A New Ankylosaurid Dinosaur from the Upper Cretaceous (Kirtlandian) of New Mexico with Implications for Ankylosaurid Diversity in the Upper Cretaceous of Western North America

A new ankylosaurid (Ankylosauria: Dinosauria), Ziapelta sanjuanensis, gen. et sp. nov., is based on a complete skull, an incomplete first cervical half ring, a possible fragment of the second cervical half ring, and additional fragmentary osteoderms. The holotype specimen is from the Upper Cretaceous (Upper Campanian, Kirtlandian Land-Vertebrate Age) Kirtland Formation (De-na-zin Member) at Hunter Wash, San Juan Basin, in northwestern New Mexico, USA. Diagnostic characters of Ziapelta include: a large, prominent triangular median nasal caputegulum; a mixture of flat and bulbous frontonasal caputegulae; ventrolaterally oriented squamosal horns with a sharp, prominent dorsal keel; and the ventral surface of basicranium with three prominent anteroposteriorly oriented fossae. A phylogenetic analysis suggests that Ziapelta is not closely related to the other ankylosaurid from the De-na-zin Member, Nodocephalosaurus, but allies it to the northern North American ankylosaurids Ankylosaurus, Anodontosaurus, Euoplocephalus, Dyoplosaurus, and Scolosaurus.


Introduction
The terrestrial deposits in the San Juan Basin of northwestern New Mexico have produced significant specimens of dinosaurs, representing most of the major groups known in the North American Upper Cretaceous [1,2]. Ankylosaurid remains from these beds, largely consisting of isolated osteoderms, vertebrae and some fragmentary appendicular elements, have long been recognized in the Fruitland and Kirtland formations [3,4]. These incomplete specimens were generally referred to North American taxa that are better known from Montana and Alberta [4,5]. More recent discoveries have shown that the ankylosaurids of New Mexico were distinct from those of Montana and Alberta.
Nodocephalosaurus kirtlandensis Sullivan, 1999 [6] was named from the De-na-zin Member of the Kirtland Formation based on a partial skull. Because it was the only diagnosable ankylosaurid known from the De-Na-Zin Member, all ankylosaurid material collected subsequently was referred to that taxon. Other specimens referred to Nodocephalosaurus included isolated cranial osteoderms, a cervical osteoderm, two free caudal vertebrae, and several partial tail club knob osteoderms [7,8].
Glyptodontopelta mimus Ford, 2000 [9] was named based on a fragment of the pelvic shield and other isolated osteoderms from the overlying Maastrichtian Naashoibito Member of the Ojo Alamo Formation in the San Juan Basin. Burns [10] supported the validity of Glyptodontopelta and reclassified it as a nodosaurid, also synonymizing Edmontonia australis Ford, 2000 [9], with Glyptodontopelta mimus. Known only from osteoderms, Glyptodontopelta is currently the only nodosaurid recognized from the San Juan Basin.
More recently, Burns and Sullivan [11] named a small ankylosaurid Ahshislepelta minor, Burns and Sullivan, 2011, from the Hunter Wash Member of the Kirtland Formation. The holotype includes a partial left humerus, right and left partial scapulocoracoids, numerous vertebral fragments, and complete and fragmentary thoracic osteoderms. The taxon is diagnosed on the dorsolateral overhang of the scapular acromion process to 25% of the dorsoventral width of the scapula. In addition, its osteoderm surface texture is characterized by uniformly distributed pitted rugosity, and sparse distribution of reticular neurovascular grooves with neurovascular foramina extending perpendicularly to obliquely into the bone [11,12]. Nodocephalosaurus osteoderms have a more prominent, projecting rugosity [12]. The surface texture of Ahshislepelta osteoderms distinguishes it from contemporaneous ankylosaurids with the exception of some specimens of Euoplocephalus tutus (Lambe, 1902) [13] (sensu stricto; [14]).
In 2011, a team led by R. M. Sullivan from the SMP and NMMNH (see Table 1) collected an ankylosaurid skull and incomplete first and second cervical half rings from the De-na-

