Supporting Information Supporting Information Corrected September 24, 2014 Scannella et al. 10.1073/pnas.1313334111 SI Text with a more posteriorly inclined NPP contributing to a low, Variation in Parietal-Squamosal Frill. Scannella and Horner (1) sinusoidal rostrum in some specimens [e.g., MOR 1120, American suggested that the number of epiossifications present in Triceratops Museum of Natural History (AMNH) 5116, National Museum of may vary stratigraphically. Often, epiossifications are unpreserved Natural History (USNM) 1201, and YPM 1820]. or are detached from the parietal-squamosal frill, which compli- The angle between the NPP and narial strut appears to increase cates testing of this hypothesis. Data currently available highlight stratigraphically in the HCF; specimens from the upper M3 and variation in epiossification number and position as the number and U3 exhibit a larger angle between the NPP and narial strut than configuration can vary between the squamosals of a single in- specimens found lower in section (Fig. 2C). To quantify this shift dividual [e.g., Museum of the Rockies (MOR) 1120]. Additionally, in morphology, the angle between the NPP and narial strut (Fig. there may be an ontogenetic component to epiossification number S2) was measured using the Ruler tool in Adobe Photoshop. and position in chasmosaurines (1–3). Hell Creek Formation This angle was measured between the approximate midlines of (HCF) specimens with the highest numbers of epiparietals (MOR each process, parallel to the direction representing the primary 1122 and MOR 3081) and episquamosals (MOR 1120, MOR trend (results presented in Dataset S1 and Fig. 2). We note that, 1122, and MOR 3081) are found lower in the formation. in some more basal taxa (e.g., Anchiceratops) (3), the NPP can be The development of fenestrae also varies within Triceratops (refs. oriented nearly perpendicular to the narial strut and as such can 1and2;butseerefs.4–6). A pronounced transition in thickness on give the rostrum a more convex appearance in lateral view. the ventral surface of the parietal surrounding this area (2) is noted The width of the NPP also affects the rostrum morphology in Triceratops from the upper unit of the HCF (U3) and at least because a wider NPP reduces the apparent sinuosity of the anterior one specimen from the upper part of the middle unit (M3) (MOR premaxilla. MOR 3027 and MOR 3045 (both recovered from 3045), but lower in the formation there appears to be a more upper M3) exhibit a more vertically inclined premaxillary articu- gradual transition in the thickness of the parietal (e.g., MOR 335, lation with the nasal than specimens found lower in the formation. MOR 1120, and MOR 2985). This finding may suggest that the MOR 3045 (collected ∼2 m above MOR 3027) exhibits the further fenestrae developed later, ontogenetically, in Triceratops found derived feature of an anteroposteriorly expanded NPP, contrib- stratigraphically higher. Alternatively, if Triceratops and Tor- uting to a rostrum that seems even more convex in lateral view. osaurus represent distinct but closely related taxa, Triceratops Rostrum length appears to vary both stratigraphically and found stratigraphically lower may express more basal parietal ontogenetically. The largest specimens from U3 (e.g., MOR 004 features, including eventual fenestration of the parietal. and MOR 1625) exhibit more elongate rostra (Fig. 2E and Da- The stratigraphically documented cranial morphological trends taset S1); however, even these large specimens do not exhibit the expressed in Triceratops are thus far consistent with the morphology strongly posteriorly inclined NPP and sinusoidal dorsal margin of specimens referred to “Torosaurus latus” (however, we note that of the rostrum exhibited in specimens referred to T. horridus. the majority of specimens exhibiting the “Torosaurus” morphology Evolutionary changes in rostrum morphology may reflect the were recovered from the lower half of the formation). Precise lo- development of an enlarged epinasal. cality data for MOR 981 are not available, but it was collected in a mudstone located above the basal sandstone. Perhaps the strati- Other Triceratopsin Taxa. Longrich (14) referred Oklahoma Mu- graphically highest known Torosaurus from the HCF of Montana, seum of Natural History (OMNH) specimen 10165, a large ce- Milwaukee Public Museum (MPM) specimen VP6841 (which, ratopsid specimen recovered from Campanian deposits of New based on study of topographic and geologic maps, appears to have Mexico and previously diagnosed as a gigantic specimen of Pen- been collected from the upper half of the formation), exhibits an taceratops sternbergi (15), to the new taxon Titanoceratops our- incomplete yet relatively narrow epinasal morphology that