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THE SMALLEST KNOWN TRICERATOPS SKULL: NEW OBSERVATIONS ON CERATOPSID
CRANIAL ANATOMY AND ONTOGENY
MARK B. GOODWIN
1
, WILLIAM A. CLEMENS
2
, JOHN R. HORNER
3
, and KEVIN PADIAN
4
1
Museum of Paleontology, University of California, Berkeley, CA 94720-4780, mark@berkeley.edu;
2,4
Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley, California 94720-3140,
bclemens@berkeley.edu; kpadian@berkeley.edu
3
Museum of the Rockies, Montana State University, Bozeman, Montana 59717-0040, jhorner@montana.edu
ABSTRACT —The discovery of the smallest Triceratops skull (UCMP 154452) provides a new ontogenetic end member
for the earliest stage of ceratopsid (Centrosaurinae plus Chasmosaurinae) cranial development. The lack of co-ossification
among the parietal, squamosals, postorbitals, quadratojugal arch, and the braincase preserves sutural contacts and bone
surfaces that later become obscured in adults. The ability to document the early development and morphology of the
horns and frill in Triceratops allows a reevaluation of their functional roles. UCMP 154452 shows that the cranial
ornamentation of the frill and the postorbital horns were not restricted to adults, but began at an early age in this species.
This evidence supports the hypothesis that the function of ceratopsid horns and frills was potentially important for visual
communication and species recognition because in this young form it could not have functioned in sexual display.
Although some features of UCMP 154452 anticipate or mimic the adult character states, some braincase characters
recapitulate the juvenile and adult stages of more basal neoceratopsians.
INTRODUCTION
Triceratops is one of the most familiar genera of Late Creta-
ceous dinosaurs; it is recognized by its distinctive skull, with
three horns and massive frill made up of the parietal and paired
squamosals. Previous assessments of ontogeny in Triceratops are
based on an isolated juvenile postorbital horn (ca. 100 mm long)
described over 60 years ago by Brown and Schlaikjer (1940a)
from the Hell Creek Formation, Montana. Two supraorbital
horn cores (95 mm and 65 mm long) from the Frenchman For-
mation of Saskatchewan were described by Tokaryk (1997) but
could not be identified beyond Chasmosaurinae. Here we report
the discovery of the smallest Triceratops skull, UCMP 154452,
from the upper Hell Creek Formation (Maastrichtian), Garfield
County, Montana. This new skull is identified as Triceratops by
the presence of two 35-mm-long postorbital horns (outgrowths
of the postorbital bones) and a highly scalloped, unfenestrated
frill (Fig. 1).
This diminutive Triceratops skull is a mere 30 cm long and is
the smallest ceratopsid skull known. Like the young of many
other kinds of dinosaurs (Carpenter et al., 1994), UCMP 154452
has large orbits relative to skull size and a foreshortened face.
The next smallest Triceratops skull is of a subadult over four
times as long (Schlaikjer, 1935), and adult skulls are six to seven
times longer (Hatcher, 1907). UCMP 154452 brings the known
growth series of Triceratops to a new small extreme and shows
that cranial ornamentation in the frill and the postorbital horns
were not restricted to adult members, but began at an early age.
UCMP 154452 provides important information on the morphol-
ogy and development of the horns and frill in Triceratops and
allows a reevaluation of their functional significance. A compre-
hensive assessment of Triceratops ontogeny based on a very
complete cranial growth series in the collections of the MOR and
UCMP (Goodwin and Horner, 2001) will follow this study (Hor-
ner and Goodwin, pers. observ.).
UCMP 154452 was discovered in strata of the Hell Creek For-
mation exposed in a small badland area located just north of the
divide separating the drainages of Snow Creek, to the north, and
Hell Creek (UCMP locality V97006, Garfield County, Mon-
tana). The skull was preserved in a bed of essentially unstratified,
medium gray siltstone that weathers to light gray. Yellow, fer-
ruginous streaks and globules as well as fragmentary plant re-
mains occur throughout the sediment. Teeth and/or skeletal frag-
ments of Tyrannosaurus,Triceratops, and Meniscoessus cf. ro-
bustus were discovered in outcrops in the immediate vicinity of
the quarry at the same or slightly (ca. 2 m) higher stratigraphic
levels and document the latest Cretaceous age of the locality
(Lancian North American Land Mammal Age). The nearest ex-
posures of the contact of the Hell Creek and overlying Tullock
formations are approximately one mile (1.6 km) to the west and
2.5 miles (4.0 km) to the southeast. In both, the contact between
these formations is at an elevation of ca. 2860 feet (875.2 m). The
current elevation of V97006 is ca. 2770 feet (847.6 m). In the
region of V97006 the strata of these formations appear to be
essentially flat lying. The difference in current elevations of the
formational contact and the fossil locality, ca. 90 feet (27.5 m),
suggests that V97006 is within the upper third of the Hell Creek
Formation, which is approximately 300 feet (91.8 m) thick in the
valley of Hell Creek (see Wilson, 2004).