Materials and Methods
The specimen (NMMNH P-64484) consists of a complete skull, the left side of the first cervical half ring, fragmentary second cervical half ring, and numerous fragmentary postcranial osteoderms. It was found in 2011 in the Bisti/De-na-zin Wilderness by R. M. Sullivan and was collected by R. M. Sullivan, J. Fry, A. K. Cantrell and T. L. Suazo, and is reposited at the New Mexico Museum of Natural History and Science in Albuquerque, New Mexico, USA.
External variation in morphology among Late Cretaceous ankylosaurid specimens was noted through measurements, obser-vations, and photographs [File S1]. All measurements were taken with digital calipers or flexible measuring tape. The stratigraphic nomenclature and age assignments of the San Juan Basin Upper Cretaceous strata follow Sullivan and Lucas [1]. Nomenclature for ankylosaurid cranial ornamentation follows Arbour and Currie [14].
The phylogenetic position of Ziapelta within the Ankylosauridae was assessed via a cladistic parsimony analysis of 154 characters and 19 taxa [File S1, File S2] including 12 ingroup taxa and the outgroup taxa Lesothosaurus diagnosticus Galton, 1978 [26] (a basal ornithischian), Scelidosaurus harrisonii Owen, 1861 [27] (a basal thyreophoran or basal ankylosaur), Stegosaurus Marsh, 1877 [28] (a stegosaur), Panoplosaurus mirus Lambe, 1919 [29], and Pawpawsaurus campbelli Lee, 1996 [30], (nodosaurid ankylosaurs), and Gastonia burgei Kirkland, 1998 [31] (a basal ankylosaur or basal ankylosaurid). Although Ahshislepelta is also known from New Mexico, it was not included in the phylogenetic analysis because only 12 characters (those related to the scapula and humerus) could potentially be coded. The character matrix [File S2] was updated from Arbour et al. [32]; changes to characters and character codings are noted in [File S1]. All characters were treated as unordered and of equal weight. The data matrix was analyzed using TNT [33], with the tree bisection reconnection (TBR) swapping algorithm and 1000 replications. Bootstrap values (using 1000 replicates) were found via a heuristic search of 1000 replicates with a random addition sequence. An analysis for safe taxonomic reduction was performed using TAXEQ3 [34]. To determine the minimum tree length if Nodocephalosaurus and Ziapelta were constrained as sister taxa, the tree file was manually edited such that Nodocephalosaurus and Ziapelta formed a clade. These taxa were then defined as a group in TNT, and the traditional search was run again with constraints enforced.

Ethics Statement
The specimen was collected on federal/public protected land known as the Bisti/De-na-zin Wilderness Study Area in New Mexico, under the United States Department of the Interior, Bureau of Land Management, Paleontological Resources Use Permit number NM11-004S NLCS BDNZ, issued 13 April 2011 and valid from 1 May 2011 through 1 May 2014. All necessary permits were obtained for the described study, which complied with all relevant regulations.

Nomenclatural Acts
The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix ''http://zoobank.org/''. The LSIDs for this publication are: urn:lsid:zoobank.org:pub: 3401469D-AB4A-4D09-9884-F547215ACD54, urn:lsid:zoobank. org:act:8FC0324E-C18B-4CD6-830B-92C75AA32A58, and urn:lsid:zoobank.org:act:96AA0FCF-D7E4-438B-9B1E-FB9F3B6532F0. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central, LOCKSS.  [56]. Dates with asterisks represented revised dates from Roberts et al. [16]. doi:10.1371/journal.pone.0108804.g001 Diagnosis. Ziapelta is unique among ankylosaurids in having the following features: prominent, large, sub-triangular median nasal caputegulum; dorsoventrally deep squamosal horns curved anteriorly at the tips; three deep fossae on the ventral surface of the basicranium. Differs from other ankylosaurids in possessing a mixture of flat and weakly bulbous frontonasal caputegulae. Differs from Nodocephalosaurus kirtlandensis in the irregular basal shape of the frontonasal caputegulae and in the manner in which the caputegulae are bulbous (conical in Nodocephalosaurus, irregularly convex in Ziapelta), and the shape of the squamosal horns, which have a sharper, more prominent dorsal keel. Distal osteoderm of the cervical half ring does not envelop the terminus of the underlying bony band, unlike the condition in Ankylosaurus magniventris, Anodontosaurus lambei, Euoplocephalus tutus, and Scolosaurus cutleri.

Skull
The skull (Fig. 2) is well preserved and nearly complete, with minimal dorsoventral compression. The ventral surface is fractured, particularly on its anterior half. In dorsal view, the skull (excluding the squamosal and supraorbital horns) has a subrectangular outline. The lateral sides of the snout are parallel in dorsal view, and there is no constriction anterior to the orbit. The widest point of the skull is at the posterior tips of the squamosal horns. In lateral view, the skull has an arched profile, with a convex dorsal surface between the orbits and nares, and a flat dorsal surface between the orbits and nuchal shelf. The left side of the skull is more complete than the right, preserving part of the ventral border of the orbit and a remnant of the left quadratojugal horn.

