Evolutionary trends in Triceratops from the Hell Creek
John B. Scannellaa,b,1, Denver W. Fowlera,b, Mark B. Goodwinc, and John R. Hornera,b
aMuseum of the Rockies andbDepartment of Earth Sciences, Montana State University, Bozeman, MT 59717; andcMuseum of Paleontology, University of
California, Berkeley, CA 94720
Edited by Gene Hunt, Smithsonian Institution, Washington, DC, and accepted by the Editorial Board June 1, 2014 (received for review July 16, 2013)
The placement of over 50 skulls of the well-known horned
dinosaur Triceratops within a stratigraphic framework for the Up-
per Cretaceous Hell Creek Formation (HCF) of Montana reveals the
evolutionary transformation of this genus. Specimens referable to
the two recognized morphospecies of Triceratops, T. horridus and
T. prorsus, are stratigraphically separated within the HCF with the
T. prorsus morphology recovered in the upper third of the forma-
tion and T. horridus found lower in the formation. Hypotheses
that these morphospecies represent sexual or ontogenetic variation
within a single species are thus untenable. Stratigraphic placement
of specimens appears to reveal ancestor–descendant relationships.
Transitional morphologies are found in the middle unit of the for-
mation, a finding that is consistent with the evolution of Triceratops
being characterized by anagenesis, the transformation of a lineage
over time. Variation among specimens from this critical stratigraphic
zone may indicate a branching event in the Triceratops lineage.
Purely cladogenetic interpretations of the HCF dataset imply greater
diversity within the formation. These findings underscore the critical
role of stratigraphic data in deciphering evolutionary patterns in
Hell Creek Formation (HCF), provides insights into the paleo-
biology and evolution of the last nonavian dinosaurs (1). Tri-
ceratops (Ceratopsidae: Chasmosaurinae) is the most abundant
dinosaur in the HCF; >50 skulls, including previously unknown
or rare growth stages, have been collected throughout the entire
formation (spanning ∼1–2 million y) (2) over the course of the
Hell Creek Project (1, 3–5). The combination of a strati-
graphically controlled robust sample from the entire ∼90-m-thick
HCF and identification of ontogenetic stages makes Triceratops
a model organism for testing hypotheses proposed for the modes
of dinosaur evolution (e.g., refs. 6–8).
Since its initial discovery (9), as many as 16 species of Tri-
ceratops were named based on variations in cranial morphology
(10, 11). Forster (12) recognized only two species, Triceratops
horridus and Triceratops prorsus, based on cranial features in-
cluding differences in relative length of the postorbital horn
cores (long in T. horridus and shorter in T. prorsus), morphology
of the rostrum (elongate in T. horridus and shorter in T. prorsus),
and closure of the frontoparietal fontanelle (sensu Farke) (13)
(open in T. horridus and closed in T. prorsus). Marsh initially
distinguished these two species by the morphology of the nasal
horn (14); the type specimen of T. horridus possesses a short,
blunt nasal horn whereas the nasal horn in T. prorsus is elongate.
Whether or not these taxa were largely biogeographically sepa-
rated or represented ontogenetic variants or sexual dimorphs
within a single species has remained unresolved (8, 10, 12, 15–
18). A record of the stratigraphic distribution of Triceratops from
the Upper Cretaceous Lance Formation of Wyoming compiled
by Lull (19, 20) suggested that these taxa overlap stratigraphically.
However, this assessment was likely based on limited stratigraphic
data (15) and “the precise stratigraphic placement of these
specimens can no longer be established” (ref. 10, p. 155). As such,
he Hell Creek Project (1999–2010), a multiinstitutional sur-
vey of the fauna, flora, and geology of the Upper Cretaceous
consideration of morphological variation in a detailed stratigraphic
context is necessary to reassess systematic hypotheses.
Stratigraphic placement of Triceratops specimens within the HCF
reveals previously undocumented shifts in morphology. The
HCF is divided into three stratigraphic units: the lower third
(L3), middle third (M3), and upper third (U3) (1, 21). The
stratigraphic separation of Triceratops morphospecies is apparent
with specimens referable to T. prorsus (following Forster) (12)
found in U3 and T. horridus recovered only lower in the HCF.
Specimens from the upper part of M3 exhibit a combination of
T. horridus and T. prorsus features (Fig. 1, SI Text, and Fig. S1).
L3 Triceratops. Triceratops from the lowermost 15–30 m of the
HCF (L3) possess either a small nasal horn (Fig. 2A and Dataset
S1) or a low nasal boss. The boss morphology appears in a
large individual that histologically represents a mature specimen
[= “Torosaurus” ontogenetic morph (ref. 3; but see also refs. 22–
24)] [Museum of the Rockies (MOR) specimen 1122] (SI Text).
The nasal process of the premaxilla (NPP) in L3 Triceratops is
narrow (Fig. 2B and Fig. S2) and strongly posteriorly inclined;
a pronounced anteromedial process is present on the nasal (Fig.
S3). The frontoparietal fontanelle remains open until late
in ontogeny (MOR 1122). Specimens from the lower unit of
the HCF bear a range of postorbital horn-core lengths (ranging
from ∼ 0.45 to at least 0.74 basal-skull length) (Fig. 2D and
The deciphering of evolutionary trends in nonavian dinosaurs
can be impeded by a combination of small sample sizes, low
stratigraphic resolution, and lack of ontogenetic (developmental)
details for many taxa. Analysis of a large sample (n > 50) of the
famous horned dinosaur Triceratops from the Hell Creek For-
mation of Montana incorporates new stratigraphic and onto-
genetic findings to permit the investigation of evolution within
this genus. Our research indicates that the two currently rec-
ognized species of Triceratops (T. horridus and T. prorsus) are
stratigraphically separated and that the evolution of this genus
likely incorporated anagenetic (transformational) change. These
findings impact interpretations of dinosaur diversity at the end
of the Cretaceous and illuminate potential modes of evolution
in the Dinosauria.
Author contributions: J.B.S., D.W.F., M.B.G., and J.R.H. designed research; J.B.S., D.W.F.,
M.B.G., and J.R.H. performed research; J.R.H. contributed new reagents/analytic tools; J.B.S.
and D.W.F. analyzed data; and J.B.S., D.W.F., and M.B.G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. G.H. is a guest editor invited by the Editorial
Data deposition: Nexus files are available at Morphobank.org (project number 1099).
1To whom correspondence should be addressed. E-mail: email@example.com.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
www.pnas.org/cgi/doi/10.1073/pnas.1313334111PNAS Early Edition
| 1 of 6
M3 Triceratops. The mean nasal-horn length increases through
M3 (Figs. 1 E and F and 2A). The University of California Mu-
seum of Paleontology (UCMP) specimen 113697 (collected ∼6 m
below the base of U3) possesses a nasal horn that is elongate
(length/width: 2.12) (Dataset S1) but retains a broad posterior
surface, giving the horn a subtriangular cross-section. Forster
(12) noted that UCMP 113697 exhibits a small nasal boss pos-
terior to the nasal horn. Disarticulated specimens (e.g., MOR
3027 and MOR 3045) reveal that this protuberance posterior to
the epinasal appears to be formed by the combination of a pos-
terior projection on the epinasal (Fig. S4) and the anteriormost
nasal. A homologous morphology is observed in specimens from
L3 and the lower half of M3 (MOR 1120, MOR 2982, and MOR
3010). UCMP 128561, from the upper half of M3, exhibits a low
nasal boss (25, 26) (SI Text). The anteromedial process of the
nasal is pronounced in Triceratops from M3, and the NPP is more
vertically inclined in specimens from upper M3, producing a
more convex rostrum morphology, which was previously found to
characterize T. prorsus (12, 23). The frontoparietal fontanelle is
open in late-stage subadults/young adults (UCMP 113697).
U3 Triceratops. Specimens from U3 exhibit the features Forster
(12) found to characterize T. prorsus. U3 Triceratops possess an
elongate, relatively narrow nasal horn (average length/width > 2)
(Fig. 2A and Dataset S1). The NPP is more vertically inclined,
producing a convex rostrum lacking the low, elongate profile
noted in T. horridus [although the largest, and presumably oldest,
known specimens (e.g., MOR 004 and MOR 1625) exhibit pro-
portionally longer rostra] (Fig. 2E and Dataset S1). The NPP is
anteroposteriorly expanded, and the anteromedial process of the
nasal is greatly reduced (Fig. S3) (27). The frontoparietal fon-
tanelle becomes constricted and eventually closed in late-stage
subadults/young adults (e.g., MOR 2923 and MOR 2979), on-
togenetically earlier than in L3 and M3. The postorbital horn
cores are short (<0.64 basal-skull length) (Fig. 2D). Further, U3
Triceratops seem to exhibit nasals that are more elongate than
Triceratops from the lower half of the HCF (Fig. 2F and Dataset S1).
Shifts in Morphology over Time. Epinasals exhibit a directional
morphologic trend; average length increases throughout the for-
mation (Fig. 2A and Dataset S1) (Spearman’s rank coefficient =
0.824, P = 4.15E−07). A protuberance just posterior to the epi-
nasal, observed in specimens from L3 and M3 (Fig. 1), is partic-
ularly pronounced in UCMP 113697 from the uppermost M3 (Fig.
1E). U3 Triceratops either do not exhibit this feature or express
only a subtle ridge in the homologous location. Concurrent with
elongation of the epinasal was an expansion of the NPP (Fig. 2B)
(Spearman’s rank coefficient = −0.969, P = 3.74E−06) and an
increase in the angle between the NPP and the narial strut of the
premaxilla (Fig. 2C and Dataset S1) (Spearman’s rank co-
efficient = 0.802, P = .000186). Nasals also become more
elongate relative to basal skull length (although only three
specimens with complete nasals have thus far been recorded
from the lower half of the formation) (Fig. 2F and Dataset S1)
(Spearman’s rank coefficient = 0.804, P = 0.00894).
Postorbital horn-core length appears to be variable throughout
L3 and M3 and is consistently short in U3 Triceratops (Fig. 2D
and Dataset S1) [Spearman’s rank coefficient is negative (−0.197)
reveals trends in cranial morphology including elon-
gation of the epinasal and change in morphology of
the rostrum. (A) HCF stratigraphic units. (B) Magne-
tostratigraphic correlation (45, 46). NS, no signal, so
precise position of C29R-C30n boundary is unknown.
(C) Stratigraphic positions of Triceratops specimens
within a generalized section. Specimens plotted by
stratigraphic position, not by facies. Relative posi-
tion of specimens from different areas are approx-
imate at the meter scale (5). See refs. 1 and 5 for
further specimens for which more precise position
(beyond HCF unit) is to be determined. Scale in
meters. (D) MOR 2702 nasal horn from U3. (E) UCMP
113697 nasal horn from upper part of M3; black ar-
row indicates epinasal-nasal protuberance. (F) MOR
2982 nasal horn from lower part of M3 (image mir-
rored). (G) MOR 1120 nasal horn from L3. (H) MOR
004 young adult Triceratops skull from U3 (cast; im-
age mirrored). Postorbital horn cores reconstructed
to approximate average length of young adult
specimens from this unit. (I) UCMP 113697 late-stage
subadult/young adult Triceratops skull from the
upper part of M3. (J) MOR 1120 (cast) subadult
Triceratops skull from L3. Figure modified from ref.
5. f, fine sand; m, medium sand. (Scale bars: 10 cm.)