Institutional Abbreviations—MOR, Museum of the Rockies,
Bozeman, Montana; UCMP, University of California Museum
of Paleontology, Berkeley; USNM, United States National Mu-
seum, Washington, D.C.; UTEP, University of Texas at El Paso;
YPM, Yale Peabody Museum, Connecticut.
DESCRIPTION
The individual cranial elements of UCMP 154452 share an
external bone texture that is striated and very porous, indicative
of fast-growing tissue (Sampson et al., 1997). All sutures are
patent and allow accurate articulation of this very young Tri-
ceratops skull. This early phase of cranial morphogenesis pre-
serves sutural contacts and bone surfaces that become hidden in
adults. The right side of the skull is more complete and the
following elements, from the right side unless noted otherwise,
are preserved: parietal, left and right squamosals, left and right
postorbitals, prefrontal, jugal, quadrate, quadratojugal, occipital
condyle, basioccipital, left and right exoccipitals, surangular, and
dentary. Less complete but identifiable fragments of the maxil-
Journal of Vertebrate Paleontology 26(1):103–112, March 2006
© 2006 by the Society of Vertebrate Paleontology
103
lary, left quadrate, left jugal, vertebral centra, ossified tendons,
and teeth were also found with the skull. Morphological descrip-
tions are based on the right side.
Parietal
The parietal is nearly “square”(Figs. 2; 3B, E). It measures
124 mm in length along the midline and has a maximum width of
127 mm. The midline is ornamented by an undulating row of five
raised bony prominences. This feature was also described by
Dodson and Currie (1988) on a 210-mm-long parietal of the
previously smallest known ceratopsid, tentatively referred to
Monoclonius. Rostrally, in UCMP 154452, each prominence be-
comes progressively narrower along the midline, but remains
consistent in height, ca. 5 mm, above the surface of the parietal.
Caudally, the parietal is about 5–7 mm thick and thins rostrally
to less than 4 mm. This rostral thinning of the parietal is also
observed in adult Triceratops skulls (Dodson and Currie, 1990).
A prominent feature of UCMP 154452 is the “scalloped”caudal
margin of the parietal. A series of three scallops on either side of
the parietal midline is bordered caudally by a central scallop.
These scallops continue onto the caudal margin of each squamo-
sal and provide a distinctive appearance to the frill. These scal-
lops are not separate ossifications but are formed by the parietal
and squamosal. Consequently, they are not homologous with the
epoccipitals that border the frill in subadult and adult Tricera-
tops, but merely mimic their shape. The dorsal and ventral sur-
faces of each scallop are smoother than the surrounding bone
and were likely covered by hard keratin before the epoccipitals
ossified. This scalloped edge becomes less pronounced and gent-
ly “wavy”in subadult Triceratops skulls when epoccipitals first
appear and ossify along the frill margin (Goodwin et al., 1997).
Squamosal
The left and right squamosals (Fig. 3A, C, D, F) are nearly
complete and are ca. 150 mm in maximum length. The squamosal
thins rostrally from 6.3 mm to 3.0 mm. It articulates rostrolater-
ally with the jugal by an overlapping sutural contact. It also
overlaps the caudal portion of the postorbital rostrodorsally. The
medial edge of the squamosal forms the border for the supra-
temporal fenestra. The squamosal and parietal contribute to the
frill along a fairly straight contact. The characteristic inward
bend of the adult squamosal is expressed in the squamosals of
UCMP 154452. This bend becomes greatly exaggerated in adult
Triceratops (Dodson and Currie, 1990; Dodson, 1993). The me-
dial edge of the squamosal curves slightly and does not appear to
overlap with the parietal rostrally as in subadult and adult skulls.
The caudal border of the squamosal has five distinct scallops. A
longitudinal series of raised prominences radiate rostrolaterally
onto the dorsal surface of the postorbitals. Ventrally, a bifurcat-
ing prominent ridge of bone serves as the articular surface for the
exoccipital and quadrate where these bones form a prominent
buttress beneath the frill (Fig. 4B).
Postorbital
The most distinctive feature of the left and right postorbitals is
the 35-mm-long postorbital horns (Fig. 5). The postorbital horns
FIGURE 1. A comparison of UCMP 154452, the smallest Triceratops
skull known, with an adult Triceratops skull in right lateral view, illus-
trates the dramatic changes in size, shape, and sutural contacts of cranial
elements that occur during ontogeny. A, restoration of UCMP 154452,
Triceratops. The frill is highly scalloped at this very young stage but is
minimally developed caudally and not fan-like compared with the adult
skull. Postorbital horn growth has already begun. The nasal region is
restored after the adult condition; the fenestra may not be present in
young individuals. B, YPM 1822, an adult Triceratops skull (1.75 m long;
modified from Romer, 1966). Abbreviations (restored bones drawn in
outline only): d, dentary; j, jugal; lac, lacrimal; m, maxilla; n, nasal; nh,
nasal horn; p, parietal; po, postorbital; poh, postorbital horn; pd, pre-
dentary; pfr, prefrontal; pm, premaxilla; q, quadrate; qj, quadratojugal; r,
rostral; sa, surangular; and sq, squamosal.
