In 1916, a centrosaurine dinosaur bonebed was excavated within the Campanian-aged deposits of what is now Dinosaur Provincial Park, Alberta, Canada. Specimens from this now-lost quarry, including two parietals, a squamosal, a skull missing the frill, and an incomplete dentary, were purchased by The Natural History Museum, London. The material was recently reprepared and identified herein as a previously unknown taxon, Spinops sternbergorum gen. et sp. nov. Based upon the available locality data and paleopalynology, the quarry lies in either the upper part of the Oldman Formation or the lower part of the Dinosaur Park Formation. The facial region of the partial skull is similar to putative mature specimens of Centrosaurus spp. and Styracosaurus albertensis, with short, rounded postorbital horncores and a large, erect nasal horncore. Parietal ornamentation is consistent on both known parietals and is unique among ceratopsids. Bilateral, procurved parietal hooks occupy the P1 (medial-most) position on the dorsal surface of the parietal and are very similar to those seen in Centrosaurus apertus. Epiparietals in the P2 or possibly P3 position (lateral to P1) manifest as extremely elongate, caudally directed spikes, unlike the condition in C. apertus, S. albertensis, or any other “derived” centrosaurine. Cladistic analysis suggests that S. sternbergorum is closely related to Centrosaurus and Styracosaurus. Historically, based upon the condition in Styracosaurus and related centrosaurines, it was assumed that the medial-most elongated spikes on centrosaurine parietals correspond to the P3 epiparietal position. The exception illustrated in the new taxon suggests that homologies of epiparietals among basal centrosaurines (e.g., Albertaceratops and Diabloceratops) and derived centrosaurines (e.g., Styracosaurus and “pachyrhinosaurs”) should be reconsidered. The medially-placed, caudally-directed “P3” process of basal centrosaurines may, in fact, be homologous with P2.
During the past four decades, Ceratopsidae (popularly known as “horned dinosaurs”) has been the subject of intensive work in such diverse areas as behavior (e.g., Currie and Dodson 1984; Sampson 1997; Farke 2004; Hunt and Farke 2010), thermal physiology (e.g., Farlow and Dodson 1975; Barrick et al. 1998) and biomechanics (e.g., Dodson and Farlow 1997; Paul and Christiansen 2000; Thompson and Holmes 2007; Fujiwara 2009; Rega et al. 2010). This interest has been driven in part by the near doubling of the number of named, valid species over the past 25 years, beginning with Avaceratops lammersi (Dodson, 1986), which was the first new ceratopsid taxon to be named since Pachyrhinosaurus canadensis (Sternberg, 1950). Although several historical taxa have been “lost” during the past 25 years (e.g., “Brachyceratops” and “Monoclonius” are now regarded as nomina dubia; Sampson et al. 1997), no fewer than 18 new taxa have been erected. Of these, only four are from Alberta: representing the ceratopsid clade Centrosaurinae, Centrosaurus brinkmani (Ryan and Russell, 2005) from the uppermost Oldman Formation and Albertaceratops nesmoi (Ryan, 2007) from the lowermost Oldman Formation; and representing the clade Chasmosaurinae, Vagaceratops irvinensis (Holmes, Forster, Ryan, and Shephard, 2001) and Mojoceratops perifania (Longrich, 2010) from the Dinosaur Park Formation.
One possible reason for the limited number of new taxa from Alberta is that most of the productive ceratopsian-bearing outcrops are restricted to the Dinosaur Park Formation within the geographically limited area (73 km2) of Dinosaur Provincial Park. These beds have been well prospected since 1898, when Lawrence Lambe made the first major collections of fossils from this area (Currie 2005). A disconformity separates the Dinosaur Park Formation from the underlying Oldman Formation, and no more than 20 m (generally much less) of the upper part of the Oldman Formation is exposed within the park (Eberth 2005).
