Oohkotokia horneri, Penkalski, 2014

Penkalski, Paul, 2014, A new ankylosaurid from the late Cretaceous Two Medicine Formation of Montana, USA, Acta Palaeontologica Polonica 59 (3), pp. 617-634 : 619-626

Oohkotokia horneri sp. nov.

Figs. 2A , 3A , 4 , 5A–E, G, H .

Etymology: The specific name honours John R. Horner for his work on dinosaurs from Montana.

Holotype: MOR 433 , a skull and fragmentary skeleton from Group 1 area of quarry.

Type locality: MOR Locality TM-034, northwest of Cut Bank, Montana, USA .

Type horizon: Upper Two Medicine Formation (upper Campanian) of northwestern Montana, about 55 m below the contact with the Bearpaw Shale. The Two Medicine Formation is about 650 m thick and consists primarily of mudstone, siltstone, and fine-grained lenticular sandstone, interbedded with bentonite layers of varying thickness ( Dawson 1885; Stebinger 1914; Rogers et al. 1993). The Upper Two Medicine has been dated by Rogers et al. (1993) using radioisotopes at 74 Ma (10 m below top of formation), an age accepted by Eberth (1997) and Trexler (2001). The top of the Dinosaur Park Formation was given an age of 74–74.5 Ma by Eberth (1997), but of 75 Ma or older by Hamblin (1994). Eberth (2005) refined this to around 74.9 Ma. Thus, the uppermost Two Medicine sediments are slightly younger than the latest Dinosaur Park beds.

Referred specimens.—MOR 363, a fragmentary skull from 60 m below the top of the Upper Two Medicine Formation (John R. Horner, personal communication 2004), with supraorbital and quadratojugal bosses identical to those of the holotype; MOR 538; NSM PV 20381, an undescribed specimen recovered in 1995 from a bonebed ( Tanoue 2005) that includes a partial skull, vertebrae, a partial pelvis, forelimb and hindlimb elements without feet, and one keeled osteoderm; FPDM V-35, another undescribed specimen from the Upper Two Medicine Formation; TMP 2001.42.19; USNM 7943; USNM 11892.

Diagnosis.—An ankylosaurine diagnosed by the following unique combination of characters: median plate on nasal area of skull roof small (<50 mm), not distinguished from surrounding osteoderms; prominent, horn-like, trihedral squamosal bosses; keel on squamosal boss flat rostrally, grading into a blunt keel dorsally; apex rounded and unkeeled, situat- ed caudally; caudal surface of squamosal boss flat to gently rounded and unkeeled; broad, smooth quadratojugal bosses with strong caudal curvature; nuchal crest not visible in lateral view; occipital condyle small (≤16% basal skull length); orbit large; osteoderms basally excavated with a smooth, weakly ornamented external surface texture; steeply-pitched, triangular caudal osteoderms. The small nasal plate, caudally curved quadratojugal bosses, and horn-like squamosal bosses distinguish O. horneri from Euoplocephalus tutus . Nuchal crest morphology, supraorbital boss shape, and vertebral morphology separate O. horneri from Dyoplosaurus acutosquameus , and osteoderm shape and texture distinguish O. horneri from D. acutosquameus and Scolosaurus cutleri . The horn-like squamosal bosses distinguish O. horneri from A. lambei .

Description

Group 1 (ankylosaurid) material

Skull.—The skull ( Figs. 1–3 ) is crushed but reasonably well preserved on its dorsal and lateral surfaces. It is cracked rostrally to the right of the midline (see Fig. 2A 1 ) where the left and right sides were split apart post-mortem. As a result, the midline rostrally (midline; Fig. 2 ) is offset to the animal’s left side. To either side of the narrowly triangular gap, the dorsal surface is well preserved. The palatal side is weathered, with most of the palate having been removed by erosion. The premaxillary beak is missing, though fragments of the premaxillae remain. The skull measures 415 mm long sagittally) as preserved. After accounting for crushing and lack of the premaxillary beak, basal length was probably around 375 mm, one of the larger Campanian North American skulls. Overall, it appears most similar to skulls that have been referred to Euoplocephalus . It is crucial to note, however, that some of those skulls do not represent E. tutus Penkalski 2001 ; Arbour 2010).

