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Cretaceous Research 32 (2011) 174e186

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Cretaceous Research

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Juvenile specimens of Pinacosaurus grangeri Gilmore, 1933 (Ornithischia:Ankylosauria) from the Late Cretaceous of China, with comments on thespecific taxonomy of Pinacosaurus

Michael E. Burns*, Philip J. Currie, Robin L. Sissons, Victoria M. ArbourDepartment of Biological Sciences, University of Alberta, 11145 Saskatchewan Drive, Edmonton, Alberta, Canada T6G 2E9

a r t i c l e i n f o

Article history:Received 24 February 2010Accepted in revised form 16 November 2010Available online 24 November 2010

Keywords:DinosaurBayan MandahuPinacosaurus

* Corresponding author. Tel.: þ1 780 492 1252.E-mail address: mburns@ualberta.ca (M.E. Burns).

0195-6671/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.cretres.2010.11.007

a b s t r a c t

Four juvenile specimens referable to Pinacosaurus grangeri (Ankylosauria: Dinosauria) are described fromthe Campanian (Upper Cretaceous) locality Bayan Mandahu in northern Inner Mongolia AutonomousRegion (People’s Republic of China). All the specimens preserve the skulls as well as, in some cases,mandibles, postcrania, and osteoderm. They are not taphonomically deformed by expanding matrixdistortion, unlike many Gobi specimens, including the holotype of P. grangeri. Bayan Mandahu is also thetype locality for Pinacosaurus mephistocephalus. The proximity in space and time of these two closelyrelated species warrants a generic and specific revision for Pinacosaurus. The distinction of the twospecies is based on characters of the squamosal dermal elaborations, cranial roof posterior to the orbits,premaxillary notch, and distal margin of the ilium. Although a relatively well-represented ankylosaurtaxon, the phylogenetic position of Pinacosaurus has not been unequivocally resolved. A new analysisrecovers Pinacosaurus as the most basal member of the Ankylosaurinae.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The genus Pinacosaurus (Gilmore, 1933) is a relatively commonfaunal component of the Late Cretaceous of Asia (Inner Mongolia inthe People’s Republic of China, andMongolia) and is one of the bestknown ankylosaurs in terms of number of specimens, supersededonly by Euoplocephalus (Lambe, 1910) fromwestern North America.Gilmore (1933) erected the genus and type species, Pinacosaurusgrangeri, based on a complete, taphonomically distorted skull(AMNH 6523), mandible, atlas, axis, and osteoderms from theDjadokhta Formation of Bayan Zag (Bayn Dzak ¼ ShabarakhUsu ¼ “The Flaming Cliffs”), Mongolia. A second species, Pinaco-saurus mephistocephalus, was named by Godefroit et al. (1999)based on a sub-adult specimen consisting of a nearly complete,undeformed, articulated skeleton, osteoderms, and a tail club (IMM96BM3/1). This specimenwas collected from the Campanian BayanMandahu Formation (correlative to the Djadokhta Formation) atBayan Mandahu, Urad Houqi Banner, Bayan Nur League, InnerMongolia, People’s Republic of China (PRC). P. mephistocephaluswasdifferentiated from P. grangeri based on anatomy of the narialregion, lacrimal shape, and length of the humeral deltopectoralcrest (Godefroit et al., 1999).

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Specimens of P. grangeri comprise most of the juvenile to sub-adult ankylosaur specimens collected to date, and it is the onlyspecies of ankylosaur represented by numerous sub-adult indi-viduals. These specimens are important to our understanding ofankylosaur ontogeny as well as anatomy that cannot be studied inadults. In all adult ankylosaurs, cranial sutures are almostcompletely obliterated due to the development of osseous orna-mentation on the dorsum of the skull roof (Vickaryous et al., 2004).Marya�nska (1971) presented the first account of a juvenile skull of P.grangeri as well as the first description of sutural contacts betweenskull elements of an ankylosaur. Numerous juvenile Pinacosaurusspecimens were subsequently collected from Bayan Mandahu(Jerzykiewicz et al., 1993) by the Sino-Canadian Dinosaur Projectand from Alag Teeg in Mongolia (Tverdochlebov and Zybin, 1974;Marya�nska, 1977; Fastovsky and Watabe, 2000; Watabe andSuzuki, 2000a,b). Most recently, Hill et al. (2003) describedanother juvenile skull of P. grangeri (MPC 100/1014) from the UpperCretaceous locality Ukhaa Tolgod, Mongolia. Additional definitivejuvenile material has been described from Euoplocephalus (Coombs,1986), a nodosaurid (Jacobs et al., 1994), Anoplosaurus (Pereda-Suberbiola and Barrett, 1999), and Liaoningosaurus (Xu et al., 2001).

Fourof thePinacosaurus skulls thatwere collectedbytheCanadaeChina Dinosaur Project at Bayan Mandahu (Campanian-age BayanMandahu Formation, Inner Mongolia, PRC) have been prepared.A fifth skull and skeleton, IVPP V16855, was collected from anunknown site at Bayan Mandahu by the Silk Road Expedition

M.E. Burns et al. / Cretaceous Research 32 (2011) 174e186 175

(Dong, 1997) but has not yet been described. Measurements fromthat specimen are included in this paper. The sub-adult cranium(IVPP V16346) had largely eroded from the sediment when it wasdiscovered, and formed the erosion-resistant top of a mushroom-shaped hoodoo. The top of the skull is poorly preserved, and the skullwas therefore prepared in palatal view. The best of the preparedjuvenile skulls (IVPP V16853) and a second skull with its skeleton(IVPPV16854)wereondisplay formanyyears aspartof the travellingexhibit known as “The Greatest Show Unearthed” (Acorn,1993). Thefinal specimen (IVPP V16283) consists of a partial juvenile skull andleft mandible. These specimens are described here for the first time.Along with an analysis of comparative material, the genus and twoincluded species of Pinacosaurus are revised, with comments on thephylogenetic position of Pinacosaurus and current status of ankylo-saur systematics.

Terminology of the skull bones herein follows that establishedby Vickaryous et al. (2004). Synonymies with the osteologicalnomenclature of previous workers are noted in the text. Thespellings of Mongolian geographic and stratigraphic names followthose of Benton (2000), and the chronostratigraphic framework isfrom Jerzykiewicz and Russell (1991).

Institutional Abbreviations. AMNH, American Museum of NaturalHistory, New York, New York, USA; IMM, Inner Mongolia Museum,Hohhot, Inner Mongolia, PRC; MPC, Paleontological Center of theMongolian Academy of Sciences, Ulaan Baatar, Mongolia; IVPP,Institute for Vertebrate Palaeontology and Paleoanthropology,Beijing; UALVP, Laboratory for Vertebrate Palaeontology, Universityof Alberta, Edmonton, Alberta, Canada; ZPAL, Zaklad Paleobiologii,Polish Academy of Sciences, Warsaw, Poland.

2. Geological setting

The Bayan Mandahu locality is roughly 40 km north of UradHouqi Banner, Bayan Nur, Inner Mongolia Autonomous Region, PRC(Fig. 1). The Bayan Mandahu Formation is temporally equivalent tothe more northern Campanian-aged Djadokhta Formation (Fox,1978; Lillegraven and McKenna, 1986; Jerzykiewicz and Russell,1991; Jerzykiewicz et al., 1993). Vertebrate remains at BayanMandahu include squamates, turtles, crocodiles, dinosaurs, andmammals. The dinosaur component of the fauna consists mostly ofProtoceratops and Pinacosaurus, but also includes theropods(Godefroit et al., 2008). The combination of low vertebrate diver-sity, relatively small size of the dinosaurs, and sedimentary dataindicates an unstable semiarid paleoclimate in an alluvial and/oraeolian environment (Eberth, 1993).