Recent work by Currie and Russell (2005), Eberth (2005), and others has allowed precise geographic and stratigraphic placement of many significant dinosaur skulls and skeletons collected within Dinosaur Provincial Park. Using these data for the Dinosaur Park Formation, Ryan and Evans (2005) identified three dinosaur faunal zones, each possessing a unique, stratigraphically restricted centrosaurine ceratopsid and lambeosaurine hadrosaurid. In ascending order the zones are: the Centrosaurus—Corythosaurus Zone (the lowest 30 m), the Styracosaurus—Lambeosaurus Zone (the successive ∼30 m), and the “pachyrhinosaur—Lambeosaurus magnicristatus— Vagaceratops irvinensis Zone (in the uppermost Lethbridge Coal Zone). Each zone also appears to have an associated chasmosaurine ceratopsid species (Holmes et al. 2001), these being Chasmosaurus russelli, C. belli, and Vagaceratops irvinensis, respectively. However the current data are not sufficiently robust to confirm that these species do not cross the proposed faunal boundaries (Ryan and Evans 2005). Both Centrosaurus apertus and Styracosaurus albertensis are known from multiple bone beds (>20 and at least two, respectively), skeletons and isolated elements, whereas an unidentified “pachyrhinosaur” (sensu Currie et al. 2008) is known from only a single specimen (Ryan et al. 2010).
Here, we describe a new centrosaurine ceratopsid, Spinops sternbergorum, from the northwestern region (“Steveville Badlands”) of Dinosaur Provincial Park (Fig. 1). The material was collected by Charles H. Sternberg and Levi Sternberg in the summer of 1916 on behalf of the British Museum (Natural History, now the Natural History Museum, London), with funding provided by the Percy Sladen Trust. During the expedition, the Sternbergs discovered and collected fossils from a ceratopsian bonebed—the first to be systematically excavated in Alberta.
Unfortunately, no known field notes are associated with the excavation, but correspondence between Charles Sternberg and staff at the Natural History Museum indicates that the bonebed was quite dense and that the skeletal material was disarticulated and preserved within a well-indurated, iron-rich matrix. Charles Sternberg believed that the material represented a new taxon closely related to Styracosaurus albertensis, but Arthur Smith Woodward (then Keeper of Geology at the museum) was greatly disappointed with the quality of the specimens. An unsigned letter to C. Sternberg in the Natural History Museum archives, dated 11 January 1918, states, “There is indeed in the Sladen Collection nothing but rubbish…”. Consequently, most of the material remained overlooked and unprepared for over 90 years. However, re-examination of the collection led several of the authors to conclude that the fossils might represent a new taxon, providing the impetus to prepare the specimens fully (Figs. 2–4).
Beyond expanding ceratopsid diversity counts, specimens of Spinops sternbergorum provide new information on the evolution of epiparietals, the marginal ossifications on the parietal bone of the frill. Epiparietal morphology varies greatly within Centrosaurinae, ranging from low, rounded crescents to elongated spikes to procurved hooks (Fig. 5). Because these ornamentations appear to have phylogenetic significance, epiparietals are central to centrosaurine systematics. Traditionally, epiparietals have been homologized on the basis of their positions relative to the midline of the parietal. The ossification immediately adjacent to the midline is termed the P1 process, the next lateral ossification the P2 process, and so forth (Sampson 1995; Fig. 5). The derived morphology for P3 in many long-known species is an elongated spike (e.g., Styracosaurus albertensis and Pachyrhinosaurus lakustai), and thus elongated spikes in a number of newly described species (e.g., Albertaceratops nesmoi) have likewise been identified as P3. Material of Spinops sternbergorum suggests that alternative homology statements are possible, with implications for centrosaurine phylogenetic hypotheses.
Institutional abbreviations.—CMN, Canadian Museum of Nature, Ottawa, Ontario, Canada; NHMUK, The Natural History Museum, London, England, United Kingdom; ROM, Royal Ontario Museum, Toronto, Ontario, Canada; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, A1berta, Canada.
Other abbreviations.—CI, consistency indices; CIC, cladistic information content; MPT, most parsimonious trees; RCI, rescaled consistency indices; SRC, strict reduced consensus; TL, tree lengths.
The specimens were prepared mechanically by one of the authors (MRG). Thick areas of matrix were scored with a rotary industrial diamond-edged cutter (to 12 cm on NHMUK R16308), and the matrix was removed by hand chisels and airpens. Rotary grinders were used nearer the bone surfaces, and fine removal of sediment particles was achieved by air abrasion with powdered aluminium oxide and sodium bicarbonate. A small sample of matrix from around NHMUK R16306 was processed for pollen, using standard techniques (see Supplemetary Online Material at http://app.pan.p1/SOM/app56-Farke_etal_SOM.pdf: SOM 1).
Dinosauria Owen, 1842
Ornithischia Seeley, 1887
Ceratopsia Marsh, 1890
Ceratopsidae Marsh, 1888
Centrosaurinae Lambe, 1915
Genus Spinops nov.
