Pachyrhinosaurus perotorum, Fiorillo & Tykoski, 2012

Fiorillo, Anthony R. & Tykoski, Ronald S., 2012, A new Maastrichtian species of the centrosaurine ceratopsid Pachyrhinosaurus from the North Slope of Alaska, Acta Palaeontologica Polonica 57 (3), pp. 561-573 : 563-568

Pachyrhinosaurus perotorum sp. nov.

Figs. 2–5 .

Etymology: In recognition of members of the Perot family (Margot and H. Ross Perot and their children), who have demonstrated a long history of supporting science and science education for the public.

Type material: Holotype: DMNH 21200 , posterior part of parietal including partial median bar, posterior median emargination, and autapomorphic anterior−facing horns . Paratypes: DMNH 22558 , partial skull missing parietals, squamosals, right cheek region ; DMNH 21201 , a short segment of right parietal transverse bar bearing an anterior rim horn .

Type locality: All specimens collected from the Kikak−Tegoseak Quarry along the Colville River, North Slope, Alaska, USA. Exact coordinates on file at the Museum of Nature and Science, Dallas. The palaeolatitude of this quarry places it in the ancient polar world and makes it one of the highest latitude centrosaurine localities known.

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Type horizon: The specimens were recovered from lower Maastrichtian rocks (69–70 Ma) within the Prince Creek Formation ( Fiorillo et al. 2010; Flaig et al. 2011). This qualifies P. perotorum as the youngest known centrosaurine.

Diagnosis.— Pachyrhinosaurus perotorum is distinguished from other centrosaurine ceratopsids by the presence of a pair of short, dorsoventrally flattened horns projecting anteromedially and slightly dorsally from the anterior (fenestral) margin of the parietal transverse bar. Potential diagnostic features of P. perotorum also include paired fossae dorsal to narial fossa; a narrow median hump on the posterior part of the nasal boss; basal sulcus of nasal boss extends ventrally to a level lower than the dorsal rim of narial fossa; and lack of a P2 horn or process on posterior margin of parietal transverse bar, but this feature may vary as seen in specimens of P. lakustai ( Currie et al. 2008) . The new taxon may also be distinguished by a blunt−tipped profile to the rostrum with an upturned premaxillary and rostral ventral border. However, this condition is so unusual for a ceratopsian that it may prove to be an individual oddity rather than a diagnostic feature. P. perotorum may also be differentiated from P. lakustai by lack of anterior pommel on nasal boss (pommel present in some specimens of P. lakustai ), and nasal boss and supraorbital bosses that nearly contact, separated only by narrow groove (shared with P. canadensis , but bosses widely separated in P. lakustai ). The new species is also differentiated from P. canadensis by the presence of a rostral comb (shared with P. lakustai , but not present in P. canadensis ).

Description

Much of the fossil material collected from the Kikak−Tegoseak site awaits preparation. This description is based mainly on DMNH 22558, a partial skull missing the parietosquamosal frill and lower right cheek area ( Figs. 2 , 3 ), and the holotype partial parietal frill section DMNH 21200 ( Fig. 4 ). The skull (DMNH 22558) is distorted in an oblique manner, such that the right side is shifted dorsally relative to the left. The sutures between most of the cranial bones are obscured by extensive fusion, a sign of relative maturity in other ceratopsids. In most respects, the skull of P. perotorum closely resembles both P. canadensis and P. lakustai . Therefore, we focus here only on cranial features pertinent to establishing phylogenetic relationships between P. perotorum and other pachyrhinosaurs. Measurements of DMNH 22558 are given in Table 2. They are based on the same landmarks used by Langston (1967, 1975) and Currie et al. (2008). Based on these measurements ( Table 2) and those of occipital condyles from the quarry ( Table 1), the cranial material of P. perotorum falls within the size range of specimens of P. lakustai ( Currie et al. 2008) .

Rostral.—The rostral bone is fused to the premaxillae, and the right side is missing its posteroventral portion. The rostral lacks an anterior point or beak−like hook ( Figs. 2A, B , 3A, B ). Instead, it has an almost continuously convex lateral profile from dorsal to ventral contacts with the premaxillae. In contrast, the rostral of some P. lakustai specimens is sharply hooked (TMP 1987.55.285, TMP 1986.55.258), and there is a lesser beak on specimens of P. canadensis (NMC 9485 and an un−numbered specimen at the Drumheller Valley Interpretive Centre informally referred to in the literature [ Currie et al. 2008] as “the Drumheller skull”). There is no evidence for pre−mortem breaking or pathology of the element. The ventral margins of the rostral have a relatively rounded, broad cross−section ( Figs. 2D , 3D ).

