A new chasmosaurine from northern Laramidia expands frill disparity in ceratopsid dinosaurs
A new taxon of chasmosaurine ceratopsid demonstrates unexpected disparity in parietosquamosal frill shape among ceratopsid dinosaurs early in their evolutionary radiation. The new taxon is described based on two apomorphic squamosals collected from approximately time equivalent (approximately 77 million years old) sections of the upper Judith River Formation, Montana, and the lower Dinosaur Park Formation of Dinosaur Provincial Park, Alberta. It is referred to Chasmosaurinae based on the inferred elongate morphology. The typical chasmosaurine squamosal forms an obtuse triangle in dorsal view that tapers towards the posterolateral corner of the frill. In the dorsal view of the new taxon, the lateral margin of the squamosal is hatchet-shaped with the posterior portion modified into a constricted narrow bar that would have supported the lateral margin of a robust parietal. The new taxon represents the oldest chasmosaurine from Canada, and the first pre-Maastrichtian ceratopsid to have been collected on both sides of the Canada–US border, with a minimum north–south range of 380 km. This squamosal morphology would have given the frill of the new taxon a unique dorsal profile that represents evolutionary experimentation in frill signalling near the origin of chasmosaurine ceratopsids and reinforces biogeographic differences between northern and southern faunal provinces in the Campanian of North America.
KeywordsChasmosaurinae Mercuriceratops gemini Campanian Judith River Formation Dinosaur Park Formation Laramidia
Ceratopsidae is a diverse clade of large bodied horned dinosaurs known from a well-sampled fossil record that spans the last 20 million years of the Mesozoic (Dodson et al. 2004). Ceratopsids are common in the Campanian–Maastrichtian deposits of western North America, where they range geographically from Coahuila, Mexico in the south (Coahuilaceratops magnacuerna; Loewen et al. 2010), to northern Alaska in the North (Pachyrhinosaurus perotorum; Fiorillo and Tykoski 2012). Marked latitudinal differences between the dinosaur faunas of the late Campanian has led to the recognition of northern (Wyoming and north) and southern (Utah and south) faunal provinces adjacent to the Western Interior Seaway during this time (Lehman 1987, 1997; Sampson et al. 2010). Ceratopsians reached their maximum diversity during the Campanian and provide some of the strongest evidence for provinciality in Late Cretaceous dinosaurs (Sampson et al. 2010, 2013; Farke 2013; Ryan 2013). Ceratopsidae consists of two major subclades that form a basal dichotomy of Centrosaurinae, or the ‘short-frilled’ ceratopsids, and Chasmosaurinae, which generally have relatively longer, less adorned frills. The two clades have traditionally been easily distinguished by the size and shape of their squamosals, which form the lateral segments of the parieto squamosal frill. Centrosaurines have broad, rectangular squamosals with typically concave contact surfaces (in adult-sized elements) for the parietal on the medial margin of the element. In contrast, chasmosaurines have an elongate, triangular squamosal with a corresponding contact surface for the parietal on the medioventral margin of the posterior squamosal. A recent morphometric study has confirmed that the distinctive squamosal shape of each subfamily is very conservative, with some minor proportional differences within each subfamily (Maiorino et al. 2013). Chasmosaurinae, in particular, exhibit little variation in squamosal shape, with little clear differentiation among taxa.
ROM, Royal Ontario Museum, Toronto, Canada
UALVP, University of Alberta, Laboratory for Vertebrate Paleontology, Edmonton, Canada
Mercuriceratops gemini sp. nov.
Same as for species, by monotypy
Mercuriceratops gemini sp. nov. urn:lsid:zoobank.org:act:70D4C099-1A46-4192-B8A1-D134E49D4861
Etymology Mercuri, in reference to winged helmet of the Roman messenger god Mercury, and ‘ceratops’, meaning horned face, a common suffix for genera of ceratopsid dinosaurs. Close carriage return.
Gemini, from mythology, the twins Castor and Pollux were transformed into the constellation Gemini, referring to the twin specimens from Alberta and Montana.
Locality and horizon
ROM 64222 was derived from the upper Judith River Formation, Fergus County, Montana, SW 1/4 Sec 9, T22N R21E (Fig. 1). Detailed locality data on file at the Royal Ontario Museum. UALVP 54559 was found on the north side of the Red Deer River, approximately 1 km east of ‘Happy Jack’s cabin’, 12 U0471467, 5624176, Dinosaur Provincial Park, Alberta, lower Dinosaur Park Formation (Fig. 1), approximately 2 m above the contact with the Oldman Formation.