Taphonomic studies have suggested that most of the Pinacosau-rus specimens discovered at this locality died in situ andwereburied

Fig. 1. Map of China and Mongolia showing location of Bayan Mandahu and detail of the Nomephistocephalus and vertebrate fossil localities 100, 101, and 106 (from Jerzykiewicz et aV16283, IVPP V16346, IVPP V16853, IVPP V16854). Modified from Jerzykiewicz et al. (1993

either during sandstorms (Jerzykiewicz et al., 1993) or rainstorms indune-sand alluvial fans (Loope et al., 1998). Many of the vertebratespecimens, including Pinacosaurus, from this and other localities intheGobi Basin exhibit trace peri- and/or post-mortemborings likelyformed by carrion insects (Kirkland et al., 1998).

3. Systematic paleontology

Dinosauria Owen, 1842Ornithischia Seeley, 1887Thyreophora Nopsca, 1915Ankylosauria Osborn, 1923Ankylosauridae Brown, 1908Ankylosaurinae Brown, 1908Pinacosaurus Gilmore, 1933

Type species. P. grangeri Gilmore, 1933Stratigraphic horizon. ?Upper Santonian to Upper Campanian of

Mongolia and PRC.Revised Diagnosis. Apomorphies of taxon: posterior embayment

of nasal dermal ornamentation dorsal to external nares and para-nasal apertures creating a shallow nasal vestibule, paranasal aper-tures unenclosed by external nares, characteristic skull roofornamentationcomposedofweakly-developeddermal elaborationsand fused osteoderms. Differing from all other ankylosaurines in:interpterygoid vacuity between palate and braincase, secondarypalate flat, occipital condyle composed of multiple elements.Differing from all other Asian ankylosaurines in: hemisphericaloccipital condyle. Differing from all North American ankylosaurinesin: cranial roof in lateral profile flat anterior to orbits, anteriorlyexcavatedquadrate, cingulapresenton teeth, posteriormarginof thepterygoids anterior to the ventral margin of the pterygoid process ofthe quadrate, fused basipterygoid process-pterygoid contact.

P. grangeri Gilmore, 1933

Pinacosaurus ninghsiensis Young, 1935:5, pls. 1e3 (originaldescription).Syrmosaurus viminocaudus Maleev, 1952:131, figs. 1e3 (originaldescription).Syrmosaurus viminicaudus Maleev, 1954:143, figs. 1e13, 16(emended spelling).

Holotype. AMNH 6523, skull and mandible of an adult specimen,atlas, axis, and several associated osteoderms. Material is dorso-ventrally crushed and has undergone expanding matrix distortion.

Type locality. Bayan Zag, Gobi Desert, Mongolia.

rth Canyon area of the Bayan Mandahu locality with the type locality for Pinacosaurusl., 1993) representing assemblages of juvenile Pinacosaurus grangeri specimens (IVPP, fig. 2).

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Stratigraphic horizon. Djadokhta Formation (Campanian, UpperCretaceous).

Revised diagnosis. As for genus. Differs from P. mephistocephalusin: weakly-developed pyramidal dermal elaborations of the squa-mosals, flat cranial roof posterior to orbits, premaxillary notch,distal margin of ilium encircles the acetabulum laterally.

New Referred Material. IVPP V16853, complete juvenile skull andneck rings from fossil locality 100 (Fig. 2); IVPP V16283, partialjuvenile skull from fossil locality 100 (Figs. 3e5); IVPP V16854

Fig. 2. IVPPV16853, Pinacosaurus grangeri, in dorsal (A, A0), anterior (B, B0) and right lateral (C, Care light grey. Cross-hatching represents brokenbone surfaces. Anterior is to the right inA, A0 , cnarial region; lac, lacrimal; lom, lateral osteodermal mass covering narial region; mo, mansquamosal; po, postorbital; prf, prefrontal; qj, quadratojugal; so, supraorbital ossification; so

(Fig. 6A), an almost complete skeleton (length from snout to back ofilia is 73 cm, tail not prepared) from fossil locality 101; IVPP V16346(Fig. 6B), incomplete sub-adult skull from fossil locality 106. Allspecimens were collected by the Sino-Canadian Dinosaur expedi-tions of 1988 and 1990 from three fossil localities that are markedon the map (Fig. 1) of the North Canyon, Bayan Mandahu(Jerzykiewicz et al., 1993); IVPP V16855, skull and skeleton from anunknown site at Bayan Mandahu (it is a slightly smaller specimenwith a total length of 1.3 m, and a skull length of less than 11 cm).

0) views.Natural orifices are shaded darkgrey,whereas borings andotherdamaged areasomingoutof thepage inB, B0 and to the right in C, C0 . Abbreviations: a, b, c, apertures in thedibular osteoderm; oqj, osteoderm covering quadratojugal; osq, osteoderm coveringa, supraorbital prominence, anterior; sop, supraorbital prominence, posterior.

Fig. 3. IVPP V16283, Pinacosaurus grangeri, skull in dorsal (A, A0) and left lateral (B, B0) views. Natural orifices are shaded dark grey, whereas borings and other damaged areas arelight grey. Anterior is up in A, A0 and to the left in B, B0 . See Fig. 2 for abbreviations.

M.E. Burns et al. / Cretaceous Research 32 (2011) 174e186 177

None of these specimens have undergone expanding matrixdistortion.

P. mephistocephalus Godefroit et al., 1999Holotype. IMM 96BM3/1, undeformed, articulated skeleton

(with in situ cervical rings and tail club), lacking only the leftforelimb, majority of the left pelvic girdle, and the left hind limb.

Type locality. Quarry SBDE 96BM3 (41�47.2690 N; 106�43.5730 E;elevation 1239 m), Bayan Mandahu, Urad Houqi Banner, Bayan Nur,Inner Mongolia Autonomous Region, PRC (Fig. 1).

Stratigraphic horizon. Bayan Mandahu Formation (Campanian,Upper Cretaceous).

Revised diagnosis. As for genus. Differing from P. grangeri in:relatively long and slender dermal elaborations of the squamosals,domed cranial roof posterior to orbits, distal margin of ilium formsa horizontal shelf dorsal to the acetabulum.

Fig. 4. Detail of left narial region of Pinacosaurus grangeri, IVPP V16283, in left anterolateral vpremaxillary foramina. Anterior is to the left. Scale bar equals 1 cm.

4. Occurrence

The Sino-Canadian Dinosaur Project discovered twelve juvenilePinacosaurus from fossil localities 100/101 and one from locality106 in Bayan Mandahu in 1988 and 1990 (Jerzykiewicz et al., 1993).The distance between quarries 100 and 101 is 19 m, which isthe distance between the closest animals in each quarry. The fivespecimens discovered in quarry 100, collected in 1998, werepartially disarticulated, probably because they were scavenged bysmall theropods (both Velociraptor and Saurornithoides teeth werefound amongst their remains). The seven skeletons from the nearbyquarry 101 had not been scavenged, however, and were lyingupright in the sediments with fore and hind limbs tucked under-neath the bodies. These animals were sub-parallel to each other,although some faced one way and others were facing in theopposite (180�) direction. Although the seven skeletons in this

iew. The external nares (a) are dorsal to the paranasal apertures (b and c) and a series of

Fig. 5. IVPP V16283, Pinacosaurus grangeri, left mandible in labial (A, A0) and lingual (B, B0) views. Natural orifices are shaded dark grey, whereas borings and other damaged areasare light grey. Anterior is to the left in A, A0 and to the right in B, B0 . See Fig. 2 for abbreviations.

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cluster were all articulated, erosion had removed the skulls andanterior regions of three before they were found. All of the twelvespecimens from fossil localities 100 and 101 are small, immatureindividuals between 1.3 and 1.5 m in length. In all probability, thetwo quarries represent the same event and further excavationcould reveal more specimens between them. Only three of theskulls from these fossil localities have been prepared, and aredescribed in this paper. The larger but still immature specimen(IVPP V16346) was collected at nearly the same horizon on theopposite side of the North Canyon (Fossil locality 106 inJerzykiewicz et al., 1993). This skull was badly eroded dorsally, andwas therefore prepared from the palatal surface.