Etymology: From the Latin spina, spine; and the Greek—ops, face; referring to the ornamentation on the face.
Type species: Spinops sternbergorum sp. nov.; see below.
Diagnosis.—As for the only species.
Etymology: The specific epithet honors Charles H. and Levi Sternberg, collectors of the original specimens.
Holotype: NHMUK R16307, a partial parietal bone, preserving most of the midline bar and a portion of the lateral rami (Fig. 3C).
Type locality: Exact locality unknown but collected in the vicinity of the Red Deer River, Steveville Badlands, Dinosaur Provincial Park, A1berta, Canada (Fig. 1). Attempts to relocate the quarry have been unsuccessful so far.
Type horizon: Unfortunately, only limited stratigraphic data are available. The locality was estimated to be at least 90 m stratigraphically below the quarry that yielded the type specimen of Styracosaurus albertensis (C. H. Sternberg, 9 July 1916 letter to A. Smith Woodward, NHMUK archives; see also Tanke 2010). The S. albertensis locality is approximately 42 m above the contact between the Dinosaur Park Formation and the underlying Oldman Formation. Because the Oldman Formation is only 40 m thick in Dinosaur Provincial Park (Eberth 2005), this would place the type locality for Spinops sternbergorum in the Foremost Formation (which underlies the Oldman Formation). However, the Foremost Formation is not exposed in the immediate region, so Sternberg's estimation of the stratigraphic position of the quarry is almost certainly incorrect. Palynological analysis of the matrix from the Spinops bonebed is most consistent with location of the quarry within either the upper few meters of the Oldman Formation or at any level in the Dinosaur Park Formation (see SOM 1). Sternberg described the quarry as at floodplain level near Berry Creek; the pattern of exposures in this vicinity suggests that the quarry is no higher than the lower Dinosaur Park Formation and quite possibly in the upper Oldman Formation, which is exposed at floodplain level there.
Definition and diagnosis.—A centrosaurine ceratopsid characterized by the following unique combination of characters: a procurving hook as the most medial-most epiparietal (P1) on the caudal margin of the parietal, with a straight, caudally-projecting spike (presumed P2 or possibly P3) with gentle dorsal curvature immediately lateral to the hook; short postorbital horncores that project dorsally; nasal horn core longer than the postorbital horncores. The prominence of the P1 hooks and their proximity to the caudally-projecting spike distinguish Spinops sternbergorum from Styracosaurus albertensis and other centrosaurines exclusive of Centrosaurus spp., and the caudally-projecting spike distinguishes S. sternbergorum from Centrosaurus spp.
Referred material.—NHMUK R16308, a partial parietal bone, preserving portions of the midline bar and lateral rami, with adhered partial dentary and unidentifiable limb elements. NHMUK R16306, an incomplete skull, preserving the dorsal portion of the skull from the rostralmost portion of the midline parietal bar to the caudalmost portion of the premaxillae, lacking all elements ventral to the ventral border of the orbit. NHMUK R16309, a partial right squamosal. Although none of this material was found in articulation, it was all closely associated in the same bone bed and no evidence suggests that any other ceratopsid taxon was present. None of the isolated elements can be assigned confidently to the individual represented by the partial skull.
Parietal (Fig. 3B, C).—The following description is based primarily upon the holotype and most complete specimen, NHMUK R16307, supplemented by information from the referred specimen NHMUK R16308. In nearly all details, the two specimens are remarkably similar.
The midline bar of the parietal is smooth and unornamented, with only extremely subtle dorsal undulations. In cross section, the bar is roughly triangular in outline, with a rounded dorsum and lateral edges that thin to a point. The bar is 119 mm wide in NHMUK R16307. The ventral surface is gently and convexly rounded, similar to the condition seen in other centrosaurines. Only a portion of the margin of the parietal fenestra is preserved on the right side of NHMUK R16307 (Fig. 3C1); although this is complete enough to indicate the presence of fenestrae, it does not allow any details of their morphology to be determined.
The lateral rami of the parietal have a broad, V-shaped embayment (253 mm wide between the bases of epiparietals P2), as seen in most other centrosaurines (e.g., Centrosaurus apertus). Well-developed epiparietals ornament this portion of the frill. Following the numbering scheme of Sampson (1995), only epiparietals P1 and P2 are preserved in each specimen (see discussion below for more on establishing homology of the processes in Spinops; the epiparietal adjacent to P1 may instead represent P3). The remainders of the lateral rami, and the associated epiparietals, are not preserved.