Stratigraphic placement of HCF Triceratops
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| www.pnas.org/cgi/doi/10.1073/pnas.1313334111Scannella et al.
and not statistically significant (P = 0.392)]. Large juvenile U3
Triceratops (e.g., MOR 1110) can possess more elongate post-
orbital horn cores (0.64 basal-skull length). Whereas U3 post-
orbital horn core length falls within the range of variation
observed lower in the formation (Fig. 2D), elongate postorbital
horn cores have thus far not been found in post-juvenile stage
Triceratops from U3. Many large Triceratops (e.g., MOR 1122
and MOR 3000) (3) exhibit evidence of postorbital horn-core
resorption, suggesting that maximum length is reached earlier in
ontogeny. Maximum postorbital horn-core length may have been
expressed later in development (or for a longer duration) in
Triceratops from lower in the formation.
Triceratops from the upper half of the HCF exhibit a more
vertically inclined NPP (Fig. 2C), which contributes to a rostrum
that appears shorter and more convex in lateral profile (a feature
Forster noted in T. prorsus) (12). However, we note that a
Spearman’s rank correlation test found apparent reduction in
rostrum length to be statistically insignificant (Spearman’s rank
coefficient 0.018, P = 0.966). Large specimens from U3 (e.g.,
MOR 004) possess a more elongate rostrum relative to basal-
skull length (Fig. 2E and Dataset S1); however, the shape of U3
rostra appears to be consistently convex.
Eotriceratops xerinsularis, found in the stratigraphically older
uppermost Horseshoe Canyon Formation (∼68 Ma) (28), ex-
presses morphologies (elongate postorbital horn cores, small
nasal horn) consistent with its stratigraphic position relative
Cladistic and Stratocladistic Analyses. Initial cladistic analyses re-
covered a polytomy of all HCF specimens, with the 50% majority
tree producing a succession of Triceratops that largely correlates
with stratigraphic placement (Fig. S5 and SI Text). Removal of
the more fragmentary material recovered Torosaurus specimens
as basal to a stratigraphic succession of Triceratops, including
a polytomy of specimens from the upper half of the formation
(Fig. 3A). A similar topology was recovered when specimens not
exhibiting codeable features of the parietal squamosal frill were
removed from the analysis (Fig. 3B). Removal of MOR 2924,
a specimen from the base of U3 that does not preserve post-
orbital horn cores (SI Text), recovers specimens from the upper
part of M3 as basal to U3 Triceratops.
In the analysis of the most reduced dataset, UCMP 113697
and MOR 3027 cluster together (Fig. 3C). These specimens
exhibit a combination of characters found in Triceratops from L3
and M3. The epinasal of UCMP 113697 is morphologically in-
termediate between L3 and U3 Triceratops (the epinasal of MOR
3027 is incomplete). These specimens each exhibit large post-
orbital horn cores (a feature expressed in some L3 Triceratops)
and a more vertically inclined NPP (found in U3 Triceratops).
MOR 3045 is recovered as being more derived than UCMP
113697 and MOR 3027 (Fig. 3) based on its possession of rela-
tively short postorbital horn cores, a more expanded NPP, and
a pronounced step bordering the “incipient fenestrae” (sensu ref.
3) (SI Text). This specimen exhibits the basal condition of the
anteromedial nasal process and expresses a pronounced upturn
of the posterior surface of the epinasal, suggesting the presence
of a protuberance in life. MOR 3045 exhibits a fairly elongate
epinasal (estimated length/width, ∼1.88), with a posterior surface
that is broader than is seen in most U3 specimens and, like
UCMP 113697, MOR 3027, and U3 Triceratops, exhibits a more
vertically inclined NPP.
Stratocladistic analyses, in which specimens were grouped into
operational units based on stratigraphic position, were per-
formed in the program StrataPhy (29). Torosaurus specimens
were initially considered separately from other specimens (SI
Text). Initial results suggested that specimens from the upper
half of the HCF represented a sequence of ancestors and
descendants but differed on the position of operational units
from the lower half of the formation (Fig. S6A). This result was
likely influenced by missing data for specimens from the lower
half of the formation; no specimens from lower M3 preserve frill
characters that can distinguish them from the Torosaurus mor-
phology. When Torosaurus specimens were incorporated into
Triceratops operational units, three topologies were produced:
a strictly cladogenetic result, a topology in which all operational
units except lower M3 were recovered in a transformational se-
quence, and a topology in which the HCF operational units were
recovered in two lineages (an upper L3/lower M3 lineage and an
upper M3/U3 lineage) that had diverged at some point in the
deposition of L3 (Fig. S6B). Pruning of Torosaurus specimens
from the dataset produced two topologies that incorporated
morphological transformation: one topology in which all HCF
operational units fell into a single lineage and another topology
presenting two HCF lineages that diverged either in L3 or before
deposition of the HCF (Fig. S6C).
Evolutionary Patterns. One of the principle questions in evolu-
tionary biology regards the modes of evolution: what evolutionary
patterns are preserved in the fossil record and how prominent are
these patterns (30–32)? Small sample sizes for most nonavian
dinosaur taxa complicate the investigation of evolutionary modes
in this group. As such, it is unknown how prominent a role ana-
genesis (the transformation of lineages over time) (Fig. 4A) (33–
37) played in their evolution or whether the majority of
morphologies recorded in the fossil record were a product of
divided into upper and lower sections. For L3, the upper part is here desig-
nated as strata above the basal sandstone. For M3, the upper part is here
considered the upper 15 m of the unit. The 10-m sandstone and above is here
considered the upper section of U3. (A) Epinasal length/width. MOR 3011
denoted by a gray diamond as the fragmentary nature of this specimen
obscures its ontogenetic status. Square represents UCMP 128561. Results of
Spearman’s rank correlation analysis in white box. (B) Nasal process of the
premaxilla (NPP) height/width. (C) Angle between the NPP and the narial strut
of the premaxilla. (D) Postorbital horn core length/basal skull length. Esti-
mated total lengths are used here for postorbital horn core length (Dataset
S1). (E) Rostrum length/basal skull length. (F) Nasal length/basal skull length.
Vertical lines denote the mean (not including juvenile or taphonomically
deformed specimens) (Dataset S1). Dark gray bars represent SE. Black rings
denote juveniles. Gray ring represents Royal Tyrrell Museum (RTMP) specimen
2002.57.7. Spearman’s rank correlation analyses exclude juveniles and
taphonomically distorted specimens (indicated by diamonds). Asterisks in-
dicate statistically significant P values. Scale on x axes are logarithmic.
Stratigraphic variation in cranial morphology. Each unit of the HCF is
Scannella et al.PNAS Early Edition
| 3 of 6
cladogenesis (evolution via branching events) (Fig. 4 B–D) (8,
32, 33, 37, 38).
Horner et al. (6) presented evidence for anagenesis in several
dinosaur clades within the Cretaceous Two Medicine Formation
of Montana. It has been suggested that the ceratopsid sample
size presented in that study was too small and that cladogenesis
was a more conservative interpretation of the data (7). A com-
bination of large sample size, ontogenetic resolution, and de-
tailed stratigraphic data makes Triceratops an ideal taxon for
testing hypotheses regarding evolutionary mode in a nonavian
Restriction of the full T. prorsus morphology to U3 renders
untenable hypotheses that T. horridus and T. prorsus represent
sexual or ontogenetic variation within a single taxon. Triceratops
from the upper part of M3 exhibit a combination of features
found in L3 and U3 Triceratops. This pattern suggests that the
evolution of Triceratops incorporated anagenesis.
Strict consensus trees produced by cladistic analyses either
recover upper M3 specimens in a polytomy with all HCF speci-
mens, in a polytomy of HCF Triceratops from the upper half of
the formation, or UCMP 113697 and MOR 3027 cluster together
whereas MOR 3045 shares more features with U3 Triceratops
(Fig. 3 and Fig. S5). We will consider four alternative hypotheses
for the morphological pattern recorded in the HCF:
i) T. prorsus evolved elsewhere and migrated into the HCF,
eventually replacing the incumbent HCF Triceratops popu-
lation by the beginning of the deposition of U3. Upper M3
specimens represent early members (or close relatives of)
this group that would come to dominate the ecosystem.
ii) Variation between MOR 3045, MOR 3027, and UCMP
113697 represents intraspecific (or intrapopulation) varia-
tion. As the HCF Triceratops lineage evolved, some individ-
uals expressed more of the features that would eventually
dominate the population. Over time, these traits were se-
lected for and characterized U3 Triceratops. This is a purely
iii) A bifurcation event is recorded in the HCF and occurred at
some point before the deposition of U3, resulting in two
lineages that differ primarily in the morphology of the epi-
nasal and rostrum (consistent with Forster’s diagnoses for
T. horridus and T. prorsus). MOR 3045 represents an early
member of a lineage that evolved into U3 Triceratops. This
scenario incorporates anagenesis (38) and is presented in
some trees produced by the stratocladistic analysis (Fig. S6).
iv) The evolution of Triceratops was characterized by a series of
cladogenetic events that produced at least five taxa over the
course of the deposition of the HCF (the L3 clade, the lower
M3 clade, the MOR 3027 clade, the MOR 3045 clade, and
the U3 clade). This strictly cladogenetic scenario suggests
that no Triceratops found lower in the HCF underwent
evolutionary transformation into forms found higher in
A Biogeographic Signal? The Hell Creek Project’s stratigraphic
record of Triceratops is primarily restricted to northeastern
Montana. It has been hypothesized that T. horridus and T. prorsus
were largely biogeographically separated, with T. prorsus generally
restricted to the Hell Creek and Frenchman Formations and
T. horridus commonly found in the more southern Lance, Laramie,
and Denver Formations (15, 17). However, this suggested bio-
geographic segregation may represent an artifact of the strati-
graphic record. Specimens that have thus far been described from
neighboring coeval formations exhibit morphologies consistent
with their stratigraphic position relative to the HCF (39, 40)
produced by analysis of HCF specimens using a heuristic search and multistate
coding once the most fragmentary specimens were removed (See SI Text and Fig.
S5 for additional results). Bootstrap support values below nodes. Bremer support
values greater than 1 above nodes. Torosaurus specimens are recovered as basal
to a stratigraphic succession of specimens. MOR 3011 does not preserve char-
acters of the parietal-squamosal frill. (SI Text). (B) Analysis (multistate coding,
branch-and-bound search) excluding specimens that could not be coded for at
least 10 cranial characters or characters of the frill. (C)Analysis (multistatecoding,
branch-and-bound search) after removal of MOR 2924 (SI Text).
Results of cladistic analysis of HCF Triceratops. (A) Strict consensus tree
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| www.pnas.org/cgi/doi/10.1073/pnas.1313334111 Scannella et al.
Anagenesis and Cladogenesis. If the morphological trends noted in
Triceratops were purely the result of cladogenetic branching
(consistent with punctuated equilibrium) (32) (Fig. 4 C and D),
we would expect to find the full U3 morphology coexisting with
Triceratops found lower in the formation, or alternatively, speci-
mens exhibiting the L3 morphology in U3. Such specimens have
yet to be discovered (SI Text). Specimens from the upper part of
M3 exhibit transitional features relative to L3 and U3 Triceratops,
a pattern consistent with anagenesis.
Some cladistic analyses distinguish MOR 3045 from other
upper M3 Triceratops based on variation in the length of the
postorbital horn cores, width of the NPP, and the thickened
regions of the parietal (Fig. 3, Fig. S5, and SI Text). Triceratops
collected from a multiindividual bonebed in U3 (MOR locality
no. HC-430) (44) show variable morphology of the premaxillae
and parietal between individuals (Fig. S2) (41). This finding
suggests that the variation between upper M3 specimens may
represent intrapopulational, not taxonomic, variation (10). Indi-
viduals exhibiting more pronounced U3 character states may
have become increasingly abundant in the HCF Triceratops
population over time until, by the end of the Cretaceous, all
Triceratops exhibited these character states (Fig. 4A). Alterna-
tively, MOR 3045 may represent an early member of a U3
(T. prorsus) lineage, with MOR 3027 representing a separate
lineage. Stratocladistic analyses suggest the possibility of two
lineages in the HCF (Fig. 4B and Fig. S6); however, this scenario
would require the independent evolution of an enlarged epi-
nasal-nasal protuberance. A purely cladogenetic interpretation
of the HCF Triceratops dataset suggests the presence of at least
five stratigraphically overlapping taxa in the formation (Fig. 4 C
and D). This scenario is possible, but we would argue that
interpretations that incorporate populational transformation (ana-
genesis) are more conservative.
