin of the splenial stretches caudodorsally to the
rostroventral end of the coronoid. The rostral half of the venâ
tral margin is nearly horizontal, whereas the caudal half is
ventrally convex, reaching the middle of the angular. The
caudodorsal margin is convex rostroventrally, forming the
rostral and ventral walls of the Meckelian foramen.
The coronoid is the smallest element of the mandible conâ
tributing to the medial side of the coronoid process. It conâ
tacts the dentary rostrally and dorsally, the splenial ventrally,
the prearticular caudoventrally and the surangular caudally.
Its rostrocaudal length measures 11 mm and height 12 mm.
Only the left coronoid is preserved in this specimen. In meâ
dial view, it is Lâshaped with its dorsal portion curved cauâ
dally. The rostral process extending from the ventral portion
of the vertical shaft is just ventral to the caudal end of the
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YOU ET AL.âCRANIAL ANATOMY OF
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193
predentary
dentary
surangular
angular
articular
predentary
coronoid
dentary
articular
surangular
angular
articular
prearticular
splenial
ventrolateral flange
50 mm
Fig. 4. Ceratopsian dinosaur
Psittacosaurus major
Sereno, Zhao, Brown, and Tan, 2007, new cranial specimen CAGSâIGâVDâ004 from the Early Cretaâ
ceous Lujiatun Bed of Yixian Formation near Beipiao City, Liaoning Province, China. Left mandible in lateral (
A
), medial (
B
), dorsal (
C
), and rostral (
D
)
views (A
1
, B
1
, C, and D are photographs and A
2
and B
2
are interpretive outlines).
tooth row. In this specimen, the coronoid does not reach the
dorsal end of the coronoid process of the dentary. The dorsal
end of coronoid is possibly missing.
The prearticular consists of a thin strip on medial side of
the caudal portion of the mandible. It is bounded by the
splenial rostrally, the coronoid rostrodorsally, the articular
caudodorsally and the angular ventrally. It is 75 mm long and
22 mm high. The rostral half of the prearticular is thinner
than its caudal half and it stretches rostrodorsally. The venâ
tral margin of the rostral half composes the dorsal and caudal
wall of the Meckelian foramen. The caudal half of preâ
articular is horizontal, with a dorsal projection just rostral to
the articular in medial view. It does not reach the caudal end
of the mandible along with articular.
The articular constitutes the dorsal side of the caudalmost
portion of the mandible. The rostral portion of the articular is
a very unusual flat, diskâshaped articular surface, which reâ
ceives the quadrate of the skull. The medial half of the articuâ
lar surface extends medially as a projection. The left articular
surface is 21 mm in length and 19 mm in width. The length
and width of the right articular surface are both 19 mm. There
is a slender retroarticular process stretching caudally from
the diskâshaped articular surface, reaching the caudal end of
the mandible.
Dentition
.âThere are ten and nine teeth in left and right
maxillary tooth rows, respectively. The tooth rows are nearly
straight in ventral view. The left tooth row is 52 mm long and
the right tooth row 49 mm (Fig. 5A). In ventral view, the
crowns are aligned at a shallow angle to the tooth row. The
mesial end of each crown lingually overlaps the distal end of
the preceding crown as in
P. meileyingensis
(Sereno et al.
1988). Although one replacement tooth each for the 9th left
maxillary tooth and the 8th right tooth are present, the outâ
lines of the two unworn maxillary tooth crowns are not fully
exposed. The crowns of the functional teeth are approxiâ
mately 4 to 6 mm long mesiodistally and 3 to 4 mm wide
linguolabially. In labial view, two deep and long grooves
separate the primary ridge from the mesial and distal lobes.
In some maxillary teeth, these grooves and primary ridges
are poorly developed. Separated by shallow grooves, some
ridges are developed in both lobes. The mesial lobe is relaâ
tively flat and long, whereas the distal lobe is convex and
short. The details of the lingual surface are obliterated by
tooth wear. The enamel layer can be observed on the labial
side of the crown, but not on the lingual side.
On both dentaries, there are 10 teeth in the tooth rows.
The tooth row is nearly straight. The left tooth row is 50 mm
long and the right tooth row 54 mm (Fig. 5B). The last two
teeth are located medial to the rostral slope of the coronoid
process. However, the caudalmost tooth is rostral to the apex
of the coronoid process unlike ceratopsids, in which the tooth
row extends caudal to the apex. Replacement teeth are presâ
ent for first, third, and fifth dentary teeth on both right and
left tooth rows. As in the maxillary teeth, there is only one reâ
placement tooth for each tooth position. The crowns of reâ
placement teeth for both left and right first dentary teeth are
well exposed, showing circular to subcircular outlines. The
crowns of functional teeth are approximately 5 to 6 mm long
mesiodistally and 3 to 4 mm wide linguolabially. On the linâ
gual view of the functional tooth, the primary ridge is more
prominent than that of maxillary tooth, both in width and
height. Rostral and caudal lobes are of about the same size.
The rostral lobe is more convex than the caudal lobe. There is
an enamel layer on lingual side, but not on the labial side.