Rostral Region
In ventral view, the paired premaxillae form a broad, square outline (Fig. 2B), and the anterior margins of the premaxillae are eroded. The boundaries of the maxillae are obscured by cranial ornamentation. The supranarial caputegulae form the dorsal border of the nasal vestibule ( Fig. 2A), which is ovoid in anterior view (Fig. 2C). They are dorsoventrally tallest near the midline of the skull, and are separated at the midline by a 0.9 cm gap. These caputegulae are arched, with the peak of the arch located at about three-quarters of the length of the caputegulum from the median sagittal plane. The supranarial caputegulae have a smooth texture, similar to the other cranial caputegulae.
Both maxillae are preserved, but are fractured and weathered. The maxillae articulate with the premaxillae anteriorly and the ectopterygoids posteriorly. In lateral view, each maxilla is mostly covered by a large, flat, rectangular loreal caputegulum, which extends onto the dorsum of the skull posterior to the supranarial caputegulae (Fig. 2E). Posterior to the loreal caputegulum and anterior to the orbit is the flat, square lacrimal caputegulum, which does not extend as far onto the dorsal surface of the skull as does the loreal caputegulum. The lacrimal caputegulum is anteroposteriorly longer than the loreal caputegulum, with a ventral length of 6.6 cm. The ventral margin of the orbit is formed by the lacrimal and jugal, but this region is damaged in NMMNH P-64484, so the contact between these bones is not visible.
Posterior to the supranarial caputegulae is a relatively large, flat, triangular median nasal caputegulum, which has a maximum width of 11.3 cm and a maximum length along the midline of 9.2 cm ( Fig. 2A). Overall, the pattern of caputegulae is bilaterally symmetrical. A nearly bilaterally symmetrical mosaic of smaller caputegulae covers the nasals and frontals, and obscures the contact between these bones. These frontonasal caputegulae are rectangular to hexagonal and 2.4 to 3.5 cm in diameter. These caputegulae form irregular sub-parasagittal rows. Frontonasal caputegulae towards the anterior and lateral sides of the skull are flat or concave, whereas frontonasal caputegulae along the midline and more posterior areas of the skull are convex. Shallow grooves between the frontonasal caputegulae make each caputegulum distinct from adjacent caputegulae.

Temporal Region
The boundaries between individual caputegulae become less distinct towards the frontoparietal region of the skull, and the boundary between the frontals and parietals is not visible ( Fig. 2A). Caputegulae are most distinct laterally, and less distinct towards the midline of the skull in this region. Two laterally keeled supraorbital caputegulae overhang the orbit ( Fig. 2A). A shallow notch separates the peak of each supraorbital. Both are triangular in dorsal view. The posterior supraorbital caputegulum is sharp and prominent, and the anterior supraorbital is less prominent. The anterior supraorbital is 6 cm long. The dorsal surface of the posterior supraorbital is concave.
Posterior to the supraorbitals, the parietals are concave and have a smooth texture. The nuchal caputegulae are small and bean-shaped and are oriented anteromedially-posterolaterally on each side of the midline of the skull ( Fig. 2A). The medial nuchal caputegulae are smaller than the lateral nuchal caputegulae.

Palatal Region
The ventral surface of the premaxilla forms part of the secondary palate (Fig. 2B). The surface of the premaxillary secondary palate is concave. Two circular foramina are preserved near the anterior edge of the left premaxilla, although the medial foramen is damaged. Corresponding foramina were probably present on the right premaxilla, but these are obscured by fractures on the ventral surface. A median sagittal slit divides the two premaxillae anteriorly. The posterior halves of the premaxillae are fractured on both sides, and there is a semi-circular depression on the ventral surface on the left side probably due to taphonomic compression. The posterior parts of the premaxillae contact the vomers at the midline. The distance between the posteriormost extent of the premaxillary slit to the anteriormost projection of the vomer is 3.5 cm.
The right tooth row is broken at the posterior end, so its total length cannot be measured. There are 18 tooth positions, and there are four poorly preserved teeth (1st, 2nd, 3rd and 5th), and the base of another (9 th ) preserved. The surfaces of all of the teeth are damaged and individual denticles are not visible. All of the alveoli are damaged on the right maxilla. The left maxilla is weathered on the lateral surface. The length of the left maxillary tooth row is 11.9 cm. The buccal emargination is marked by a tear-drop shaped erosional pit lying midway along the surface. The left maxillary tooth row is broken at the anterior end, and only 16 tooth positions are preserved. The posterior end is damaged. There is a partial tooth at what appears to be position 6, and there is a nearly complete tooth at position 9. The latter tooth has a vertical fracture that splits the tooth in half through its apex. The alveoli are damaged throughout the maxillary tooth row, particularly at the posterior end. The medial borders of both the right and left maxillae form the lateral walls of the paired choanae.
The narrow, sheet-like vomer is preserved along its ventral anterior surface for a distance of 6.0 cm. The anterior extension of the vomer joins the premaxillae at their midline. The vomer is largely missing posteriorly, and only fragments of the vomer have been preserved in situ to allow the reconstruction of this element.
The palatines form a triangular surface on each side of the posterior end of the vomer. The extent of the palatines cannot be determined due to breakage of the vomer medially and of the pterygoids posteriorly. The ventral surface of each palatine forms a single triangular fossa. Although damaged, the palatines lack palatal apertures.
Both ectopterygoids are preserved. The left ectopterygoid is bordered anteriorly by a fenestra measuring 1.0 cm at its widest mediolateral extent. The right ectopterygoid is broken medially and along the anterior part where it contacts the right maxilla. The posterior border of both ectopterygoids forms the anteriormost parts of both post-temporal fenestrae. Only the traces of the pterygoids are visible.