A procurving bony hook (P1) occurs on both sides of the midline, similar to those seen in Centrosaurus apertus. The base of the hook projects directly dorsally, and the bone curves rostrally towards its distal end (Fig. 3C3). The distal ends of both processes in the holotype were broken and lost during collection, as indicated by the cancellous bone texture visible along fresh breaks. The left hook, which is 120 mm in mediolateral width and 47 mm in craniocaudal length, is more completely preserved than the right. The ventral surface of the hook is smooth, but the dorsal surface has a very deep sulcus laterally (approximately 15 mm deep), with a second, shallower sulcus (5 mm deep) positioned immediately lateral to this. The remainder of the hook was abraded post-collection.
In the referred specimen (NHMUK R16308; Fig. 3B) the right hook is approximately 90 mm in width, but its length cannot be reliably measured as other bones cover this area. The hook on the left side projects at least 80 mm from the dorsal surface of the parietal and measures at least 115 mm in length along the outer curve (approximately 100 mm straight length). A portion of the cross section is preserved, allowing estimation of the degree of taper for the hook, which suggests the complete hooks might have been up to 200 mm long.
In contrast to Centrosaurus apertus and Styracosaurus albertensis, Spinops sternbergorum lacks an inward-curving or projecting P2 epiparietal. The fact that both known parietals for Spinops exhibit excellent preservation of the frill in this region, and that both specimens consistently lack this process, indicates that this is a genuine feature of the taxon.
An elongated spike, here identified as P2 (although also possibly homologous to P3; see Discussion) occurs just lateral to the procurving hook at P1 (Fig. 3B, C). Unlike other centrosaurines that possess large parietal spikes (e.g., Styracosaurus albertensis) the spikes in both known specimens of Spinops sternbergorum show a gentle dorsal curvature along their entire lengths. The consistency of the morphology between the specimens, as well as the generally uncrushed preservation of the parietals, indicates that this morphology is not a result of post-burial distortion. The left spike of NHMUK R16307 has a deep longitudinal sulcus inscribed dorsally and a shallower one just lateral to this. Another sulcus is on the medial surface, but the ventral surface is unsulcated. The left and right spikes are 275 mm and 260 mm long, respectively (as preserved), but the distal ends are missing, which would add up to an additional 10% to the overall lengths of the processes. Each spike is approximately 100 mm wide at its base. In NHMUK R16308, the left and right spikes are 290 and 245 mm long as preserved, with basal widths of approximately 90 mm. The medial edges of their bases are separated by 270 mm.
The P2 spike changes in cross-sectional morphology along its length. It is much wider than deep at the base (93 mm wide by 62 mm deep on the right P2 spike of NHMUK R16307), with a roughly flat dorsal surface, strongly rounded ventral surface, and a flattened medial surface. Distal to the base, this process deepens dorsoventrally and narrows mediolaterally (49 mm deep and 39 mm wide on the right P2 spike of NHMUK R16307). This occurs bilaterally. The medial sulcus deepens distally. The P2 morphologies are consistent between the two known parietal specimens. In specimen NHMUK R16308, the right spike has two sulci parallel to the long axis dorsally, each no more than 10 mm deep, and at least one sulcus laterally on the left side (incomplete preservation obscures the rest of the morphology).
The surface texture of the bone on the holotype is difficult to discern, because the adhering ironstone matrix is hard to remove. Towards the distal end of the midline parietal bar, some faint neurovascular impressions are visible, and other eroded impressions are visible elsewhere. In all, the texture is consistent with this being an adult individual (Sampson et al. 1997).
Squamosal (Fig. 3A).—The isolated right squamosal (NHMUK R16309) preserves nearly the complete “blade” of the element, but the rostral portion that would have articulated with the facial bones is not preserved. Five marginal undulations ornament the lateral margin of the squamosal, but the parietal-squamosal contact is not sufficiently well preserved to determine whether an ossification spanned this suture. The overall shape, proportions, and ornamentation of the element are similar to those in other centrosaurines. The squamosal measures 280 mm from the distal end of the parietosquamosal contact to the rostral corner of the free blade just caudal to the jugal notch.
