Zuul is estimated to have been long. The describing authors of
Zuul,
Victoria Arbour and
David Evans, noted some distinguishing traits of
Zuul. Some of these were
autapomorphies, unique characters which set
Zuul apart from all other known ankylosaurids. The
caputegulae, armour tiles of the head, that lay on the nasal bones, the frontals and the parietals, are imbricated, overlapping, and pointed on top. The squamosal horns, on the rear corners of the skull roof, have conspicuous longitudinal grooves on their side surfaces. The osteoderms on the side of the tail, the knob itself excepted, have a front edge with a strongly hollow profile, while their points are off-set to the rear. The tail knob is vertically flattened with a height less than a fifth of the length. Other traits were not unique but distinguished
Zuul from various relatives in the
Ankylosaurinae. The
caputegulae on the prefrontals, the frontoparietals and the middle supraorbitals are pyramid-shaped, in contrast to the conical
caputegulae with
Nodocephalosaurus and
Talarurus. The squamosal horn protrudes to behind the rear edge of the skull roof, just as with
Scolosaurus but different from
Anodontosaurus,
Euoplocephalus or
Ziapelta. The
caputegulae behind the eye socket are small and sparsely distributed, again like
Scolosaurus but differing from
Anodontosaurus,
Euoplocephalus or
Ziapelta. The osteoderms on the handle of the tail club are relatively larger and more pointed than those of Asian ankylosaurines of the
Nemegt Formation of
Mongolia.
Skull With a length of about , the skull of
Zuul is of considerable size, being only surpassed among the ankylosaurs of
Laramidia (western North America) by the three known skulls of
Ankylosaurus, specimens AMNH 5214, AMNH 5895 and CMN 8880. The skull is also rather flat but this is partly caused by compression. The snout is wide and truncated at the front. The bony nostrils are pointing to the front. In each nostril only a single nasal opening is visible, perhaps homologous to "Aperture A" in related species. Behind a narrow pair of nasal armour plates, rows of
caputegulae stretch to the rear. The two front rows consists of pairs of rectangular plates. Part of the third row is a small central hexagonal
caputegula. More to the back, the osteoderms all become square or hexagonal in profile and more strongly imbricating. Above the eye socket the front and rear supraorbitals form a sharp edge, protruding sideways. Deep in each eye socket, a bony plate is present. Similar plates in specimens of
Euoplocephalus and
Dyoplosaurus were reported by
Walter Preston Coombs as bony eyelids. The squamosal horns on the rear corners of the skull roof are robust and pyramidal in shape. They have a sharp keel on top and deep grooves running towards the tip. The quadratojugal horns on the lower cheeks are also robust, with a convex front edge and a straight rear edge. They were seen as being mainly outgrowths of the
quadratojugal bones themselves. The lower jaw is long and low. At its rear side, a large armour plate is present. According to Arbour and Evans, this is not a fused osteoderm, but instead an outgrowth of the jaw bones themselves. The adductor fossa, the opening through which the muscles closing the jaw entered its hollow inside, is relatively small and shallow. The dentary, the bone bearing the teeth, at the front curves sharply to the inside. This way both dentaries together had a profile fitting in the broad snout. The tooth row has a length of about . At the rear underside of the right lower jaw numerous ossicles, small bony scales, are visible. The largest of these are rectangular and up to high. They lie directly below the large armour plate. Even lower, rows of smaller hexagonal or diamond-shaped ossicles are positioned, of about in diameter, grouped in rosettes. The teeth are placed in tooth rows which together have a fluted profile. The tooth rows of the upper jaws bear about eighteen to twenty teeth. Those of the lower jaws bear twenty-eight teeth. The teeth are small, leaf-shaped and transversely flattened. The front teeth of the dentary have a base width, measured from the front to the rear, of about . Their crown height is about . The teeth have in total twelve to fourteen cusps on their edges. The cusp forming the tip of the tooth is off-set to behind. Around the neck of the crown, a thickened
cingulum, or shelf, is present.
Tail Of the remainder of the skeleton only the tail had been prepared in 2017. The tail has a total length of . The tail is divided into "free" front caudal vertebrae of the base and a "tail club" at the rear. In the latter organ Coombs distinguished again a "handle" and a "knob". The tail club, including the handle, has a length of , a record among North American ankylosaurids. In the handle, thirteen vertebrae are present. Probably another three are covered by the knob. To increase the chance of a damaging
plastic deformation on impact of the knob, the handle is a stiff structure, the lack of flexibility caused by special connections between the vertebrae. The paired front joint processes, the
prezygapophyses, are strongly elongated and overlap half of the preceding vertebra. They form a V-like structure, the branches angled at about 20°. Their joint facets are rotated to above and embrace the
neural spine of the preceding vertebra. This spine is wedge-shaped and bent to behind with a flat top surface to fit into the V. This way a series of interlocking connections is formed, covering the entire top surface of the handle. The handle is further stiffened by bundles of ossified tendons, closely appressed to the vertebral sides. The tendons are over long and in diameter, with tapering ends. Along the sides of the handle runs a series of five pairs of osteoderms.
Zuul is the first American ankylosaurid in which such handle osteoderms have actually been discovered; they had only been assumed for other species. On the free vertebrae of the tail base another three pairs are present. Between the large osteoderms smaller ossicles are positioned. The side tail osteoderms are flattened and have a triangular profile in top view, with a sharp point. Those more at the front have a sharper point, being longer than wide. They are almost shaped as an
equilateral triangle with straight front and rear edges. More to behind the triangles are lower and wider. From the fifth pair onwards the front edges become longer and strongly concave, off-setting the point to the rear. The last pair, immediately before the knob, is strongly rounded. The front three side osteoderm pairs are covered by a black film that could be the remnant of their original keratin sheaths. If so, these added to their length. The sheaths show clear grooves and ridges directed towards the point. At the left osteoderm of the third pair, a layered structure is visible at its base, resembling the construction of horn bases in
bovids. At the rear osteoderms, the film covering is incomplete, showing that the underlying bone structure does not conform to the sheath riles as it is much smoother. The knob is relatively large with a length of , a width of , and a vertical height of . The bulk of the knob is formed by a pair of large osteoderms, each positioned at the side. In the holotype individual, the left side osteoderm is distinctively longer than the right one, giving the knob as whole a rather asymmetrical profile. This osteoderm also overlaps the last left triangular spike. The general profile of the knob is oval as spikes or keels are lacking. The side osteoderms almost touch each other at the top surface; at the underside a midline hiatus is present. At the rear of the knob, a cluster of small osteoderms is positioned. The outer corners of this cluster are formed by two trapezium-shaped elements. The very rear is formed by a covering triangular midline osteoderm. As a whole, the rear cluster has a straight posterior edge in top view. The bone tissue of the knob has a sponge-like texture with many pits. Apart from these bony structures, the tail also preserves non-bony scales. These are not skin impressions but remains of the keratin skin tissue itself. Such fossil scale remains are exceedingly rare. In between the third pair of tail osteoderms a transverse row of five large scales is present. Behind it a further row is visible, running more to below, and containing two scales. The scales are relatively large with a diameter of between . In cross-section, they have the form of a truncated cone with a rounded top pointing somewhat to behind. More dispersed scales are visible at the base of the fourth osteoderm pair and the front base of the fifth pair. ==Classification==