Occipital/Basicranial Region
The basicranium is composed of the basioccipital posteriorly and the basisphenoid anteriorly. They are separated by a suture that is visible in both left and right lateral views. The suture is oriented in a posterodorsal to anteroventral direction. The Abbreviations: III, opening for the oculomotor nerve; VII, opening for the facial nerve; V, opening for the trigeminal nerve; IX-X-XI, opening for the glossopharyngeal, vagus, and accessory nerves; XII, opening for the hypoglossal nerve; bas, basisphenoid; bocc, basioccipital; bpt, basipterygoid process; bt, basal tubera; fo, foramen ovale; fos, fossa in basioccipital; oc, occipital condyle. doi:10.1371/journal.pone.0108804.g003 basicranium length is 9.4 cm (Fig. 3). The cranial foramina are best preserved on the right side.
The basioccipital has a reniform condyle. The condyle was originally found broken off of the basioccipital, but it has since been rejoined to the skull. Consequently, the present orientation of the condyle may not reflect the precise life position. The maximum width of the condyle is 5.4 cm. The articular surface of the condyle is smooth and surrounded by a rim. The exoccipitals do not contribute to the occipital condyle. In ventral view, the basioccipital has three prominent fossae, flanked by four well-developed walls (Fig. 3B). The medial fossa bears the basioccipital foramen, located at the posterior margin of the fossa. Anteriorly, the basioccipital joins the basisphenoid. A prominent suture between the two elements occurs both ventrally and laterally on each side.
The basisphenoid contacts the midline of the pterygoids anteriorly and the basioccipital posteriorly (Fig. 2B). It has an isosceles triangular shape ventrally, with an anterior apex. In right lateral view the suture between the basisphenoid and the basioccipital is oriented posterodorsally-anteroventrally and extends posterior to three cranial foramina (III, VII and the foramen ovale) on the right side of the basicranium (Fig. 3B). In addition to these cranial foramina, cranial foramina V, IX-X-XI, and XII are also visible. The foramina of the left side of the basicranium are difficult to determine due to poor preservation. The pattern of cranial foramina appears to be largely consistent with those of other ankylosaurids (e.g. [39,40]), but breaks and suboptimal preservation in this region of the skull make more detailed comparisons difficult.
The exoccipitals are complete but are fractured along their dorsal and distal ends. The distal ends are distinguished by blunt, ventrally directed processes, anterior to which parts of the both the left and right quadrates are fused. The maximum width between the distal ends of the left and right paroccipital processes is 19.0 cm. The supraoccipital lies directly dorsal to the foramen magnum and is fused to the left and right paroccipital processes.
Both the left and right quadrates are largely missing except for the fragmentary portions fused to the distal ends of the left and right paroccipital processes, and to the medial portions of both the quadrate condyles (Fig. 2B, D). The quadratojugals are also missing except for a remnant of the left quadratojugal.

First Cervical Half Ring
The first cervical half ring (Fig. 4A) preserves the medial part, along with the left and right medial osteoderms, left lateral osteoderm, and the medial basal part of the right lateral osteoderm, coossified to the underlying bony band. The anterior edge of the cervical half ring is largely eroded and missing on the left side, with only the posterior edge preserved. All osteoderms of Ziapelta (including cervical and post-cervical) share a common external surface texture characterized by dense, uniform pitting and sparse to no neurovascular grooves. Three of the osteoderms have tall, well-developed keels, although the keels are damaged in places. The bases of the osteoderms are sub-oval in shape. The right medial osteoderm measures 15.5 cm long by 10.4 cm wide and, although the keel is damaged, was at least 9.1 cm tall. The left medial osteoderm has similar proportions, at 15.3 cm long, 7.3 cm wide and 9.5 cm tall. The left lateral osteoderm measures more than 14.6 cm long, 5.8 cm wide, and 10.9 cm tall. Smaller interstitial osteoderms are present between the left and right medial osteoderms. They are distributed irregularly between the basal edges of the left medial and lateral osteoderms. A fractured conical interstitial osteoderm is located along the midline at the anterior edge of the cervical half ring. A partial isolated osteoderm was reconstructed from fragments found near the right side. This appears to be the left distal osteoderm from the first cervical half ring.

Second Cervical Half Ring
A broken ribbon of bone lies adjacent to the ventral surface of the left medial osteoderm of the first cervical half ring, and extends toward the midline (Fig. 4A). This structure is separated by a wedge of matrix between the osteoderm fused on this element and the ventral side of the left lateral osteoderm of the first cervical half ring. We interpret this structure to be the anterior portion of the left lateral side of the second cervical half ring, which must be offset posteriorly. The inner surface of the half ring is broken and fractured. Two osteoderms are fused to this element, one lying adjacent to the left medial osteoderm of the first cervical half ring and the other near the opposite end of the second cervical half ring. Both these osteoderms are poorly preserved, but each has a discernible outline. They both have an apex; the one lying adjacent to the left lateral osteoderm is directed laterally. This osteoderm measures approximately 9 cm apicobasally. The other osteoderm has an oval base, and the apex is located more toward the midline. It measures 8.6 cm long and 4.8 cm wide. The height of the apex is 2.4 cm. Both osteoderms are situated along the anterior edge of the second cervical half ring. The medial osteoderm has an oval base and a straight anteroposterior keel; although the top of the keel is broken, it does not appear that it had a prominent apex. The lateral osteoderm also has a narrower oval base compared to the medial osteoderm, and has a sigmoidal longitudinal keel. The distal osteoderm has the narrowest base and is dorsoventrally compressed. The keel has a central apex, giving this osteoderm a triangular outline in dorsal view. The dorsal surface of the distal osteoderm is slightly convex. The keels of all osteoderms are directed anteromedially-posterolaterally, with the distal edges of the keels curving dorsomedially. The basal bony band is wider between the paired medial osteoderms than it is between the medial and lateral or lateral and distal osteoderms. Interstitial osteoderms (sensu Arbour and Currie [14]) are present between the medial osteoderms, but between the medial and lateral, or lateral and distal osteoderms.