Differs from all other chasmosaurines in having a squamosal with a constricted posterior ramus that is rod-shaped rather than having a tapering, obtuse triangular shape (most chasmosaurines) or a broadly rounded triangular shape (Diceratops, Ojoceratops, and Triceratops) in dorsal view. The expanded anterolateral flange (posterior to the otic notch in dorsal view and posterior to the quadrate groove ventrally) is similar to the same region in most ceratopsids and bears four large, tab-shaped episquamosals. Elongate episquamosals also extend along the lateral margin of the posteriorly projecting bar.
Of note is the presence of a fragment of what would have been an elongate, robust postorbital horncore found in the wash channel below UALVP 54559. Although this element cannot be definitely associated with the UALVP 54559, multiple fragments of the squamosal were collected from the same area, suggesting a possible association between the squamosal and the horncore fragments.
The new taxon is represented by two incomplete right squamosals, ROM 64222 (Fig. 2a–d) and UALVP 54559 (Fig. 3a and b). The specimens are similar in size and would have been derived from large, adult-sized animals. ROM 64222 is relatively gracile compared to UALVP 54559. ROM 64222 preserves most of the relatively thin anterior (prequadrate groove) blade that contacts the postorbital anteriomedially and the jugal anterolaterally, and both specimens preserve at least a portion of the jugal projection. Each of the squamosals are apomorphic in being constricted just posterior to the quadrate groove such that most of the posterior blade is modified from a dorsoventrally thin, posteriorly tapering blade seen on all other chasmosaurines as a mediolaterally compressed shaft with a subrectangular cross section. The preconstriction, anterior portion of the squamosal with four large, well-fused episquamosals present on UALVP 54559 superficially resembles the flange-like, posterior portion of a centrosaurine squamosal. The posterolateral margin of this flange is broken away on ROM 64222, but loci for (or the fused) episquamosals 1 and 2 are preserved. More of the posterior squamosal shaft is preserved on ROM 64222, with at least one low, long-based episquamosal present on the lateral edge. None are preserved on the posterior part of the shaft of UALVP 54559, but this shaft is broken just anterior to where the first episquamosal on the shaft would be expected based on the morphology of ROM 64222. The medioventral portion of the shaft of UALVP 54559 has deeply incised, longitudinal grooves that would have tightly interdigitated with the parietal. The same surface of ROM 64222 is relatively smooth and flat and lacks any deep grooving, although it does preserve the elongate contact surface for the parietal.
ROM 64222 (Fig. 2a–d) is an almost complete right squamosal (maximum preserved length = 793 mm) that preserves most of the anterior contact for the postorbital, the medial surface that forms the margin of the supraorbital fenestra anteriorly and contacts the parietal posteriorly, and most of the lateral margin. The posterior portion of the modified shaft is missing, as is part of the posterior margin of the expanded anterolateral flange. Although the specimen is somewhat fractured, both dorsal and ventral surfaces are well preserved.
In dorsal view, the medial margin of the element forms a broad concave arc with the constricted posterior shaft forming the posterior one half of the element. The base of the shaft (111 mm in length and 31.6 mm in thickness) is medially offset from the anterolaterally projecting flange portion of the squamosal by a deep notch that gives the body of the element a hatchet shape. Although the posterior margin of this flange is partially broken, loci for episquamosals 1 and 2 are present. The jugal notch is wide and most of the jugal flange is preserved. The anterodorsal margin of the infratemporal fenestra appears to preserve a contact surface for the jugal suggesting that it formed most of the anterior margin of this opening. Ventrally, the element resembles other ceratopsids in the arrangements for the contacts with the quadrate and the exoccipital. The relatively narrow, elongate contact surface for the parietal is lightly inscribed along the medioventral margin of the posterior shaft.
UALVP 54559 (Fig. 3a and b) is a partial right squamosal (maximum preserved length = 470 mm) missing the anterior portion of the blade, and most of the elongate squamosal ‘bar’ that contacts the parietal medially. The element is robust and most likely came from a larger, more mature individual than ROM 64222. The proximal portion of the jugal notch (base of the jugal flange) is partially preserved and indicates that the notch would have resembled those of most chasmosaurines. The dorsal surface of the anterolateral flange is convex, similar to the dorsal surface of most ceratopsid squamosals. This surface is rugose with several deeply inscribed vascular grooves.
The anterolateral flange has four dorsally reflected scallops (loci) each capped by a dorsoventrally compressed, well-fused episquamosal. The narrow, crescent-shaped episquamosal 1 is the largest episquamosal with a basal length of 188 mm and the height of 45 mm at midpoint. Its base is only visible on the ventral surface. The almost indistinguishable episquamosals 2 and 3 are both low and relatively long-based (61.5 and 51 mm basal lengths, 15 and 20 mm heights, respectively). The large, tab-like scallop forming episquamosal locus 4 is capped by a crescentic episquamosal that covers the apex and anterior margin on the dorsal surface. The episquamosal 4 process has a basal length of 65 mm and a height of 43 mm; the base of episquamosal cannot be identified on the ventral surface. The entire lateral margin of the anterolateral flange is thick, but the blade thins medially towards the midpoint of the body of the flange. There is a pronounced, small, concave depression (~45 mm in diameter) on its dorsal surface proximal to the base of episquamosal 3 and between episquamosals 2 and 3, possibly due to crushing. The ventral surface of each episquamosal is rugose and crossed by several deeply imprinted vascular grooves.