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Premaxilla.—The premaxillae are fused to one another, and most of the ventral margin of the right premaxilla has eroded away. A small part of the ventral margin of the left has been reconstructed. The premaxillae give rise to a broad narial septum as in other centrosaurines. A pronounced ventral angle protrudes below the alveolar margin of the maxilla. Dorsally, the premaxillae form part of the narial bridge and contribute to a rostral comb like those seen in some specimens of P. lakustai ( Currie et al. 2008) , but not in P. canadensis ( Sternberg 1950; Langston 1967, 1975). The rostral comb includes a large, overhanging protrusion even with the anterior margin of the narial fossa. A second but smaller, transversely oriented protrusion is posterodorsal to the first. The lightly eroded dorsal surface of the rostral comb shows a faint median line that marks the sutural contact between premaxillae. The line continues posterodorsally and stops at another transverse ridge that marks the anterior margin of the nasal boss.

Nasal.—The nasal boss in DMNH 22558 is very large, extending from above the anterior part of the narial fossa to above the middle of the orbit ( Figs. 2A–C , 3A–C ). The boss has no anterior pommel, unlike some specimens of P. lakustai ( Currie et al. 2008) . The basal sulcus ( Hieronymus et al. 2009), a system of small grooves and foramina that defines the ventrolateral margin of the boss, curves low on the lateral surface of the face to a level below the dorsal rim of the narial fossa. The lateral surfaces of the boss are marked by extensive dorsoventral grooves and ridges, which produce the palisaded texture typical of Pachyrhinosaurus bosses. The sides of the boss are otherwise relatively flat and expand dorsolaterally, making the boss the widest part of the skull anterior to the parietosquamosal frill. There is a large, sub−spherical cavity that deeply invades the bone near the posterior end of the right side of the boss ( Figs. 2B , 3B ). The edges of this apparent pathology are well rounded, finished bone. Within the cavity are several osseous structures, including multi−layered, onionskin−like bone, the broken edges of which show they are comprised of coarse fibers generally oriented perpendicular to the outside surfaces.

The dorsal surface of the nasal boss is irregular and pitted as in other Pachyrhinosaurus ( Figs. 2C , 3C ). In dorsal view it is widest at a point over the centre of the maxillae. The anterior part of the boss is relatively flat, and slopes anteroventrally to meet the premaxillae where it contributes to the rostral comb. The asymmetrical dorsolateral edges are raised above the middle of the dorsal surface, producing a concavity in the center of the boss. A low median ridge in the concavity probably marks the union of the two nasals.

Posteriorly, the boss bears a conspicuous median hump, or dome. The hump is sub−triangular in dorsal view, narrows anteriorly, and is the highest point on the skull anterior to the frill ( Figs. 2A, C , 3A, C ). It is mediolaterally narrow, with lower surfaces of the boss lateral to it. The anterior half of the hump is pitted like the rest of the boss surface, but the posterior half bears anteroposteriorly oriented ridges, or "fins" ( Hieronymus et al. 2009) ( Figs. 2C , 3C ). This matches the texture seen on the posterior surfaces of nasal bosses in both P. canadensis and P. lakustai ( Langston 1967, 1975; Currie et al. 2008; Hieronymus et al. 2009).

The nasal boss extends posteriorly to at least over the midpoint of the orbit. It stops abruptly at a sharp ridge backed on each side by narrow transverse grooves reminiscent of the transverse tunnel described in the same place in P. canadensis ( Langston 1967, 1975). The grooves are shallow laterally between the nasal and supraorbital bosses, but then enter a deep triangular fossa on the midline ( Figs. 2C , 3C ). The groove on the left side is interrupted at one point where the nasal and supraorbital bosses actually contact. The trace of the right−side groove can be followed anteroventrally where it is continuous with the basal sulcus. The groove on the left side may also be continuous with the basal sulcus, but poor surface preservation makes it difficult to discern.