Postcervical Osteoderms
Two isolated osteoderms were found lying adjacent to the cervical half rings (Fig. 4A), one between the right side of the skull and anterior border of the first cervical half ring. We identify this as a displaced pectoral osteoderm. The other osteoderm is incomplete but had a relatively large base. This osteoderm also has a well-developed sigmoidal keel and may represent a left lateral osteoderm from the second cervical half ring. This osteoderm is 16.9 cm long with a maximum keel height of 10.3 cm.
A single isolated osteoderm was found posterior and adjacent to the basal surface of the posterior border of the first cervical half ring (Fig. 4A). It is 11.8 cm long and 6.8 cm wide. The keel rises 5 cm to an apex and declines in height 7 cm anteroposteriorly. This osteoderm is compressed and has a sigmoidal keel, which creates a concave dorsal surface on the osteoderm. It is further distinguished by a prominent groove that encircles the entire osteoderm, and is situated about 1 cm from the periphery. We interpret this as a right thoracic osteoderm that was displaced prior to burial.
Lastly, a number of smaller, sub-circular osteoderms were recovered in association with the first and second cervical half rings. Most are relatively complete; three are conical, whereas a few are flat. All have a pitted surface texture. The smallest of these osteoderms measures 2 cm across the width of its base, and the largest measures approximately 4 cm. These may be loose interstitial osteoderms from the cervical half rings or may be from a more posterior body region.

Referred specimen NMMNH P-66930
An isolated complete first cervical half ring, from the upper part of the Hunter Wash Member of the Kirtland Formation, is referable to Ziapelta sanjuanensis based on the presence of interstitial osteoderms and the morphology of the major osteoderms coossified to the band (Fig. 4C). This cervical half ring shares several of the unique features found in the holotype of Ziapelta, including proportionately tall and narrow osteoderms compared to those of Anodontosaurus, Euoplocephalus and Scolosaurus, interstitial osteoderms like in Anodontosaurus but unlike in Euoplocephalus, and a distal osteoderm that does not envelop the terminus of the band as completely as does the distal osteoderm in Anodontosaurus, Euoplocephalus, and Scolosaurus.

