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. 

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... 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 clusters 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 specimens 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. S5 G ) (multistate coding: MPT 7036, 53 steps, CI 0.7358, HI 0.3774, RI 0.8082) (Fig. 3 B ) 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 recovered 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. S5 H ) (multistate coding: MPT 282; 52 steps; CI 0.7500; HI 0.3654; RI 0.8116) (Fig. 3 C ) recovered MOR 3045 as basal to U3 specimens and as more derived 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 specimens 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, alternatively, 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 Torosaurus 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. S6 A ). 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 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 specimen MOR 3081. Incorporation of Torosaurus specimens into Triceratops operational units (total debt = 67, nine trees, three topologies) (Fig. S6 B ) produced a single tree suggesting that all operational units arose via cladogenesis and two additional topologies 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 bifurcation 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. S6 C ). Four trees indicate that all HCF operational units represent a single anagenetic lineage with specimens exhibiting the T. horridus morphology evolving into T. prorsus (Fig. 4 A ). 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. 4 B ). Characters Incorporated in Cladistic Analysis. The first use in a cladistic 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 pronounced ventral keel. Specimens for which apparently laterally 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 modified; (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 phylogenetic 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 [sensu 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 ...

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... 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. S5 G ) (multistate coding: MPT 7036, 53 steps, CI 0.7358, HI 0.3774, RI 0.8082) (Fig. 3 B ) 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 recovered 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. S5 H ) (multistate coding: MPT 282; 52 steps; CI 0.7500; HI 0.3654; RI 0.8116) (Fig. 3 C ) recovered MOR 3045 as basal to U3 specimens and as more derived 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 specimens 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, alternatively, 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 Torosaurus 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. S6 A ). 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 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 specimen MOR 3081. Incorporation of Torosaurus specimens into Triceratops operational units (total debt = 67, nine trees, three topologies) (Fig. S6 B ) produced a single tree suggesting that all operational units arose via cladogenesis and two additional topologies 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 bifurcation 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. S6 C ). Four trees indicate that all HCF operational units represent a single anagenetic lineage with specimens exhibiting the T. horridus morphology evolving into T. prorsus (Fig. 4 A ). 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. 4 B ). Characters Incorporated in Cladistic Analysis. The first use in a cladistic 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 pronounced ventral keel. Specimens for which apparently laterally 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 modified; (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 phylogenetic 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 [sensu 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 ...

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... 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 se- lected 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). 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. 4 A ). 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. 4 B 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 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 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 ...

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... from lower in the formation. Triceratops from the upper half of the HCF exhibit a more vertically inclined NPP (Fig. 2 C ), 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. 2 E 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. 3 A ). A similar topology was recovered when specimens not exhibiting codeable features of the parietal squamosal frill were removed from the analysis (Fig. 3 B ). 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 . In the analysis of the most reduced dataset, UCMP 113697 and MOR 3027 cluster together (Fig. 3 C ). 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 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. S6 A ). 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. S6 B ). 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. S6 C ). 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. 4 A ) (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). 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 se- lected 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). 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. 4 A ). Alternatively, MOR 3045 may represent an early member of ...

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... 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. S6 B ). 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. S6 C ). 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. 4 A ) (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). 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 se- lected 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). 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. 4 A ). 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. 4 B 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 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 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 ...

Context 6

... 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 se- lected 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). 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. 4 A ). 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. 4 B 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 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 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 ...

Context 7

... record were a product of 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 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 se- lected 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). 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. 4 A ). 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. 4 B 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 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 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 ...

Context 8

... 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. 2 C ), 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. 2 E 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. 3 A ). A similar topology was recovered when specimens not exhibiting codeable features of the parietal squamosal frill were removed from the analysis (Fig. 3 B ). 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 . In the analysis of the most reduced dataset, UCMP 113697 and MOR 3027 cluster together (Fig. 3 C ). 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 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. S6 A ). 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. S6 B ). 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. S6 C ). 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. 4 A ) (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). 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 se- lected 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). Indi- viduals exhibiting more pronounced U3 character states may have become increasingly abundant in the HCF ...