Skull roof (Fig. 4).—NHMUK R16306 preserves much of the skull roof, from the region rostral to the nasal horn to the rostralmost portion of the parietal bar. The skull is well preserved, but is skewed slightly to the left by post-burial deformation (Fig. 4B). Sutures between individual elements were not visible even after detailed preparation, so some structural relationships cannot be described. Based on bone surface texture, the morphology of the postorbital horncores, and the fusion of cranial elements, the animal was an adult. The specimen cannot be articulated with either parietal or the squamosal; because the specimens were collected from a bonebed, it is possible that none of the elements belongs to the same individual.
The prominent, caudally recurved nasal horncore is complete and centered just over the caudal margin of the ectonaris (Fig. 4A). The horn's base measures 115 mm long and 45 mm wide, and it extends 251 mm above the top of the external naris, or 201 mm above the dorsum of the nasal bones.
A portion of the premaxillary septum is preserved, indicating that the septum extended for the full distance to the top of the nasals, as is typical for centrosaurines (Fig. 4A). The distance from the front of the orbit to the caudal margin of the external naris is 231 mm. A complete Centrosaurus apertus skull, ROM 767, measures 223 mm at this point, suggesting that NHMUK R16306 came from an individual of similar skull size (ROM 767 measures 724 mm in basal skull length).
The postorbital horncores are short (69 mm tall above the orbit on the right side) and have relatively blunt tips, as is typical of many adult individuals of Centrosaurus apertus and Styracosaurus albertensis (Fig. 4A, B). A small, broad resorption pit occurs on the rostral surface of the left horncore, also similar to pits in adult individuals of C. apertus and S. albertensis. The lateral surfaces of the horns are flattened and the medial surfaces are convex. Despite the slight crushing of the specimen, there is no evidence that the horncores curved laterally (as in Centrosaurus brinkmani; Ryan and Russell 2005). The horns are centered just caudal to the midpoint of the orbits, which are roughly circular (98 mm tall by 112 mm long on the right side). An antorbital buttress projects over the rostrodorsal quadrant of the orbit (Fig. 4A).
The narrow and elongate frontoparietal fontanelle (126 mm long by 22 mm wide, as preserved; Fig. 4C) is typical for centrosaurines. The margins leading to the opening taper gently. The fontanelle extends up to the plane defining the rostral third of the orbit and the caudal end extends approximately 5 to 10 cm caudal to the orbit. The supracranial sinus complex extends laterally to the medial base of the postorbital horncores. This condition is typical for Centrosaurus and Styracosaurus, but less extensive than the condition in Pachyrhinosaurus lakustai (Farke 2010).
Bilaterally positioned dorsotemporal channels, lined by smooth bone, extend into the caudal end of the frontoparietal fontanelle from the dorsotemporal fenestrae. The channels join at their rostral ends to form a common channel that then slopes ventrally into the caudal margin of the fontanelle. A midline pocket into the dorsum of the parietal is placed just caudal to the common channel.
The entire medial margins and portions of the dorsal margins of the dorsotemporal fenestrae are preserved, showing the sharp emargination typical of ceratopsids. The median bar of the parietal, where preserved, is unremarkable and shows typical adult centrosaurine bone texture.
Dentary (Fig. 3B).—The rostral end of a left dentary, preserving the articular surface for the predentary, adheres to the partial parietal NHM R16308. The morphology of this fragment does not differ markedly from that of other centrosaurine dentaries.
Methodology.—In order to investigate the phylogenetic relationships of Spinops sternbergorum relative to other centrosaurine ceratopsids, a data matrix including 18 taxa (Protoceratops andrewsi as the outgroup, Turanoceratops tardabilis, Zuniceratops Christopheri, two chasmosaurine ceratopsids, and 13 centrosaurine ceratopsids) and 97 morphological characters was assembled (see Appendix 1, SOM 5). Because cranial characters are the most useful for elucidating phylogenetic relationships among these taxa, the matrix focused on this character subset. Spinops sternbergorum was coded on the basis of all available bonebed material. With the exception of Turanoceratops, which was coded from the literature, all other taxa were examined firsthand.