None of the skulls described in this paper have undergonetaphonomic crushing or shearing, nor have they been subjected toexpanding matrix distortion as have some other specimens fromUpper Cretaceous Gobi deposits (see section on TaphonomicAnalysis). However, the right and posterior portions of IVPPV16283 have been heavily weathered, and many bones have beenlost from these regions. All of these specimens also have multiplepost-mortem insect borings (Kirkland et al., 1998), which in somecases continue downward into the sediment as bone-lined tubes.

The Pinacosaurus in fossil localities 100 and 101 may representa family group(s), or crèche(s) formed by conspecifics of similar age.Regardless, they were contemporaneous and part of the samepopulation. These specimens therefore represent an excellent andrare opportunity to study variation among individuals of the samespecies and same ontogenetic stage in an extinct taxon. Althoughhistological work has not been done on these specimens, unpub-lished work on similar-sized individuals from Alag Teeg inMongolia (Fastovsky and Watabe, 2000) suggests that they areseveral years old (Greg Erickson pers. comm., 2007).

5. Anatomical description

5.1. Overview

In all four of the specimens (IVPP V16346, V16853, V16854, andV16283), the skull is roughly triangular in dorsal view. They alsolack antorbital, supratemporal, and infratemporal fenestrae (theregion of the temporal opening is characteristically closed inankylosaurs; Vickaryous et al., 2004). The orbit is roughly circular,but is somewhat longer than high in all specimens. This is clearlybecause of dorsoventral crushing in IVPP V16346. The maximumanteroposterior diameter of the orbit is only 33mm in IVPP V16853(Fig. 2C), but is an estimated 55 mm in IVPP V16346. Although theanimals from quarries 100 and 101 were all approximately the

same size, there is some variability in the measurements andproportions of the skulls (Table 1). Although there is some sculp-turing over the nasals, it does not obscure the internasal sutures inany of the skulls. Osteoderms are fused to the posterior portions ofeach skull roof, but the sutural boundaries are still identifiable. Theleft side of the skull of IVPP V16283 is preserved posterior to thesquamosal, though a secondary dermal ossification of the squa-mosal is not present. The right side is preserved only as far back asthe prefrontal.

The descriptions of individual bones are composites of all fourspecimens divided into anatomical regions. As each element is notnecessarily preserved on every specimen, the bones that have beenlost taphonomically are noted within each region.

5.2. Rostral region

The paired premaxillae are well preserved in all four of thespecimens. The posterodorsal premaxillary process can be seen onthe right side of IVPP V16854 and left side of IVPP V16283, where itprotrudes between the maxilla and nasal. The premaxillary palateof IVPP V16283 is wider (4.0 cm) than long (2.5 cm), rounded alongits anterior margin, and relatively flat. The same dimensions in IVPPV16346 are 10.7 cm by 7 cm, which suggest that the rostrummaintains the same proportions as the animals become older andlarger. The tomial crest in each of the toothless premaxillae of IVPPV16283 and V16346 extends posteriorly across the premaxillary-maxillary suture to terminate lateral to the front of the maxillarytooth row. The maxillary portion of the tomial crest of these twospecimens is partially overlain by a co-ossified osteoderm, unlike inadult specimens of Euoplocephalus. The interpremaxillary suture(a median sagittal butt joint) is visible in dorsal and palatal views inIVPP V16283 and V16346, but neither a premaxillary notch nora paired incisive foramen (generally directly posterior to the notch)is visible .

The external nares and apertures are visible on the right side ofIVPP V16853 and both sides of IVPP V16283 and V16346. Ventralto the external nares (“a” in Fig. 4, according to Hill et al., 2003), twosets of paired paranasal apertures (“b” and “c” in Fig. 4) lead tosinuses within the premaxilla/maxilla (Marya�nska, 1977; Godefroitet al., 1999; Vickaryous et al., 2001, 2004; Hill et al., 2003;Vickaryous and Russell, 2003). Aperture B (sensu Hill et al., 2003;¼gland opening of Marya�nska, 1977; and Godefroit et al., 1999), themost ventral, opens on both sides into the oral cavity of IVPPV16283 directly anterior to the premaxillary-maxillary suture andanterolateral to the internal choanae. Although there are openingsin roughly the same position in IVPP V16346, there are at least four

Table 1Cranial measurements for several specimens of Pinacosaurus. Measurements from IMM 96BM3/1 taken from Godefroit et al. (1999). Two skeletons (IVPP 050790-1a and050790-1b) came from Quarry 63 (Fig. 1), but have not been assigned final numbers because they have not been prepared yet. Abbreviations: BM¼ BayanMandahu, BD¼ BaynZag. Locality numbers are shown in Fig. 1. The four specimens described in this paper are in bold. All measurements in mm.

Specimen Locality Max.length

Length fromfont of orbits tofront of skull

Width betweensquamosalhorns

Width betweenquadratojugalhorns

Width betweensupraorbitalhorns

Max width ofpremaxilla

Orbitallength

Orbitalwidth

Max mandiblelength

IVPP V16283,P. grangeri

BM 100/101 157 107 82 72

IVPP V16346,P. grangeri

BM 106 255 130 280 210 108 55 56 190

IVPP V16853,P. grangeri

BM 100/101 130 62 110 126 120 49 33 28 99

IVPP V16854,P. grangeri

BM 100/101 151 86 114 141 114 51 38 28 107

IVPP V16855,P. grangeri

BM 109 109 39 33

ZPAL MgD-II/1,P. grangeri

BD 185 95 210 202 203 90 45 45 135

IMM 96BM3/1,P. mephistocephalus

BM 238 116 308 375 240 83 54 54 210

IVPP 050790-1a BM 160IVPP 050790-1b BM 240IVPP Quarry 101

#7 (A)BM 101 136 125

IVPP Quarry 101#10 (G)

BM 101 129 127

IVPP Quarry 130 BM 130 185

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borings through each premaxilla, making it impossible to deter-mine whether or not the openings connecting with Aperture B arepresent. Aperture C (Fig. 4; sensu Hill et al., 2003) does not havea discernible terminus in any specimen. In IVPP V16283, there arethree anteroposteriorly aligned foramina anteroventral to ApertureB, the posteriormost of which extends into an anteroventrally-directed vascular furrow. There are only two openings in eachpremaxilla of IVPP V16853 and V16346, so the number is individ-ually variable.

The maxillae are present and well preserved in all specimens,although they are incomplete in IVPP V16283, and the left maxillaand left jugal of IVPP V16346 have rotated slightly out of position.Anteriorly, each meets the premaxilla in a butt joint laterally, butcurves anteromedially to invade a depression in the back of thepalatal portion of the premaxilla. The raised alveolar ridge extendsto the front of this anteromedial process, but does not extend ontothe premaxilla. The maxilla forms the posterior rim of the externalnaris, and has a relatively short anterodorsal contact with the nasal.The posteroventrally sloping suture with the lacrimal is pierced bya foramen close to the orbital margin on the right side of both IVPPV16853 (Fig. 2) and V16346. The anterior tapering tip of the jugalinserts between the posterior ends of the maxilla and lacrimal. Thestraight tooth row is medially inset (as is apomorphic for theAnkylosauria), forming a deeply emarginated cheek.

The paired nasals are the largest bones of the skull roof and arevisible in all four specimens. The median sagittal suture betweenthe nasals is not obscured by sculpturing. The naso-frontal sutureextends transversely, anterior to the mid-point of the orbit.A prominent, sharp-edged ridge extends posterolaterally across thenaso-prefrontal suture in both IVPP V16283 and V16853 (Fig. 2).

