The median epiparietal of Regaliceratops is similar, and likely homologous, to the median epiparietal of Triceratops spp., previously termed P0 (e.g., [] and K.E. Clayton et al., 2010, Soc. Vertebrate Paleontol., abstract), in terms of (1) a median position, (2) being of unpaired morphology, and (3) projecting caudal to the parietal margin. Additionally, based on (1) dorsally offset position from the plane of the frill, (2) rostrally offset position from the caudal margin of the frill, and (3) roughly triangular cross-section, the median epiossification of Regaliceratops is likely also homologous with the laterally curved hooks of Anchiceratops, which have previously been termed P1 []. To test this homology across Chasmosaurinae, we shifted the identity of the epiparietals of Anchiceratops, Pentaceratops, and Utahceratops one position medially ( Figure S3 ; see Supplemental Experimental Procedures, “Phylogenetic Methods” ). The phylogenetic analysis of this revised homology scheme for the epiossifications results in a simpler and better-resolved evolutionary history (i.e., a shorter, better-resolved tree), indicating support for the revised homology hypothesis proposed here ( Figure S1 ; see Supplemental Experimental Procedures, “Phylogenetic Methods” ).

The pattern of homology of the parietosquamosal epiossifications is integral to an understanding of both taxonomy and phylogeny of Ceratopsidae. A homology scheme numbering the epiossifications from medial to lateral was derived initially for the diverse array of spike- and horn-like epiossifications seen in centrosaurines [] and was subsequently applied across Ceratopsidae [] and modified to include more basal taxa (K.E. Clayton et al., 2009, 2010, Soc. Vertebrate Paleontol., abstracts). These studies have shown that for the parietal of Centrosaurinae, the number of epiossifications is relatively constant, but with more medial loci showing more variation in development and more phylogenetic signal than lateral ossifications []. Contrasted with Centrosaurinae, epiossification homology in Chasmosaurinae is less well established and has featured less prominently in inferring phylogeny. New data derived from the epiossifications of Regaliceratops have prompted a review of epiossification homology across Chasmosaurinae, specifically those taxa with a median epiparietal ( Figure S3 ).

Two new horned dinosaurs from the Upper Cretaceous Two Medicine Formation of Montana: With a phylogenetic analysis of the Centrosaurinae (Ornithischia: Ceratopsidae).

The discovery of Regaliceratops, although well nested within Chasmosaurinae, shows the reciprocal pattern to the basal centrosaurines above, where the nasal horn and epiossifications are exaggerated relative to the postorbital horns and frill length. A plot of postorbital horncore length as a function of nasal horncore length results in Regaliceratops falling far outside the morphospace occupied by other Maastrichtian chasmosaurines, and within the morphospace of Campanian centrosaurines ( Figure 4 ). This is an unexpected pattern for Maastrichtian Chasmosaurinae, and one that is evolutionarily convergent with Centrosaurinae. Convergent evolution of horns, or other display/signaling structures, between sister clades or more distantly related groups has been well documented in mammals [] and recently found in hadrosaurid dinosaurs [], but this marks the first occurrence within the diverse clade of horned dinosaurs. Based on disparate patterns of cranial ornamentation, the use of these horns and frills as display or sociosexual signaling structures has been suggested to be distinct between these two subfamilies of horn dinosaurs []. Convergent horn evolution in mammals often correlates with convergent social behaviors []. It may be hypothesized that Regaliceratops converged not only morphologically with Centrosaurines but also behaviorally, following the early Maastrichtian extinction of Centrosaurinae.

