Fig. S1. Triceratops from upper M3. ( A ) MOR 3027 (cast), a large subadult. ( B ) MOR 3045, subadult recovered from ∼ 2 m stratigraphically higher than MOR 3027. These specimens exhibit a combination of primitive and derived features. Both specimens exhibit a more convex rostrum than Triceratops found stratigraphically lower. MOR 3045 represents the lowest occurrence of a wide NPP in the HCF dataset. Parietal, squamosal, postorbital, nasal, and epinasal of MOR 3045 mirrored. Orbit is crushed. (Scale bars: 10 cm.) 

Context in source publication

Context 1

... Hell Creek Project (1999 – 2010), a multiinstitutional sur- vey of the fauna, flora, and geology of the Upper Cretaceous Hell Creek Formation (HCF), provides insights into the paleo- biology 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. Stratigraphic placement of Triceratops specimens within the HCF reveals previously undocumented shifts in morphology. The HCF is divided into three stratigraphic units: the lower third (L3), middle third (M3), and upper third (U3) (1, 21). The stratigraphic separation of Triceratops morphospecies is apparent with specimens referable to T. prorsus (following Forster) (12) found in U3 and T. horridus recovered only lower in the HCF. Specimens from the upper part of M3 exhibit a combination of T. horridus and T. prorsus features (Fig. 1, SI Text , and Fig. S1). L3 Triceratops. Triceratops from the lowermost 15 – 30 m of the HCF (L3) possess either a small nasal horn (Fig. 2 A 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. 2 B 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. 2 D and Dataset S1). M3 Triceratops. The mean nasal-horn length increases through M3 (Figs. 1 E and F and 2 A ). 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. 2 A and Dataset S1). The NPP is more vertically inclined, producing a convex rostrum lacking the low, elongate profile noted in T. horridus [although the largest, and presumably oldest, known specimens (e.g., MOR 004 and MOR 1625) exhibit pro- portionally longer rostra] (Fig. 2 E 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. 2 D ). Further, U3 Triceratops seem to exhibit nasals that are more elongate than Triceratops from the lower half of the HCF (Fig. 2 F and Dataset S1). Shifts in Morphology over Time. Epinasals exhibit a directional morphologic trend; average length increases throughout the formation (Fig. 2 A 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 partic- ularly pronounced in UCMP 113697 from the uppermost M3 (Fig. 1 E ). 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. 2 B ) (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. 2 C 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. 2 F 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. 2 D 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. 2 D ), 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 ...