The prefrontals contribute to the dorsal and lateral walls of theskull roof. The left prefrontal of IVPP V16283 is complete, whereasthe right prefrontal is missing in the posterior half, and both pairedprefrontals are complete on the remaining specimens. They arerectangular in shape, unlike the quadrangular prefrontals ofCedarpelta and the polygonal prefrontals of Minmi .

A complete left and the posterior portions of a right lacrimal ofIVPP V16283 are preserved. Only the right lacrimal is preserved inIVPP V16854 and neither is visible in IVPP V16346. In IVPP V16853,

the region of contact with the prefrontal is damaged on the rightside (Fig. 2C), but is well preserved on the left side. The lacrimalsare square and anterodorsally angled.

5.3. Temporal region

The temporal regions of IVPP V16853 and V16854 are completebut somewhat variable in appearance, suggesting there was a greatdeal of plasticity in the development of this region of the skull. Thetemporal region in IVPP V16283 is less complete but preserves thefrontals, left supraorbitals, partial left postorbital, and left jugal. InIVPP V16346, this area is embedded in matrix and its field jacket soit is not visible except for the jugals.

The paired frontals, separated by a distinct midline suture, areroughly rectangular indorsal view, buthave small anteriorprocesseson either side of the midline that insert between the posterior endsof the nasals (Fig. 2B). In IVPP V16283, the contacts are visibleanteriorly with the nasals, anterolaterally with the left prefrontal,and laterally with a supraorbital. An “ethmoid ossification” or“ethmoideum” (Marya�nska, 1971, 1977) is not visible in any of thespecimens. Note that these bones have no homology to an ethmoidand should be referred to as “extra-sutural bones” (sensu White,2000).

The parietals in some sub-adult P. grangeri specimens (ZPALMgD-II/1) remain paired elements and preserve the posteriorextension of the median sagittal suture. In both IVPP V16283 andV16854 (Fig. 2A) they are fused into one element, as is the case inMinmi, Cedarpelta, P. mephistocephalus, and most other ornithis-chians (Romer, 1956; Sereno, 1991; Vickaryous and Russell, 2003).

The squamosals are preserved only in IVPP V16853, althoughthe left squamosal is heavily bored. This bone contacts the parietaland frontal medially and postorbital laterally. It is flanked by thesupraorbitals anteriorly and by an osteoderm posteriorly, whichforms the characteristic squamosal “horn” of adult ankylosaurids.

Three pairs of supraorbital bones (¼palpebrals of Coombs, 1972)are present in IVPP V16853, V16854, and V16283 (only on theleft side in this specimen). They are somewhat variable in theirshapes both among specimens and from one side of the skull tothe other. The two more lateral supraorbital bones form the dorsal

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and posterior orbital margins. The anterior supraorbital(¼presupraorbital of Marya�nska, 1977) meets the prefrontal ante-riorly and the lacrimal ventrally. The medial supraorbital(¼postfrontal of Marya�nska, 1977) contacts the prefrontal anteri-orly, frontal medially, and postorbital posteriorly. The posteriorsupraorbital (¼postsupraorbital of Marya�nska, 1977) posteriorlycontacts the postorbital on the lateral wall of the skull roof. Thiscomplex of irregularly shaped elements is characterized by orna-mental elaborations. The ornamentation on the anterior andposterior supraorbitals forms a dorsolaterally projected supraor-bital protuberance on each element.

The postorbitals are visible in both IVPP V16853 and V16854,but only a portion of the left postorbital is preserved in IVPPV16283. The dorsal ramus contacts the two supraorbital bonesanteriorly, whereas the lateral ramus forms part of the posteriormargin of the orbit. The suture on the frontoparietal process for thesquamosal is difficult to discern in IVPP V16853, the only specimenfor which it is visible.

IVPP V16853 and V16346 preserve both jugals. Both are weath-ered away in IVPPV16854 and only the left jugal is preserved in IVPPV16283. All of the jugals preserved are unornamented, each formingthe ventral orbital border and suborbital arch. They contact thelacrimal anterodorsally and maxilla anteriorly. The long slopingcontact with the quadratojugal and somewhat interdigitating buttjointwith thepostorbital arewell preserved in IVPPV16853 (Fig. 2C).

5.4. Palatal region

The palatal region of IVPP V16346 (Fig. 6B) is partially obscuredby the right mandible, whereas IVPP V16283 preserves only theanteriormost portion of the paired vomers. This region is not visiblein IVPP V16853 or V16854 due to the presence of articulatedmandibles.

The vomers contact the premaxillae anteriorly, but the largesizes of the choanal recesses separate them from the maxillae.The vomers bisect the region in a median sagittal plane, creating

Fig. 6. Pinacosaurus grangeri, IVPP V16346, skull in dorsal (A, A0) view and, IVPP V16854, skullgrey, whereas borings and other damaged areas are light grey. Anterior is up in A, A0 and to theregion; mo, mandibular osteoderm; oqj, osteoderm covering quadratojugal; pmx, premaxilla

the medial borders of the choanal recesses and forming the verti-cally-oriented nasal septum. The longitudinal intervomeral sutureis visible on the vomerine keel. This keel also extends ventrallybeyond the level of the maxillary tooth row and is rounded inlateral view. The contact of the vomers with the skull roof isobscured by matrix.

Both pterygoids are preserved in IVPP V16346 (they are eitherweathered away or embedded in matrix in the remaining threespecimens) and are taphonomically displaced and the right ptery-goid is somewhat weathered. The anterolateral pterygoid flangeand posterolateral quadrate ramus of the pterygoid are roughlyequal in size.

5.5. Mandible

The mandibles are still in close association in IVPP V16853 andV16854, but only the lateral surfaces can be seen because of themanner in which they have been prepared (Figs. 2 and 6). The rightjaw of IVPP V16346 has twisted medially and underlies the brain-case. All of the bones except the articular are visible in this spec-imen. The predentary lies behind the skull, and the left mandible isincomplete. The partial left mandible in IVPP V16283 preserves thedentary, splenial, angular, anterior portion of the surangular,a fragment of the prearticulars, mandibular osteoderm, andseventeen teeth.

In labial view the dentary of IVPP V16283 contacts the angularventrally and the surangular posteriorly. In lingual view, it ispartially overlain by the splenials and contacts the angular ante-roventrally. The mandibular symphysis and predentary contact arenot well preserved in IVPP V16283 and are obscured by matrix.Several small foramina are visible along the labial alveolar border.This border houses the mandibular tooth row and is displacedmedially and dorsally. The mandibular osteoderm is restricted tothe ventral margin of the angular in this specimen. The dentary inIVPP V16346 is similar to V16283; however, the mandibularosteoderm is more fully developed so that it extends anteriorly

and in situ cervical half-rings in right lateral (B, B0) view. Natural orifices are shaded darkright in B, B0 . Abbreviations: ang, angular; lom, lateral osteodermal mass covering narial; pt, palatine; q, quadrate; suran, surangular; vpt, vomerine process of the palatine.

M.E. Burns et al. / Cretaceous Research 32 (2011) 174e186 181

onto the dentary across the angularedentary contact. In IVPPV16854, the mandible is weathered posteriorly and preserves onlya partial dentary and angular, which match IVPP V16283 inmorphology.

The splenial in IVPP V16283 labioventrally overlays portions ofthe dentary, surangular and angular, displaying a clear ventral buttsuture with the latter. It forms the posterior portion of themandibular sulcus, which is infilled with matrix.

The angular is on the lingual surface of the mandible and on thelabial side contacts the dentary anterodorsally and the surangularposterodorsally in IVPP V16283, V16346, and V16854. A small(1.7 cm long) osseous ornamentation is positioned at the poster-oventral terminus of the preserved portion of the angular in IVPPV16283. An anterior portion of the surangular is also preserved inlabial view on this specimen.