There has been a historical dichotomy in ceratopsid cranial ornamentation where individual species tend to emphasize either (1) the nasal horn and frill epiossification (e.g., Styracosaurus, Centrosaurus) or (2) the postorbital horns and length of the frill (e.g., Anchiceratops, Anchiceratops, Pentaceratops), but not both [] ( Figure 4 ). These contrasting patterns of cranial ornamentation have been thought to be evolutionary hallmarks of two independent evolutionary lineages, Centrosaurinae (emphasizing nasal horn and frill epiossifications) and Chasmosaurinae (emphasizing the postorbital horns and length of the frill) []. Recent discovery of the basal centrosaurines Albertacertops [], Diabloceratops [], and Nasutoceratops [] have illustrated higher disparity in cranial ornamentation than previously thought and indicate that this dichotomy evolved not at the split between Chasmosaurinae and Centrosaurinae, but within the Chasmosaurinae and Centrosaurinae lineages. Additionally, Sampson et al. [] reported the discovery of a clade within Centrosaurinae (Nasutoceratops and Avaceratops) showing cranial ornamentation similar to those of Chasmosaurinae, i.e., an emphasis on the size of the postorbital horns and retention of a less complex frill.

Convex hulls (colored polygons) show morphospace occupation by Centrosaurinae (red), Campanian Chamsosaurinae (light blue), and Maastrichtian Chasmosauridae, excluding Regaliceratops (dark blue). Diagonal dotted lines indicate ratio of postorbital horn length to nasal horn length. Basal centrosaurines (e.g., Albertaceratops, Diabloceratops) are not plotted. Data are derived from specimen measurements (C.M.B.) as well as previously published data; see Table S2

The cladistic analysis of relationships within Chasmosaurinae recovers Regaliceratops in a polytomy with Eotriceratops and Ojoceratops, as sister taxa to the remaining Triceratopsini (Triceratops, Torosaurus, Nedoceratops, Titanoceratops) ( Figures 3 and S1 ). This phylogenetic position hypothesized for Regaliceratops is consistent with its temporal occurrence, being roughly time equivalent to Eotriceratops ( Figure 3 ). This suggests that Regaliceratops is part of an evolutionary trend showing both an increase in body size and solidification of the frill, with an extreme example being the latest Maastrichtian Triceratops. Despite being recovered within the Triceratopsini, Regaliceratops shows ornamentation morphology distinct from all other members of this clade, as well as the more inclusive clade including Anchiceratops and Arrhinoceratops. Relative to all other taxa in this clade, Regaliceratops has the smallest postorbital horncores ( Figure S2 ), as well as the tallest nasal horncore relative to skull size. Campanian chasmosaurines show plasticity in the size of the postorbital horncore, with the evolution of short horncores occurring at least three times ( Figure S2 ). However, prior to the discovery of Regaliceratops, Maastrichtian chasmosaurine taxa appeared to have stabilized on a long-horned postorbital, and this new taxon represents the only evolution of a short-horned postorbital within Maastrichtian chasmosaurines ( Figure S2 ).

Time-calibrated strict consensus tree of five most parsimonious trees for Chasmosaurinae utilizing the new epiossification homology scheme (for tree details, see Figure S1 B). For comparison of results and support indices, see Figure S1 . Black bars indicate confident stratigraphic occurrence, whereas gray bars indicate less confidence. Stratigraphic information is derived from []. Bottom right: oblique view of the holotype of Regaliceratops peterhewsi, TMP 2005.055.0001.

Chasmosaurine Evolution

12 Eberth D.A.

Evans D.C.

Brinkman D.

Therrien F.

Tanke D.H.

Russell L.S. Dinosaur Biostratigraphy of the Edmonton Group (Upper Cretaceous), Alberta, Canada: Evidence for Climate Influence. 40 Ryan M.J.

Evans D.C. Ornithischian dinosaurs. 41 Mallon J.C.

Evans D.C.

Ryan M.J.

Anderson J.S. Megaherbivorous dinosaur turnover in the Dinosaur Park Formation (upper Campanian) of Alberta, Canada. 42 Campione N.E.