Comparisons with other ankylosaurids
Three ankylosaurids have been recovered from the Kirtland Formation: Nodocephalosaurus and Ziapelta from the De-na-zin Member, and Ahshislepelta from the stratigraphically lower Hunter Wash Member. Ahshislepelta and Ziapelta cannot be directly compared because there little overlapping material -Ahshislepelta is known only from osteoderms and axial and appendicular material, whereas Ziapelta is known only from a skull and cervical half rings. Although their external surface textures are similar, consisting of uniform pitting with a sparse pattern of neurovascular grooves, the osteoderms of Ahshislepelta are characteristically smoother [11] than either Nodocephalosaurus [12] or Ziapelta.
Numerous features differentiate Ziapelta from Nodocephalosaurus, the other ankylosaurid known from the De-na-zin Member of the Kirtland Formation. The holotype of Nodocephalosaurus kirtlandensis is a laterally crushed, asymmetric, partial skull [6]. The frontonasal caputegulae of Nodocephalosaurus have circular bases, whereas these caputegulae on Ziapelta have square, rectangular, or hexagonal bases (Fig. 5). The frontonasal caputegulae in Nodocephalosaurus are conical, and none are flat. In Ziapelta, these caputegulae can be convex, but many are flat or even concave. The grooves between the caputegulae are more prominent in Ziapelta than in Nodocephalosaurus, which makes them more distinct. These differences are unlikely to represent ontogenetic changes in a single taxon. In Pinacosaurus grangeri Gilmore, 1933 [41], the extent of the frontonasal ornamentation changes from juveniles [42,43], to adults (AMNH 6526), but the form of the ornamentation does not appear to change substantially. No small juvenile skulls are yet known for Anodontosaurus or Euoplocephalus, but in the relatively large sample sizes for these taxa [14], which probably include at least some differences in ontogenetic stage, the basal shape of the frontonasal caputegulae does not include the range of variation represented by the holotypes of Nodocephalosaurus and Ziapelta.
In Nodocephalosaurus, the supraorbital caputegulae have distinct peaks, but these are less sharp than the supraorbital caputegulae in Ziapelta. Nodocephalosaurus has a prominent, bulbous loreal caputegulum (maxillary osteoderm sensu Sullivan [6]) that extends onto the dorsal surface of the skull. This caputegulum in Ziapelta is anteroposteriorly broader, and is less convex on its lateral surface and flat on its dorsal surface. In Nodocephalosaurus, the squamosal horns are curved ventrally, but in Ziapelta the squamosal horns are curved anteriorly. Also, the squamosal horns of Ziapelta have a sharper, more prominent dorsal keel compared to those of Nodocephalosaurus.
The cranial caputegulum pattern of Ziapelta is most similar to that of Ankylosaurus, Anodontosaurus, Dyoplosaurus, Euoploce-phalus, and Scolosaurus from Montana and Alberta ( [14]: Fig. 5). Like these taxa, Ziapelta has flat, irregularly shaped cranial caputegulae separated by distinct grooves. Tsagantegia longicranialis Tumanova, 1993 [44], from the Bayanshiree Formation of Mongolia, also has flat cranial caputegulae, but the skull differs in many ways from that of Ziapelta; for example, the skull of Tsagantegia is longer than wide, the squamosal horns are shorter and rounder, and the cranial caputegulae are larger and more rectangular. The presence of convex frontonasal caputegulae distinguishes Ziapelta from Ankylosaurus, Anodontosaurus, Euoplocephalus, and Scolosaurus, and a mixture of convex, flat, and concave frontonasal caputegulae is unknown in any other ankylosaurid. The arrangement of frontonasal caputegulae in Ziapelta is more bilaterally symmetrical than in any other North American ankylosaurid taxon.
The supranarial caputegulae of Ziapelta are similar in shape to those of Euoplocephalus and Anodontosaurus. In many Euoplocephalus skulls (e.g. AMNH 5337, AMNH 5405), the supranarial caputegulae are rugose and pitted and have an irregular surface texture, which differs from the texture of the other cranial caputegulae. In contrast, the supranarial caputegulae of Ziapelta are smooth, having a similar texture compared to the other cranial caputegulae.
Ziapelta shares with Ankylosaurus, Anodontosaurus, and Euoplocephalus a large median nasal caputegulum, a feature that is absent in the Asian ankylosaurids. However, in Ankylosaurus, Euoplocephalus and Anodontosaurus, the median nasal caputegulum is hexagonal, not triangular (Fig. 5). The median nasal caputegulum occupies a greater proportion of the snout width in Ziapelta compared to Euoplocephalus and Anodontosaurus (50% in Ziapelta versus about 40% in Euoplocephalus and Anodontosaurus). In Euoplocephalus and Anodontosaurus, there is always a pair of long, horizontally-oriented caputegulae (postnarial caputegulae) between the supranarial caputegulae and median nasal caputegulum, which are absent in Ziapelta.
The taxonomic significance of squamosal horn shape in ankylosaurids has been controversial [6,[45][46][47]. Squamosal horn shape can vary within Euoplocephalus [14]. In dorsal view, squamosal horns can be triangular and pointed (or with a pit at the apex), as in UALVP 31, can be blunt triangles (e.g., AMNH 5405, ROM 1930), or can be nearly indistinguishable from the skull and simply form a rugose posterior corner of the skull (e.g., AMNH 5404, AMNH 5337). These differences could represent ontogenetic changes in Euoplocephalus; however, the overall triangular shape is present in all but the largest specimens.
The dorsoventral orientation at which the squamosal horns project from the skull can also vary, but Arbour and Currie [48] suggest that much of this variation can be attributed to dorsoventral compaction during burial and fossilization. Aspects of the squamosal horns that appear to be taxonomically significant include the angle of lateral projection from the skull, curvature of the horn, and overall shape (narrow cylindrical versus pyramidal). Ziapelta has unique squamosal horns that are curved anteriorly at the tips, a morphology not observed in any other ankylosaurid. Scolosaurus has backswept squamosal horns with ventrally curved tips (e.g. USNM 11892; [14]), and Nodocephalosaurus may have slightly ventrally curved tips to the squamosal horns, although this may be the result of plastic deformation in the only known skull. In Anodontosaurus, Ankylosaurus, and Euoplocephalus, the squamosal horns are not lateroventrally ventrally-oriented, nor are they curved in the Asian ankylosaurids [32].
The skull of Ziapelta is not constricted anterior to the orbits as seen in Tarchia kielanae and Pinacosaurus grangeri [32]. Although some of the frontonasal caputegulae of Ziapelta are bulbous, they differ from the tall, pyramidal, sharp-edged caputegulae of Tarchia and Saichania chulsanensis Maryańska, 1977 [39]. In lateral view, the antorbital region of the skull roof is convex in Ziapelta, which is similar to the condition in Anodontosaurus, Euoplocephalus, and Ankylosaurus, but unlike the condition in the Asian ankylosaurids.
Ziapelta appears to lack palatal apertures in the palatine bones, which are present in Anodontosaurus and Euoplocephalus ( [14]: fig. 6). The braincase also differs from those of Anodontosaurus and Euoplocephalus. Ziapelta has a deep longitudinal groove along the ventral surface of the basioccipital, and paired fossae on the lateral sides of the basioccipital. These are absent in Anodontosaurus and Euoplocephalus, and are not likely to be the result of taphonomic compression of NMMNH P-64484, because the fossae are not present even in severely dorsoventrally compressed specimens of Euoplocephalus such as AMNH 5403.
The cervical half rings are most similar to those of Anodontosaurus and Euoplocephalus. The medial osteoderm has a central longitudinal keel, the lateral osteoderm has a sigmoidal keel, and the distal osteoderm is transversely compressed. The cervical half ring osteoderms of Ziapelta have more rectangular bases compared to Ankylosaurus, Anodontosaurus, and Euoplocephalus, in which the osteoderms are more elliptical and transversely broader. Scolosaurus has low circular medial osteoderms with central apices on the first and second cervical half rings, unlike the condition in Ziapelta. An unnamed ankylosaurid (UMNH VP 20202, from the Kaiparowits Formation of Utah, [49]) also has low, indistinct (or absent) medial osteoderms on the first cervical half ring. Ziapelta has interstitial osteoderms (two morphotypes) between the major osteoderms of the first cervical half ring, which are absent in Euoplocephalus but are present in Anodontosaurus. The interstitial osteoderms of Anodontosaurus tend to be more circular or square, whereas the interstitial osteoderms of Ziapelta are either in the form of a single conical osteoderm, or flat clusters in a sub-trapezoidal shape. The distal osteoderms of the cervical half ring in Ziapelta barely overlap the terminus of the band, which differs from the condition in Ankylosaurus, Anodontosaurus, Euoplocephalus, and Scolosaurus, in which the distal osteoderms entirely envelop the terminus of the band [14].
At present, Ziapelta is not easily compared to any of the ankylosaurid material described from the Kaiparowits Formation of Utah [49]. UMNH VP 19473 includes a cervical half ring, vertebrae, and appendicular elements [49]. Based on the figured material, the cervical half ring may be a second cervical half ring missing the medial osteoderms and preserving the lateral osteoderms. Unfortunately, the poor preservation of the second cervical half ring in Ziapelta precludes comparison with UMNH VP 19473. UMNH VP 20202 is a partial skeleton that includes a skull, mandibles, both cervical half rings, vertebrae (including the tail club), and appendicular elements [49]. This specimen has not yet been described or figured in detail, and the skull has not been figured, so comparisons with Ziapelta are not possible at present. Loewen et al. [49] also reference an additional undescribed skull from the Kaiparowits Formation that may be distinct from UMNH VP 20202.