5.6. Teeth

Seventeen teeth (Fig. 7) of IVPP V16283 are preserved: ten in themandibular tooth row, six ventral to this row as replacement teeth,one dissociated from its alveolus, and two dissociated roots withtheir crowns missing. The only remnants of the maxillary teeth inIVPP V16283 are three in situ roots in the anteriormost alveoli of theright maxilla (only a 1.0 mm long portion of the ventrally-directedalveolar ridge is preserved). In IVPP V16853, one mandibular toothand three maxillary teeth are preserved in situ on the right side ofthe skull. IVPP V16346 preserves no teeth but only eight sequentialalveoli on the right maxilla.

In IVPP V16283 the average anteroposterior basal crown lengthis 3.5 mm. Tooth crowns are laterally compressed, each exhibitingthe phylliform shape characteristic of Ornithischia, and show nosigns of wear. Each tooth possesses an apical cusp, seven marginaldenticles (four mesial and three distal relative to the apical cusp),and a swollen base. The denticles are separated by distinct notches.The crown is marked by longitudinal ridges extending ventrally

Fig. 7. The mandibular teeth of juvenile Pinacosaurus grangeri (IVPP V16283): (A) threeconsecutive in situ teeth 6e8 in lingual view (mesial is to the right); (B) isolated dis-placed tooth in labial view; (C) tooth 4 in lingual view (mesial is to the right); (D)replacement tooth 3 in lingual view (mesial is to the right). Tooth counts are frommesial to distal and represent only teeth preserved in situ (i.e., do not correspond toabsolute tooth position in the living animal). Scale bars (lower right of each pane)equal 1 mm.

from the apical notches, disappearing before reaching the base. Thisdenticle count is relatively low for ankylosaurs, which generallyhave 8e17 per tooth (Coombs and Marya�nska, 1990). Eight denti-cles are found in each of the teeth ofMPC 100/1014 (Hill et al., 2003)and ZPAL MgD-II/1, both of which are young individuals but stilllarger than IVPP V16283. It is possible, therefore, that the number ofdenticles represents an ontogenetic trait, increasing as the animalgets older

6. Taphonomic analysis

The holotype specimen of P. grangeri (AMNH 6523, the onlyadult skull published to date), and Pinacosaurus from the Alag Teegbonebed in Mongolia has undergone expanding matrix distortion(VMA, pers. obs.). Expanding matrix distortion (EMD) occurs whensmall fragments of an element are separated bymatrix-filled cracks(White, 2003). The bone remains intact, but the true shape can bedramatically altered as a result of different amounts of crackenlargement. An estimate of the amount of crack-infill can beobtained by measuring linear transects across the specimen(White, 2003); this could be used as a measure of deformation.Unfortunately, there is currently no way of correcting for EMD indistorted specimens (White, 2003). The fracturing and volumetricincrease experienced by vertebrate fossils from a semiarid paleo-environment in Brazil indicates that fossilization probably occurredat a shallow depth, because breakage by crystallization of calcitecould only occur under low lithostatic pressures (Holz and Schultz,1998). The presence of expanding matrix distortion in some Pina-cosaurus specimens may suggest that these specimens werefossilized at relatively shallow depths as well.

To quantify the amount of expanding matrix distortion in theholotype, transects across dorsal and ventral photographs of AMNH6523 were measured using ImageJ (Rasband, 2009). If a scale ispresent in the image, ImageJ can be calibrated to measuredistances, angles, and areas. Evenly spaced lines were overlain onthe dorsal and ventral photographs in Adobe Photoshop to createthe transects. The total length of each transect, and the lengths ofmatrix infill, were measured using the line tool in ImageJ, and thenthe percentage of matrix in each transect was calculated (Tables 2and 3). This provides an estimate of the amount of extra lengthand width that may result from expanding matrix distortion.

Expanding matrix distortion produced an approximately 35%increase in the dimensions of AMNH 6523. Expansion appears tohave been slightly greater in the anteroposterior compared to themediolateral direction, although this difference is not statisticallysignificant (t ¼ 1.57, df ¼ 37, a ¼ 0.05). Although the overallincrease in dimensions is 35%, transects varied between 19 and65%. As such, the shape of the Pinacosaurus adult skull AMNH 6523is highly distorted relative to juvenile specimens, and is artificiallylarger (and possibly longer) because of expanding matrix distor-tion. By comparison, a cast of an undescribed adult skull (UALVP49399, Gaston Design) has a skull length equal to its width.Although deformed, the holotype of P. grangeri provides useabledata on the overall proportions of the adult skull, an essential datapoint when considering the ontogeny of this genus.

7. Phylogenetic analysis

The phylogenetic positions of P. grangeri and P. mephistocephaluswithin Ankylosauria were assessed by a cladistic analysis of 23 taxa(21 ingroup and two outgroup) and 66 characters (Table 4). Cranialcharacters (50) were modified from Hill et al. (2003) and post-cranial and osteoderm characters (16) were modified fromVickaryous et al. (2004) (Appendix 1). Cranial characters werechosen from Hill et al. (2003) as they not only included and revised

Table 2Expanding matrix distortion in dorsal view, showing the average percent of matrixin each transect. All measurements in mm.

Transect Total length Matrix length total Percentage of matrixin each transect.

Length transects1 190 56 2932 260 99 3833 272 146 5354 280 166 5915 314 92 2926 326 161 4957 331 93 2808 334 126 3769 337 119 35110 336 76 22511 306 73 240

Mean 369Standard Deviation 123

Width Transects1 135 32 2372 144 37 2593 147 41 2804 143 28 1925 165 70 4216 190 72 3787 223 60 2708 249 84 3399 249 58 23310 239 111 463

Mean 288Standard Deviation 85

M.E. Burns et al. / Cretaceous Research 32 (2011) 174e186182

some of the characters of Vickaryous et al. (2001), but also incor-porated data from other studies. Outgroup taxa included Huayan-gosaurus and Scelidosaurus. The scorings for Scelidosaurus weretaken from Parsons and Parsons (2009) and postcranial charactersfor Nodocephalosaurus were coded by MEB. All characters weretreated as unordered and of equal weight. Bootstrap values werefound via a heuristic search of 1000 replicates with a randomaddition sequence. For Bremer support analysis, cladistic analyseswere run retaining suboptimal tree lengths to collapse the ingroup.

Table 3Expanding matrix distortion in ventral view, showing the average percent of matrixin each transect. All measurements in mm.

Transect Total length Matrix length total Percentage of matrixin each transect.

Length transects1 37 08 2122 45 10 4303 56 27 4874 60 39 6535 81 15 1876 72 25 3417 73 25 3348 75 36 4759 73 33 45210 68 34 49911 38 14 371

Mean 404Standard deviation 134

Width transects1 32.25 6.57 2042 69 21.7 3143 81.87 24.42 2984 118.94 53.42 4495 130.27 56.08 4306 108.64 44.48 4097 115.46 52.24 452

Mean 365Standard deviation 95

The data matrix was analyzed using PAUP* 4.0b10 (Swofford, 1999)with the tree bisection reconnection (TBR) swapping algorithm and1000 replications. There were 21 most parsimonious trees retainedwith a branch-length score of 153, consistency index of 0.497, andretention index of 0.686. A 50% majority rule consensus tree wasgenerated within a branch and bound search, though a heuristicsearch produced the same tree.

From the resulting tree produced via this analysis (Fig. 8), thefollowing can be determined. The Ankylosauria is well-supportedas monophyletic. Gargoyleosaurus is positioned as a basal ankylo-saur. There is support for a monophyletic Nodosauridae and itsinternal relationships. Pinacosaurus is the most basal member ofthe Ankylosaurinae, and there is strong support for a pairing of thetwo Pinacosaurus species, contrary to Vickaryous et al. (2004).There is also strong support for amonophyletic Ankylosauridae, butthe alternative hypothesis of a third major ankylosaur clade, eitherPolacanthinae or Polacanthidae, is not supported.