FIGURE 2. Parietal of UCMP 154452, Triceratops, in dorsal view.
Note the scalloped caudal margin and median row of bony ornamenta-
tion.
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 1, 2006104
are oriented rostrally at ca. 10°and do not show the caudally
directed curvature of older juvenile and subadult Triceratops
postorbital horns (Goodwin et al., 1997). Indented vascular
grooves on the exterior surface of the postorbitals indicate that
they were covered by a keratinous sheath (Horner and Marshall,
2002). The grooves are deepest on the surface of the horns. A
row of raised prominences radiates caudolaterally and continues
onto the squamosals. The right postorbital confirms that the ven-
tral part of the cornual sinus at the basal region of the horncore
formed early (Fig. 5B). The rostral face of the postorbital horn
(Fig. 5C) has a prominent rugose sutural surface for the prefron-
tal. This roughened sutural surface covers nearly the entire ros-
tral surface of the postorbital horn. Laterally, the caudodorsal
region continues as a thin wedge of bone that articulates with the
squamosal by sliding beneath its rostral edge.
Prefrontal
We identify a 39.6 mm long semi-lunate bone as the right
prefrontal (Fig. 6). It is ca. 7 mm thick. The medial and caudal
edges are dominated by a rugose sutural surface. Caudally, the
prefrontal thickens where it meets the rostral face of the post-
orbital. Three prominent foramina are on the anterodorsal sur-
face and a single central foramen penetrates the prefrontal. The
prefrontal forms the anterodorsal margin of the orbit and en-
ables the reconstruction of the front of the skull.
Jugal
The jugal (Fig. 7A, B) forms the ventral border of the orbit
and the dorsal rim of the lateral temporal fenestra. This sutural
pattern of the jugal bordering the rim of the lateral temporal
fenestra dorsally is considered plesiomorphic by Forster (1996a:
261, character 1; also see Fig. 1). In the derived state, the squa-
mosal forms the dorsal rim of the lateral temporal fenestra and
does not extend across the top of the jugal in adult Triceratops.
In UCMP 154452, the squamosal remains excluded from most of
the dorsal margin of the lateral temporal fenestra by a caudally
directed jugal spur. Individual variation may be the cause of this
slight anatomical difference, but it could also be an expression of
the primitive condition retained in the adult Triceratops and at
this early stage of ontogeny. In all ceratopsids, the jugal expands
caudally and the squamosal enlarges caudoventrally. As a result,
the infratemporal fenestra is compressed and reduced in size.
The jugal-squamosal contact excludes the postorbital from the
infratemporal fenestra in adult Triceratops (Dodson and Currie
1990:600). This is also observed in UCMP 154452. A squamosal
process ventral to the temporal opening is undeveloped at this
early stage of ontogeny. Deep vesicle grooves are present around
the ventral rim of the orbit and become shallower on the remain-
ing jugal dorsally. A prominent feature of the jugal is the ven-
trally directed “wedge”of bone that covers the anterolateral
surface of the quadratojugal along an overlapping sutural con-
FIGURE 3. The fill of UCMP 154452, Triceratops, in dorsal (A–C) and ventral (D–F) views. Right squamosal (A,F), parietal (B,E), and left
squamosal (C,D). The caudal margin of the frill is highly scalloped.
GOODWIN ET AL.—SMALLEST TRICERATOPS SKULL 105
tact. Ventrally, the sutural surface of the jugal is slightly concave
and thinner (ca. 4 mm) compared to ⱖ6 mm in overall thickness.
The distal tip of the jugal is flared but does not show any evi-
dence for an epijugal, which evidently forms later in ontogeny.
Quadrate
The quadrate (Fig. 8) is 98 mm in maximum length dorsoven-
trally and is nearly totally excluded from the caudal margin of
the infratemporal fenestra by the overlapping quadratojugal.
The quadrate articulates with the quadratojugal along a vertical
axis laterally. This articulation is further supported by a spur of
bone that rises from the ventral sutural surface of the quadrate to
meet a gentle depression on the lower medial surface of the
quadratojugal. The quadrate expands transversely dorsally, a
precursor to the significant dorsal expansion in adult Triceratops
(Forster, 1996b). The flange of bone meets the underside of the
squamosal along a pronounced V–shaped ridge of bone (see Fig.
4B). It is overlain by the squamosal and underlain by the exoc-
cipital. This arrangement is also observed in adult skulls
(Hatcher, 1907; Ostrom and Wellnhofer, 1986). The surface for
articulation with the lower jaw lies at the rostral-most region of
the quadrate. It is robust and round, but does not form a trans-
verse articular surface or distinct double condyle, bisected by a
trough, for articulation with the mandible of adults (Hatcher,
1907; Ostrom and Wellnhofer, 1986). This ventral condyle is
rostral to the more caudally directed dorsal flange.