Specimens from upper M3 exhibit a combination of primitive
and derived characters, as well as more developed states of
characters expressed in L3 Triceratops. Forster (12) noted that,
whereas T. prorsus exhibited derived characters, no autapomor-
phic characters were recognized in T. horridus. This finding is
consistent with the hypothesis that the evolution of Triceratops
incorporated anagenesis and illustrates the potential difficulties
with defining species in evolving populations (6, 35). The HCF
dataset underscores the importance of considering morphologies
in a populational, rather than typological, context (42).
The documented changes in Triceratops morphology occurred
over a geologically short interval of time (1-2 million y) (2).
High-resolution stratigraphy is necessary for recognizing fine-
scale evolutionary trends. If cladogenesis is considered the pri-
mary mode of dinosaur evolution, a problematic inflation of
dinosaur diversity occurs.
Current evidence suggests that the evolution of Triceratops
incorporated anagenesis as there is currently no evidence for bio-
geographic segregation of contemporaneous Triceratops morpho-
species and there is evidence for the morphological transformation
of Triceratops throughout the HCF. This dataset supports hy-
potheses that the evolution of other Cretaceous dinosaurs may
have incorporated phyletic change (6, 43, 44) and suggests that
many speciation events in the dinosaur record may represent
bifurcation events within anagenetic lineages (38).
Materials and Methods
Most specimens in this study were placed in section relative to either the
upper and/or lower formational contacts, respectively, marked by the
overlying Fort Union or underlying Fox Hills Formations (5). For some
specimens, stratigraphic precision was increased by measuring position rel-
ative to marker sandstones (1). The base of each unit in the HCF near Fort
Peck Lake is marked by a prominent amalgamated channel sandstone that
consistently occurs in the same stratigraphic position (1). The Basal sand, Jen-
rex sand, and Apex sand mark the bases of the Lower, Middle, and Upper
units, respectively (1, 21) (Fig. 1C). Each sandstone complex fines upwards
into overbank mudstones and siltstones. MOR 981 was collected from
a mudstone horizon above the basal sandstone; more detailed stratigraphic
data are unavailable for this specimen.
The boundary between the C30N and C29R magnetozones occurs either at
the base, or in the middle of the Apex sand (base of U3) (Fig. 1). Samples
taken within the sandstone do not produce a signal, but samples from above
the sandstone are of reversed polarity (C29r) whereas those below are
normal polarity (C30n) (45, 46).
A cladistic analysis of HCF Triceratops specimens was conducted in PAUP*
4.0b10 (47) (SI Text), initially using the heuristic search command with
Arrhinoceratops (48) designated as the outgroup. The matrix was assembled
in Mesquite 2.75 (49), and cladograms were displayed using FigTree (50).
Analyses were conducted using the random addition sequence (SI Text). The
most complete post-juvenile stage specimens were included in the analysis
(SI Text) (51). Characters found to vary within Triceratops, and between
Triceratops and Eotriceratops, (Fig. S7 and SI Text), were included. Strongly
ontogenetically influenced characters (e.g., postorbital horn core orienta-
tion; frill epiossification shape) were excluded from the analysis; however,
characters often used to distinguish Triceratops and Torosaurus (e.g., pari-
etal fenestrae, epiossification position) were retained. Specimens exhibiting
multiple character states (e.g., differing numbers of episquamosals on each
squamosal) were coded as polymorphic. All characters were left unordered;
maxtrees was set to 250,000. Bremer support indices were calculated using
TreeRot v3 (52). Analyses of the reduced dataset were conducted using the
branch-and-bound search command. Stratocladistic analyses were per-
formed in the program StrataPhy (29), with maxtrees set to 250,000. Speci-
mens with some ambiguity regarding stratigraphic position (MOR 981, MOR
1604, and MOR 2978) were excluded from the analysis (SI Text). Spearman’s
rank correlation analyses were performed in R (53) using the “cor.test”
function [cor.test(x,y, method = “spear”, exact = FALSE)]. Juvenile specimens
were excluded from these analyses as were specimens exhibiting significant
taphonomic distortion (Fig. 2 and Dataset S1). Additional calculations were
performed in Microsoft Office Excel 2007.
ACKNOWLEDGMENTS. We thank D. Barta, R. Boessenecker, N. Campione,
T. Carr, W. Clemens, W. Clyde, P. Dodson, D. Evans, A. Farke, C. Forster,
E. Fowler, J. Frederickson, J. Hartman, L. Hall, J. Hoganson, M. Holland,
F. Jackson, S. Keenan, M. Lavin, M. Loewen, J. Mallon, K. Olson, C. Organ,
K. Padian, A. Poust, D. Pearson, P. Renne, D. Roberts, K. Scannella,
J. Stiegler, D. Varricchio, D. Woodruff, H. Woodward, and B. Zorigt for
helpful discussions, without implying their agreement with our conclu-
sions. Comments and suggestions from G. Hunt, P. D. Polly, and three
anonymous reviewers were very helpful. We thank D. Fox, A. Huttenlocker,
and J. Marcot for help using StrataPhy. D. Evans, K. Seymour, and B. Iwama
provided access to the holotype of Arrhinoceratops at the Royal Ontario
Museum. B. Strilisky and G. Housego provided access to the holotype of
Eotriceratops at the Royal Tyrrell Museum and additional images of its
transformation of a lineage over time. (B) Bifurcation of an anagenetic lin-
eage in which the ancestor becomes pseudoextinct through evolution (sensu
ref. 38). Wagner and Erwin (38) distinguish bifurcation from “true” clado-
genesis, which includes no anagenetic component. (C and D) True clado-
genesis implies the coexistence of ancestral and descendant clades for at
least a short period. The presented modes represent points on a spectrum of
potential evolutionary patterns. The gray and white asterisks represent
potential positions for MOR 3045 and MOR 3027, respectively; black X rep-
resents MOR 2982. The x and y axes represent morphospace.
Potential patterns of HCF Triceratops evolution. (A) Anagenesis, or
Scannella et al.PNAS Early Edition
| 5 of 6
premaxilla. J. Sertich provided access to specimens at the Denver Museum
of Nature and Science. P. Sheehan provided access to, and locality data
for, Milwaukee Public Museum (MPM) specimen VP6841. R. Scheetz and
B. Britt provided access to a cast of Museum of Western Colorado (MWC)
specimen 7584 at Brigham Young University Museum of Paleontology;
W. Clemens shared stratigraphic data for this specimen. The Bureau of
Land Management, Charles M. Russell National Wildlife Refuge, and the
Department of Natural Resources and Conservation provided access to
land under their management and provided collection permits. We are
grateful to the Twitchell, Taylor, Trumbo, Holen, and Isaacs families for
land access. We thank Museum of the Rockies (MOR) volunteers, staff, and
all participants in the Hell Creek Project. We thank the MOR and the
University of California Museum of Paleontology (UCMP) for support
of this research. Funding for the Hell Creek Project was provided by
donations from J. Kinsey, C. B. Reynolds, H. Hickam, and Intellectual
Ventures. The Windway Foundation and the Smithsonian Institution pro-
vided grants (to J.R.H.). UCMP provided funding (to M.B.G.). The Theodore
Roosevelt Memorial Fund of the American Museum of Natural History, the
Fritz Travel Grant of the Royal Ontario Museum, and the Jurassic Foundation
provided grants (to J.B.S.). The Evolving Earth Foundation and the Doris O.
and Samuel P. Welles Research Fund of the UCMP provided grants (to J.B.S.
1. Horner JR, Goodwin MB, Myhrvold N (2011) Dinosaur census reveals abundant Ty-
rannosaurus and rare ontogenetic stages in the Upper Cretaceous Hell Creek Formation
(Maastrichtian), Montana, USA. PLoS ONE 6(2):e16574, 10.1371/journal.pone.0016574.
2. Hicks JF, Johnson KR, Obradovich JD, Tauxe L, Clark D (2002) The Hell Creek Formation
and the Cretaceous-Tertiary Boundary in the Northern Great Plains: An Integrated
Continental Record of the End of the Cretaceous, eds Hartman JH, Johnson KR,
Nichols DJ (Geological Society of America, Boulder, CO), Special Paper 361, pp 35–55.
3. Scannella JB, Horner JR (2010) Torosaurus Marsh, 1891, is Triceratops Marsh, 1889
(Ceratopsidae: Chasmosaurinae): Synonymy through ontogeny. J Vertebr Paleontol
4. Horner JR, Goodwin MB (2006) Major cranial changes during Triceratops ontogeny.
Proc Biol Sci 273(1602):2757–2761.
5. Scannella JB, Fowler DW (2014) Through the End of the Cretaceous in the Type Lo-
cality of the Hell Creek Formation in Montana and Adjacent Areas, eds Wilson GP,
Clemens WA, Horner JR, Hartman JH (Geological Society of America, Boulder, CO),
Special Paper 503, pp 313–332.
6. Horner JR, Varricchio DJ, Goodwin MB (1992) Marine transgressions and the evolution
of Cretaceous dinosaurs. Nature 358:59–61.
7. Sampson SD (1995) Two new horned dinosaurs from the Upper Cretaceous Two
Medicine Formation of Montana: With a phylogenetic analysis of the Centrosauriane
(Ornithischia: Ceratopsidae). J Vertebr Paleontol 15(4):743–760.
8. Sampson SD, Loewen MA (2010) New Perspectives on Horned Dinosaurs: The Royal
Tyrell Museum Ceratopsian Symposium, eds Ryan MJ, Chinnery-Allgeier BJ, Eberth DA
(Indiana Univ Press, Bloomington, IN), pp 405-427.
9. Marsh OC (1889) Notice of gigantic horned Dinosauria from the Cretaceous. Am J Sci
10. Ostrom JH, Wellnhofer P (1986) The Munich specimen of Triceratops with a revision of
the genus. Zitteliana 14:111–158.
11. Ostrom JH, Wellnhofer P (1990) Dinosaur Systematics: Approaches and Perspectives,
eds Carpenter K, Currie PJ (Cambridge Univ Press, New York), pp 245-254.
12. Forster CA (1996) Species resolution in Triceratops: Cladistic and morphometric ap-
proaches. J Vertebr Paleontol 16(2):259–270.
13. Farke AA (2010) Evolution, homology, and function of the supracranial sinuses in
ceratopsian dinosaurs. J Vertebr Paleontol 30(5):1486–1500.
14. Marsh OC (1890) Description of new dinosaurian reptiles. Am J Sci 3(39):81–86.
15. Farke AA (1997) Dinofest International Proceedings Volume, eds Wolberg DL,
Stump E, Rosenberg GD (Philadephia Academy of Natural Sciences, Philadelphia),
16. Dodson P (1996) The Horned Dinosaurs (Princeton Univ Press, Princeton).
17. Happ JW, Morrow CM (1996) Separation of Triceratops (Dinosauria:Ceratopsidae)
into two allopatric species by cranial morphology. J Vertebr Paleontol 16:40A.
18. Lehman (1998) A gigantic skull and skeleton of the horned dinosaur Pentaceratops
from New Mexico. J Paleontol 72(5):894–906.
19. Lull RS (1915) The mammals and horned dinosaurs of the Lance formation of Niobrara
County, Wyoming. Am J Sci 4(40):319–348.
20. Lull RS (1933) A revision of the Ceratopsia or horned dinosaurs. Yale Peabody Mu-
seum Memoir 3:1–175.