The crushing is unusual. Rather than being more-or-less unidirectional, there is a roughly triangular area around which the skull has been splayed outward, as if it were stepped on. This flattened area is bordered caudally by the fracture or escarpment along the nuchal crest, laterally by the right supraorbital + preorbital bosses, and rostromedially by the crack that extends from the rostral triangular break toward the left squamosal boss ( Fig. 2A 1 ). The overall distortion suggests dinoturbation.

The dorsal surface has numerous small, flat, polygonal osteoderms with weak overall bilateral symmetry, an ankylosaurid character ( Coombs 1978; Thompson et al. 2012). The borders of the osteoderms are clearly delineated rostrally but become less well defined in the area between the supraorbital bosses; caudally, the borders are again well defined. This, too, is typical of ankylosaurids.

Rostrally, there is no large, median osteoderm or plate “large sagittally positioned polygon” of Vickaryous and Russell 2003; “internarial ornamentation” of Carpenter et al. 2011) in the nasal region as there is in most ankylosaurid taxa. Despite crushing, the dorsal surface here is well preserved, and the outlines of most or all nasal region osteoderms can be traced with confidence; only along the crack to the right of the midline are the borders untraceable. Characteristically small osteoderms plainly surround a similarly small central nasal osteoderm (nasal plate; Fig. 2 ). In Euoplocephalus tutus , this plate is prominent, having a raised central area and spanning almost half the width of the snout. Some basal forms and earlier Asian ankylosaurids also lack this feature, although the Campanian Saichania and Tarchia both have a prominent nasal plate. The plate’s absence—or more properly, its small size—in Oohkotokia is presumably reversal. In Oohkotokia , the plates in the rostral region sensu Vickaryous and Russell 2003) are presumed to be fused osteoderms; however, in some ankylosaurids at least, the cranial sculpturing arises instead from dermatocranial elaboration by the periosteum ( Carpenter et al. 2001).

The skull is striking in several ways. For one, it has a relatively smooth overall surface texture compared to most other late Campanian ankylosaurid skulls. Ankylosaurus magniventris is similarly smooth, as is the holotype of Euoplocephalus tutus and several referred skulls. However, the majority are rugose, pitted, and more highly sculptured. In the present specimen, most of the osteoderms have light, subparallel nonvascular grooves and a pitted texture that is less pronounced than in most other skulls.

The squamosal and quadratojugal bosses (horn-like protuberances at the caudal corners of the skull) are prominent. These have variously been referred to as horns, bosses, and ornamentation, but Vickaryous et al. (2001) gave a good rationale for use of the term boss. The squamosal bosses are large and trihedral ( Figs. 2 , 3 ), with a degree of caudal curvature. The boss has a flat, narrowly triangular area rostrodorsally that grades into a blunt keel toward the caudally-situated apex of the boss. The keel ends 20–30 mm short of the apex; distal to this, the boss is unkeeled or very weakly keeled, rounded, and suboval in cross section. This morphology is unique and is also present in the referred specimens TMP 2001.42.19 and USNM 11892 (see Gilmore 1930: pl. 9). The right squamosal boss in MOR 433 is better preserved than the left, but the flat rostral area and unkeeled, subconical apex are present in both. Both bosses are missing fragments dorsally; the apices were not pitted. The overall morphology is similar in other specimens, including some from Asia, but the boss is relatively large in MOR 433. Medially, the boss is concave, while laterally and caudally it is relatively flat. These three surfaces form the three sides of the “trihedron”. The only Campanian specimens from North America that have a larger boss are others from the Two Medicine Formation that are referred herein to Oohkotokia .