The shaft-like posteromedial extension of the anterior blade is separated from the anteromedial flange by a deep embayment, as in ROM 64222. The preserved shaft is approximately rectangular in cross-section, although the medial and ventromedial surfaces have been modified by several long, deep, longitudinal grooves representing the contact surfaces for the parietal that are up to one half the thickness of the shaft. Based on the size and depth of the grooves, and the overall robustness of the shaft, the contacting lateral portion of the parietal can be inferred to have been massive. These grooves narrow and converge anteriorly onto the thin ventromedial margin of the anterior blade (margin of the supratemporal fenestra) at the level of the perpendicular quadrate groove. On the dorsal surface of the shaft, three narrow vascular grooves anastomose adjacent to the break and form a single groove parallel to the margin of the remaining shaft. On the ventral surface of the flange, a thin sheet of bone adjacent to the quadrate groove extends anteriorly forming the ventral wall of a deep, broad cavity that would have been confluent with the supratemporal fenestra. The preserved dorsal surface of the anteromedial flange adjacent to the postorbital is thick and has two low, rounded bumps, as in most ceratopsids.
Mercuriceratops can be unequivocally referred to Chasmosaurinae based on the elongate posterior region of the squamosal, with a distinct medioventral contact for the lateral parietal process. These characters have been shown to unambiguously diagnose Chasmosaurinae in all recent phylogenetic analyses of the group (e.g., Sampson et al. 2010; Mallon et al. 2011), and occur in both ROM 64222 and UALVP 54449. Mercuriceratops is distinct from all other known chasmosaurines and is diagnosed by the apomorphic, hatchet-shaped lateral margin of the squamosal, which differs considerably from the typically straight-margined, triangular squamosals of most chasmosaurines, and the more rounded lateral margins of Nedoceratops, Ojoceratops, and Triceratops. Whereas the anterior blade (anterolateral flange) of the squamosal of Mercuriceratops is of typical ceratopsid shape, a strap-like posterolateral bar offset from the lateralmost margin of the anterior blade by a deep embayment is unique within chasmosaurines. The lateral margin of the anterior flange has four low, long-based episquamosals, and at least one well-fused episquamosal is preserved at the base of the posterolateral bar on ROM 64222 followed by the broken base of at least one more; a complete squamosal may well have had at least eight episquamosals per side, typical of most chasmosaurines.
The unusual morphology of the squamosal cannot be explained by pathology or modification through bone resorption. Although pathological elements are not uncommon on putatively old, mature individuals (Rega et al 2010), there is no indication on either element of injury and rehealing, or bone loss/growth due to pathology. Some chasmosaurines are known to develop fenestrae in their squamosals (Tanke and Farke 2007; Tanke and Rothschild 2010), but no known specimens that are pathological have morphologies similar to that manifested in ROM 64222 and UALVP 54559. The presence of episquamosals on the lateral margin of the posterior bar of ROM 64222 in the area of the notch confirms that this bar is not a result of the modification or loss of the lateral margin of the squamosal. The presence of almost identical morphologies on squamosals from two different formations from two widely separated geographic areas strongly suggests that ROM 64222 and UALVP 54559 represent a previously undescribed taxon of chasmosaurine ceratopsid.
Although ROM 64222 and UALVP 54559 are strikingly similar, the two specimens do show variation in several features including the size (angle) of the jugal notch, the shape and size of the parietal contact, and the orientation of the posterolateral bar; however, this is conservatively interpreted as individual and/or ontogenetic variation within a single taxon. The angle of the jugal notch is highly variable in all ceratopsid taxa (Maiorino et al. 2013) and probably does not have any taxonomic utility (contra Sullivan and Lucas 2010). The size and thickness of the posterior bar on UALVP 54559 is consistent with size- and shape-related changes seen in progressively older Marginocephalia growth stages (e.g., Scannella and Horner 2010; Horner and Goodwin 2009). Although both specimens pertain to individuals that were similar in size to adult Chasmosaurus specimens from Alberta, we consider UALVP 54559 to be from a more mature, more robust individual. This would explain the long, deep, interdigitating suture for the parietal that, by inference, must have also been robust. The more gracile ROM 64222, while close to, or possibly at, full adult size, may not have reached full maturity at the time of death. The presence of a well-fused episquamosal near the base of the shaft of ROM 64222 does not contradict this suggestion because the ontogenetic pattern of episquamosal fusion is anterior to posterior on chasmosaurines (Sampson et al. 1997), and the unpreserved posteriorly positioned episquamosals of this specimen may not have been fused. Given the limited material available, and the presence of considerable individual and ontogenetic variation in frill morphology within ceratopsids, ROM 64222 and UALVP 54559 are both referred to the same taxon; however, the differences in the specimens may be recognised as taxonomically significant when more material is collected.