21. Hartman JH, Butler RD, Weiler MW, Schumaker KK (2014) Through the End of the
Cretaceous in the Type Locality of the Hell Creek Formation in Montana and Adjacent
Areas, eds Wilson GP, Clemens WA, Horner JR, Hartman JH (Geological Society of
America, Boulder, CO), Special Paper 503, pp 89–122.
22. Farke AA (2011) Anatomy and taxonomic status of the chasmosaurine ceratopsid
Nedoceratops hatcheri from the upper Cretaceous Lance Formation of Wyoming,
U.S.A. PLoS One 6(1):e16196.
23. Longrich NR, Field DJ (2012) Torosaurus is not Triceratops: Ontogeny in Chasmo-
saurine Ceratopsids as a case study in dinosaur taxonomy. PLoS One 7(2):e32623.
24. Maiorino L, Farke AA, Kotsakis T, Piras P (2013) Is torosaurus triceratops? Geometric
morphometric evidence of late maastrichtian ceratopsid dinosaurs. PLoS One 8(11):
25. Cobabe EA, Fastovsky DE (1987) Ugrosaurus olsoni, a new ceratopsian (Reptilia: Or-
nithischia) from the Hell Creek Formation of eastern Montana. J Paleontol 61(1):
26. Forster CA (1993) Taxonomic validity of the ceratopsid dinosaur Ugrosaurus olsoni
(Cobabe and Fastovsky). J Paleontol 67(2):316–318.
27. Horner JR, Goodwin MB (2008) Ontogeny of cranial epi-ossifications in Triceratops.
J Vertebr Paleontol 28(1):134–144.
28. Wu X, Brinkman DB, Eberth DA, Braman DR (2007) A new ceratopsid dinosaur (Or-
nithischia) from the uppermost Horseshoe Canyon Formation (upper Maastrichtian),
Alberta, Canada. Can J Earth Sci 44:1243–1265.
29. Marcot JD, Fox DL (2008) StrataPhy: A new computer program for stratocladistic
analysis. Palaeontol Electronica 11:5A.
30. Simpson GG (1944) Tempo and Mode in Evolution (Columbia Univ Press, New York).
31. Simpson GG (1953) The Major Features of Evolution (Columbia Univ Press, New York).
32. Eldredge N, Gould SJ (1972) Models in Paleobiology, ed Schopf TJM (Freeman,
Cooper, San Francisco,), pp 82–115.
33. Rensch B (1947) Neure Probleme der Abstammungslehre. Die transspezifische evo-
lution (Ferdinand Enke, Stuttgart).
34. Malmgren BA, Berggren WA, Lohmann GP (1983) Evidence for punctuated gradual-
ism in the Late Neogene Globorotalia tumida lineage of planktonic foraminifera.
35. Gingerich PD (1985) Species in the fossil record: Concepts, trends, and transitions.
36. Macleod N (1991) Punctuated anagenesis and the importance of stratigraphy to pa-
leobiology. Paleobiology 17(2):167–188.
37. Benton MJ, Pearson PN (2001) Speciation in the fossil record. Trends Ecol Evol 16(7):
38. Wagner PJ, Erwin DH (1995) New Approaches to Speciation in the Fossil Record, eds
Erwin DH, Anstey RH (Columbia Univ Press, New York), pp 87–122.
39. Tokaryk TT (1986) Ceratopsian dinosaurs from the Frenchman Formation (Upper
Cretaceous) of Saskatchewan. Can Field Nat 100(2):192–196.
40. Carpenter K, Young DB (2002) Late Cretaceous dinosaurs from the Denver Basin,
Colorado. Rocky Mt Geol 37(2):237–254.
41. Keenan SW, Scannella JB (2014) Through the End of the Cretaceous in the Type Lo-
cality of the Hell Creek Formation in Montana and Adjacent Areas, eds Wilson GP,
Clemens WA, Horner JR, Hartman JH (Geological Society of America, Boulder, CO),
Special Paper 503, pp 349–364.
42. Simpson GG (1951) The species concept. Evolution 5:285–298.
43. Evans D, Currie P, Eberth D, Ryan M (2006) High-resolution lambeosaurine dinosaur
biostratigraphy, Dinosaur Park Formation, Alberta: Sexual dimorphism reconsidered.
J Vertebr Paleontol 26:59A.
44. Campione NE, Evans DC (2011) Cranial growth and variation in edmontosaurs
(Dinosauria: Hadrosauridae): Implications for latest Cretaceous megaherbivore diversity
in North America. PLoS One 6(9):e25186.
45. Lerbekmo JF, Braman DR (2002) Magnetostratigraphic and biostratigraphic correla-
tion of late Campanian and Maastrichtian marine and continental strata from the Red
Deer Valley to the Cypress Hills, Alberta, Canada. Can J Earth Sci 39:539–557.
46. LeCain R, Clyde W, Wilson GP, Riedel J (2014) Through the End of the Cretaceous in
the Type Locality of the Hell Creek Formation in Montana and Adjacent Areas, eds
Wilson GP, Clemens WA, Horner JR, Hartman JH (Geological Society of America,
Boulder, CO), Special Paper 503, pp 137–148.
47. Swofford DL (2003) PAUP*: Phylogenetic Analysis Using Parsimony (*and Other
Methods) (Sinauer Associates, Sunderland, MA), Version 4.
48. Parks WA (1925) Arrhinoceratops brachyops, a new genus and species of Ceratopsia
from the Edmonton Formation of Alberta. University of Toronto Studies (Toronto
University Library, Toronto), Geological Series No. 19, pp 5–15.
49. Maddison WP, Maddison DR (2011) Mesquite: A modular system for evolutionary
analysis. Version 2.75. Available at http://mesquiteproject.org/.
50. Rambaut A (2012) FigTree. Version 1.4.0. Available at http://tree.bio.ed.ac.uk/software/
51. Campione NE, Brink KS, Freedman EA, McGarrity T, Evans DC (2013) ’Glishades
ericksoni’, an indeterminate juvenile hadrosaurid from the Two Medicine Formation
of Montana: Implications for hadrosauroid diversity in the latest Cretaceous (Cam-
panian-Maastrichtian) of western North America. Palaeobio Palaeoenv 93:65–75.
52. Sorenson MD, Franzosa EA (2007) TreeRot (Boston University, Boston, MA), Version 3.
53. R Core Team (2013) R: A language and environment for statistical computing
(R Foundation for Statistical Computing, Vienna, Austria). Available at http://www.
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Scannella et al. 10.1073/pnas.1313334111
Variation in Parietal-Squamosal Frill. Scannella and Horner (1)
suggested that the number of epiossifications present in Triceratops
may vary stratigraphically. Often, epiossifications are unpreserved
or are detached from the parietal-squamosal frill, which compli-
cates testing of this hypothesis. Data currently available highlight
variation in epiossification number and position as the number and
configuration can vary between the squamosals of a single in-
dividual [e.g., Museum of the Rockies (MOR) 1120]. Additionally,
there may be an ontogenetic component to epiossification number
and position in chasmosaurines (1–3). Hell Creek Formation
(HCF) specimens with the highest numbers of epiparietals (MOR
1122 and MOR 3081) and episquamosals (MOR 1120, MOR
1122, and MOR 3081) are found lower in the formation.
The development of fenestrae also varies within Triceratops (refs.
1 and 2; but see refs. 4–6). A pronounced transition in thickness on
the ventral surface of the parietal surrounding this area (2) is noted
in Triceratops from the upper unit of the HCF (U3) and at least
one specimen from the upper part of the middle unit (M3) (MOR
3045), but lower in the formation there appears to be a more
gradual transition in the thickness of the parietal (e.g., MOR 335,
MOR 1120, and MOR 2985). This finding may suggest that the
fenestrae developed later, ontogenetically, in Triceratops found
stratigraphically higher. Alternatively, if Triceratops and Tor-
osaurus represent distinct but closely related taxa, Triceratops
found stratigraphically lower may express more basal parietal
features, including eventual fenestration of the parietal.
The stratigraphically documented cranial morphological trends
expressed in Triceratops are thus far consistent with the morphology
of specimens referred to “Torosaurus latus” (however, we note that
the majority of specimens exhibiting the “Torosaurus” morphology
were recovered from the lower half of the formation). Precise lo-
cality data for MOR 981 are not available, but it was collected in
a mudstone located above the basal sandstone. Perhaps the strati-
graphically highest known Torosaurus from the HCF of Montana,
Milwaukee Public Museum (MPM) specimen VP6841 (which,
based on study of topographic and geologic maps, appears to have
been collected from the upper half of the formation), exhibits an
incomplete yet relatively narrow epinasal morphology that is consis-
tent with its stratigraphic position. The observation that a nasal boss
morphology appears to occur in relatively mature specimens of Tri-
ceratops (Torosaurus morph; e.g., MOR 1122 and MOR 981) (7)
suggests that development of the boss morphology was ontogenetic,
as isseeninsome centrosaurineceratopsids(e.g.,refs.8and9.).The
nasal boss morphology is not exhibited in all Torosaurus specimens
[MOR 3081, MPM VP 6841, Yale Peabody Museum (YPM) 1830,
and YPM 1831], and thus the degree to which this feature is de-
veloped may vary individually or stratigraphically. University of Cal-
boss (10, 11); however, due to the fragmentary nature of the speci-
men, it is unclear whether it represents the Torosaurus morphology.
Morphology of the Rostrum. Forster (12) recognized rostrum
morphology as one of the features that distinguish Triceratops
horridus from Triceratops prorsus. T. horridus exhibits a low,
elongate rostrum with a sinusoidal dorsal margin whereas, in
T. prorsus, the rostrum is shorter and more convex. Longrich and
Field (5) noted that specimens of T. prorsus have a more verti-
cally oriented nasal process [= ascending nasal process of the
premaxilla (sensu ref. 13)], here referred to as the nasal process
of the premaxilla (NPP)] compared with T. horridus. Rostrum
morphology appears to be tied to the orientation of the NPP,
with a more posteriorly inclined NPP contributing to a low,
sinusoidal rostrum in some specimens [e.g., MOR 1120, American
Museum of Natural History (AMNH) 5116, National Museum of
Natural History (USNM) 1201, and YPM 1820].
Theanglebetween the NPPandnarialstrut appearstoincrease
stratigraphically in the HCF; specimens from the upper M3 and
U3 exhibit a larger angle between the NPP and narial strut than
specimens found lower in section (Fig. 2C). To quantify this shift
in morphology, the angle between the NPP and narial strut (Fig.
S2) was measured using the Ruler tool in Adobe Photoshop.
This angle was measured between the approximate midlines of
each process, parallel to the direction representing the primary
trend (results presented in Dataset S1 and Fig. 2). We note that,
in some more basal taxa (e.g., Anchiceratops) (3), the NPP can be
oriented nearly perpendicular to the narial strut and as such can
give the rostrum a more convex appearance in lateral view.
The width of the NPP also affects the rostrum morphology
because a wider NPPreducesthe apparent sinuosity oftheanterior
premaxilla. MOR 3027 and MOR 3045 (both recovered from
upper M3) exhibit a more vertically inclined premaxillary articu-
lation with the nasal than specimens found lower in the formation.
MOR 3045 (collected ∼2 m above MOR 3027) exhibits the further
derived feature of an anteroposteriorly expanded NPP, contrib-
uting to a rostrum that seems even more convex in lateral view.
Rostrum length appears to vary both stratigraphically and
ontogenetically. The largest specimens from U3 (e.g., MOR 004
and MOR 1625) exhibit more elongate rostra (Fig. 2E and Da-
taset S1); however, even these large specimens do not exhibit the
strongly posteriorly inclined NPP and sinusoidal dorsal margin
of the rostrum exhibited in specimens referred to T. horridus.