The scythe-like quadratojugal boss ( Fig. 3 ) curves caudally, tapering to a point. This boss varies considerably in both size and shape among late Campanian ankylosaurids. However, in undisputed Euoplocephalus specimens (TMP 1991.127.1 and UALVP 31; see Penkalski 2001; Arbour et al. 2009; Burns and Sullivan 2011), the quadratojugal boss is more symmetrical, with a central apex. In Nodocephalosaurus , the quadratojugal bosses curve rostrally, and Sullivan (1999) found the curvature to be taxonomically useful within Ankylosauridae . Thus, although the caudal curvature is not exclusive to Oohkotokia , it is one of a unique suite of diagnostic characters.

On the lateral surface of each side of the skull between the squamosal and quadratojugal bosses is a small (25 mm) but distinct, caudally situated, bluntly pointed osteoderm or ossicle ( Figs. 2 , 3 ). Many referred Euoplocephalus specimens are smooth here, while others have small osteoderms between the bosses; however, in none is there a keeled or pointed ossicle as in MOR 433.

The orbit is large, measuring at least 63 mm horizontally on the better preserved left side. This diameter is as large as in any skull currently referred to Euoplocephalus ; however, it is relatively smaller than in three other skulls referred herein to Oohkotokia : USNM 11892, TMP 2001.42.19, and NSM PV 20381. Hill and Norell (2008) concluded that the orbit was relatively larger in juvenile ankylosaurs. Yet, none of the three referred Oohkotokia specimens are juveniles; the smallest measures about 340 mm in basal length—90% the size of MOR 433. Thus Oohkotokia apparently had a larger orbit than other taxa from Montana and Alberta. The antorbital margin is correspondingly narrower.

In the palatal region, crushing and erosion make detailed description impossible. Additionally, after preparation was finished, the skull was loaned for an exhibit and in the process was damaged (John R. Horner, personal communication 2011). Several fragments, e.g., portions of the right quadrate and left maxilla, are present in a cast ( Fig. 3A 2 ) but no longer in the original skull ( Fig. 3A 1 ).

Portions of both maxillae remain, but the alveolar borders are eroded and no teeth are preserved. Importantly, the nasal septum (or vomer) is present dorsally in the nasal cavity, making it possible to locate the midline on the dorsal surface. Note that the vomer is broken and offset along the diagonal fracture in the skull roof, showing how much the rostral portion of the skull is displaced to the animal’s left side.

The occipital region is mostly intact although crushed, with the left paroccipital process incomplete distally. Both quadrates are nearly complete, but both are broken mid-shaft and the condyles are abraded. As in E. tutus and USNM 11892 ( O. horneri ), the paroccipitals were not fused to the quadrate, as evidenced by the contact on the left quadrate for the incomplete paroccipital process (paroccipital surface; Fig. 3 ). On the right side, the quadrate ramus of the pterygoid overlaps and is fused to the pterygoid wing of the quadrate, plausibly forming a scarf joint as described by Vickaryous and Russell (2003) for E. tutus .

The braincase is complete, but crushing and weathering make interpretation of nerve openings problematic. The large internal carotid foramen is visible piercing the basisphenoid, as in FPDM V-35, with a crescentic nerve III (oculomotor) foramen dorsal to that at the laterosphenoid border. Moving caudally, there is a smaller opening that may be the nerve V (trigeminal nerve) opening, but little else can be discerned with any certainty. Rostrally, the basioccipital tubera are preserved but distorted. The occipital condyle is chipped but otherwise well preserved and measures 61 mm in diameter.

The nuchal crest (“scutellum” of Tumanova 1987; “nuchal ridge” of Carpenter et al. 2011; “nuchal shelf” of Penkalski 2001)—that is, the caudal border of the skull roof excluding the squamosal bosses—has four distinct osteoderms as in many ankylosaurid taxa. However, it is not deeply sculptured and is not visible in lateral view as it is in many of the Campanian specimens (compare Fig. 3B–G ). This is due in part to the larger squamosal horns in MOR 433 (and the other Two Medicine specimens) but is also related to the width and caudodorsal expanse of the crest. In AMNH 5403, a referred Euoplocephalus specimen, the nuchal crest is elevated and rugose and is prominent in lateral view ( Fig. 3G ). The crest is also visible in TMP 1991.127.1 (= E. tutus ; Penkalski 2001) and possibly UALVP 31 (= E. tutus ; Arbour and Currie 2012) after accounting for crushing, as well as in the Asian Pinacosaurus , Saichania , Tarchia , Talarurus , Tianzhenosaurus , and Crichtonsaurus benxiensis Lü, Ji, Gao, and Li, 2007 .