Results of the phylogenetic analysis
Five equally most parsimonious trees were recovered, with a branch-length score of 267 hit 424 times out of 1000, a consistency index of 0.633, and a retention index of 0.722 (Fig. 6). In all trees, Pinacosaurus was monophyletic, and a clade containing Saichania, Tarchia, and a new Mongolian ankylosaurid was recovered. Four of the trees include a clade of North American ankylosaurids (Ankylosaurus, Anodontosaurus, Euoplocephalus, and Scolosaurus), three of which also include Ziapelta, although the relationships among the North American ankylosaurids are somewhat labile. Ziapelta is the sister taxon of Scolosaurus in three of the five trees. The relationships of Tsagantegia, Talarurus plicatospineus Maleev, 1952 [50], and Nodocephalosaurus relative to the other taxa in this analysis are most variable, but an analysis of the dataset in TAXEQ3 showed that none of these taxa could be safely removed. Broader taxon sampling in future analyses may help stabilize the position of these ankylosaurids. Nevertheless, Ziapelta appears to be most closely allied with the North American taxa Ankylosaurus, Anodontosaurus, Euoplocephalus and Scolosaurus, and does not appear to be closely related to the North American species Nodocephalosaurus, which is recovered in various Mongolian clades in four of the five trees.
When Nodocephalosaurus and Ziapelta were constrained as sister-taxa, the minimum tree length was 268, an increase of only one step compared to when Nodocephalosaurus and Ziapelta are not constrained. In the resulting 8 most parsimonious trees, Nodocephalosaurus and Ziapelta were never recovered in derived clades containing the North American ankylosaurids.