Evolutionary changes in rostrum morphology may reflect the
development of an enlarged epinasal.
Other Triceratopsin Taxa. Longrich (14) referred Oklahoma Mu-
seum of Natural History (OMNH) specimen 10165, a large ce-
ratopsid specimen recovered from Campanian deposits of New
Mexico and previously diagnosed as a gigantic specimen of Pen-
taceratops sternbergi (15), to the new taxon Titanoceratops our-
anos. Longrich proposed that Titanoceratops represents the oldest
member of the Triceratopsini, the clade that includes Triceratops,
Torosaurus, “Nedoceratops,” and Ojoceratops (14). This specimen
exhibits several features consistent with its stratigraphic position
relative to HCF Triceratops. It has a relatively short epinasal,
short arched nasals, a posteriorly inclined NPP, and elongate
postorbital horn cores. Given the degree of ontogenetic trans-
formation noted in several marginocephalians (1, 2, 16, 17), it is
possible that many of the features considered to distinguish Ti-
tanoceratops from Pentaceratops (including large size, broad epi-
ossifications, extensive cornual sinuses, strongly anteriorly curved
postorbital horn cores, elongate premaxilla) (14) may instead
represent ontogenetic or individual variation within the latter
taxon (18), which would be consistent with the original diagnosis
by Lehman (15). Further assessment of this specimen and its
phylogenetic position is beyond the scope of the current study.
Triceratopsin material from the southern region of the western
interior of North America includes specimens that have been re-
ferred to Ojoceratops fowleri and Torosaurus utahensis (14, 19–23).
Ojoceratops, from the Ojo Alamo Formation of New Mexico,
appears to be closely related to Triceratops (3, 19, 24) and has
been suggested to be synonymous with the latter taxon (14).
Material referred to Ojoceratops thus far consists of isolated
or fragmentary elements. Due to the missing morphological
Scannella et al. www.pnas.org/cgi/content/short/1313334111 1 of 11
information for much of this material, specimens of O. fowleri were
not included in the current cladistic analysis of HCF Triceratops.
A nasal horn referred to this taxon [State Museum of Pennsyl-
vania (SMP) specimen VP-1828] exhibits a morphology similar to
that observed in several lower unit (L3)/lower M3 Triceratops, which
is consistent with its stratigraphic position relative to the Hell Creek
Formation of Montana. The holotype squamosal (SMP VP-1865)
has a greatly reduced anterolateral projection of the squamosal,
which has been used to distinguish it from Torosaurus utahensis. The
degree to which this feature can distinguish Ojoceratops from other
taxa is unclear; the HCF dataset demonstrates that the morphology
of this projection varies within Torosaurus and Triceratops, and even
within a single individual (MOR 2999). Variation in this feature has
previously been noted by Hunt and Lehman (23).
The incomplete or fragmentary nature of specimens that have
been referred to Torosaurus utahensis has engendered debate
regarding what material is referable to this taxon, its stratigraphic
and biogeographic range, and which morphologic features, if any,
distinguish it from other chasmosaurine taxa (22, 23). This ma-
terial was not included in the current study of HCF specimens.
Tatankaceratops sacrisonorum is represented by a fragmentary
partial skull from the upper (∼20 m below the K/Pg boundary)
HCF in South Dakota (25). The specimen exhibits an enlarged
nasal horn and very small postorbital horn cores. As noted by
Longrich (14), this specimen may represent T. prorsus, which
would be consistent with its stratigraphic position.
Triceratops Biogeography. Triceratops in the Frenchman Formation
(Saskatchewan, Canada) and Laramie Formation (Colorado) ap-
pear to exhibit morphologies consistent with those expressed by
specimens in the Hell Creek Formation (HCF), Montana. The base
of the Frenchman Formation occupies the uppermost C30n
magnetozone, with the majority of the unit residing in C29r up to
the K-Pg boundary (26, 27), which indicates that the Frenchman
Formation correlates largely to U3 of the HCF (28). Thus, we
predict that most Triceratops skulls from the Frenchman Formation
will exhibit T. prorsus morphologies. Diagnostic specimens pub-
lished to date have been referred to T. prorsus (29). Conversely, the
uppermost Laramie Formation exhibits reversed magnetic polarity,
aligning it with magnetochron C30R (Castle Pines core) (30, 31)
and making it slightly older than the HCF (28). Thus, specimens
from the Laramie Formation should exhibit cranial morphologies
similar to L3 Triceratops, and to date this hypothesis remains un-
falsified (32). The Denver Formation is partly coeval with the HCF
of Montana (33) and is predicted to yield a similar stratigraphically
segregated Triceratops record. Thus far, specimens collected outside
of Montana present morphologies that are consistent with their
stratigraphic positions relative to the HCF sample.
Formation of Wyoming and other coeval formations will permit
further testing of biogeographic hypotheses. The historical record
remains unresolved and of limited utility.
Stratigraphic Correlations of Specimens from Upper M3. MOR 3027,
MOR 3045, and UCMP 113697 were all recovered from high in
the middle unit of the HCF. The localities that produced MOR
3027 and MOR 3045 (Fig. S2) are within a mile of one another,
which facilitates their relative stratigraphic placement. MOR
3027 was collected ∼5.5 m below the Apex Sandstone (the base
of U3). MOR 3045 was collected from ∼7.5 m below this marker
bed; thus, initially, it appeared that MOR 3045 was found
stratigraphically lower than MOR 3027. However, the Apex
Sandstone is thicker and cuts down further into the underlying
strata just above the quarry that produced MOR 3027. There is
a prominent organic-rich horizon that can be laterally traced
above both quarries. MOR 3027 was found 5.3 m below this
organic-rich bed whereas MOR 3045 was ∼3.3 m below. Thus,
the quarry that produced MOR 3045 is higher stratigraphically
relative to MOR 3027. UCMP 113697 was discovered 21.5 km to
the east of these localities. Locally, the Apex Sandstone is ∼6 m
above the base of the quarry that produced this specimen. An
organic-rich horizon that may correlate with the organic-rich bed
found above the two MOR localities is ∼3 m above the quarry,
and thus UCMP 113697 appears to have been collected from
roughly the same stratigraphic level as MOR 3045.
Estimation of Basal-Skull Length. In this study, basal-skull length
was considered the distance from the anteriormost point of the
rostrum to the posterior surface of the occipital condyle (fol-
lowing previous researchers) (7, 12). Skull-length measurements
for some specimens were taken from reconstructions (Dataset
S1). For some largely complete specimens that do not preserve
the occipital condyle or in which it is obscured (e.g., MOR 004),
this distance was approximated by measuring the distance from
the anteriormost point of the rostrum to the posterior margin of
the lateral temporal fenestra (Dataset S1). For less complete, or
disarticulated specimens, basal-skull length was estimated using
linear regressions of basal-skull length against preserved cranial
elements. Linear models relating basal-skull length to dentary
length (measured from the anteriormost point to the posterior
surface of the coronoid process), maxilla length (measured along
the lateral surface), occipital condyle area (following ref. 34), and
jugal length (measured from the base of the orbit to the distal tip)
produced R2values of 0.995, 0.979, 0.944, and 0.980, respectively
(Dataset S1). The use of multiple elements allowed more speci-
mens to be included in quantitative comparisons. If multiple el-
ements were preserved in a specimen (for example, MOR 2982
has a dentary and jugal), the estimated values for basal-skull
length produced by the regression analyses were averaged.
Cladistic Analysis. A cladistic analysis of cranial variation in HCF
Triceratops initially used the heuristic search strategy of the pro-
gram PAUP* 4.0b10 (35). Nexus files are available on Morpho-
Bank (36) as project 1099. Analyses used the random addition
sequence with tree-bisection-reconnection (TBR) branch swap-
ping and 1,000 replicates; all most parsimonious trees were saved.
Characters were unordered and unweighted. Maxtrees was set to
250,000. Analyses were initially performed using binary coding
for morphological characters (37, 38). Additional analyses were
performed using multistate coding that combined binary charac-
ters 10 and 11 (development of the epinasal-nasal protuberance),
25 and 26 (development of the anterolateral projection of the
squamosal), and 29 and 30 (number of epiparietals). Support for
clades was determined using nonparametric bootstrap resampling
(39) in PAUP* 4.0b10; 10,000 bootstrap replicates were analyzed,
with one tree retained per replicate. Application of bootstrap
resampling to data in which multistate characters have been dis-
tilled to binary characters is problematic (39) but was performed
for comparative purposes. In addition, Bremer support indices
were calculated using TreeRot.v3 (40) and PAUP* 4.0b10 (34).
This analysis focused on features found to vary within the HCF
Triceratops dataset. Eotriceratops was included in the analysis to
test the hypothesis that it represents a taxon distinct from Tri-
ceratops. As such, characters found to distinguish Eotriceratops by
Sampson et al. (24) and characters describing the relative height
of the narial process and the morphology of the epijugal (41)
were examined. Forster (12) noted five cranial characters that
vary within Triceratops. Four of these characters were included in
this analysis (Forster’s character 4, which describes rostrum
shape, was modified in this analysis to reflect the influence of
NPP orientation) (5). Forster’s character 1 (describing the post-
orbital, jugal, squamosal suture pattern) was not found to vary in
the HCF dataset. Either all coded specimens exhibited the
“primitive” state of the jugal contributing to the dorsal margin of
the lateral temporal fenestra, or sutural relationships of this region
were unpreserved or were obscured by fusion.
Scannella et al. www.pnas.org/cgi/content/short/1313334111 2 of 11
Initially, specimens that were collected or stratigraphically
relocated during the Hell Creek Project and that were largely
complete or exhibited morphologies not otherwise found within
their respective stratigraphic units (e.g., MOR 2552 and UCMP
128561) were included in the cladistic analysis. Only post-juvenile
stage specimens were included in the analyses (42). MOR 981
exhibits the Torosaurus morphology and was collected from
a mudstone above the basal sand of the formation; however,
detailed stratigraphic data are unavailable for this specimen.
The initial strict consensus tree produced using binary coding
[most parsimonious trees (MPT) 250,000, 55 steps, consistency
index (CI) 0.7091, homoplasy index (HI) 0.4000, retention index
(RI) 0.8400] produced a polytomy of all HCF specimens (Fig.
S5A). The holotype of Eotriceratops [Royal Tyrrell Museum
(RTMP) 2002.57.7] was recovered as being basal to the HCF
dataset, consistent with the initial hypothesis proposed by
Wu et al. (41). The 50% majority tree revealed a succession of
specimens that were consistent with stratigraphic position, aside
from some specimens that were missing a large portion of
codeable characters (e.g., MOR 2552 and MOR 3010). Speci-
mens exhibiting the Torosaurus morphology clustered together as
basal to the rest of the HCF dataset as these specimens exhibit
several features (including a fenestrated parietal) that are ob-
served in more basal taxa. MOR 3011, which preserves relatively
thick sections of parietal-squamosal frill but is too fragmentary to
be coded for features of these elements, was not distinguished
from the Torosaurus group.
Rerunning the analysis using multistate rather than binary
characters produced a polytomy in the strict consensus tree
(MPT 250,000,54 steps, CI 0.7222,HI 0.3889, RI0.8469), andthe
50% majority-rule tree similarly produced a sequence of speci-
mens consistent with stratigraphic position aside from the most
fragmentary specimens (Fig. S5B).
specimens (individuals that did not exhibit at least seven codeable
features). This analysis resulted in a strict consensus (MPT
250,000, 55 steps, CI 0.7091, HI 0.4000, RI 0.8161) in which
specimens were largely recovered in stratigraphic succession
(except for MOR 3011, which, as noted above, grouped with
Torosaurus specimens). MOR 1120 from L3 was found to be the
most basal non-Torosaurus HCF specimen, and MOR 2982 from
the lower M3 was recovered as the next most basal. Above MOR
2982 is a large polytomy consisting of specimens from the upper
half of the formation. The identical topology was recovered
when the multistate matrix was analyzed (MPT 250,000, 54 steps,
CI 0.7222, HI 0.3889, RI 0.8214) (Fig. 3); however, a bremer
decay value of 2 was recovered for the upper M3–U3 polytomy
when the binary matrix was used (as opposed to a value of 1
when the multistate matrix was used).