Axial skeleton.—One partial mid-cervical vertebra ( Fig. 4A ) was found just behind the skull (see Fig. 1 ). The dorsal half of the neural spine is missing, but judging from its base, the spine was grooved caudally but not compressed craniocaudally into a flange as in Ankylosaurus and cf. Euoplocephalus (AMNH 5337, AMNH 5403). The neural canal is taller than wide and D-shaped (wider dorsally) as in Ankylosaurus and as in the cervical described below with the Group 2 material. In referred Euoplocephalus specimens (e.g., AMNH 5403, AMNH 5404), the cervical neural canal is quadrangular. The narrowly oval diapophyses are more strongly inclined than in AMNH 5337 or AMNH 5404.

Only one other isolated vertebra was found in Group 1: a well-preserved ankylosaurid caudal ( Fig. 4B ), probably the fourth or fifth caudal judging by the presence of the chevron and the length of the transverse processes. The zygapophyseal surfaces are strongly inclined, making an acute angle between them of about 70°. This contrasts with the condition in Dyoplosaurus (ROM 784) where the zygapophyseal surfaces of the proximal tail form an obtuse angle of 120–150°. The neural spine is relatively taller than in cf. Euoplocephalus (AMNH 5404; Coombs 1971) or Dyoplosaurus (Arbour et al. 2009: fig. 4).

In addition to these two vertebrae, parts of the sacrum were found in Group 1, as discussed below. There are also several L-shaped thoracic ribs ( Fig. 1 ), including the distal third of a rib (rib fragment; Fig. 1 ) with an uncinate process attachment virtually identical to that figured by Brown (1908: fig. 14) for Ankylosaurus .

Forelimb.—A poorly preserved scapula ( Fig. 4F ) consists only of the central portion of the scapular blade; the distal and proximal ends including all but the caudalmost corner of the glenoid are missing. The piece measures 12 cm at its narrowest width. There is no sign of the transverse pseudoacromial process present in most nodosaurids.

Osteoderms.—All of the basally-excavated, ankylosaurid-type osteoderms were found in Group 1, several in close proximity to the skull (see Fig. 1 ). They all have a similar external surface texture consisting of fine pitting, particularly on the keel, with ordered (sensu Hieronymus et al. 2009) neurovascular channels over much of the surface ( Fig. 4 ). The degree of vascularization and pitting is variable, but all of the osteoderms are of the “smooth” type (sensu Arbour et al. 2009). One is a steeply-pitched, triangular osteoderm ( Fig. 4C ) that resembles the curved, lateral caudal “plates” (sensu Blows 2001) of some primitive ankylosaurs (e.g., Polacanthus foxii ; Blows 2001). However, in Oohkotokia , this osteoderm is basally excavated and thin-walled, only being thicker along the keel. Similar osteoderms are present laterally in cf. Saichania on the flanks and tail ( Carpenter et al. 2011).

Another osteoderm ( Fig. 4G ) is conical and weakly keeled cranially ( Fig. 4G 2 ), with an oval base that is wider (transversely) than long. Plate-like osteoderms (sensu Penkalski 2001) are represented by the one with a damaged keel shown in Fig. 4E and by a smaller, flat fragment with a preserved edge. Many of the osteoderms ( Fig. 4C 2 , E, G 1 ) have a double-edged perimeter (sensu Penkalski 2001) and some (e.g., Fig. 4D ) have incipient radial ribbing as well.

A neural

1 2 F 10 cm H

Sacral elements.—There appear to be two partial synsacra in the quarry ( Fig. 5 ), supporting the count of two animals. Most of the synsacral material is poorly preserved, so conclusions regarding overall pelvic morphology must be made with caution. The majority of it appears to be ankylosaurid.