The matrix was analyzed using a heuristic search within PAUP* 4.0b10 (Swofford 2003); all characters were equally weighted and most were unordered (with the exception of ordered characters 20 and 38, which were polarized using ontogenetic data). The analysis was conducted with TBR branch swapping and branches with minimum lengths of zero set to collapse. Both bootstrap (with 1,000 replications) and Bremer support values were calculated, using TNT 1.1 (Goloboff et al. 2008), in addition to strict consensus and 50% majority rule consensus trees. In order to evaluate the effects that different interpretations of epiparietal homology would have upon the results, two matrices were generated. The first assumed, based upon the positions of the ossifications, that the large spike-like processes in Spinops, Albertaceratops, and Diabloceratops were homologous to the P2 position of other centrosaurines (see Discussion and Fig. 5, character codings 58–68 in Appendix 1 and SOM 5). The second assumed, based upon morphology of the ossifications, that the elongated, spike-like process in the aforementioned taxa was homologous to the P3 position of other centrosaurines (as traditionally homologized, e.g., Ryan 2007; Fig. 5, character codings 98–108 in Appendix 1 and SOM 5). The strict reduced consensus (SRC) method was also applied in order to identify “wildcard” taxa that were causing lack of resolution within the tree (Wilkinson 1995). These analyses were implemented using RadCon (Thorley and Page 2000), which generated a number of SRC trees that each pruned various unstable taxa. Preferred SRC trees were selected on the basis of their cladistic information content (CIC).
Results.—The first analysis (using new interpretations of epiparietal homology) yielded 90 most parsimonious trees (MPTs) with tree lengths (TL) of 130 steps, consistency indices (CI) of 0.7615, and rescaled consistency indices (RCI) of 0.5905. The second analysis (using traditional interpretations of epiparietal homology) yielded 4,536 MPTs with TL of 128 steps, CI of 0.7578, and RCI of 0.5798.
Resolution for the strict consensus tree is poor in both analyses. Centrosaurinae are not resolved in either analysis, and Spinops sternbergorum either forms a polytomy with Styracosaurus albertensis and Centrosaurus spp. (“new” codings) or forms a polytomy with other centrosaurines and chasmosaurines (“traditional” codings; SOM 2). Bremer support and bootstrap values are comparatively low for all clades. In the 50% majority rule trees (SOM 3), S. sternbergorum belongs to a clade including Centrosaurus spp. and Styracosaurus albertensis (in 100% and 67% of the trees using “new” and “traditional” codings of epiparietals, respectively). These clades are nested well within Centrosaurinae.
Application of strict reduced consensus to the analysis using the “new” set of epiparietal character codings generated three SRC trees: of these SRC 3 had the highest CIC value (45.38) and also retained 17 of the original 18 taxa. This tree (SOM 4A) excludes only Sinoceratops, and this one deletion results in a substantial increase in the resolution of centrosaurine interrelationships. A “pachyrhinosaur” clade (Achelousaurus, Einiosaurus, and Pachyrhinosaurus) sister to Rubeosaurus is recovered, which is in turn sister to an unresolved clade comprising Centrosaurus spp., Spinops, and Styracosaurus. SRC 2 has the next highest CIC (37.53) and excludes Centrosaurus brinkmani only (SOM 4B). SRC 2 also recovers the “pachyrhinosaur” + Rubeosaurus clade and a “centrosaur” clade containing Centrosaurus apertus, Styracosaurus, and Spinops. However, these clades and other ceratopsids together form an unresolved polytomy. SRC 1 (CIC = 38.58) is identical to the strict consensus tree. Sinoceratops and C. brinkmani were identified as the most unstable taxa, so the original analysis was re-run with the same settings, but these two “wildcards” were deleted a posteriori. The strict consensus tree obtained (Fig. 6B) following their deletion is much better resolved and recovers the “pachyrhinosaurs” (inclusive of Rubeosaurus) as the sister-group of the “centrosaurs,” and Spinops was recovered as sister to Styracosaurus + C. apertus. Albertaceratops and Avaceratops are outgroups to this “derived centrosaur” clade and more basal nodes within the analysis are also more highly resolved, including a monophyletic Centrosaurinae.
Strict reduced consensus generated five SRC trees on the basis of the “traditional” epiparietal character codings. Of these, SRC 5 has the highest CIC value (40.52) and excludes Sinoceratops and Rubeosaurus. This tree (Fig. 6A) recovers the “pachyrhinosaur” clade and a “centrosaur” clade that includes Centrosaurus spp., Spinops and Styracosaurus. In all other respects the tree topology is identical to that of the modified strict consensus tree described above for the analysis based on “new” epiparietal codings.