The analysis was next run after removing specimens that could
not be coded for features of the parietal-squamosal frill. A
branch-and-bound search was used with the furthest addition
sequence implemented. The strict consensus tree produced using
the binary matrix (MPT 218,972, 55 steps, CI 0.7091, HI 0.4000,
RI 0.8000) (Fig. S5C) recovered a polytomy of Torosaurus speci-
mens as basal to other specimens. MOR 1120 and MOR 2982
from the lower half of the formation were recovered together as
basal to a large polytomy of specimens from the upper half of the
formation. The multistate analysis (MPT 189,820, 54 steps, CI
0.7222, HI 0.3889, RI 0.8077) (Fig. S5D) resulted in greater res-
olution; MOR 1120 was recovered as basal to the stratigraphically
higher MOR 2982. MOR 1122 and MOR 981 clustered together.
These specimens both exhibit a nasal boss and do not exhibit an
epiossification or crenulation spanning the parietal-squamosal
margin whereas the third Torosaurus specimen (MOR 3081)
possesses a narrow dorsal surface of the epinasal and a parietal-
squamosal crenulation. As these features appear to exhibit
a large degree of variation within Triceratops (2), intraspecific
variation appears more likely than these differences being tax-
onomic in nature. We note that, in this analysis, a midline epi-
parietal (character 32) was coded as absent in MOR 1122 as the
element is not present and there does not appear to be a pro-
nounced crenulation on the midline. Scannella and Horner (1)
suggested the presence of a midline epiparietal in this specimen
based on vascular patterns observed on the parietal. The 50%
majority tree for both analyses (Fig. S5 E and F) found MOR
3045 to be more derived than MOR 3027. UCMP 113697 clus-
ters with MOR 2924 (U3) in the binary analysis, and with MOR
2924 and MOR 2999 in the multistate analysis. These topologies
suggest that UCMP 113697 is more derived than other speci-
mens from upper M3; however, we note that this result may be
influenced by missing data. MOR 2924 (recovered from the
sandstone at the base of U3) preserves a broader posterior
surface of the epinasal than other specimens from U3 but does
not preserve postorbital horn cores. The anteromedial processes
of nasals of MOR 2924 are unobservable due to articulation with
the premaxillae. The morphology of the anteromedial processes
on the nasals of UCMP 113697 are currently obscured due to the
mounting of the disarticulated skull elements for display.
When specimens that did not preserve at least 10 codeable
features (in the multistate matrix) were removed from the
analysis, the strict consensus trees (binary coding: MPT 7036; 54
steps; CI 0.7222; HI 0.3889; RI 0.8000) (Fig. S5G) (multistate
coding: MPT 7036, 53 steps, CI 0.7358, HI 0.3774, RI 0.8082)
(Fig. 3B) exhibited an identical topology. Torosaurus specimens
were recovered as basal to MOR 1120 and MOR 1982, and
specimens from the upper half of the formation were again re-
covered in a large polytomy. When MOR 2924 was removed
from the analysis, both analyses (binary coding: MPT 282; 53 steps;
CI 0.7358; HI 0.3774; RI 0.8028) (Fig. S5H) (multistate coding:
MPT 282; 52 steps; CI 0.7500; HI 0.3654; RI 0.8116) (Fig. 3C)
recovered MOR 3045 as basal to U3 specimens and as more de-
rived than UCMP 113697 and MOR 3027, which cluster together.
Stratocladistic Analysis. Stratocladistics incorporates stratigraphic
data into cladistic analyses (see, for example, refs. 43–48). A
stratocladistic analysis was performed using the program StrataPhy,
which produces trees that can indicate possible ancestor-
descendant relationships (49). The multistate dataset was used
for the analysis, with the specimens MOR 981, MOR 1604, and
MOR 2978 removed from the analysis due to ambiguity over
their precise stratigraphic position. Rather than coding speci-
mens separately, specimens from the lower M3, upper M3, lower
U3, and upper U3 were combined into operational units based
on stratigraphic position. MOR 3081 and MOR 3005 were
considered separately from other specimens from the same
stratigraphic zones due to the distinct ontogenetic [(2) or, al-
ternatively, taxonomic (4–6)] morphological differences between
these specimens. MOR 3005 is a fragmentary specimen, but
preserves thin sections of frill and thus may represent the Tor-
osaurus morphology. A single stratigraphic character was added
[stratigraphic position: (position 0) stratigraphically below the
HCF; (position 1) lower L3; (position 2) upper L3; (position 3)
lower M3; (position 4) upper M3; (position 5) lower U3; (position
6) upper U3]. Arrhinoceratops (ROM 796) was designated the
outgroup. MAXTREES was set to 250,000, and all other pa-
rameters were StrataPhy’s default settings (49).
The initial analysis produced 61 trees with nine topologies
(total debt = 64) (Fig. S6A). Aside from one tree that suggests all
operational units arose via cladogenesis, specimens from the
upper half of the formation were consistently found to represent
an anagenetic succession. The position of operational units from
the lower half of the formation varied and were not always
consistent with stratigraphic position. This result is likely
influenced by the fact that specimens from the lower half of M3
do not preserve features of the parietal-squamosal frill that
Scannella et al. www.pnas.org/cgi/content/short/13133341113 of 11
would allow them to be distinguished from the Torosaurus
morphology. MOR 2982 preserves an anterolateral projection of
the squamosal, which is consistent with the morphology expressed
in several other HCF specimens, including the Torosaurus spec-
imen MOR 3081. Incorporation of Torosaurus specimens into
Triceratops operational units (total debt = 67, nine trees, three
topologies) (Fig. S6B) produced a single tree suggesting that all
operational units arose via cladogenesis and two additional to-
pologies that include ancestor-descendant relationships. In four
trees, all operational units were recovered within an anagenetic
lineage except the lower M3 group. This operational unit was
recovered as basal to the upper L3 operational unit, suggesting
a cladogenetic event. The remaining four trees exhibited a bi-
furcation event in L3 giving rise to two lineages.
Given the lack of frill characters for the lower M3 operational
unit, the influence of Torosaurus specimens on the results was
examined by pruning all Torosaurus from the analysis. This
pruning resulted in reduced total debt (57) and 12 trees (Fig.
S6C). Four trees indicate that all HCF operational units repre-
sent a single anagenetic lineage with specimens exhibiting the
T. horridus morphology evolving into T. prorsus (Fig. 4A). Eight
trees recovered two lineages suggested to diverge at some point
in L3 or before the deposition of the HCF. One lineage gave rise
to lower M3 specimens and the other to U3 specimens. This
result suggests that two anagenetic lineages, one comprising
specimens referable to T. horridus and the other giving rise to
T. prorsus, coexisted in the HCF (for at least some time) (Fig. 4B).
Characters Incorporated in Cladistic Analysis. The first use in a cla-
distic study is cited.
1) Postorbital horn-core length: (code 0) long (postorbital
horn-core/basal-skull length ratio: ≥0.64); (code 1) short
(postorbital horn-core/basal-skull length ratio: <0.64). [(50)
character 58 modified; (12) character 2 modified].
2) Cross-section of postorbital horn core: (code 0) circular to
subcircular; (code 1) narrow. The postorbital horn cores of
some specimens of Triceratops (e.g., MOR 2702 and MOR
2923) exhibit a markedly narrow morphology that does not
appear to be a product of taphonomic distortion. MOR
2923 exhibits no evidence of lateral compression, and yet
the postorbital horn cores of this specimen have a pro-
nounced ventral keel. Specimens for which apparently lat-
erally compressed postorbital horn cores are likely a result
of taphonomic processes (e.g., MOR 2982 and MOR 3027)
have been coded as “?”.
3) Rostrum shape: (code 0) primary axis of nasal process of
premaxilla (NPP) is strongly posteriorly inclined; (code 1)
NPP vertical or nearly vertical [(12, 50) character 4 modi-
fied; (5) Fig. S2].
4) Frontoparietal fontanelle: (code 0) open fontanelle; (code 1)
closed or constricted due to fusion of frontals and parietals.
[(50) characters 49 and 50 modified; (12) character 3 modified].
5) Epijugal: (code 0) comes to a pronounced peak; (code 1)
low and blunt [(51) character 102 modified; (24) character
50 modified). Epijugal morphology has been used in phylo-
genetic studies of chasmosaurines (e.g., refs. 14 and 24) and
as a diagnostic feature of some taxa. In most specimens of
Triceratops, the epijugal is a low, blunt element. Specimens
exhibiting the Torosaurus morphology exhibit an epijugal
that comes to a pronounced peak, similar to the condition
noted in more basal taxa such as Eotriceratops (41). At least
one large Triceratops with a nonfenestrated parietal (MOR
1625) also exhibits a peaked epijugal.
6) Quadratojugal notch: (code 0) present; (code 1) absent [sen-
su ref. 52, character 71; and (53) character 16]. The quadrates
of Triceratops exhibit a pronounced ridge on the anterolateral
surface. In many specimens, this ridge is interrupted by
a pronounced notch; however, in some specimens, this
notch is not present.
7) Nasal-horn length: (code 0) short (length/width ratio <1.85);
(code 1) long (length/width ratio >1.85). [(50) character 28
modified; (12) character 5 modified].
8) Dorsal surface of epinasal: (code 0) narrow to peaked;
(code 1) broad. The posterior surface of the epinasal varies
from being quite broad to nearly flat in some specimens, to
being narrow and coming to a pronounced peak in others.
The peaked condition is observed in the holotypes of Arrhi-
noceratops and Eotriceratops.
9) Nasal: (code 0) short, arched; (code 1) elongate, straight. A
short, arched nasal is observed in the holotype of T. horridus
(YPM 1820) and several other specimens referred to this
taxon. Specimens from U3 of the HCF exhibited a more
elongate nasal morphology that lacks pronounced arching
of the lateral margin.
10) Anterior nasals and posterior portion of epinasal fused to
form a protuberance posterior to epinasal: (code 0) present;
(code 1) subtle or absent. Forster (12) noted a pronounced
bump or boss posterior to the nasal horn in UCMP 113697. A
similar structure is present in the holotype of Triceratops “cal-
icornis” (USNM 4928) as noted by Ostrom and Wellnhofer
(54). The structure appears to be formed by a combination
of the anterior nasals and the posterior portion of the epinasal.
Forster (12) suggested that this feature was due to the incom-
plete fusion of the epinasal to an underlying boss or horn core;
disarticulated nasals reveal no underlying boss (13) but the
anterior nasal can be somewhat thickened relative to the mid-
dle segment of this element. Presence of a homologous struc-
ture in mature individuals (MOR 1122) suggest that its
presence is not a result of incomplete fusion although the de-
gree to which this feature varies throughout ontogeny is cur-
rently unknown. Development of this feature may be tied to
evolutionary elongation of the epinasal.
11) Epinasal-nasal protuberance: (code 0) reduced or absent;
(code 1) developed into pronounced boss.
12) Anteromedial process on nasal: (code 0) present, pronounced;
(code 1) reduced, constricted or absent (Fig. S3). Triceratops
from the lower half of the HCF appear to exhibit a distinct
process on the anteromedial surface of the nasal, medial to
the rostroventral process (following the terminology of
Fujiwara and Takakuwa) (55). In specimens from U3 in
which this process is visible, it is greatly reduced.