Evans D.C. Cranial growth and variation in edmontosaurs (Dinosauria: Hadrosauridae): implications for latest Cretaceous megaherbivore diversity in North America. The occurrence of a chasmosaurine ceratopsid with centrosaurine-like ornamentation following the early Maastrichtian extinction of the Centrosaurinae illustrates chasmosaurine ornamentation increasing in disparity and exploration of novel morphospace previously occupied by the Centrosaurinae ( Figure 4 ). Prior to this discovery, Maastrichtian-aged ceratopsid and hadrosaurid taxa exhibited a pattern of lower diversity and longer duration than Campanian-aged taxa [] and may also have exhibited a pattern of lower morphological disparity [] and slowing of evolutionary rates. Regaliceratops not only increases the diversity of Maastrichtian ceratopsids but also greatly increases the known morphological disparity.

14 Longrich N.R. Titanoceratops ouranos, a giant horned dinosaur from the late Campanian of New Mexico. The present study finds cladistic support for a deep split within the Chasmosaurinae into an older Chasmosaurus clade and a younger Triceratops clade based on features of the premaxilla and parietal shape (exclusive of the epiossifications). If supported by future analyses, this deep split within Chasmosaurinae has two major implications for chasmosaurine evolution. First, because these two clades are nearly non-overlapping (the Chasmosaurus clade being of largely Campanian age, and the Triceratops clade being of largely Maastrichtian age), a long (3 Ma) ghost lineage is implied at the base of the Triceratops clade ( Figure 3 ). A similar (5 Ma) ghost lineage is also implied for many taxa within the Triceratopsini [], and these ghost lineages taken together suggest that there is an undiscovered diversity of Chasmosaurine taxa in the Campanian and Maastrichtian. Much of the ghost lineage within the Triceratops clade is inferred due to Titanoceratops being well nested within Triceratopsini yet occurring in the Campanian, a combination that is stratigraphically inconsistent relative to the other taxa.

The second implication is that the Chasmosaurus clade appears to go extinct in the early Maastrichtian, at approximately the same time as the extinction of the Centrosaurinae. The dynamics of the centrosaurine extinction are not well understood, so whether a common mechanism may be responsible cannot be determined. We speculate that the extinction of two diverse clades of horned dinosaurs in the early Maastrichtian may have allowed for the diversification of the Triceratopsini just prior to the end-Cretaceous mass extinction.

12 Eberth D.A.

Evans D.C.

Brinkman D.

Therrien F.

Tanke D.H.

Russell L.S. Dinosaur Biostratigraphy of the Edmonton Group (Upper Cretaceous), Alberta, Canada: Evidence for Climate Influence. 12 Eberth D.A.

Evans D.C.

Brinkman D.

Therrien F.

Tanke D.H.

Russell L.S. Dinosaur Biostratigraphy of the Edmonton Group (Upper Cretaceous), Alberta, Canada: Evidence for Climate Influence. 43 Roberts E.M.

Sampson S.

Deino A.

Bowring S.A.

Buchwaldt R. The Kaiparowitz Formation: A remarkable record of Late Cretaceous terrestrial environments, ecosystems, and evolution in western North America. More than 100 years of fossil collecting and stratigraphic work on the upper Campanian and Maastrichtian strata of the Red Deer River Valley, specifically the Horseshoe Canyon and Scollard formations, has revealed distinct faunal turnovers in ornithischian taxa through this interval []. This is perhaps best illustrated by the Chasmosaurinae, with Anchiceratops occurring in the Horsethief, Morrin, and Tolman members; Anchiceratops occurring in the Horsethief and Morrin members; Eotriceratops occurring in the Carbon member; and Triceratops occurring in the Scollard Formation []. This high-resolution dinosaur biostratigraphy, particularly across the Campanian-Maastrichtian boundary, is in contrast to much of the Upper Cretaceous record elsewhere in North America, where terrestrial stratigraphic sections are neither as complete and continuous nor as rich in dinosaurs (e.g., Figure 6.11 in []).

The unexpected discovery of Regaliceratops reinforces the plasticity in cranial ornamentation expressed in Ceratopsidae and highlights how much of their true diversity still remains unknown. Ongoing research will likely continue to reveal both increased taxonomic diversity and increased morphological disparity (particularly in cranial ornamentations) within the group and may continue to blur the suite of ornamentation features between the two subfamilies.