Homology of epiparietal ossifications.—For the purposes of phylogenetic analysis, the marginal ossifications on the parietals of ceratopsids (often called epoccipitals, but more correctly termed epiparietals; Hatcher et al. 1907, Horner and Goodwin 2008) have been numbered starting from their most medial position (Sampson 1995, Sampson et al. 1997). Process Pl occurs in Centrosaurus apertus as a strongly procurved hook, but is typically manifested as a more muted knob or bump of bone in Styracosaurus albertensis and other taxa (Sampson et al. 1997). Homology across taxa is inferred on the basis of its unique position (dorsal surface of the parietal) relative to other epiparietal positions. Position P1 unambiguously occurs in Spinops sternbergorum.
In all “derived” centrosaurines (exclusive of Avaceratops, Albertaceratops, and Diabloceratops), position P2 is a medially directed hook or small tab of bone in the plane of the frill, emanating from the caudal margin of the frill just lateral to the midline of the parietal. In taxa with Pl hooks, the P2 epiparietals occur immediately lateral to the P1 position. No epiparietal consistent with this pattern occurs in Spinops, and some centrosaurines (Albertaceratops and Diabloceratops) were previously assumed to lack P2 (e.g., Ryan 2007; Kirkland and DeBlieux 2010). It should be noted that the P2 process is variable in Styracosaurus albertensis, developed as either a medially recurved P2 process or a small, dorsoventrally depressed, medially oriented, tab-shaped process similar to that seen in Achelosaurus and Einiosaurus. When present as a hook (e.g., the right side of ROM 1436; TMP 86.126.1) the process tends to be shorter, more gracile and more lightly textured than P2 hooks in C. apertus. The tab-like P2 morphology occurs in many specimens of S. albertensis, including the holotype, CMN 344, and on isolated partial parietals such as TMP 66.10.4, TMP 81.19.157, TMP 81.19.160, TMP 81.19.209, TMP 91.36.254, and TMP 99.55.2. In the last specimen, the small right P2 process is positioned at what appears to be the base of the P3 spike (see Ryan et al. 2007: fig. 9D).
Finally, many centrosaurines (Styracosaurus, Pachyrhinosaurus, Achelousaurus, and Einiosaurus) possess an elongated spike at the P3 position, immediately lateral to P2. For taxa lacking clear epiparietals positioned more medially (e.g., Albertaceratops, Diabloceratops) it is typically assumed that medially placed, prominent epiparietals are homologous to P3 (Ryan 2007; Kirkland and DeBlieux 2010). However, new data provided by Spinops suggest an alternative set of epiparietal homologies (see also Clayton et al. 2009). If confirmed, this new information would have important consequences for the homology statements that form a core component of most phylogenetic analyses of ceratopsid dinosaurs.
The topographic relationship between the procurving hook (P1) and the elongate spike of Spinops is compatible with the elongate spike representing position P2, rather than P3 (but, see below for an alternative interpretation favored by some of the authors). In Centrosaurus apertus, the base of process P2 (a medially-curving hook in the plane of the parietal) partially overlaps the base of the procurving P1 process. Even in species that substitute a prominent Pl hook with a subtle swelling on the dorsum of the parietal, such as Styracosaurus albertensis, the swelling is immediately rostral to the medial edge of the relatively short P2 process (Ryan et al. 2007). In Spinops sternbergorum, the same topographic relationship occurs between the procurving P1 process and the spike-like process. Thus, based on positional criteria, the spike may represent the P2 epiparietal rather than the P3 epiparietal.
Assuming that this hypothesis of homology is correct, it then motivates a re-evaluation of previously published homology statements for epiparietals in other centrosaurine ceratopsids. For instance, Ryan (2007) hypothesized that the most medially placed, laterally hooked process on the parietal of Albertaceratops nesmoi represented epiparietal P3. Under the new scheme, it is then possible that the process actually represents P2, as in Spinops. Thus, the true P3 is a short, triangular process. Similarly, Kirkland and DeBlieux (2010) hypothesized that a spike-like, medially placed epiparietal in Diabloceratops also represents P3. Again, an alternative interpretation holds that the elongated epiparietal represents P2, with the consequence that P3 is a rather short, unremarkable process in this taxon also.
We posit that the medially placed spikes on the parietals of centrosaurine ceratopsids might represent P2 and P3 in different taxa. Given the extremely plastic nature of ceratopsid epiparietals throughout their evolution, this hypothesis is not entirely unexpected.