Ankylosaurid sacrum.—The preserved elements include three broken segments ( Fig. 5A, E, G ) representing at least six vertebrae. There is no positive fit between the three segments, suggesting that one or more centra are missing. Where the segments are unbroken, the centra are fused. On each end, the face has a notochordal prominence. Two of the segments Fig. 5A, G ) are crushed laterally. The ribs and their transverse processes are fused and T-shaped in cross-section. The caudalmost is fused to its centrum; the others seem to have been unfused or poorly fused.

The segment with two vertebrae only ( Fig. 5A ) is poorly preserved dorsally but appears to be the cranial part of a presacral rod. The wider segment ( Fig. 5E ) consists of two fused centra missing their arches. This piece is obliquely crushed with the?cranial centrum broken and incomplete. On each side are two large attachment points for sacral ribs, both intervertebral and facing dorsolaterally. The unit is interpreted as the last two dorsosacrals. The two largest ribs are the caudalmost dorsosacral and first sacral rib from the animal’s left side compare Coombs 1978: fig. 13). Most of one sacral (i.e., s1) is apparently missing from between the wide segment ( Fig. 5E ) and the third segment ( Fig. 5G ) which then consists of the caudal bit of s1 plus s2 and s3 (sensu Coombs 1986). The presumed presacral rod ( Fig. 5A ) and medium-sized rib ( Fig. 5H ) were found close to the skull (see Fig. 1 ).

Another piece ( Fig. 5B ) consists of two fused neural arches and appears to match the wide centrum segment. The cranialmost arch was poorly fused to its centrum on one side (upper left in Fig. 5B 1 ). Dorsally, there is a broken longitudinal surface where a bladelike neural process was broken off. It appears that the pre- and post-zygapophyses were fused, but incomplete fusion with the centra—and the partial fusion of the ribs—suggests that this animal was immature. The caudal face of the presumed s3 centrum (bottom in Fig. 5G ) is an articular face, so no sacrocaudals were fused to the synsacrum (yet).

Nodosaurid sacrum.—The preserved nodosaurid sacral piece ( Fig. 5F ) comprises the?caudal end of a synsacrum. It is badly weathered dorsally, and only the ventral surface remains ( Fig. 5F ). It preserves part of an articular face and the medial groove described by Gilmore (1930) for E. rugosidens . Such a groove is also present in the nodosaurid Peloroplites and the basal ankylosaurid Cedarpelta ( Carpenter et al. 2008) . The?ankylosaurid synsacrals described above lack the ventral groove, so even if there is no overlap, i.e., no duplication of elements, this piece is likely from a different animal. Judging by its length (18 cm), the piece represents two vertebrae. Its location was not recorded on the field map or in field notes, but if it was found near similarly numbered items, it was located between Group 1 and Group 2.

Group 2 material

Most or all of these elements likely belong to an edmontoniid (sensu Bakker 1988) that was deposited near the Oohkotokia holotype. However, some elements differ from typical Edmontonia , and there is the possibility, however remote, that they are from the same animal as the Group 1 material.

Vertebrae.—Four vertebrae were found in Group 2: a mid-cervical and three caudals ( Figs. 1 , 6A–C ). The base of the cervical’s broken neural spine is abraded but is solid and not a laterally expanded flange as in some ankylosaurids. An incomplete proximal caudal ( Fig. 6C ) is similar to the third caudal of E. rugosidens described by Gilmore (1930) and is nominally referable to that genus. The neural canal is excavated into the centrum, resulting in a heart-shape to the centrum in axial view. The preserved caudals of cf. Euoplocephalus (AMNH 5404, ROM 1930) do not show this morphology, but those of Nodocephalosaurus do ( Sullivan and Fowler 2006) as does the one preserved caudal of CMN 8530 ( Anodontosaurus ). There is some positional variation in the shape of the centrum and neural canal. Another caudal ( Fig. 6B ) is unremarkable except that the neural spine is mostly complete and shorter than in the ankylosaurid caudal ( Fig. 4B ) described above. The fourth Group 2 vertebra is a poorly preserved mid-caudal centrum.