Mercuriceratops is the oldest known chasmosaurine in the well-documented succession of chasmosaur taxa from the Campanian of Alberta (Godfrey and Holmes 1995; Holmes et al. 2001; Ryan and Evans 2005; Mallon et al. 2012). UALVP 54559 was collected from the Dinosaur Park Formation of Alberta, approximately 2 m above the contact with the Oldman Formation making it approximately 77 Ma (Eberth 2005). The Canadian Campanian chasmosaurine taxa are well established, with most taxa known from multiple skulls (Ryan and Evans 2005); UALVP 54559 is clearly morphologically distinct from all chasmosaurines recovered from this formation including Chasmosaurus belli, Chasmosaurus russelli, and Vagaceratops irvinensis, which all have long, triangular squamosals (Maiorino et al. 2013).
ROM 64222 was collected from a locality above the SD2 disconformity (Rogers and Kidwell 2000) of the upper Judith River Formation of Montana. The locality of ROM 64222 is approximately time equivalent to the lower portion of the Dinosaur Park Formation (Rogers, personal communication), allowing us to infer an approximate time equivalency for the two specimens. The Campanian record of Chasmosaurinae in Montana is relatively depauperate, with only two putative taxa being based on a small amount of material from the Judith River Formation. The fragmentary, nonassociated specimens referred to Judiceratops tigris (Longrich 2013) come from the lower portion of the formation, making the material significantly older than Mercuriceratops. The material probably represents a distinct taxon, although it is equivocal whether the reported diagnostic characters are supportable. The squamosal YPM VPPU 023262 referred to Judiceratops is characterised as having a typical chasmosaurine shape, thus eliminating its referral to Mercuriceratops. Medusaceratops lokii (Ryan et al. 2010) was described based on material collected from a bone bed in the lower Judith River Formation of Montana in the Kennedy Coulee system just south of the Alberta border. Material from the bone bed was originally referred to the contemporaneous Albertaceratops nesmoi from equivalent beds in the lowermost Oldman Formation of Alberta (Ryan 2007) making it approximately 1 Ma older than UALVP 54559. Ryan et al. (2010) referred two anomalous parietals from the locality to the new chasmosaurine M. lokii, while leaving the remainder of the material tentatively referred to Albertaceratops. Although squamosals are poorly represented from the Montanan bone bed, the most complete specimens possess an elongate, concave sutural surface for the parietal on the medial surface of the posterior blade that is typically centrosaurine; no typical chasmosaurine or Mercuriceratops-like squamosals are known from the locality (Ryan 2007; Ryan et al. 2010, Ryan unpublished data). Ceratops montanus Marsh 1888 (nomen dubium), based on an occipital condyle and a pair of large postorbital horncores (USNM 2411), may represent a chasmosaurine, but is currently regarded as a nomen dubium due to lack of diagnostic characters in the holotype specimen (Dodson et al. 2004).
Campanian dinosaurs have been hypothesised to occur in two distinct paleobiogeographic provinces within the Western Interior of North America (Lehman 1987, 1997; Sampson and Loewen 2010; Farke 2013; Loewen et al. 2013; Ryan 2013). The northern zone is typified by fossils collected in the Belly River Group of southern Alberta (Eberth and Hamblin 1993) and the Two Medicine/Judith River clastic wedge in Montana. The southern zone is less well characterised, due, in part, to a less intense and shorter collection history, but includes taxa from the Kaiparowits, Fruitland, and Aguja formations of Utah, New Mexico, and Texas, respectively. Centrosaurine ceratopsids are well known from both the north and south biogeographic zones and provide some of the strongest evidence in support of provinciality (Sampson et al. 2010, 2013). The recognition of the new, anatomically unique chasmosaurine, Mercuriceratops, in the well-sampled Dinosaur Park and Judith River formations will have important implications for characterising dinosaur provinciality within Laramidia.
The specimen was acquired by the ROM from Triebold Paleontology and prepared by Ian Morrison. UALVP 54559 was discovered by Susan Owen-Kagen on July 1, 2012, collected by SO-K and PJC on July 3, 2012 and subsequently prepared by SO-K. Danielle Dufault drew the line illustrations in Figs. 2 and 4. Clive Coy photographed UALVP 54559. DCE was supported by a Natural Sciences and Engineering Research Council of Canada Discovery Grant. Reviews by Rob Holmes and two anonymous reviewers improved the manuscript.
Conflict of interest
The authors declare that they have no conflict of interest.
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