Within the nasal vestibule, the size and orientation of the narial process (= intranarial process) is notable. The narial process is well preserved on the right side, but partially reconstructed on the left. The process is large and robust, with a thick proximal base and triangular, flat−bottomed cross−section. A lateral groove on the lower part is a continuation of faint grooves and foramina that may mark the nasal−premaxillary suture. If so, the premaxilla forms the ventral part of the process, as reported in TMP 2002.76.1 and Diabloceratops ( Kirkland and DeBlieux 2010; Ryan et al. 2010). The narial process angles anteromedially toward the midline, similar to the condition in P. canadensis ( Figs. 2E , 3E ) ( Langston 1975).

Above the narial fossa and posterolateral to the rostral comb is a shallow depression best seen on the right side ( Figs. 2B , 3B ). The fossa is defined dorsally and posteriorly by a curved ridge and a series of grooves and foramina that are continuous with the basal sulcus. The ridges from each side meet on the midline to mark the anterior tip of the fused nasals, and together form the third horizontal protrusion that makes up the rostral comb. The fossa on each side is located in the same place as the “tumescent bulge” or “supranasal” boss ( Langston 1975: 1582) in P. canadensis ( Sternberg 1950; Langston 1967, 1975). There is no “fist−sized knob” ( Sternberg 1950: 111) posterodorsal to the narial fossa. No supranasal boss or fossa is present in P. lakustai ( Currie et al. 2008) .

Supraorbital bosses.—The supraorbital bosses of DMNH 22558 are large and asymmetrical. Both are highest along the medial and posteromedial edges, and slope anterolaterally. They are separated from one another by a midline groove that rises from the same deep median fossa as the grooves that separate the nasal and supraorbital bosses. The T−shaped pattern of grooves and ridges created by the transverse and median grooves matches that in P. canadensis , but which are reportedly absent in P. lakustai ( Sternberg 1950; Langston 1967, 1975; Currie et al. 2008).

The right supraorbital boss is almost circular in dorsal view ( Figs. 2C , 3C ), with clearly defined edges. The anterior edge is sharp and steep walled with a deeply recessed surface posterior to it. The posterior and posteromedial margins are raised abruptly above the adjacent skull surface. The dorsal surface is nearly planar and marked by pits and ridges, some of which are weakly aligned anterolaterally to posteromedially. The left boss is well defined medially and posteriorly, but has only a low anterior rim and almost indiscernable lateral and posterolateral margins ( Figs. 2C , 3C ). The dorsal surface bears pits and ridges, but the center is much lower than that of the right boss. An unusual mass of dark−coloured, pitted material was found in the center of the left supraorbital boss during preparation ( Figs. 2C , 3C ). The dark material averages 10 mm or less in thickness, and appears to follow the contours of the underlying bone ridges.

Parietal.— The holotype parietal ( DMNH 21200 ) consists of the posterior part of the median parietal bar, part of the right−side transverse parietal bar, and a horncore from the left side of the transverse parietal bar found in position but separated from the rest of the parietal by an interval of missing bone ( Fig. 4 ). The dorsal surface of the parietal is marked by faint vascular grooves, while the ventral surface is nearly smooth. The median bar is relatively broad with an asymmetrical cross−section. The right side of the median bar is dorsally excavated by a broad fossa where it curves to become the transverse bar. The fossa overlaps the midline. There is no such fossa on the left side of the median bar .

We follow the convention of Sampson (1995) when referencing parietal horns and processes. The posterior edge of the parietal has only a very shallow median emargination. There is no P1 horn or enlargement, a character shared with other pachyrhinosaurs. There is little development of a process at the P2 locus, in contrast to most other centrosaurines which bear at least rudimentary P2 spines or hooks (Sampson

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Pachyrhinosaurus perotorum

Pachyrhinosaurus lakustai Pachyrhinosaurus canadensis

TMP 2002.76.1

Achelousaurus horneri Einiosaurus procurvicornis

Rubeosaurus ovatus Centrosaurus apertus Styracosaurus albertensis

Centrosaurus brinkmani

Albertaceratops nesmoi

Diabloceratops eatoni Chasmosaurus belli

Triceratops Zuniceratops christopheri Protoceratops

1995; Currie et al. 2008; McDonald and Horner 2010). This resembles some parietals referred to P. lakustai ( Currie et al. 2008) . Another incomplete parietal still under preparation bears a large, triangular P3 horn that would project posterolaterally from a complete frill ( Fig. 4D ). The location and size of this horn matches the P3 horn in other Pachyrhinosaurus , especially P. canadensis ( Langston 1975; Currie et al. 2008). It does not exhibit the anteroventral twist described in many P3 horns of P. lakustai ( Currie et al. 2008) .