Quadratojugal
The quadratojugal (Fig. 8) is wedged between the quadrate
and the overlapping ventral jugal flange. The quadratojugal is
thick ventrally, thin dorsally, and wrapped around the caudal
portion of the quadrate. The medial ventral condyle is slightly
concave where it contacts the quadrate. Laterally, the quadrato-
jugal is marked by a ridge and faceted surface where the jugal
flange overlaps it. This arrangement is consistent with the adult
condition (Ostrom and Wellnhofer, 1986). All of the sutural sur-
faces are open and overlapping. The quadratojugal forms nearly
the entire ventral and caudal border of the lateral temporal fe-
nestra. In adult Triceratops, the squamosal forms this caudal
border of the infratemporal fenestra (Dodson and Currie, 1990).
In lateral view the quadratojugal is largely obscured by the jugal
in most adult skulls but not in UCMP 154452, particularly adja-
cent to the infratemporal opening.
Lateral Temporal Fenestra
The lateral temporal fenestra (Fig. 1) lies beneath and caudal
to the orbit. The opening is nearly oval and bordered by the jugal
FIGURE 4. Right squamosal of UCMP 154452, Triceratops, in dorsal
(A) and ventral (B) views. Note the well-developed transverse buttress
for articulation of the exoccipital and quadrate and support of the fill.
Abbreviations:b, buttress; ex, exoccipital articulation; q, quadrate
articulation.
FIGURE 5. Postorbitals of UCMP 154452, Triceratops. Right postorbital in lateral (A,E), medial (B,F), and rostral (C,G) views; left postorbital
in lateral (D,H) view. Postorbital horns are present at this early stage of ontogeny. Indented vascular grooves cover their dorsal surface. Formation
of the corneal sinus is evident at the base of the horncore ventrally in (B). A rugose sutural surface for the prefrontal is preserved on the rostral face
of the right postorbital horn in (C). Abbreviations:pf, sutural surface for the prefrontal; sq, sutural surface for the right squamosal.
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 1, 2006106
rostrally, the squamosal caudodorsally, and by the quadratojugal
caudoventrally.
Braincase
The braincase of UCMP 154452 (Fig. 9) is well preserved and
reveals the substantial amount of ontogenetic transformation
that takes place into adulthood. The bones of the braincase are
unfused and are loosely coalesced by overlapping rugose or
tongue-and-groove sutures. Important differences with the adult
ceratopsid braincase in the arrangement and articulation of the
basioccipital, exoccipitals, and supraoccipital are noted below.
Occipital Condyle—The occipital condyle is 31.9 mm medio-
laterally and 28.9 mm vertically. It is nearly oval and unfused and
is formed by a one-third contribution from the basioccipital and
one-third from each ventral exoccipital (Fig. 9). This arrange-
ment is typical of all ceratopsids but is often obscured by fusion
in adult skulls (Lehman, 1989; Chinnery, 2004). The dorsal su-
tural surface is irregularly grooved where it accepts the exoccipi-
tal. The short pedunculate neck of the condyle is constricted
dorsoventrally and transversely.
Basioccipital—The basioccipital is 48 mm long and 26.5 mm
wide. Caudally it is dominated by its contribution to the occipital
condyle. A median ridge divides the basioccipital. A distinctive
midline hourglass-shaped ridge and grooved sutural surface for
the alisphenoid are preserved. The rostrodorsal surface is rugose
where it articulates with the descending process of the exoccipi-
tal. Well-developed basioccipital tubera extend rostrolaterally.
The basioccipital is excluded from the foramen magnum by the
paired exoccipitals. A small portion of the left basisphenoid is
preserved in close contact along the rostroventral surface of the
basioccipital. The foramen magnum is 23.6 mm wide.
FIGURE 8. Right quadrate and quadratojugal of UCMP 154452, Tri-
ceratops, in caudal view. The quadratojugal overlaps the lateral quadrate
along a close-fitting, broad, sutural contact. Abbreviations:c, quadrate
condyle articulates with the surangular; j, sutural surface for the over-
lapping jugal spur; q, quadrate; qj, quadratojugal.
FIGURE 6. Right prefrontal of UCMP 154452, Triceratops. Right pre-
frontal in dorsal (A), ventral (B), lateral (C), and medial (D) views.
Postorbital and frontal sutural surfaces are heavily rugose. Abbrevia-
tions:f, sutural surface for frontal; po, sutural surface for postorbital.
FIGURE 7. Right jugal of UCMP 154452, Triceratops, in lateral (A)
and medial (B) views. Abbreviations:qj, sutural surface for the quadra-
tojugal; sq, sutural surface for the squamosal.