Discussion
Psittacosaurus
is a well defined clade, and includes perhaps as
many as 10 species:
Psittacosaurus mongoliensis
(Osborn
1923),
P. sinensis
(Young 1958)
, P. meileyingensis
(Sereno et
al. 1988),
P. xinjiangensis
(Sereno and Chao 1988),
P. neiâ
mongoliensis
(Russell and Zhao 1996),
P. ordosensis
(Russell
and Zhao 1996),
P. mazongshanensis
(Xu 1997),
P. sibiricus
(Leschinskiy et al. 2000; Averianov et al. 2006);
P. lujiatuâ
nensis
(Zhou et al. 2006), and
P. major
(Sereno et al. 2007).
Among them, four have been discovered from the Lower Creâ
taceous Jehol Group in western Liaoning Province, China:
two (
P. lujiatunensis
and
P. major
) from the Yixian Formation
near the Town of Shangyuan, and the other two (
P. meileyinâ
gensis
and
P. mongoliensis
) from the overlying Jiufotang Forâ
mation near the Town of Shengli. These two localities are
about 80 km apart, and both in the vicinity of Chaoyang City,
Liaoning Province. Historically, many described species of
Psittacosaurs
are based on single specimens and/or very inâ
complete material, a practice that needs to change as progress
is made in understanding
Psittacosaurus
.
Psittacosaurus
is
one of the most abundant dinosaurs known (Dodson 1990;
Sereno 1990a; Russell and Zhao 1996). Juvenile specimens
are very common (e.g., Meng et al. 2004). Despite a history of
194
ACTA PALAEONTOLOGICA POLONICA 53 (2), 2008
10 mm
10 mm
Fig. 5. Ceratopsian dinosaur
Psittacosaurus major
Sereno, Zhao, Brown,
and Tan, 2007, new cranial specimen CAGSâIGâVDâ004 from the Early
Cretaceous Lujiatun Bed of Yixian Formation near Beipiao City, Liaoning
Province, China.
A
. Right maxillary tooth row in labial view.
B
. Right denâ
tary tooth row in lingual view.
study stretching back more than 80 years, it is only in the past
year (Zhou et al. 2006; Averianov et al. 2006; Sereno et al.
2007) that giant specimens of
Psittacosaurus
have been disâ
covered. There are skulls of 190 mm in length or more, comâ
pared to 150 mm for a typical skull of
Psittacosaurus
(i.e.,
P.
mongoliensis
,
P. sinensis
,
P. neimongoliensis
,
P. meileyinâ
gensis
). Not only is absolute skull size very large, but Sereno
et al. (2007) have demonstrated that skull size relative to body
size is enlarged in
P. major
.
Several features of CAGSâIGâVDâ004 support its placeâ
ment in
P. major.
According to Sereno et al. (2007),
P. major
is diagnosed by its large skull relative to its postcranial skeleâ
ton, transversely narrow dorsal skull roof, very prominent
dentary flange, ventrolaterally projecting jugal horn, absence
of the external mandibular fenestra, and seven sacral verteâ
brae. CAGSâIGâVDâ004 has a large skull, which is 192 mm
long (203 mm in LH PV1, the holotype of
P. major
); however,
the relative length of its skull compared to the postcranial skelâ
eton is unknown. Nevertheless, large skull size seems to be a
feature shared by
P. major, P. lujiatunensis, and P. sibiricus
.
CAGSâIGâVDâ004 also has a transversely narrow dorsal skull
roof as in
P. major
, in striking contrast to the condition in
P.
lujiatunensis
and
P. sibiricus
. In
P. lujiatunensis
, the nasal is
transversely wide, about twice the width of its prefrontal. As in
P. major
, CAGSâIGâVDâ004 has a prominent dentary flange
about oneâthird the depth of the mandibular ramus. Dentary
flanges also exist in
P. lujiatunensis
,
P. sibiricus
, and
P.
meileyingensis
, but they are not as prominent as in
P. major
.
We do not consider the ventrolaterally projecting jugal horn in
P. major
a reliable feature, because the jugal horn always proâ
jects laterally in
Psittacosaurus
; and the caudal and/or ventral
components are easily affectedly by preservation. However,
the position of the jugal horn is a worthy feature. As noticed by
Zhou et al. (2006), the caudallyâplaced jugal horn on the
infratemporal bar is diagnostic of
P. lujiatunensis
, and this is
also present in
P. major
. Closure of external mandibular
fenestrae is another feature of
P. major
, but its condition in
CAGSâIGâVDâ004 cannot be verified. The large openings
where the external mandibular fenestrae should be in CAGSâ
IGâVDâ004 may either be real or the result of breakage of thin
bone. Therefore, two out of six of
P. major
features (Sereno et
al. 2007) are uniquely shared by LH PV1 and CAGSâIGâVDâ
004: the transversely narrow dorsal skull roof and the very
prominent dentary flanges. Other autapomorphies of
P. major
noticed here include skull longer than wide, and the ventrally
narrowing infratemporal fenestra.