Forelimb.—Two partial humeri are present. The left ( Fig. 6I ) is more complete than the right but is missing the proximal end and the ulnar condyle. It is unclear whether the deltopectoral crest extended distally past the midpoint of the humerus as it typically does in ankylosaurids and Gastonia ( Kirkland 1998) . The radial condyle is weathered, but the shape is roughly correct, and the rest of the bone is well preserved. This humerus was certainly very large, comparable in size to that of Edmontonia rugosidens or Ankylosaurus .

near the apex; along the keel are one or two small, additional points. Carpenter (1990) referred the spine to E. rugosidens . Cervical armour.—Most of one-half of the first cervical half-ring is present ( Fig. 6E ) including a distal end. The two complete osteoderms are oval and keeled, with apices that overhang the caudal border. The lateral osteoderm (sensu Penkalski 2001) is in close contact with the medial one— typical for nodosaurids—but does not contact the distal osteoderm, and the underlying bony half-ring is visible where their curvature forms an angle cranially and caudally ( Fig. 6 , arrows). The osteoderms are crushed and were evidently hollow underneath.

There is also a large (210 mm) polygonal, keeled cervical osteoderm preserved. It is damaged along one edge, while the opposite border (to the right in Fig. 6G ) is a complete edge that abutted another plate (the bony layer can just be seen protruding at lower right). The keel is damaged?cranially. The bony layer has a woven texture (underneath) that runs roughly parallel to the lower (straight) edge in Fig. 6G , indicating that this is the transverse direction. The element is likely a medial plate from the second half-ring.

The largest osteoderm is a distal cervical or pectoral spine ( Fig. 6H ) measuring 41 cm long. The end opposite the point has a rugose, undulating sutural contact for the half-ring segment medial to the spine. The spine is crushed and was either basally hollow or filled with cancellous bone as in cf. Edmontonia ( Hayashi et al. 2010) . The dorsal side is keeled for most of its length but becomes rounder in cross section Referred specimens

USNM 11892.—This specimen ( Fig. 3D ) from the Upper Two Medicine Formation consists of a partial skull and five teeth. It was described in detail by Gilmore (1930), so here I add only a few comments relevant to its taxonomic status. Unfortunately, Gilmore’s field notes and map do not give the elevation at which it was collected, and the quarry has not been relocated (John R. Horner, personal communication 2012).

Gilmore (1930) referred the specimen to Dyoplosaurus acutosquameus based primarily on tooth morphology. Although dentition may not be taxonomically useful for ankylosaurs below the familial level ( Coombs and Demere 1996; personal observations), the teeth of USNM 11892 do represent a morphological extreme among teeth of late Campanian ankylosaurid specimens from Montana and Alberta. The teeth have a shelf-like labial cingulum and a unique Z-shape to the carina in occlusal view. They also lack the severe, random fluting described by Vickaryous and Russell (2003) for E. tutus . Conversely, the preserved teeth of Dyoplosaurus (ROM 784) lack the sharp cingula and Z-shaped carinae.

Gilmore (1930) believed that the skull lacked the large nasal plate (he called them scutes) while acknowledging that he had “great difficulty” making out the shapes due to fracturing and poor preservation in this region ( Gilmore 1930: 32); however, it appears that Gilmore (1930) was correct, as discussed below. Gilmore (1930) also noted two large plates on the side of the snout and claimed that in Euoplocephalus (UALVP 31) there were three; in fact, most or all known ankylosaurines including Euoplocephalus have two, not including the narrow lacrimal ossification just rostral to the orbit. Even the Asian forms Saichania and Tarchia (and probably Tianzhenosaurus ) have a very similar morphology here (e.g., Tumanova 1987).

USNM 11892 has the largest squamosal bosses—indeed, they could reasonably be called horns—of any known Upper Campanian/Lower Maastrichtian ankylosaurid specimen from North America. The bosses are very similar to those of the holotype, although the medial surface of the “trihedron” is not concave. It is noteworthy that Vickaryous and Russell (2003) did not mention USNM 11892 in their redescription of the skull of Euoplocephalus , nor did Arbour et al. (2009) sustain Gilmore’s (1930) referral to Dyoplosaurus .