Alternatively, it is possible that the elongated spike in Spinops sternbergorum (as well as in Albertaceratops and Diabloceratops) is indeed homologous to the P3 process of Styracosaurus. This would imply that the P2 process is missing, or that a frame shift in the gene expression of the epiparietal morphology occurred. The discovery and description of additional specimens is necessary in order to further clarify and revise these hypotheses of homology. Pending resolution of this issue, we recommend that phylogenetic analyses should consider the effects of both coding schemes on reconstructions of centrosaurine relationships.
Comparison with other centrosaurines.—The facial region, including both the nasal and postorbital horns, is nearly identical to that of Centrosaurus apertus and Styracosaurus albertensis (Sampson et al. 1997; Ryan et al. 2007). Within the frill, Spinops sternbergorum displays a mosaic of features seen in the latter two species. The elongated P1 hooks are virtually identical to those in C. apertus, whereas the elongate spike is similar to the condition in S. albertensis. Based on this anatomy, it is arguable that S. sternbergorum represents an anagenetic intermediate between C. apertus and S. albertensis. This hypothesis is unlikely, however, assuming that Spinops is indeed from low in the Dinosaur Park Formation or high in the Oldman Formation (thus predating or coincident with the occurrence of C. apertus). Alternatively, Spinops could represent the sister taxon to Centrosaurus, Styracosaurus, or both, as suggested by some consensus trees (Fig. 6B; SOM 2B, 3B). Unfortunately, the lack of stratigraphic constraint for the Spinops bonebed as well as poor phylogenetic resolution and weak support for most of the nodes within Centrosaurinae prevents definitive evaluation of these conflicting hypotheses. Furthermore, the limited nature of the S. sternbergorum material does not allow an investigation of how ontogenetic changes or sexual dimorphism may differ in this taxon versus other centrosaurines.
Interestingly, the putative Chinese centrosaurine Sinoceratops zhuchengensis was not consistently recovered as a centrosaurine by our analyses (in contrast to the analysis presented by Xu et al. 2010). This may be driven at least in part by the taphonomic loss of the premaxilla in the known specimens, which is an important diagnostic element for Centrosaurinae, as well as poor differentiation of sutures on the material that prevents coding of many critical characters. Clearly, additional research is needed here. As more species are discovered, achieving satisfactory resolution within the phylogeny of ceratopsids has become increasingly difficult. The higher degree of resolution presented by previous centrosaurine phylogenies probably resulted from a general consensus on epiparietal homologies (now undermined by the condition in Spinops) in addition to the completeness of the included taxa. Many new taxa are based on more fragmentary material, increasing the proportion of missing data in the data matrices.
Regardless of its evolutionary affinities, or the homologies of its cranial structures, Spinops sternbergorum is an unexpected addition to the Campanian fauna of Alberta, particularly given the intensive collecting of ceratopsids there in recent years. A total of six centrosaurine taxa are now recognized from the Oldman/Dinosaur Park Formation sequence, representing a variety of morphologies. Much additional work is needed, inside and outside Dinosaur Provincial Park, to address a number of remaining unanswered questions. Do the ceratopsians preserved here document anagenesis or cladogenesis? How are the taxa of Alberta related to those from elsewhere? Was Spinops a rare element of the Campanian fauna, or will more remains be recognized?
Discussions with William Blows (City University, London, UK), David Eberth (TMP), and Scott Sampson (Natural History Museum of Utah, Salt Lake City, Utah, USA) were helpful. We thank Sandra Chapman (NHMUK) for facilitating collections access, Phil Hurst (NHMUK Image Resources) for taking photographs for publication and Polly Parry (NHMUK archives) for access to archival material. Brenda Chinnery-Allgeier (University of Texas, Austin, USA) and Peter Dodson (University of Pennsylvania, Philadelphia, USA) are gratefully acknowledged for their insightful reviews. This work was funded in part by NSF grant EAR 0819953 to AF and ML, with Scott Sampson (Utah Museum of Natural History) and Catherine Forster (George Washington University, Washington, D.C., USA), and the TNT software was made available with the sponsorship of the Willi Hennig Society. Lukas Panzarin (Venice, Italy) prepared the reconstruction in Fig. 2. Ben Creisler (Seattle, Washington, USA) offered etymological assistance.