Paleobiogeography and biostratigraphy
Ziapelta (Fig. 7) provides important new information for understanding the paleobiogeography of ankylosaurid dinosaurs. Sullivan [6] considered Nodocephalosaurus to be closely related to the Mongolian ankylosaurids Saichania and Tarchia, although this was based only on the presence of bulbous caputegulae on the skull, and not the result of a phylogenetic analysis. The presence of an ankylosaurid closely related to Mongolian taxa in the late Campanian of New Mexico in turn suggested some palaeogeographic connection between Asia and western North America at or somewhat before that time [6]. Thompson et al. [51] recovered the North American Dyoplosaurus acutosquameus as the sister taxon to the Chinese Pinacosaurus mephistocephalus Godefroit, Pereda-Suberbiola, Li, and Dong, 1999 [52] from the Bayan Mandahu Formation (Campanian), Inner Mongolia, People's Republic of China, which at first may seem to provide support for an intercontinental exchange of ankylosaurids. However, Arbour and Currie [14] found the characters uniting these two taxa to represent inaccurate character codings: revising the codings of several characters, including the ''doming'' of the parietal surface, resulted in a loss of the close relationship between Dyoplosaurus acutosquameus and Pinacosaurus mephistocephalus. Although Ziapelta and Nodocephalosaurus occur in the same formation, Ziapelta is more closely related to northern North American ankylosaurids than to Nodocephalosaurus.
Ziapelta shares several features with Ankylosaurus, Anodontosaurus, Euoplocephalus, and Scolosaurus, such as the flat, square-to-hexagonal based cranial caputegulae and a convex antorbital region of the skull roof. However, it also has some bulbous, convex cranial caputegulae, which are otherwise known only in Nodocephalosaurus and the derived Asian ankylosaurids. Based on the results of the phylogenetic analysis, bulbous cranial caputegulae appear to have evolved independently in Nodocephalosaurus and Ziapelta. Additional taxon sampling of Early Cretaceous Asian ankylosaurids may help resolve the relationships of Nodocephalosaurus and the Campanian-Maastrichtian Asian ankylosaurids, and to address questions about ankylosaurid dispersal between North America and Asia.
Does the presence of Ziapelta in the southern portion of Laramidia support current hypotheses of distinct southern and northern North American dinosaur faunas (e.g. [53,54])? Ziapelta is not known from Montana or Alberta, and Anodontosaurus, Euoplocephalus, Dyoplosaurus, and Scolosaurus have not been recovered south of Montana [14]. However, identifying northern and southern faunal provinces requires that the representative faunas are coeval. The Dinosaur Park Formation, upper portions of the Judith River and Two Medicine formations, and Kaiparowits Formation overlap in time, but the Kirtland Formation represents a younger time interval [1,16,55]. The holotype of Ziapelta sanjuanensis was collected from the De-nazin Member of the Kirtland Formation, approximately 12 m below Ash H, which has recently been recalibrated to 72.9860.18 Ma by Roberts et al. [16]. In Alberta, the equivalent time is represented by the lower portion of the Drumheller Member of the Horseshoe Canyon Formation [56]. No identifiable ankylosaurids have been recovered from this part of the Horseshoe Canyon Formation (Fig. 1); Anodontosaurus is known from a few specimens in the upper part of the Dinosaur Park Formation (,75 Ma), and from the Horsethief Member (71.5 Ma [56]), Morrin Member, and Tolman Member of the Horseshoe Canyon Formation [14]. Dyoplosaurus, Euoplocephalus, and Scolosaurus occur in older sediments than Ziapelta, and Ankylosaurus is known from younger sediments. As a result, the presence of Ziapelta in the Kirtland Formation, outside of the Kaiparowits-Dinosaur Park-upper Two Medicine taphozone (sensu Roberts et al. [16]), cannot be used to support hypotheses of dinosaur provincialism at this time; additional diagnostic ankylosaurid specimens from the Drumheller Member of the Horseshoe Canyon Formation, or other time-equivalent strata, are needed to clarify the paleobiogeography of Campanian-Maastrichtian ankylosaurids from Laramidia.
Since 1999, Nodocephalosaurus has been considered the only ankylosaurid from the De-na-zin Member of the Kirtland Formation. The small Ahshislepelta minor is known only from the holotype from the slightly older Hunter Wash Member [11]. The nodosaurid Glyptodontopelta had been considered restricted to the Maastrichtian Naashoibito Member of the Ojo Alamo Formation [18], and a taxon characteristic of the Alamo Wash local fauna [2,9,10]. The diversity of ankylosaurids in the San Juan Basin appears to be higher than that of nodosaurids based on the specimens currently known, and all of the named ankylosaurid taxa are characteristic of the Kirtlandian Land Vertebrate Age [1,55]. Nodocephalosaurus may be a taxon unique to the Willow Wash local fauna, and Ahshislepelta unique to the older Hunter Wash local fauna.

Conclusions
Ziapelta sanjuanensis, gen. et sp. nov., is a new anklyosaurid based on a well-preserved skull, incomplete first and second cervical half rings and isolated body and cervical half ring osteoderms. It was found low in the De-na-zin Member of the Kirtland Formation, stratigraphically below, and close to, the holotype locality of Nodocephalosaurus kirtlandensis, yet it appears to be more closely related to Scolosaurus from the chronostratigraphically older Dinosaur Park Formation of Alberta, than to Nodocephalosaurus, based on three of the five most parsimonious trees. Additional character and taxon sampling are needed in order to further resolve ankylosaurid relationships, but our analysis suggests that the two ankylosaurids from the De-na-zin Member of the Kirtland Formation are not most closely related to each other.

Supporting Information
File S1 Specimens examined and character definition statements.