USNM 7943.—This is a partial first cervical half-ring found north of Milk River, near Landslide Butte, Glacier County, Montana, USA. Gilmore (1917: 44) states that it was found “a short distance from the place where the type of Brachyceratops montanensis was discovered, at a slightly higher horizon”. According to Sampson (1995), Brachyceratops was found about 60 m below the Bearpaw contact; hence, USNM 7943 must be from about the same horizon as MOR 433. The specimen ( Fig. 7A ) includes both medial osteoderms and a partial lateral osteoderm (sensu Penkalski 2001), solidly

1

2

coossified with that section of the underlying bony half-ring. The medial osteoderms are subrectangular and unkeeled, with low, blunt central apices. The lateral osteoderm is steeply-pitched and keeled, although the exact shape is uncertain as the half-ring is dorsoventrally crushed. The shapes match those of the first half-ring of TMP 2001.42.19.

TMP 2001.42 .19.— An undescribed specimen from the Two Medicine Formation of Montana, USA , TMP 2001.42 .19 includes a partial skull without teeth ( Fig. 3F ), axial and limb elements, a tail club, and some armour. The specimen was found southwest of Cut Bank, Montana, USA by a private collector, but detailed stratigraphic data were not recorded. The specimen is from a bonebed (Brandon Strilisky, personal communication 2011) so the association of elements is uncertain. The skull has been heavily reconstructed; only the caudal skull roof and occipital regions are well preserved. In overall shape, the skull is similar to that of USNM 11892. The squamosal bosses are horn-like and unkeeled caudally and have the subconical apex present in MOR 433 and USNM 11892 .

Most of the axial and appendicular elements are of limit- ed value as they are largely reconstructed. The proximal third of the humerus is missing, but the element appears to match that of NSM PV 20381 in being small but very robust. The one well-preserved pedal ungual is typically hooflike and unlike the wide, triangular unguals of Dyoplosaurus .

The right half of the first cervical half-ring is preserved ( Fig. 7B ), including all three osteoderms from that side. The half-ring is distorted craniomedially, but the overall morphology is comparable to the preserved portion in USNM 7943 ( Fig. 7A ), although in the latter the medial osteoderm is more rectangular. Several free osteoderms are also present; these are keeled and have a smooth (sensu Arbour et al. 2009) texture, with sparse, subparallel neurovascular grooves.

There is a well-preserved tail club, the first to be described from the Two Medicine Formation. The club ( Fig. 8 ) includes both major osteoderms and an almost complete handle but is damaged and incomplete distally. It is a round (sensu Coombs 1995), average-sized knob measuring 320 mm wide. The keeled major osteoderms are offset to the dorsal side of the knob, giving the club a somewhat “elf-eared” (dorsolaterally pointed) outline in distal view, though not as pronounced as in some other clubs, e.g., AMNH 5245 and UALVP 16247 ( Coombs 1995: fig. 2C; Arbour and Snively 2009: fig. 2D). In dorsal view, the major osteoderms are not laterally pointed as in AMNH 5216 and AMNH 5245 from the Horseshoe Canyon Formation. The surface has a hummocky (sensu Hieronymus et al. 2009), cauliflower-like texture that may in part be a taphonomic artefact, although the major osteoderm of another partial club (MOR 538) from the Upper Two Medicine has a similar texture and is about the same size as TMP 2001.42.19.

Figures

Fig. 2. Skull of ankylosaurid dinosaur Oohkotokia horneri gen. et sp. nov. from the late Campanian Upper Two Medicine Formation of Montana, compared with related taxa. A. MOR 433, holotype; stereo pair of high fidelity cast in dorsal view (A 1) with interpretive drawing (A 3); oblique rostrodorsal view of the actual skull (A 2). B. CMN 8530, holotype of Anodontosaurus lambei from the late Campanian–early Maastrichtian Horseshoe Canyon Formation of Alberta. C. CMN 0210, holotype of Euoplocephalus tutus from the late Campanian Dinosaur Park Formation of Alberta. D. TMP 1991.127.1, Euoplocephalus tutus from the late Campanian Dinosaur Park Formation of Alberta. E. AMNH 5403,?Euoplocephalus tutus from the late Campanian Dinosaur Park Formation of Alberta. B–E. Interpretive drawings.