8. Discussion

8.1. Taphonomy, taxonomy, and diagnostic revisions

P. grangeri represents the second most common dinosaurspecies at BayanMandahu after Protoceratops andrewsi. The localityis unique because ankylosaurs, especially juveniles and fully artic-ulated skeletons, are a relatively rare dinosaurian component inthe faunas of other localities (Coombs, 1986; Jacobs et al., 1994;Pereda-Suberbiola and Barrett, 1999; Xu et al., 2001). Specimenarticulation, consistent posturing, and their preservation withinstructureless sandstone suggest that the Pinacosaurus from BayanMandahu died in situ during a sandstorm, at least within Quarry101. At Quarry 100, specimens showed no preferred orientation andother evidence of scavenging in the form of associated Velociraptorteeth (Jerzykiewicz et al., 1993). Other possible postmortem alter-ations include borings made by necrophagous insects (Kirklandet al., 1998) and EMD. The former does not greatly affect interpre-tations of anatomy because the insects destroy bones withoutaltering their overall shapes. EMD, on the other hand, can alter thetrue shape of an element and has done so in the holotype skull of P.grangeri according to this analysis; however, none of the BayanMandahu specimens exhibit evidence of this type of taphonomicmodification.

The Campanian-aged deposits at the Bayan Mandahu localitypresent a problem with respect to the generic validity of Pinaco-saurus. The four specimens described herein (IVPP V16346, V16853,V16854, and V16283) were collected between 1988 and 1990, andassigned to P. grangeri (Jerzykiewicz et al., 1993) as Pinacosauruswasmonospecific at that time. The naming of P. mephistocephalus in1999 (Godefroit et al., 1999) from roughly the same locality (Fig. 1)and stratigraphic horizon, complicate this assignment. However,these new specimens have not to date enjoyed thorough descrip-tion or illustration. It is important to know whether one or twospecies of Pinacosaurus are found so close together in geographicalspace and stratigraphic time.

These four new specimens are assigned to Pinacosaurus basedon the presence of two bilaterally symmetrical paranasal aperturesand a posterior embayment of nasal dermal ornamentation dorsalto the external nares and paranasal apertures. They are assigned toP. grangeri based on the presence of weakly-developed pyramidaldermal elaborations of the squamosals, a flat cranial roof posteriorto orbits, and a premaxillary notch. There are several charactersthat have previously been used to differentiate P. mephistocephalusfrom P. grangeri. However, it cannot be excluded that many of thesemay reflect ontogenetic variation. Those that are useful for specific-and generic-level taxonomy are discussed here.

Table 4Character-taxon matrix used for phylogenetic analysis. Cranial and dental characters (1 through 50) are modified from Hill (2005). Postcranial and osteoderm characters (51through 66) are modified from Vickaryous et al. (2004; characters 48 through 63, available as supplementary material from http://dinosauria.ucpress.edu). All characters aretreated as unordered.

Taxon 5 10 15 20 25 30 35 40 45 50 55 60 65

Scelidosaurus 00??? ????? ?00?0 0???0 ??00? ?00?? ???00 00000 00000 0100? 11?10 00000 0000? 0Huayangosaurus 00000 00000 000?0 ?1??? 0?00? 01010 00000 00000 00000 00000 0??00 000?0 1?00? 0Gargoyleosaurus 00000 00100 01001 ?1000 ???1? 1111? 01010 00000 22010 1010? ???11 ????? ????? ?Gastonia 01100 00111 010?1 ?1??1 1??1? ?0100 01010 10000 11000 ????? ???21 ?2?11 11??? 1Gobisaurus 01011 10101 11011 ?1?12 ?0101 1110? 11011 10000 110?0 ????? ????? ????? ????0 1Shamosaurus 01010 ?0021 110?1 ????1 ????? 1110? ??111 10000 1100? 1110? ???21 ?211? ???1? ?Tsagantegia 01111 ?0021 11011 ?1??? ?1?1? 1010? ?1111 10000 11001 ????? ????? ????? ????0 1Talarurus 01?11 ???0? ????1 1???? ????? ????? ?1011 10000 21001 1?11? ?2121 12?12 10111 1Minmi 11??? ????? 010?1 ????? ???0? ?0101 00011 00000 21001 10?0? ????1 ???0? ?0?1 ?P. grangeri 02111 10101 11011 01111 10101 00111 01011 10000 21011 11101 02121 12012 10000 0P. mephistocephalus 02111 10001 11011 01111 10101 00111 01011 10000 23011 11101 ??121 1??11 ???1? 1Tarchia 11111 ?0021 110?1 01112 11101 00101 00011 10011 23000 11101 ??0?1 12?12 1011? ?Saichania 11111 11021 01011 11112 1111? 10101 01111 10011 23001 11101 12121 12?12 10111 1Tianzhenosaurus 11111 00?21 110?1 ?1??? ????? 00100 01111 10011 22111 ????? 0?1?1 1???1 ?0??? 1Nodocephalosaurus ?1?1? ?1??1 ?10?1 11??? ????? ???11 0??11 10001 221?? ????? ????? ????? ????? ?Ankylosaurus 12110 01001 11011 11112 11110 00101 01111 10000 22110 1110? ?11?1 11?1? 10??? ?Euoplocephalus 12111 10001 11011 11112 11110 10101 01111 10000 22011 11101 0?121 11112 101?0 1Pawpawsaurus 00000 00000 011?1 010?0 10111 11111 11010 10100 10100 ????? ????? ????? ????? ?Silvisaurus 00000 0??10 010?1 1?2?0 ?1111 ?11?1 11011 1??00 10??1 1??1? ?1011 ????? ??01? 1Sauropelta 00?10 0???? 01??1 1?2?0 ?1111 11111 11010 11100 10?01 1011? 0?0?1 03001 11?1? ?Panoplosaurus 00010 00000 011?1 1?2?1 ?1111 11111 11011 11100 10101 11101 1??11 0300? 11?10 1E. longiceps 00010 0?000 011?1 1?2?1 ?1111 11111 01010 11000 10101 1011? 1??11 ????? ????? 1E. rugosidens 00010 0?000 011?1 1?2?2 ?1111 11111 01010 11000 10101 1011? 11111 ??1?? ????? 1

Fig. 8. Phylogenetic relationships of Ankylosauria based on a branch and bound searchof 23 taxa and 66 characters (60 parsimony-informative) taken from Vickaryous et al.(2004) and Hill et al. (2003) using PAUP* 4.0b10. Huayangosaurus and Scelidosaurus areoutgroup taxa. From 30248625 tree arrangements, a 50% majority rule consensus treewas generated with a consistency index of 0.497 and retention index of 0.686. Twentytrees retained. Best score tree length 153. Named clades are labelled as follows: A,Ankylosauria; B, Nodosauridae; C, Ankylosauridae; D, Ankylosaurinae. Numbers abovebranch are Bootstrap support values (only values >50% shown); numbers belowbranches are Bremer support values (symbol ‘þ’ represents “more than”).

M.E. Burns et al. / Cretaceous Research 32 (2011) 174e186 183

Although regarded as “indubitable specific characters”(Godefroit et al., 1999:29), the number of nasal apertures in spec-imens assigned to Pinacosaurus is highly variable. All specimensshow external nares and have Aperture B, but the presence ofAperture C is inconsistent. IVPP V16283 exhibits the samearrangement of external nares and paranasal apertures as ZPALMgD-II/1, a juvenile individual of P. grangeri (Marya�nska, 1977, fig.3) and AMNH 6523, the holotype of P. grangeri. In MPC 100/1014(another sub-adult specimen of P. grangeri slightly older than ZPALMgD-II/1) Aperture C is divided into three distinct subsidiaryopenings. In IMM 96BM3/1 (holotype of P. mephistocephalus), it issubdivided into two. Aperture C opens into a relatively largepremaxillary air sinus system (Marya�nska, 1977; Tumanova, 1987;Witmer, 1999; Hill et al., 2003). This combined with the inherentvariability of such systems (Witmer,1997a,b) led Hill et al. (2003) tosuggest that the variation seen in Aperture C is individuallyinconsistent and of little systematic value.