GOODWIN ET AL.—SMALLEST TRICERATOPS SKULL 107
Supraoccipital—In all adult ceratopsids, the supraoccipital is
excluded from the foramen magnum by the exoccipitals, which
unite above the foramen magnum (Hatcher, 1907; Dodson and
Currie, 1989; Forster, 1996b). UCMP 154452 does not share this
condition. At this very early stage of ontogeny, UCMP 154452
exhibits the primitive state in which the supraoccipital articulates
between the exoccipitals dorsomedially (see Fig. 9). This ar-
rangement is seen in the more basal Protoceratops andrewsi
(Brown and Schlaikjer, 1940; Dodson and Currie, 1989; Hailu
and Dodson, 2004). Contribution of the supraoccipital to the
foramen magnum also occurs in the neoceratopsian Leptocera-
tops gracilis (Sternberg, 1951) and Bagaceratops rozhdestvenskyi
(Maryanska and Osmólska, 1975). Chinnery and Weishampel
(1998) confirm that the paired exoccipitals meet above the fora-
men magnum and exclude the supraoccipital from any contribu-
tion to the foramen magnum in a juvenile braincase of Montan-
aceratops cerorhynchus (MOR 542). Fusion of the supraoccipital
with the exoccipitals obscures this arrangement in an isolated
braincase of M. cerorhynchus (AMNH 5244) described by Ma-
kovicky (2001). In UCMP 154452, the supraoccipital is posi-
tioned firmly between the exoccipitals and forms the dorsal roof
of the foramen magnum and the rostral endocranial cavity. Later
in ontogeny, the supraoccipital is pushed upward and becomes
completely underlain by the exoccipitals along a broad sutural
contact (Dodson et al., 2004) in adult Triceratops.
The caudal surface of the supraoccipital is abraded and broken
sagittally. A ridge of bone forms a midline crest. Small depres-
sions are present on either side of the crest. This area serves as
the attachment site for the epaxial musculature of the neck and
is significantly more developed in adult ceratopsids, where these
deep, paired depressions are separated by a thin vertical septum
of bone (Hatcher et al. 1907; Ostrom and Wellnhofer, 1986). The
ventral surface of the supraoccipital is rugose along the sutural
surface for articulation with the exoccipitals. The roof of the
braincase dominates the internal surface. The entrance and exit
foramina for the auditory nerve are preserved.
Exoccipital—The exoccipitals extend as a “wing”of bone lat-
erally from the foramen magnum, forming a buttress that con-
tacts the ventral surface of the squamosal (see Fig. 4B). Accord-
ing to Ostrom and Wellnhofer (1986:123), this configuration pro-
vides major support for the entire frill along this junction of the
quadrate and squamosal contact. This arrangement remains con-
sistent into adulthood. The right exoccipital is more complete
than the left and preserves the relatively large exit foramina for
cranial nerves IX–XI and XII with a septum of bone between the
foramina (Fig. 9).
Surangular
The right surangular (Fig. 10) is 62.5 mm in length. It varies in
thickness from 3.5 mm at the most rostral edge where it meets
the dentary to 10.7 mm caudally along the articular surface for
the quadrate. Rostromedially, the surface is striated where it
articulates with the dentary. The mandibular foramen is pre-
served along the upper portion of the dorsal surface. The suran-
gular is laterally convex and slightly concave medially. The sur-
angular thickens and develops a flat curved shelf caudally. This
shelf is deflected laterally, flattened dorsally, and articulates with
the quadrate. Caudomedially, the surangular is sharply concave
where it meets the articular.
Dentary
The dentary (Fig. 11) is 160 mm long and allows determination
of the maximum skull length and restoration of the skull in
Fig. 1. The coronoid process is robust, offset laterally, and curves
rostrally. Ostrom and Wellnhofer (1989) interpreted the sturdy
coronoid process as a critical attachment site for powerful ad-
ductor musculature. The dentary is straight, convex laterally and
concave medially except for the dental battery, which is nearly
vertical. The rostral edge is relatively thicker (6.5–10.4 mm)
where it meets the predentary bone. Caudally it is 4.4 mm thick.
Medially, the symphyseal surface is indicated by horizontal stria-
tions on the thickened bony facet. Vertically, the coronoid pro-
cess is offset about 30 degrees to the axis of the dentary. A strong
lateral ridge runs along the length of the dentary ventrolaterally.
Below this ridge, the ventral surface of the dentary is flattened
and striated where the splenial would lie longitudinally. A deep
adductor fossa is present caudomedially below the coronoid pro-
cess. Even at this young age, the fossa is relatively large and
sufficient as a major insertion site for the M. adductor posterior
(Ostrom and Wellnhofer, 1989). The Meckelian groove extends
ca. 48 mm along the rostromedial surface of the adductor fossa
from the caudal edge of the dental battery. A shallow longitu-
dinal groove lies along the ventral border of the fossa caudally,
indicating the place of attachment for the M. intramandibularis
(Ryan and Currie, 1998). The dental battery is 107 mm long and
FIGURE 10. Right surangular of UCMP 154452, Triceratops,inob-
lique view. Abbreviations:ar, articular articulation; d, dentary articula-
tion; mf, mandibular foramen; q, quadrate articulation.