Our cladistic analysis of
Psittacosuarus
recovered
P. maâ
jor
at the base of a clade including
P. neimongoliensis
,
P.
ordosensis
,
P. meileyingensis
, and
P. mongoliensis
, while this
clade is the sister group of
P. lujiatunensis
+
P. mazongâ
shanensis
; they together in turn constitutes the sister group of
P. sinensis
+
P. sibiricus
(Fig. 6). Although both
P. major
and
P. lujiatunensis
are from the same stratigraphic horizon (Luâ
jiatun Bed of Yixian Formation) and same locality (Town of
Shangyuan),
P. major
represents a long and narrowâskulled
psittacosaur, and
P. lujiatunensis
represents a large broadâ
skulled psittacosaur. Although some may doubt the need for a
second species of largeâskulled
Psittacosaurus
from the Lujiâ
atun beds of the Yixian Formation, our study supports the vaâ
lidity of
P. major
. Additional specimens appear referable to
P.
major
, including one recently described by LĂŒ et al. (2007).
Acknowledgements
We are grateful to Qiang Ji (Chinese Academy of Geological Sciences,
Beijing, China) for supporting the project, YuâQing Zhang (Chinese
Academy of Geological Sciences, Beijing, China) for preparing the specâ
imen, and KeâQin Gao and ChangâFu Zhou (Peking University, Beijing,
China) for arranging to access specimens at Peking University. Review
comments from David Norman (Sedgwick Museum, Department of
Earth Sciences, University of Cambridge, Cambridge, UK) and Donald
Brinkman (Royal Tyrrell Museum of Palaeontology, Drumheller, Canâ
ada) improved the manuscript and are greatly appreciated. Funding was
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YOU ET AL.âCRANIAL ANATOMY OF
PSITTACOSAURUS
195
Psittacosaurus
Psittacosaurus
sinensis
Psittacosaurus
sibiricus
Psittacosaurus
lujiatunensis
Psittacosaurus
mazongshanensis
Psittacosaurus
major
Psittacosaurus
neimongoliensis
Psittacosaurus
ordosensis
Psittacosaurus
meileyingensis
Psittacosaurus
mongoliensis
G
H
A
B
C
D
E
F
Fig. 6. The most parsimonious tree (length 58, consistency index 0.534, reâ
tention index 0.603) of
Psittacosaurus
obtained using the implicit enumeraâ
tion option of the T.N.T. software (Goloboff et al. 2007). The data matrix of
Averianov et al. (2006) was supplemented by coding 31 characters for
Psittacosaurus major
and
P. lujiatunensis
(see Appendix 1).
P. xinjiangensis
and
Psittacosaurus
specimen L0001 from the Yixian Formation of Liaoning
(Xu and Wang 1998) were excluded due to large amount of missing data.
Nodes are supported by the following unambiguous synapomorphies (in paâ
renthesis are character numbers from Averianov et al.âs (2006) matrix). Node
A: preorbital segment less than 40% of skull length (1), nasal extends venâ
trally beyond the external naris (5), ventral border of external naris is dorsal to
the maxillary dorsal end (6), premaxillary teeth absent (8), and premaxillaâ
lacrimal contact present (9). Node B: lateral surface of the mandible straight
(21). Node C: skull profile rounded (2). Node D: no synapomorphies. Node
E: ventral margin of premaxillaâmaxilla contact incised (11) and external
mandibular fenestra present (20). Node F: âmaxillary processâ of maxilla
present (14) and primary ridge on maxillary teeth caudoventrally angled (25).
Node G: skull width exceeds skull length (3), premaxilla contacts jugal cauâ
dally (10), ventral postorbital horn present (15), rostral ramus of squamosal
extends as far as to rostral wall of the supratemporal fenestra (17), quadrate
shaft strongly arched in lateral view with caudal margin deeply excavated
(19), and primary ridge on maxillary teeth weakly developed or absent (24).
Node H: âmaxillary processâ of maxilla present (14) and denticles number on
maxillary teeth equal to or more than 14 (26).
provided by the Ministry of Science and Technology of China (973 Proâ
ject: 2006CB701405), the National Natural Science Foundation of China
(40672007) and the Hundred Talents Project of Ministry of Land and Reâ
sources of China (to H.âL.Y.). K.T. was funded by Summer Research Stiâ
pends in Paleontology (University of Pennsylvania), School of Arts and
Sciences Dissertation Research Fellowship (University of Pennsylvaâ
nia), and Jurassic Foundation Research Grant. P.D. thanks his chairman,
Narayan Avadhani, of the Department of Animal Biology, School of
Veterinary Medicine, University of Pennsylvania for support.
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Appendix 1
Character states of
Psittacosaurus lujiatunensis
and
Psittacosaurus major
as coded in the data matrix of Averianov et al. (2006).
Psittacosaurus lujiatunensis
1 0 1 1 1 1 1 1 1 0 1 1 0 1 0 1 0 1 1 1 1 1 1 1 0 1 ? ? ? ? ?
Psittacosaurus major
1 1 0 1 1 1 1 1 1 0 1 1 0 0 0 1 0 1 0 1 1 1 1 1 1 ? ? ? ? ? ?
196
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