In the description of P. mephistocephalus, the lacrimal ofP. grangeri was described as an “elongate rod of bone” (Godefroitet al., 1999:23), apparently based on ZPAL MgD-II/1. Examinationof ZPAL MgD-II/1 reveals that the lacrimal is in fact of the samemorphology as in P.mephistocephalus and IVPP V16283. Each of thespecimens of Pinacosaurus possesses a relatively square lacrimal(when viewed laterally), although that ofMinmi is described as rod-like, slender, and vertically elongate (Molnar, 1996). In Cedarpelta,the lacrimal is rectangular, and is relatively longer anteroventrally(Carpenter et al., 2001). Therefore, this lacrimal character is notdiagnostic at the species level.

Recent phylogenetic studies on ankylosaurs have not generallybeen performed at the species level (Carpenter, 2001; Hill et al.,2003). An analysis done by Vickaryous et al. (2004) differentiatedP. grangeri from P. mephistocephalus based on a posteriorly-directed(as opposed to posteroventrally-directed) foramen magnum.P. mephistocephalus, however, formed a more deeply nested cladewith Tianzhenosaurus, united by the distal margin of the ilium,which forms a horizontal shelf dorsal to the acetabulum as opposedto partially encircling it laterally. The only autapomorphy found forP. mephistocephalus was a domed cranial roof in lateral profileposterior to orbits.

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The phylogenetic analysis conducted by Hill et al. (2003)returned several character state changes that distinguished Pina-cosaurus from other ankylosaurids. The highest point of the skullroof was coded as occurring anterior to the orbits (Character 2).Whereas this is true for P. grangeri, in P. mephistocephalus the skullis domed posterior to the orbits so that the highest point occurspostorbitally, and this is no longer a valid diagnostic character forthe genus. The morphology of the projecting squamosal dermalossifications (¼“horns”; Character 42), considered polymorphic forPinacosaurus by Hill et al. (2003) and as noted in their reviseddiagnosis of P. grangeri, is informative at the species level. MPC 100/1014 and ZPAL MgD-II/1 (it is unknown in AMNH 6523) haverelatively weakly-developed pyramidal squamosal ornamenta-tions, whereas in IMM 96BM3/1 they are slender and relativelylong. Unfortunately, these elements are not preserved in IVPPV16283.

The revised diagnoses presented in this study include onlyunambiguous autapomorphies for Pinacosaurus and the twoincluded species. Some generic apomorphies from Vickaryous et al.(2004) are incorporated into this revision. The “shallow nasalvestibule” (Vickaryous et al., 2004:389) is subjective and enhancedby the presence of a “dorsal embayment over the narial region”.Also note that the nasal opening and paranasal apertures are notprotected by encompassing external nares as they are in otherankylosaurs. These are herein identified as an apomorphy forPinacosaurus. Characters that separate Pinacosaurus from otherankylosaurines and North American ankylosaurines are taken fromVickaryous et al. (2004) and Hill et al. (2003).

It remains unclear if most of the diagnostic characters listed byGodefroit et al. (1999) for P. mephistocephalus reflect ontogenetic ortaxonomic variation, which leads to confusionwhen assigning newspecimens. They are for that reason omitted in this revision.Characters that are retained here include relatively long andslender dermal elaborations of the squamosals, a domed cranialroof posterior to orbits, and the distal margin of ilium forminga horizontal shelf dorsal to the acetabulum.

8.2. Phylogenetic position of Pinacosaurus

Although it is a relatively well-represented and well-knownankylosaur taxon, there have been different interpretationsregarding the phylogenetic position of Pinacosaurus within theAnkylosauria. It has traditionally been considered a highly derivedankylosaurine (Coombs and Marya�nska, 1990; Kirkland, 1998;Carpenter, 2001). The analysis conducted by Hill et al. (2003)recovered a more basal position for the taxon, between Minmiand Talarurus. Conversely, Vickaryous et al. (2004) found it to bemore derived. The addition of newcharacters and taxa (Parsons andParsons, 2009; Arbour et al., 2009), however, disrupt thisphylogeny, so the position of either species of Pinacosaurus is notunequivocally resolved. Hill et al. (2003) explain the discrepancy bynoting that many characters for Pinacosaurus have been scoredfrom the many well-preserved juvenile specimens (namely ZPALMgD-II/1), not the adult skull of the holotype (AMNH 6523).However, it has yet to be determined whether this approach is notsomewhat valid in light of the extreme taphonomic distortion ofAMNH 6523 that has been demonstrated here.

It is interesting to note that some of the diagnostic characters ofPinacosaurus exhibit affinities to more primitive states within theAnkylosauria and Thyreophora. Advanced ankylosaurids possessa system of palatal shelves that contribute to a complex respiratorychamber within the skull (Witmer and Ridgely, 2009). Pinacosaurushas a flat secondary palate, a character shared with derived nodo-saurids and Gastonia (Coombs and Marya�nska, 1990; Lee, 1996;Carpenter et al., 1998; Kirkland, 1998; Carpenter, 2001; Hill et al.,

2003). Pinacosaurus also shares an interpterygoid vacuity withthe primitive nodosaurid Pawpawsaurus and the ankylosauridGobisaurus (Lee, 1996; Sereno, 1991; Vickaryous et al., 2001; Hillet al., 2003). These two characters also distinguish Pinacosaurusfrom the rest of the Ankylosaurinae.

Several ankylosaurian relationships that result from the phylo-genetic analysis are worthy of note. For example, Shamosaurus isplaced sister to Tsagantegia within the Ankylosaurinae despite thefact that it shares many anatomical affinities with the more basalGobisaurus, which also lived at the same time (AptianeAlbian). Inaddition, the consistently problematic Tianzhenosaurus is placedwithin a North American clade of three anatomically disparate taxa,including Ankylosaurus and Nodocephalosaurus. It is unclear whythere is no stronger attraction between Ankylosaurus and Euoplo-cephalus within this clade, as has been recovered by others(Vickaryous et al., 2004). Whereas Tianzhenosaurus may at firstseem misplaced within a group of otherwise North Americananimals, it is interesting to note that Nodocephalosaurus has manyanatomical affinities traditionally considered characteristic of Asianankylosaurs (Sullivan, 1999). There is perhaps more interactionbetween Asian and North American ankylosaur taxa than previ-ously suggested. At present, this hypothesis is considered morelogical than a strong affinity for Tianzhenosaurus (stronger thanP. grangeri) to P. mephistocephalus.

Confusion in the phylogenetic placement of Pinacosaurus stemsfrom a more general disorder in ankylosaur systematics. This isa product of several factors including lack of consensus amongworkers on character coding and an exclusion of possible post-cranial and osteoderm characters from analyses. The solution tothese problems is a revision and reformulation of traditionalphylogenetic characters coupled with a more vigorous investiga-tion of possible diagnostic characters in the postcrania and osteo-derms of all ankylosaurs. Whether or not the addition of suchcharacters will provide more resolution to this phylogeny remainsto be tested, although evidence exists for both a derived and basalposition within Ankylosaurinae (Carpenter et al., 1998; Kirkland,1998; Hill, 2005).

9. Conclusions

The prevalence of well-preserved juvenile specimens of Pina-cosaurus has added to our understanding of ankylosaur anatomy,taxonomy, and ontogeny. These specimens, however, have alsocreated some potential confusion, as most of what we know aboutthe genus is not based on adult material. The description presentedhere of several such specimens, coupledwith a review of previouslyreferred material allows for a revision of the two known Pinaco-saurus species. An overview of the diagnostic characters for thegenus reveals that Pinacosaurus may possess a more basal positionwithin the Ankylosauridae (as suggested by Hill et al., 2003) thanpreviously thought. A thorough revision of the characters used inankylosaur systematics will hopefully increase the resolution of thephylogeny of the clade as it is currently understood.