FIGURE 9. Braincase of UCMP 154452, Triceratops, in occipital view.
In all ceratopsids, the supraoccipital is excluded from the foramen mag-
num as the exoccipitals unite above the foramen magnum, but not at this
early ontogenetic stage in UCMP 154452. Here, UCMP 154452 exhibits
the primitive state: the supraoccipital articulates between the exoccipitals
dorsomedially. The occipital condyle is formed by a one-third contribu-
tion from the basioccipital and one-third from each exoccipital. Exit
foramen for cranial nerves IX–XI and XII are noted. Abbreviations:bo,
basioccipital; exo, exoccipital; fm, foramen magnum; oc, occipital con-
dyle; so, supraoccipital.
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 1, 2006108
occupies a large extent of the dentary. At least 20 alveoli are
present along the length of the dental battery. No teeth are
preserved in the jaw. Laterally, the dentary bears two rows of
foramina. According to Lehman (1989), these foramina probably
communicate with the mandibular fossa, carrying branches of
the mandibular ramus of the trigeminal nerve, the mandibular
artery, and veins to the tissues of the cheek and predentary.
Rostrally, an upper third row of foramina is present.
Maxillary
A 55.3-mm-long left maxillary fragment was recovered with
the skull. Seven dental grooves are preserved on the interior
surface. No maxillary teeth are preserved in position. The dental
magazine is relatively delicate, and the bone becomes more ro-
bust along the caudodorsal margin.
Teeth
Isolated leaf-shaped teeth were found associated with the
skull. They are double-rooted with a strong median ridge and a
lingual covering of enamel.
Vertebrae
At least three isolated, fragmentary vertebrae were recovered
with the skull. Fragments of their centra reveal a very spongy
interior surrounded by a relatively thin periosteal exterior.
Toothed sutures remain open on the dorsal surface of the centra,
indicating that the neural spines are unfused. This is not un-
expected in such a small, young individual.
Ossified Tendons
Fragments of ossified tendons were closely associated with the
skull. The largest piece is 38.1 mm long and ca. 4 mm in diameter.
The medial surface is incompletely striated and the remaining
surface is smooth. One fragment is triangular and broad crani-
ally, like the adult tendon.
DISCUSSION
In adult Triceratops, the large, solid, saddle-shaped frill is 65–
70% of the basal skull length (⳱caudal surface of basioccipital
to tip of rostral bone; Forster, 1996b). In UCMP 154452, the frill
is only 48% of the estimated basal skull length. Although it
already has small postorbital horns and a solid, scalloped frill
that closely resembles the epoccipital-bordered adult skull,
UCMP 154452 had proportions very different from those of the
adult skull. The parietosquamosal frill is short and square,
whereas in adults it is elongate, fan-like, and more concave. Fea-
tures of the braincase in UCMP 154452 recall the adult condition
of more basal neoceratopsians (Brown and Schlaikjer, 1940b).
Here, the exoccipitals unite below but not above the foramen
magnum, allowing the supraoccipital to form the dorsal margin
of the foramen magnum and contribute to the roof of the brain-
case (Fig. 9). This condition is shared with Protoceratops, even as
adults (Brown and Schlaikjer, 1940b, Dodson and Currie, 1990;),
but is lost in adult Triceratops where the exoccipitals unite above
the foramen magnum, excluding the supraoccipital (Hatcher et
al., 1907; Brown and Schlaikjer, 1940b). In this sense, some ju-
venile features of Triceratops recapitulate a character state of
more basal neoceratopsians as might be expected. Evidence pre-
FIGURE 11. Right dentary of UCMP 154452, Triceratops, in lateral (A) and medial (B) views. The dental battery occupies a large area of the lower
jaw. Abbreviations:a, alveoli; af, adductor fossa; c, coronoid process; d, dentary; pd, predentary articulation; and sym, symphyseal surface.
GOODWIN ET AL.—SMALLEST TRICERATOPS SKULL 109
sented by Gilmore (1917) and more recently by Lehman (1989)
suggests that this arrangement of the supraoccipital may be a
juvenile ceratopsid character. Gilmore (1917:fig. 11) determined
that the supraoccipital of the type of Brachyceratops montanensis
(USNM 7951) contributes to the formation of the foramen mag-
num in this immature ceratopsid. Lehman (1989:fig. 6B, C) ob-
served that the supraoccipital forms at least the caudal roof of
the endocranial cavity in an incomplete juvenile braincase re-
ferred to Chasmosaurus mariscalensis (UTEP P.37.7.068).
