Spinops sternbergorum, Farke & Ryan & Barrett & Tanke & Braman & Loewen & Graham, 2011

Farke, Andrew A., Ryan, Michael J., Barrett, Paul M., Tanke, Darren H., Braman, Dennis R., Loewen, Mark A. & Graham, Mark R., 2011, A new centrosaurine from the Late Cretaceous of Alberta, Canada, and the evolution of parietal ornamentation in horned dinosaurs, Acta Palaeontologica Polonica 56 (4), pp. 691-702 : 694-696

Spinops sternbergorum sp. nov.

Figs. 2–4 .

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, Alberta, 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 bor− der 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.

Description

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. 3C 1 ); 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. 3C 3 ). 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.

http://dx.doi.org/10.4202/app.2010.0121

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.

Figures

Fig. 2. Reconstruction of the skull of Spinops sternbergorum gen. et sp. nov. from the Campanian of Dinosaur Provincial Park, southern Alberta, in right lateral view. Preserved elements are stippled; missing portions are dotted and modeled after Centrosaurus apertus.

Fig. 3. Centrosaurine ceratopsid Spinops sternbergorum gen. et sp. nov. from the Campanian of Dinosaur Provincial Park, southern Alberta. A. Partial right squamosal in lateral view, NHMUK R16309. B. Partial parietal with adherent bone fragments in dorsal view, NHMUK R16308. C. Partial parietal in dorsal (C1), rostral (C2), and left lateral (C3) views, holotype NHMUK R16307.

Fig. 4. Partial skull of the centrosaurine ceratopsid Spinops sternbergorum gen. et sp. nov. from the Campanian of Dinosaur Provincial Park, southern Alberta, NHMUK R16306; in right lateral (A), rostral (B), and dorsal (C) views.

Fig. 1. Map showing the presumed location of the Spinops sternbergorum gen. et sp. nov. type locality within the area informally called the “Steveville badlands,” Dinosaur Provincial Park, Alberta Canada. Charles Sternberg (unpublished data in NHMUK archives) indicated that the bone bed was one mile below the mouth of Berry Creek, and the estimated area that this covers is indicated by the grey semi−circle. Intense prospecting on the east side of the river has failed to relocate the quarry, and badlands on the west side are outside of the Park boundary and currently inaccessible for prospecting. The indicated southeast Park boundary does not include the margins of two major coulees in this region that are also within the Park. Note that the quarry for the holotype of Styracosaurus albertensis Lambe, 1913 (CMN 344) is in the southeast part of the Park. The inset photograph, courtesy of David Eberth, shows a typical view of the contact between the Dinosaur Park Formation (DPF) and Oldman Formation (OF) near the Steveville badlands.