Evolutionary trends in Triceratops from the Hell Creek Formation, Montana

Significance

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 Formation of Montana incorporates new stratigraphic and ontogenetic findings to permit the investigation of evolution within this genus. Our research indicates that the two currently recognized 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.

Abstract

The placement of over 50 skulls of the well-known horned dinosaur Triceratops within a stratigraphic framework for the Upper 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 formation 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 formation, 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 the Dinosauria.

The Hell Creek Project (1999--2010), a multiinstitutional survey of the fauna, flora, and geology of the Upper Cretaceous Hell Creek Formation (HCF), provides insights into the paleobiology and evolution of the last nonavian dinosaurs (1). Triceratops (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 stratigraphically 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 Triceratops 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 including 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 separated 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, consideration of morphological variation in a detailed stratigraphic context is necessary to reassess systematic hypotheses.

Results

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).

Fig. 1.

Stratigraphic placement of HCF Triceratops reveals trends in cranial morphology including elongation of the epinasal and change in morphology of the rostrum. (A) HCF stratigraphic units. (B) Magnetostratigraphic 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 position of specimens from different areas are approximate 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 arrow indicates epinasal-nasal protuberance. (F) MOR 2982 nasal horn from lower part of M3 (image mirrored). (G) MOR 1120 nasal horn from L3. (H) MOR 004 young adult Triceratops skull from U3 (cast; image 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.)

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 Dataset S1).

Fig. 2.

Stratigraphic variation in cranial morphology. Each unit of the HCF is divided into upper and lower sections. For L3, the upper part is here designated 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. Estimated 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 indicate statistically significant P values. Scale on x axes are logarithmic.

M3 Triceratops.

The mean nasal-horn length increases through M3 (Figs. 1 E and F and 2A). The University of California Museum 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 posterior 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 posterior 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 proportionally 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 fontanelle becomes constricted and eventually closed in late-stage subadults/young adults (e.g., MOR 2923 and MOR 2979), ontogenetically 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 formation (Fig. 2A and Dataset S1) (Spearman's rank coefficient = 0.824, P = 4.15E−07). A protuberance just posterior to the epinasal, observed in specimens from L3 and M3 (Fig. 1), is particularly 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 coefficient = 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) and not statistically significant (P = 0.392)]. Large juvenile U3 Triceratops (e.g., MOR 1110) can possess more elongate postorbital horn cores (0.64 basal-skull length). Whereas U3 postorbital 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), expresses morphologies (elongate postorbital horn cores, small nasal horn) consistent with its stratigraphic position relative to Triceratops.

Cladistic and Stratocladistic Analyses.

Initial cladistic analyses recovered 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 postorbital horn cores (SI Text), recovers specimens from the upper part of M3 as basal to U3 Triceratops.

Fig. 3.

Results of cladistic analysis of HCF Triceratops. (A) Strict consensus tree 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 characters 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 (multistate coding, branch-and-bound search) after removal of MOR 2924 (SI Text).

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 intermediate between L3 and U3 Triceratops (the epinasal of MOR 3027 is incomplete). These specimens each exhibit large postorbital 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 relatively 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 performed 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 morphology. 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 sequence, 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).

Discussion

Evolutionary Patterns.

One of the principle questions in evolutionary 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 anagenesis (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 cladogenesis (evolution via branching events) (Fig. 4 B–D) (8, 32, 33, 37, 38).

Fig. 4.

Potential patterns of HCF Triceratops evolution. (A) Anagenesis, or transformation of a lineage over time. (B) Bifurcation of an anagenetic lineage in which the ancestor becomes pseudoextinct through evolution (sensu ref. 38). Wagner and Erwin (38) distinguish bifurcation from “true” cladogenesis, which includes no anagenetic component. (C and D) True cladogenesis 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 represents MOR 2982. The x and y axes represent morphospace.

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 combination of large sample size, ontogenetic resolution, and detailed stratigraphic data makes Triceratops an ideal taxon for testing hypotheses regarding evolutionary mode in a nonavian dinosaur.

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 specimens, 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 population 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) variation. As the HCF Triceratops lineage evolved, some individuals expressed more of the features that would eventually dominate the population. Over time, these traits were selected for and characterized U3 Triceratops. This is a purely anagenetic scenario.
• 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 epinasal 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 the formation.

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 biogeographic segregation may represent an artifact of the stratigraphic 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) (SI Text).

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, specimens 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). Individuals 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). Alternatively, 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 epinasal-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 (anagenesis) 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 autapomorphic 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).

Conclusions

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 primary 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 biogeographic segregation of contemporaneous Triceratops morphospecies and there is evidence for the morphological transformation of Triceratops throughout the HCF. This dataset supports hypotheses 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 relative 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 orientation; frill epiossification shape) were excluded from the analysis; however, characters often used to distinguish Triceratops and Torosaurus (e.g., parietal 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 performed in the program StrataPhy (29), with maxtrees set to 250,000. Specimens 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.