In contrast, certain juvenile features appear to anticipate the
structural condition of the adult Triceratops skull. First, although
no teeth were preserved in place in the dentary or the small
fragment of the maxillary, isolated teeth recovered from the sedi-
ment around the tiny skull share the crown pattern and double-
rooted form of adults (contra Carpenter, 1982). Second, the
lateral wings of the exoccipital expand from either side of the
occipital condyle into broad flanges that contact the ventral sur-
face of each squamosal. As in adults, this portion of the exoc-
cipital forms an expansive brace that provides primary support
for the overlying frill (Ostrom and Wellnhofer, 1986; Forster,
1996b). Third, a parasagittal row of low bosses ornaments the
superior surfaces of the postorbitals, squamosals, and the pari-
etal midline. The number of scallops on the caudal margin of the
squamosals and parietal equals the number of epoccipitals that
border the adult frill (Hatcher, 1907; Forster, 1996b). If the ep-
occipital-ornamented adult frill and postorbital horns served as
an important visual sign in species communication, then perhaps
the scalloped frill and horns of very young individuals served the
same function.
It has often been suggested that the ornamental skull features
of ceratopsids (horns and frills) reflect a role in mate competition
or species recognition (Forster and Sampson, 2002). Dimorphism
can be recognized in adequate population samples of taxa by a
divergence of biometric characters during ontogeny (Darwin,
1871). Among non-avian dinosaurs, sexual dimorphism has been
suggested in theropods (Colbert, 1990; Raath, 1990), hadrosau-
rids (Dodson, 1975; Hopson, 1975; Molnar, 1977), and ceratop-
sians (Ostrom and Wellnhofer, 1986; Dodson, 1996; Forster,
1996b) but in each case it is not extreme and has not been dem-
onstrated statistically. Only the basal ceratopsian Protoceratops
shows statistically significant dimorphism (Dodson, 1976), but it
is minor. Sexual dimorphism has been inferred for various cen-
trosaurines and chasmosaurines (Lehman, 1990; Forster, 1996b;
Sampson et al., 1997; Ryan et al., 2001), but again, this variation
has not been established statistically and has not been differen-
tiated from ontogenetic or within-normal-populational variation
(Padian et al., 2004). Dimorphism can be expressed early in on-
togeny, or as a late pulse that reflects maturity and agonistic
sexual behavior, usually among males, resulting in an extended
growth trajectory (Weckerly, 1998). This by itself does not con-
firm sexual display or associated mating behavior as the principal
function of low-level dimorphism; the morphology of horns and
frills may have served different functions at different times in an
individual’s life. We suggest visual communication and species
recognition, perhaps involving complex signaling (Ord et al.,
2001) as an alternative but not exclusive function of these cranial
ornaments.
In general, dinosaurian cranial display features, such as horns,
spikes, and bony pads in ceratopsids (Forster, 1996a, Sampson
et al., 1997), crests on hadrosaurid skulls (Horner and Currie,
1994), and the frontoparietal domes of pachycephalosaurids
(Goodwin et al., 1998; Williamson and Carr, 2002; Goodwin and
Horner, 2004) did not appear until later stages of development.
Immature centrosaurines of different genera have similar horn-
core ontogenies (Sampson et al., 1997); adult features of horns
and frills appear only late in ontogeny, suggesting a function in
sexual display or species/mate recognition. Sexual dimorphism
has also been inferred for some chasmosaurines (Dodson, 1996),
but is not generally accepted for either chasmosaurines or cen-
trosaurines (Dodson et al., 2004). The new tiny Triceratops
shows that the normally late-developing features often associ-
ated (if dimorphic) with sexual display began to be expressed at
a very early age. This pattern appears likely for Chasmosaurus
(Lehman, 1989; 1990), though at relatively larger size and pre-
sumably later age. This clearly derived condition within chasmo-
saurines, given current knowledge of ceratopsian ontogeny and
phylogeny, suggests a heterochronic shift of the expression of
these characters. However, hypotheses of heterochrony can only
be tested by comparative ontogenies, which at present are insuf-
ficient.
The basal neoceratopsian Protoceratops expresses some fea-
tures, such as a nasal boss and a vertical tilt to the frill, only late
in life, suggesting that the centrosaurine pattern may be primi-
tive. However, specimens referred to Zuniceratops (Wolfe and
Kirkland, 1998) appear to show long-developing horns that are
present in juveniles, like chasmosaurines but unlike centrosau-
rines. The phylogenetic placement of cf. Zuniceratops outside
Ceratopsidae (see Dodson et al., 2004) suggests that the function
of cranial structures in species recognition may have preceded
the divergence of centrosaurines and chasmosaurines. If so, then
the late-developing structures of centrosaurines would be de-
rived, heterochronically shifted features and could perhaps be
exaptively linked to sexual display (Lehman, 1990; Ryan et al.,
2001), if significant dimorphism can be established.