13) Posterior projection on epinasal: (code 0) present; (code 1)
absent (Fig. S4). The posterior surface of some epinasals
exhibits a small but pronounced posterior projection or
shelf. The projection appears to be absent in observed speci-
mens from U3. The projection may contribute to formation
of the epinasal-nasal protuberance (see character 10).
14) Nasal process of the premaxilla: (code 0) narrow; (code 1)
expanded (Fig. S2). In some specimens of Triceratops, the
NPP is narrow, exhibiting only slight anteroposterior expan-
sion. The premaxilla of the holotype of Eotriceratops exhibits
an extremely narrow NPP. In many specimens of Triceratops
from relatively high in the HCF, this process is expanded into
a wide, nearly square structure (Dataset S1).
15) Midline peak on nasal process of the premaxilla: (code 0)
absent; (code 1) present. The nasal process of MOR 3045
exhibits a pronounced dorsal peak anterior to its posterior
margin (Fig. S2E). This process appears to be absent or greatly
reduced in other specimens but is clearly present in juvenile
specimens from U3 (MOR 1110 and MOR 2951). The degree
to which this feature varies ontogenetically in specimens from
the lower half of the formation is currently unknown.
16) Prominence immediately anterior to or descending from the
narial strut, directed into interpremaxillary fenestra: (code
0) absent; (code 1) present (Fig. S7A).
Scannella et al. www.pnas.org/cgi/content/short/1313334111 4 of 11
17) Premaxilla, accessory strut in septal fossa: (code 0) no ac-
cessory strut; (code 1) strut present [(24) character 12].
Many specimens of Triceratops appear to exhibit two prom-
inences or struts directed into the interpremaxillary fenestra
(characters 16 and 17) (Fig. S7A). The degree to which
these features are developed varies between specimens.
18) Premaxilla, triangular process recess: (code 0) shallow;
(code 1) deep [(56) character 12 (modified)].
19) Triangular [“narial,” sensu Wu et al. (41)] process of pre-
maxilla: (code 0) dorsal margin (at point of contact with
narial strut) positioned roughly at or below the ventral margin
of the interpremaxillary fenestra; (code 1) dorsal margin of
narial process (at point of contact with narial strut) positioned
well above ventral margin of interpremaxillary fenestra (41).
20) Ventromedial foramina of the premaxilla positioned (code
0) close together or (code 1) far apart (more than 1.5 times
the width of anterior foramen) (Fig. S7B). The large ante-
rior foramen was highlighted in the description of the ho-
lotype of Eotriceratops by Wu et al. (41).
21) Posteroventral surface of the posterior “prong” of premax-
illa (sensu ref. 41): (code 0) comes to a narrow ridge; (code
1) broad posterior surface. The prominent prong of the
posteriormost premaxilla exhibits a narrow ridge on its pos-
terior surface in some specimens of Triceratops. Specimens
from U3, including juveniles (MOR 1110 and MOR 2951),
appear to exhibit a much broader posterior surface of this
element. The degree to which this feature might vary ontoge-
netically lower in the formation is currently unknown (Fig. S7).
22) Posterior prong of premaxilla: (code 0) broad surface for
articulation with nasal; (code 1) exhibits a pronounced ridge
on the lateral surface and a constricted area for articulation
with the nasal (Fig. S6).
23) Episquamosal or squamosal crenulation number [(57) charac-
ter 55 modified]: (code 0) seven or more; (code 1) six or fewer.
24) Convex margin of squamosal (code 0) absent; (code 1) pres-
ent (5). Longrich and Field (5) noted that specimens of
T. prorsus tend to exhibit a strongly convex margin of the
squamosal. This study finds the shape of the squamosal
to vary within Triceratops, with some specimens that exhibit
T. horridus morphologies (MOR 1120) possessing more con-
vex squamosals than other specimens that exhibit T. prorsus
morphologies (MOR 2702) (Dataset S1). This variation is
likely tied to ontogenetic elongation of this element (2).
Specimens from higher in the HCF appear to exhibit the
convex morphology for a longer period, ontogenetically. In
this study, specimens were coded as possessing a convex squa-
mosal if the ratio of sqamosal length to the distance to the
squamosal’s lateral margin (measured from and perpendicu-
lar to the line representing length) was ≤4 (Dataset S1).
25) Anterolateral projection on squamosal (22): (code 0) pres-
ent; (code 1) greatly reduced or absent.
26) Anterolateral projection on squamosal: (code 0) pronounced,
forming strongly concave anterior margin of the squamosal;
(code 1) reduced or absent (22). Sullivan et al. (22) noted
that Torosaurus latus specimens exhibit a greatly pronounced
projection of the anterolateral surface of the squamosal that
causes the otic notch to become constricted. This projection
is present to various degrees in many specimens of Tricera-
tops; however, in some, it is greatly reduced (nearly absent).
27) Squamosal bar (code 0) present; (code 1) absent [(50) char-
acter 90 modified; (24) character 64 modified].
28) Ventral surface of parietal in areas surrounding fenestrae/
incipient fenestrae: (code 0) smooth transition in thickness;
(code 1) thickness transitions in pronounced step from
thicker to thinner bone. Scannella and Horner (2) noted
distinct thinning regions on the ventral surface of the pari-
etal of many Triceratops specimens. This region is often
rimmed by a pronounced transition in thickness, from thick
bone posteriorly to far thinner bone within the depression
(“incipient fenestra”; but see refs. 4, 5, and 58). However, in
some specimens of Triceratops, the transition in thickness in
these regions is more gradual and lacks the pronounced step
between thicker and thinner bone. MOR 1122, a specimen
with fenestrae, exhibits a trace of this step along the edge of
a fenestra. The holotype of “Nedoceratops hatcheri” (USNM
2412) also appears to exhibit a slight step around its reduced
parietal fenestra (1). A distinct ventral parietal step appears
to be more common in specimens found higher in section.
29) Number of epiparietals or epiparietal crenulations: (code 0)
four or fewer; (code 1) five or more [(59) character 28
modified; (50) character 46 modified].
30) Number of epiparietals or epiparietal crenulations: (code 0)
5 or fewer; (code 1) 6 or more [(59) character 28 modified;
(50) character 46 modified].
31) Parietal fenestrae: (code 0) present; (code 1) absent [(50)
character 84 modified)].
32) Epiossification or crenulation on midline of parietal: (code 0)
present; (code 1) absent [(12, 24) character 95 modified]. Scan-
nella and Horner (1) presented evidence suggesting the
presence of a midline epiparietal on MOR 1122. For the
purposes of the present analysis, epiossification positions
were coded based either on presence of the element or a
pronounced marginal crenulation indicating position on the
33) Epiossification or crenulation spanning parietal-squamosal
contact: (code 0) present; (code 1) absent [(57) character 43
Data Matrix. Specimen codings for this analysis. ROM 796 is the
holotype of Arrhinoceratops brachyops; RTMP 2002.57.7 is the
holotype of Eotriceratops xerinsularis.
MATRIX (BINARY CODING)
1000?0010000?00111?100(0 1)(0 1)001000?10
10?0?1??1??1??????????1(0 1)(0 1)11100?10
Scannella et al. www.pnas.org/cgi/content/short/1313334111 5 of 11
Alternative Multistate Characters. Character 10: protuberance
posterior to epinasal: (code 0) very subtle or absent; (code 1)
boss (12, 54).
Character 24: anterolateral projection on squamosal: (code 0)
present, projects anteriorly producing strongly concave anterior
margin of the squamosal; (code 1) anterior projection present but
does not project strongly anteriorly; (code 2) greatly reduced or
Character 27: number of epiparietals or parietal crenulations
per side of parietal: (code 0) four or fewer; (code 1) five; (code 2)
six or more [(59) character 28 modified; (50) character 46
modified; (24) character 93 modified].
1. Scannella JB, Horner JR (2011) ‘Nedoceratops’: Anexample of a transitional morphology.
PLoS One 6(12):e28705.
2. Scannella JB, Horner JR (2010) Torosaurus Marsh, 1891, is Triceratops Marsh, 1889
(Ceratopsidae: Chasmosaurinae): Synonymy through ontogeny. J Vertebr Paleontol
3. Mallon JC, Holmes R, Eberth DA, Ryan MJ, Anderson JS (2011) Variation in the skull of
Anchiceratops (Dinosauria,Ceratopsidae) from the Horseshoe Canyon Formation
(Upper Cretaceous) of Alberta. J Vertebr Paleontol 31(5):1047–1071.
4. Farke AA (2011) Anatomy and taxonomic status of the chasmosaurine ceratopsid
Nedoceratops hatcheri from the upper Cretaceous Lance Formation of Wyoming, U.S.A.
PLoS One 6(1):e16196.
5. Longrich NR, Field DJ (2012) Torosaurus is not Triceratops: Ontogeny in Chasmosaurine
Ceratopsids as a case study in dinosaur taxonomy. PLoS One 7(2):e32623.
6. Maiorino L, Farke AA, Kotsakis T, Piras P (2013) Is torosaurus triceratops? Geometric
morphometric evidence of late maastrichtian ceratopsid dinosaurs. PLoS One 8(11):e81608.
7. Farke AA (2007) Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs, ed
Carpenter K (Indiana Univ Press, Bloomington, IN), pp 235–257.
8. Sampson SD (1995) Two new horned dinosaurs from the Upper Cretaceous Two
Medicine Formation of Montana: With a phylogenetic analysis of the Centrosauriane
(Ornithischia: Ceratopsidae). J Vertebr Paleontol 15(4):743–760.
9. Currie PJ, Langston W, Tanke DH (2008) A New Horned Dinosaur from an Upper
Cretaceous Bone Bed in Alberta (NRC Research Press, Ottawa, ON, Canada).
10. Cobabe EA, Fastovsky DE (1987) Ugrosaurus olsoni, a new ceratopsian (Reptilia:
Ornithischia)fromtheHell CreekFormation ofeastern Montana. J Paleontol61(1):148–154.
11. Forster CA (1993) Taxonomic validity of the ceratopsid dinosaur Ugrosaurus olsoni
(Cobabe and Fastovsky). J Paleontol 67(2):316–318.
12. Forster CA (1996) Species resolution in Triceratops: Cladistic and morphometric
approaches. J Vertebr Paleontol 16(2):259–270.
13. Horner JR, Goodwin MB (2008) Ontogeny of cranial epi-ossifications in Triceratops. J
Vertebr Paleontol 28(1):134–144.
14. Longrich NR (2011) Titanoceratops ouranos, a giant horned dinosaur from the late
Campanian of New Mexico. Cretac Res 32:264–276.
15. Lehman TM (1998) A gigantic skull and skeleton of the horned dinosaur Pentaceratops
sternbergi from New Mexico. J. Paleo 72(5):894–906.
16. Horner JR, Goodwin MB (2006) Major cranial changes during Triceratops ontogeny.
Proc Biol Sci 273(1602):2757–2761.
17. Horner JR, Goodwin MB (2009) Extreme cranial ontogeny in the upper cretaceous
dinosaur pachycephalosaurus. PLoS One 4(10):e7626.
18. Fowler DW, Scannella JB, Horner JR (2011) Reassessing ceratopsid diversity using
unified frames of reference. J Vertebr Paleontol 31(5):111A (abstr).
19. Sullivan RM, Lucas SG (2010) New Perspectives on Horned Dinosaurs: The Royal Tyrell
Museum Ceratopsian Symposium, eds Ryan MJ, Chinnery-Allgeier BJ, Eberth DA
(Indiana Univ Press, Bloomington, IN), pp 169–180.
20. Gilmore CW (1946) Reptilian fauna of the North Horn Formation of central Utah (US
Geological Survey, Washington, DC), Professional Paper 210C, pp 29–51.
21. Lawson DA (1976) Tyrannosaurus and Torosaurus, Maestrichtian dinosaurs from
Trans-Pecos, Texas. J Paleo 50(1):158–164.