is consis- anos. Longrich proposed that Titanoceratops represents the oldest tent with its stratigraphic position. The observation that a nasal boss member of the Triceratopsini, the clade that includes Triceratops, morphology appears to occur in relatively mature specimens of Tri- Torosaurus, “Nedoceratops,” and Ojoceratops (14). This specimen ceratops (Torosaurus morph; e.g., MOR 1122 and MOR 981) (7) exhibits several features consistent with its stratigraphic position suggests that development of the boss morphology was ontogenetic, relative to HCF Triceratops. It has a relatively short epinasal, as is seen in some centrosaurine ceratopsids (e.g., refs. 8 and 9.). The short arched nasals, a posteriorly inclined NPP, and elongate nasal boss morphology is not exhibited in all Torosaurus specimens postorbital horn cores. Given the degree of ontogenetic trans- [MOR 3081, MPM VP 6841, Yale Peabody Museum (YPM) 1830, formation noted in several marginocephalians (1, 2, 16, 17), it is and YPM 1831], and thus the degree to which this feature is de- possible that many of the features considered to distinguish Ti- veloped may vary individually or stratigraphically. University of Cal- tanoceratops from Pentaceratops (including large size, broad epi- ifornia Museum of Paleontology (UCMP) 128561 exhibits a low nasal ossifications, extensive cornual sinuses, strongly anteriorly curved boss (10, 11); however, due to the fragmentary nature of the speci- postorbital horn cores, elongate premaxilla) (14) may instead men, it is unclear whether it represents the Torosaurus morphology. represent ontogenetic or individual variation within the latter taxon (18), which would be consistent with the original diagnosis Morphology of the Rostrum. Forster (12) recognized rostrum by Lehman (15). Further assessment of this specimen and its morphology as one of the features that distinguish Triceratops phylogenetic position is beyond the scope of the current study. horridus from Triceratops prorsus. T. horridus exhibits a low, Triceratopsin material from the southern region of the western elongate rostrum with a sinusoidal dorsal margin whereas, in interior of North America includes specimens that have been re- T. prorsus, the rostrum is shorter and more convex. Longrich and ferred to Ojoceratops fowleri and Torosaurus utahensis (14, 19–23). Field (5) noted that specimens of T. prorsus have a more verti- Ojoceratops, from the Ojo Alamo Formation of New Mexico, cally oriented nasal process [= ascending nasal process of the appears to be closely related to Triceratops (3, 19, 24) and has premaxilla (sensu ref. 13)], here referred to as the nasal process been suggested to be synonymous with the latter taxon (14). of the premaxilla (NPP)] compared with T. horridus. Rostrum Material referred to Ojoceratops thus far consists of isolated morphology appears to be tied to the orientation of the NPP, or fragmentary elements. Due to the missing morphological Scannella et al. www.pnas.org/cgi/content/short/1313334111 1of11 information for much of this material, specimens of O. fowleri were relative to MOR 3027. UCMP 113697 was discovered 21.5 km to not included in the current cladistic analysis of HCF Triceratops. the east of these localities. Locally, the Apex Sandstone is ∼6m A nasal horn referred to this taxon [State Museum of Pennsyl- above the base of the quarry that produced this specimen. An vania (SMP) specimen VP-1828] exhibits a morphology similar to organic-rich horizon that may correlate with the organic-rich bed that observed in several lower unit (L3)/lower M3 Triceratops,which found above the two MOR localities is ∼3 m above the quarry, is consistent with its stratigraphic position relative to the Hell Creek and thus UCMP 113697 appears to have been collected from Formation of Montana. The holotype squamosal (SMP VP-1865) roughly the same stratigraphic level as MOR 3045. has a greatly reduced anterolateral projection of the squamosal, which has been used to distinguish it from Torosaurus utahensis.The Estimation of Basal-Skull Length. In this study, basal-skull length degree to which this feature can distinguish Ojoceratops from other was considered the distance from the anteriormost point of the taxa is unclear; the HCF dataset demonstrates that the morphology rostrum to the posterior surface of the occipital condyle (fol- of this projection varies within Torosaurus and Triceratops, and even lowing previous researchers) (7, 12). Skull-length measurements within a single individual (MOR 2999). Variation in this feature has for some specimens were taken from reconstructions (Dataset previously been noted by Hunt and Lehman (23). S1). For some largely complete specimens that do not preserve The incomplete or fragmentary nature of specimens