A function in sexual display (Farlow and Dodson, 1975; Mol-
nar, 1977; Sampson, 1997) or resisting predators (Colbert, 1948,
1961; Molnar, 1977) has long been the dominant model for cra-
nial ornamentation in dinosaurs, despite little evidence for sex-
ual dimorphism. However, it is difficult to support a hypothesis
of sexual display when the sexes show little or no evidence of
discrete morphological features apart from size (Darwin, 1871);
even so, hypotheses of a function in sexual display must be tested
by evidence beyond simple morphologic difference. Conversely,
species recognition is simply tested by the presence of low sexual
dimorphism with species-specific morphology that is apparent to
intra- and interspecific individuals (Vrba, 1984). Non-directional
morphologic trends in phylogeny and the presence of several
related sympatric or parapatric species are two tests of species
recognition as a factor that structures morphological diversity
(Padian et al., 2004). Ceratopsids pass these tests.
Low sexual dimorphism in ceratopsians supports our hypoth-
esis that the early ontogenetic expression of horn and frill mor-
phology in some ceratopsids reflects a visual cue for communi-
cation and species recognition; clearly these features appeared
well before sexual maturity in UCMP 154452. Extant African
bovids use an effective visual communication system that in-
volves horn morphology and body color (Vrba, 1984). These
explicit species differences have long histories of divergence and
sorting in bovid subclades (Vrba, 1984). Similar visual cues based
on horn and frill morphology may have stimulated greater spe-
cies diversity earlier in ceratopsian evolutionary history. Farlow
and Dodson (1974), Hopson (1975), and Sampson (1997) have
acknowledged the potential importance of species recognition in
dinosaur diversity.
The many forms of sexual dimorphism among birds are con-
ventionally split into body size and overall plumage-color attrib-
utable to melanins, carotenoids, and structural colors (Owens
and Hartley, 1998). Structural colors arise from the scattering of
ultraviolet light by collagen fibers. Prum et al. (1994, 1999) report
how skin color in the face and head is used by an assortment of
extant birds for visual communication. Deep vesicle grooves in
the horns and frill of Triceratops indicate that their skull was
covered with hard, keratinous skin (Horner and Marshall, 2002).
Paired with the visually dominating frill and horns, skin color
may have enhanced intraspecific communication. We propose
that species recognition is at least as plausible as sexual display in
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 26, NO. 1, 2006110
explaining the diversity of horn and frill structures in Ceratop-
sidae (Centrosaurinae plus Chasmosaurinae) and their near rela-
tives, and that substantial sexual dimorphism has not yet been
established in ceratopsids.
Nearly all anatomical and behavioral studies of Triceratops
have been based on adult skulls. The development of larger,
more visible postorbital horns, a massive nasal horn, and the
ossification of epoccipitals along the frill margin may have sig-
naled sexual maturity and the onset of mating in adult Tricera-
tops. Functional analyses of these cranial features in adult Tri-
ceratops have often restricted their role to sexual display and
mating behavior, based on the presumption that these features
were not present in very young individuals and did not express
themselves until adulthood (Sampson et al., 1997; Dodson and
Currie, 1990; Sampson, 2001; but see Farke, 2004). To the con-
trary, UCMP 154452 demonstrates that in very young Tricera-
tops, these species-specific cranial characters began to be formed
early in life—ostensibly earlier than in centrosaurine ceratop-
sids—and may have been important for visual communication
and species recognition even at this early stage.
CONCLUSION
UCMP 154452 documents the youngest ontogenetic stage of
Triceratops and illuminates a great transformation in size, shape,
and rearrangement of cranial elements that occurred in the skull
during growth into adulthood. As the smallest ceratopsid skull
known, it provides a new end member for the youngest stage of
cranial ontogeny in Triceratops. The patent cranial sutures reveal
morphology that is often concealed in adult skulls. Juvenile fea-
tures of the braincase in UCMP 154452 recapitulate the primitive
character state of more basal neoceratopsians. On the other
hand, cranial ornamentation in the frill and the postorbital horns
of Triceratops were not restricted to adult members, but began at
an early age. The appearance of horns and a scalloped frill at this
small size and early age is support for the hypothesis that cranial
ornaments in ceratopsids were at least as important as a visual
organ for species communication as they may have been for
sexual display or agonistic encounters.
ACKNOWLEDGMENTS
We thank Mr. Harley J. Garbani, who discovered the skull and
skillfully prepared the delicate specimen. We also thank Laura
Cunningham for her illustration of the skull; Karen Klitz for her
final illustration of the skull, pencil drawings of the individual
cranial elements, and figure layout; Jim Hendel and Patricia Hol-
royd for photographic assistance; Sterling Nesbitt and J. Howard
Hutchison for helpful discussion. Michael Holland and Jane Ma-
son undertook final restoration, molding and casting of the skull.
We thank Brenda Chinnery, Peter Dodson, Thomas Lehman,
and Scott Sampson for helpful comments on earlier versions of
this manuscript. We appreciate the editorial assistance of Nicho-
las Fraser, David Weishampel, and Mark Wilson. Financial sup-
port from the University of California Museum of Paleontology
is gratefully acknowledged. This is University of California Mu-
seum of Paleontology contribution no. 1883.
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Submitted 4 January 2005; accepted 13 June 2005.
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