22. Sullivan RM, Boere AC, Lucas SG (2005) Redescription of the ceratopsid dinosaur
Torosaurus utahensis (Gilmore, 1946) and a revision of the genus. J Paleo 79(3):564–582.
23. Hunt RK, Lehman TM (2008) Attributes of the ceratopsian dinosaur Torosaurus and new
material from the Javelina Formation (Maastrichtian) of Texas. J Paleo 82(6):1127–1138.
24. Sampson SD, et al. (2010) New horned dinosaurs from Utah provide evidence for
intracontinental dinosaur endemism. PLoS One 5(9):e12292.
25. Ott CJ, Lawson PL (2010) New Perspectives on Horned Dinosaurs: The Royal Tyrell
Museum Ceratopsian Symposium, eds Ryan MJ, Chinnery-Allgeier BJ, Eberth DA
(Indiana Univ Press, Bloomington, IN), pp 203–218.
26. Lerbekmo JF (1999) Magnetostratigraphy of the Canadian continental drilling
program Cretaceous-Tertiary (K-T) boundary project core holes, western Canada. Can
J Earth Sci 36:705–715.
27. Lerbekmo JF, Braman DR (2002) Magnetostratigraphic and biostratigraphic correlation
of late Campanian and Maastrichtian marine and continental strata from the Red Deer
Valley to the Cypress Hills, Alberta, Canada. Can J Earth Sci 39:539–557.
28. Lerbekmo JF (2009) Glacioeustatic sea level fall marking the base of supercycle TA-1
at 66.5 Ma recored by the kaolinization of the Whitemud Formation and the Colgate
Member of the Fox Hills Formation. Mar Pet Geol 26:1299–1303.
29. Tokaryk TT (1986) Ceratopsian dinosaurs from the Frenchman Formation (Upper
Cretaceous) of Saskatchewan. Can Field Nat 100:192–196.
30. Hicks JF, Johnson KR, Obradovich JD, Miggins DP, Tauxe L (2003) Magnetostratigraphy
of Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver Basin,
Colorado. Rocky Mt. Geol. 38:1–27.
31. Raynolds RG, Johnson KR (2003) Synopsis of the stratigraphy and paleontology of the
uppermost Cretaceous and lower Tertiary strata in the Denver Basin, Colorado. Rocky
Mt. Geol. 38:171–181.
32. Carpenter K, Young DB (2002) Late Cretaceous dinosaurs from the Denver Basin,
Colorado. Rocky Mt. Geol. 37:237–254.
33. Hicks JF, Johnson KR, Obradovich JD, Tauxe L, Clark D (2002) The Hell Creek
Formation and the Cretaceous-Tertiary Boundary in the Northern Great Plains: An
Integrated Continental Record of the End of the Cretaceous, eds Hartman JH,
Johnson KR, Nichols DJ (Geological Society of America, Boulder, CO), Special Paper
361, pp 35–55.
34. Anderson J (1999) Occipital condyle in the ceratopsian dinosaur Triceratops, with
comments on body size variation. Contrib Mus Paleontol Univ Mich 30(8):215–231.
35. Swofford DL (2003) PAUP*. Phylogenetic Analysis Using Parsimony (*and Other
Methods) (Sinauer Associates, Sunderland, MA), Version 4.
36. O’Leary MA, Kaufman SG (2008) MorphoBank 2.5: Web application for morphological
phylogenetics and taxonomy. Available at www.morphobank.org.
37. Pleijel F (1995) On character coding for phylogeny reconstruction. Cladistics 11:309–315.
38. Frederickson JA, Tumarkin-Deratzian AR (2014) Craniofacial ontogeny in Centrosaurus
apertus. PeerJ 2:e252.
39. Felsenstein J (1985) Confidence limits on phylogenies: An approach using the
bootstrap. Evolution 39(4):783–791.
40. Sorenson MD, Franzosa EA (2007) TreeRot (Boston University, Boston, MA), Version 3.
41. Wu X, Brinkman DB, Eberth DA, Braman DR (2007) A new ceratopsid dinosaur
(Ornithischia) from the uppermost Horseshoe Canyon Formation (upper Maastrichtian),
Alberta, Canada. Can J Earth Sci 44:1243–1265.
42. Campione NE, Brink KS,Freedman EA,McGarrityT, EvansDC (2013) ’Glishades ericksoni’,
an indeterminate juvenile hadrosaurid from the Two Medicine Formation of Montana:
Maastrichtian) of western North America. Palaeobio Palaeoenv 93:65–75.
43. Fisher DC (1994) Interpreting the Hierarchy of Nature: From Systematic Patterns to
Evolutionary Process Theories, eds Grande L, Rieppel O (Academic Press, San Diego),
in the latestCretaceous(Campanian-
1000?001010?00111?100(0 1)(0 1)0100?10
10?0?1??1?1??????????1(0 1)(1 2)110?10
Scannella et al. www.pnas.org/cgi/content/short/13133341116 of 11
44. Fisher DC (2008) Stratocladistics: Integrating temporal data and character data in
phylogenetic inference. Annu Rev Ecol Evol Syst 39:365–385.
45. Polly PD (1997) Ancestry and species definition in paleontology: Stratocladistic
analysis of Paleocene-Eocene Viverravidae (Mammalia, Carnivora) from Wyoming.
Contrib Mus Paleontol Univ Mich 30(1):1–53.
46. Pardo JD, Huttenlocker AK, Marcot JD (2008) Stratocladistics and evaluation of
evolutionary modes in the fossil record: An example from the ammonite genus
Semiformiceras. Palaeontology 51(4):767–773.
47. Rook DL, Hunter JP (2011) Phylogeny of the Taeniodonta: Evidence from dental
characters and stratigraphy. J Vertebr Paleontol 31(2):422–427.
48. Campione NE, Reisz RR (2010) Varanops brevirostris (Eupelycosauria: Varanopidae)
from the Lower Permian of Texas, with discussion of varanopid morphology and
interrelationships. J Vertebr Paleontol 30(3):724–746.
49. Marcot JD, Fox DL (2008) StrataPhy: A new computer program for stratocladistic
analysis. Palaeontol Electronica 11:5A.
50. Forster CA (1990) The cranial morphology and systematics of Triceratops with
a preliminary analysis of ceratopsid phylogeny. PhD dissertation (Univ of Pennsylvania,
51. Longrich NR (2010) Mojoceratops perifania, a new chasmosaurine ceratopsid from
the Late Campanian of Western Canada. J Paleontol 84(4):681–694.
52. Gates TA, Sampson SD (2007) A new species of Gryposaurus (Dinosauria: Hadrosauridae)
from the Late Campanian Kaiparowits Formation, southern Utah, USA. Zool J Linn Soc
53. McDonald AT, Wolfe DG, Kirkland JI (2010) A new basal hadrosauroid (Dinosauria:
Ornithopoda) from the Turonian of New Mexico. J Vertebr Paleontol 3(3):799–812.
54. Ostrom JH, Wellnhofer P (1986) The Munich specimen of Triceratops with a revision of
the genus. Zitteliana 14:111–158.
55. Fujiwara S, Takakuwa Y (2011) A sub-adult growth stage indicated in the degree of
suture co-ossification in Triceratops. Bull Gumma Mus Nat Hist 15:1–17.
56. Dodson P, Forster CA, Sampson SD(2004) Ceratopsidae. The Dinosauria, eds Weishampel
DB, Dodson P, Osmólska H (Univ of California Press, Berkeley, CA), 2nd Ed, pp 494–513.
57. Farke AA, et al. (2011) A new centrosaurine from the Late Cretaceous of Alberta,
Canada, and the evolution of parietal ornamentation in horned dinosaurs. Acta
Palaeontol Pol 56(4):691–702.
58. Tsuihiji T (2010) Reconstructions of the axial muscle insertions in the occipital region
of dinosaurs: Evaluations of past hypotheses on marginocephalia and tyrannosauridae
using the extant phylogenetic bracket approach. Anat Rec (Hoboken) 293(8):1360–1386.
59. Holmes RB, Forster C, Ryan M, Shepherd KM (2001) A new species of Chasmosaurus
(Dinosauria:Ceratopsia) from the Dinosaur Park Formation of Southern Alberta. Can J
Earth Sci 38:1423–1438.
3027. These specimens exhibit a combination of primitive and derived features. Both specimens exhibit a more convex rostrum than Triceratops found
stratigraphically lower. MOR 3045 represents the lowest occurrence of a wide NPP in the HCF dataset. Parietal, squamosal, postorbital, nasal, and epinasal of
MOR 3045 mirrored. Orbit is crushed. (Scale bars: 10 cm.)
Triceratops from upper M3. (A) MOR 3027 (cast), a large subadult. (B) MOR 3045, subadult recovered from ∼2 m stratigraphically higher than MOR
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collected from the lower part of M3. (D) MOR 3027, collected from upper M3. (E) MOR 3045, collected from upper M3. This specimen exhibits a pronounced
peak on the nasal process (arrow) that is anterior to the posterior margin, a feature that is observed in juveniles from U3. (F) MOR 2574, collected from the
lower U3. (G) MOR 2702, collected from the lower U3 (image mirrored for comparison). Specimens from U3 (F and G) exhibit a wider NPP; MOR 3045 represents
the stratigraphically lowest occurrence of a wide NPP. MOR 2574 and MOR 2702 were collected from a multiindividual bone bed and exhibit variation in the
morphology of the NPP. A trend toward an increased angle between the NPP and NS is noted in the HCF sample (Dataset S1). NPP, nasal process of the
premaxilla. NS, narial strut. (Scale bars: 10 cm; B–G are to the same scale.)
Variation in the nasal process of the premaxilla. (A) RTMP 2002.57.7, the holotype of Eotriceratops. (B) MOR 1120, collected from L3. (C) MOR 3011,
nasal. (B) MOR 2999 (from U3); this process is greatly reduced. (Scale bar: 5 cm.)
Variation in the anteromedial process of the nasal. (A) MOR 3027 (from upper M3) expresses a prominent process on the anteromedial surface of the
stratigraphic position to be determined. (B) MOR 3045 from upper M3. (C) MOR 2924 from U3. (Scale bars: 5 cm.)
Some specimens exhibit a pronounced shelf or projection on the posterior surface of the epinasal (indicated by arrow; character 13). (A) MOR 989,
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Fig. S5. Download full-text
Bootstrap support values below nodes. Percent occurrence for nodes are reported above horizontal lines. Specimens group according to relative stratigraphic
position; however, several fragmentary specimens are recorded in positions inconsistent with stratigraphic position. MOR 981, MOR 1122, MOR 3081, and MOR
3005 exhibit the Torosaurus morphology. (B) A 50% majority-rule tree produced by initial analysis using multistate coding. (C) Strict consensus tree produced
once specimens that could not be coded for characters of the parietal-squamosal frill are removed (binary coding, branch-and-bound search). Bremer decay
values greater than one reported above nodes. (D) Strict consensus tree produced once specimens that could not be coded for characters of the parietal-
squamosal frill are removed (multistate coding, branch-and-bound search). (E) A 50% majority-rule consensus tree for analysis in which specimens that could
not be coded for characters of parietal-squamosal frill are removed (binary coding, branch-and-bound search). (F) A 50% majority-rule consensus tree for
analysis in which specimens that could not be coded for characters of parietal-squamosal frill are removed (multistate coding, branch-and-bound search). (G)
Strict consensus tree for analysis including only specimens exhibiting at least 10 cranial characters (in the multistate matrix); binary codings (branch-and-bound
search). (H) Strict consensus tree for analysis after MOR 2924 is removed from the matrix; binary codings (branch-and-bound search).
Additional results of cladistic analyses of HCF Triceratops. (A) A 50% majority-rule consensus tree produced by initial analysis using binary coding.
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