The holotype parietal bears small but well−developed horns on the anterior rim of transverse parietal bar ( Fig. 4 ), which is a diagnostic character for Pachyrhinosaurus perotorum . This anterior horn is best preserved on the right hand side of the holotype. It is dorsoventrally flattened with a narrower lateral margin and bears numerous longitudinal grooves on both dorsal and ventral surfaces. It arises from the anterior rim of the transverse bar lateral to the P2 position on the posterior rim of the parietal, curves anterodorsally and medially, and projects dorsally only a little above the plane of the frill ( Fig. 4C ). This differs from the P1 spikes and processes in Centrosaurus or Styracosaurus , which rise from the dorsal surface of the parietal nearer the midline ( Sampson 1995; Sampson et al. 1997). We refrain from applying or modifying the convention of Sampson (1995) for describing these processes because of the unique position of this horn in P. perotorum .

A large foramen penetrates the dorsal surface of the anterior parietal rim horn of Pachyrhinosaurus perotorum near the base. The foramen leads to a canal that passes through the horn core and exits through a more anteromedially positioned foramen on the ventral surface ( Fig. 4A, B ). Smooth sulci issue from both foramina, suggesting a large vascular element passed through the horn in life. The dorsolateral edge of the horn is marked by a wider longitudinal sulcus broader than the other grooves on the surface, which results in a weak lateral keel for the horn.

The left horn is much smaller than the right, has fewer longitudinal grooves, but retains a broad lateral sulcus like that on the right horn. Its base is also penetrated by large foramina on the dorsal and ventral surfaces in the same relative positions as those on the right side horn. Asymmetry of parietal ornamentation in the same individual was evidently not uncommon in centrosaurines ( Sampson et al. 1997; Ryan et al. 2001; Currie et al. 2008).

Another specimen ( DMNH 21201 ) is identified as a short section of a second right side parietal transverse bar that also bears an anterior rim parietal horn ( Fig. 5 ). It shares the presence of foramina on the dorsal and ventral sides of the horn in the same locations as in the holotype. The horn on DMNH 21201 also has a broad sulcus along the dorsolateral edge, although it is smaller and restricted to the proximal end of the horn. The axis of the horn does not angle as far dorsally as in the holotype, and projects almost directly anteriorly and medially. The presence of at least two individuals from the Kikak−Tegoseak quarry that bear anterior parietal horns is strong evidence against either specimen being from an aberrant individual. Instead, it suggests that these specimens preserve a taxonomically diagnostic frill autapomorphy that differentiates Pachyrhinosaurs perotorum from all other centrosaurines .

Geographic and stratigraphic range.— Type locality and horizon only.

Figures

Fig. 2. Centrosaurine ceratopsid Pachyrhinosaurus perotorum sp. nov., paratype (DMNH 22558); skull in left lateral (A), right lateral (B), dorsal (C), ven− tral (D), and anterior (E) views. Scale bars 10 cm.

Fig. 3. Labelled line drawings of Pachyrhinosaurus perotorum paratype (DMNH 22558) skull in left lateral (A), right lateral (B), dorsal (C), ventral (D), and anterior (E) views. Scale bars 10 cm.

Fig. 4. Centrosaurine ceratopsid Pachyrhinosaurus perotorum sp. nov., holotype (DMNH 21200); parietal in dorsal (A), ventral (B), and right lateral (C) views. D. Reconstruction of the posterior parietal frill based upon DMNH 21200 and a yet to be catalogued partial parietal with a P3 horn. Scale bars 10 cm.

Fig. 5. Centrosaurine ceratopsid Pachyrhinosaurus perotorum sp. nov., paratype (DMNH 21201); partial parietal preserving portion of right transverse parietal bar and anterior rim horn in dorsal (A), ventral (B), and right lateral (C) views. Scale bars 5 cm.