Acknowledgements

We thank Zheng Fang (IVPP), Tsogtbaatar (MPC), andMagdalenaBorsuk-Bia1ynicka (ZPAL) for access to specimens at their respectiveinstitutions. IVPP V16853 was skillfully prepared by Kevin Aulen-back, who at that time was employed by the Royal Tyrrell Museumof Palaeontology (Drumheller, Alberta). The other two specimenswere prepared by staff of the Ex Terra Foundation for exhibition inthe “Greatest Show Unearthed”. IVPP 16346 was found by thesecond author, and the concentration of juvenile Pinacosaurus atBayan Mandahu was found by Brian Noble (now at Dalhousie

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University in Halifax, Nova Scotia). Funding toMEBwas provided bythe University of Alberta Department of Biological Sciences and theDinosaur Research Institute; funding to PJC was provided by NSERCand the Alberta Ingenuity Fund; funding to RLS and VMA isprovided by the National Sciences and Engineering ResearchCouncil and (to VMA) the Alberta Ingenuity Fund. Figs. 2, 6A and Bwere done by PJC; all other figures by MEB.

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Appendix 1

Description of characters used in phylogenetic analysis of Ankylosauria and twooutgroup taxa, Scelidosaurus and Huayangosaurus. Cranial and dental characters(1 through 50) are modified from Hill (2005). Postcranial and osteoderm characters(51 through 66) aremodified fromVickaryous et al. (2004; characters 48 through 63).

1. Maximum skull width relative to maximum skull length: less than (0); greaterthan (1).

2. Highest point of skull roof: posterior to orbits (0); dorsal to orbits (1); anteriorto orbits (2).

3. Premaxillary palate wider than long: absent (0); present (1).4. Premaxillary teeth: present (0); absent (1).5. External nares facing anteriorly: absent (0); present (1).6. Accessory openings in the narial region: absent (0); present (1).7. Fused osteoderms present on premaxilla: absent (0); present (1).8. Anterior edge of premaxilla with broad ventrally concave notch in anterior

view absent (0); present (1).9. Ventral margin of premaxilla in lateral view: flat (0); concave (1); convex,

resulting in a sharp premaxillary beak (2).10. Continuous edge formed by the premaxillary beak and maxillary tooth rows:

present (0); absent (1).11. Anteriormost maxillary teeth obscured in lateral view by processes of the

premaxilla: absent (0); present (1).12. Maxillarytoothrowsdeeply inset fromlateraledgeofskull: absent (0);present (1).13. Maxillary tooth rows deeply concave laterally, outlining and hourglass shape:

absent (0); present (1).14. Nasal septum dividing the respiratory passage into two separate bony canals:

absent (0); present (1).15. Closure of antorbital fenestra: absent (0); present (1).16. Accessory antorbital ossification(s) completely separating orbit and antorbital

cavity: absent (0); present (1).17. Median palatal keel composed of the vomer and pterygoid: absent or weakly

developed (0); extending ventrally to the level of themaxillary tooth crowns (1).18. Extension of the vomerine septum: incomplete (0); extending to palatal

shelves (1); extending to skull roof (2).19. Paired premaxillary, maxillary, and nasal sinuses: absent (0); present (1).20. Secondary palate: incomplete or absent (0); present and flat, reaching as far as

the second or third maxillary tooth (1); present and composed of two palatalshelves, describing S-shaped respiratory route (2).

21. Pterygoid foramen: absent (0); present (1).22. Spacebetweenpalate andbraincase (interpterygoidvacuity):open (0); closed (1).23. Dorsoventrallynarrowpterygoids ramusof thequadrate: absent (0); present (1).24. Quadrate shaft angled strongly rostroventrally: absent (0); present (1).25. Quadrate excavated anteriorly: absent (0); present (1).26. Quadrate fused to paroccipital process: absent (0); present (1).27. Paroccipital process projecting posterolaterally: absent (0); present (1).28. Occiput rectangular and wider than high: absent (0); present (1).29. Hemispherical occipital condyle: absent (0); present (1).30. Occipital condyle formedexclusively by thebasioccipital: absent (0); present (1).31. Occipital condyle set off from the ventral braincase by a distinct neck: absent

(0); present (1).

32. Occipital condyle angled ventrally from place of maxillary tooth rows: absent(0); present (1).

33. Occipital condyle and paroccipital processes obscured in dorsal view by over-hanging skull roof: absent (0); present (1).

34. Closure of supratemporal fenestra: absent (0); present (1).35. Lateral temporal fenestra: open (0); closed (1).36. Obliteration of cranial sutures in adults, involving fusion anddermal sculpturing

of the outer surfaces of most of the dermal skull roof: absent (0); present (1).37. Large subcircular dermal ossification covering most of the skull roof between

the orbits: absent (0); present (1).38. Anteroposteriorly narrow dermal ossification along the posterior border of the

skull roof: absent (0); present (1).39. Pair of large, subrectangular osteoderms at posterior edge of skull roof: absent

(0); present (1).40. Raised, polyhedral dermal ossifications on skull roof: absent (0); present (1).41. Secondary dermal ossification, projecting ventrolaterally from the quad-

ratojugal region: absent (0); present and rounded (1); present and wedge-shaped (2).

42. Secondary dermal ossification, projecting posterolaterally from the squamosalregion: absent (0); present as weakly-developed pyramid (1); present asprominent, wedge-shaped or pyramidal structure (2); present as narrow,elongated spines (3).

43. Median dermal ossification overlying dorsum of nasal region: absent (0);present (1).

44. Two pairs of dermal ossifications bordering the external nares: absent (0);present (1).

45. Tooth crown with cingulum: absent (0); present (1).46. Closure of external mandibular fenestra: absent (0); present (1).47. Coronoid process low and rounded, projecting only slightly dorsal to the level

of the dentary tooth row: absent (0); present (1).48. Elongate osteoderm fused to the ventrolateral side of the mandible in adults:

absent (0); present (1).49. Sinuous ventral margin of mandible, which parallels the sinuosity of the dorsal

margin in lateral view: absent (0); present (1).50. Predentary ventral process short (relative to other thyreophorans): absent (0);

present (1).51. Atlas and axis fused to form a syncervical: absent (0); present (1).52. One or more postaxial cervical centra in profile: anterior and posterior ends

parallel and aligned (0); anterior and posterior ends parallel, anterior enddorsal to posterior end (1); anterior and posterior ends parallel, posterior enddorsal to anterior end (2).

53. Fusion of dorsal ribs to centra: absent (0); present (1).54. Multiple parasagittal rows of osteoderms on dorsal surface of neck region:

absent (0); present, fused together (1); present, fused to quarter/half ring (2).55. Multiple parasagittal rows of post cervical osteoderms: absent (0); present (1).56. Tail club: absent (0); present (1).57. Acrominon: absent (0); present, crest at anterior margin (1); present, bladelike

flange perpendicular to long axis (2); present, knoblike process (3).58. Ventral border of coracoid in profile: rounded (0); straight (1).59. Length of deltopectoral crest relative to humerus: less than 50% (0); approxi-

mately equal to or greater than 50% (1).60. Distal margin of ilium: oriented vertically (0); forms a horizontal shelf dorsal to

the acetabulum (1); partially encircles the acetabulum laterally (2).61. Acetabulum: open (0); closed (1).62. Shaft of ischium: little to no curvature (0); pronounced curvature (1).63. Pubis contributiontoacetabulum:one-quarterormore (0); virtuallyexcluded (1).64. Ossified tendons in region of tail: absent (0); present (1).65. Bilateral sternal element contact: not fused (0); fused (1).66. Synsacrum of co-ossified dorsal, sacral and caudal vertebrae: absent (0);

present (1).