Fig. 3. Skull of ankylosaurid dinosaur Oohkotokia horneri gen. et sp. nov. from the late Campanian Upper Two Medicine Formation of Montana, compared with other late Campanian specimens. A. MOR 433, holotype, in ventral (A 1, A 2) and lateral (A 3, A 4) views. The ventral line drawing is based on a cast and shows fragments that are now missing from the actual fossil (see text for explanation). B. CMN 8530, Anodontosaurus lambei from the Horseshoe Canyon Formation of Alberta. C. NSM PV 20381, Oohkotokia horneri from the Two Medicine Formation of Montana. D. USNM 11892, Oohkotokia horneri (reversed) from the Two Medicine Formation, Montana. E. TMP 1991.127.1, Euoplocephalus tutus from the Dinosaur Park Formation of Alberta. F. TMP 2001.42.19, Oohkotokia horneri from the Two Medicine Formation, Montana. G. AMNH 5403,?Euoplocephalus tutus from the lower Dinosaur Park Formation, Alberta.

Fig. 4. Postcranial material of Oohkotokia horneri gen. et sp. nov. (MOR 433, holotype) from the late Campanian Upper Two Medicine Formation of Montana. A. Partial cervical vertebra in cranial view. B. Well-preserved fourth or fifth free caudal in distal and left lateral view.C. Lateral plate in?ventral and?dorsal view (C1, C2). D. Weakly ribbed subconical osteoderm. E. Oval, low-keeled osteoderm in external view. F. Partial scapula in?lateral view. G. Conical osteoderm in caudal stereo (G1) and cranial (G2) views. H. High-keeled osteoderm in dorsal stereo. I. Smaller conical osteoderm.

Fig. 5. Sacrum (or sacra) associated with ankylosaurid dinosaur Oohkotokia horneri gen. et sp. nov. (MOR 433, holotype) from the late Campanian Upper Two Medicine Formation of Montana. A. Presacral rod (?) in ventral view. B. Fused neural arches in ventral (B1) and dorsal (B2) views. C, D, H. Sacral ribs in cranial view. E. Dorsosacral (?) centra in dorsal (E1) and ventral (E2) views. F. Nodosaurid sacral fragment.G. Posterior sacrals with reattached?s3 rib in ventral view.

Fig. 1. Quarry map for holotype of Oohkotokia horneri gen. et sp. nov. (MOR 433) from the late Campanian Upper Two Medicine Formation of Montana, USA.

Fig. 6. Nodosaurid (Group 2; see text) material from the quarry, catalogued as MOR 433, including cervical armour and vertebrae referable to Edmontonia sp. A. Cervical vertebra in cranial (A) and lateral (A) views. B. Ca1 2 dual vertebra in distal view. C.?Third caudal in?distal (C 1) and lateral (C 2) views. D. Phalanx. E. Part of the first cervical half-ring including one distal end, in ventral (E 1) and dorsal (E 2) views; arrows point to bony half-ring layer (see text). F. Partial osteoderm fused to underlying bone. G. Plate from?second half-ring. H. Cervical/pectoral spine in external view. I. Left humerus in caudal view.

Fig. 7. Drawings of first cervical half-rings of Oohkotokia horneri sp. et gen. nov. from the late Campanian Two Medicine Formation of Montana. A. USNM 7943 in dorsal (A) and caudal (A) views. B. TMP 2001.42.19 1 2 in dorsal (B ) and caudal (with left half mirrored) (B ) views.

Fig. 8. Well preserved tail club of ankylosaurid dinosaur TMP 2001.42.19, referred specimen of Oohkotokia horneri sp. et gen. nov. from the late Campanian Two Medicine Formation of Montana, in dorsal (A) and ventral (B) views. The complete tail club (C); rectangle shows area of photos.