Akainacephalus (meaning "thorn head") is a monospecific genus of ankylosaurid dinosaur from southern Utah that lived during the Late Cretaceous (late Campanian, 76.26 Ma) in what is now the Horse Mountain Gryposaur Quarry of the Kaiparowits Formation. The type and only species, Akainacephalus johnsoni, is known from the most complete ankylosaur specimen ever discovered from southern Laramidia, including a complete skull, tail club, a number of osteoderms, limb elements and part of its pelvis, among other remains. It was described in 2018 by Jelle P. Wiersma and Randall B. Irmis. It is closely related and shares similar cranial anatomy to Nodocephalosaurus.
Discovery and naming
An almost complete skeleton of an ankylosaurid was excavated during the 2008, 2009, and 2010 field seasons from the Horse Mountain Gryposaur (HMG) Quarry in the Grand Staircase–Escalante National Monument, Kane County, Utah. The Horse Mountain Gryposaur Quarry represents a multitaxic and multidominant bonebed that has produced a mostly complete, partially articulated skeleton of the hadrosaurid Gryposaurus, the holotype of the baenid turtle Arvinachelys, an articulated skeleton of a small alligatoroid, and a partial skull of a small theropod. The bonebed is deposited in a fine- to medium-grained sandstone crevasse splay that is located within the lower section of the middle unit of the Kaiparowits Formation. The age of the layer is 76.26 ± 0.10 million years based on zircon dating. In 2014, the skull was subjected to a CT scan in order to reveal the internal anatomy. The specimen was subsequently described in 2018 by Jelle P. Wiersma and Randall B. Irmis.
The holotype specimen, UMNH VP 20202, consists of a skull, both mandibles, predentary, dorsal vertebrae, dorsosacral vertebrae, sacral vertebrae, caudosacral vertebra, caudal vertebrae, dorsal ribs, a complete tail club, both scapulae, coracoid, humerus, ulna, partial ilium, femur, tibia, fibula, phalanx, partial cervical osteoderm half rings, and dorsal and lateral osteoderms of various sizes and morphologies. It represents about 45% of the skeletal elements and is part of the collection of the Natural History Museum of Utah in Salt Lake City.
The generic name, Akainacephalus, is derived from the Greek words "akaina" (thorn or spine) and "kephalè" (head), in reference to the thornlike cranial caputegulae of the holotype skull. The specific name, johnsoni, honours Randy Johnson, a volunteer preparator at the Natural History Museum of Utah, who prepared the skull and lower jaws of the holotype.
Description Distinguishing traits
The describing authors indicated several distinguishing traits. Some of these are autapomorphies, unique derived characters. The supraorbital bosses are massive in side view, forming a high back-swept ridge, also extending sideways over the eye socket, while encompassing the front top corner and the rear edge of the eye socket. The cheek horns are triangular, pointing almost vertically to below. On the frontal bones, a central large flat hexagonal osteoderm is present. The zone spanning the frontal bones and nasal bones is covered by symmetrically positioned, closely packed, pyramid-shaped and conical caputegulae. The nasal bones exhibit a distinct central row of conical caputegulae, symmetrically separated from the osteoderms above and to the sides of them. At the rear of the skull, the part of the foramen magnum formed by the basioccipital is located obliquely above and in front of the occipital condyle.
Cranium
The skull of Akainacephalus has a unique suite of cranial ornamentation that consists of several symmetrical rows of small pyramidal and conical caputegulae along the dorsolateral surface of the skull. As in Nodocephalosaurus, the external nares are oriented towards the sides. The external nares are fully obscured in front view, a condition not seen in many Asian and Laramidian ankylosaurids, and are relatively small, tear-shaped openings. Positioned on the rear margin of the left nares is a small bony fragment as in Pinacosaurus and Minotaurasaurus which suggests the presence of internarial apertures. The external nares have upper and front margins that are bounded by the supranarial caputegulum. The orbits are almond-shaped and both jugals are slightly above the lacrimals due to anteroposterior deformation. The premaxillae slightly taper towards the front and form a narrow, U-shaped beak. The maxillae preserves 16 alveolar cavities each. As in other ankylosaurids and nodosaurids, the maxillae are concave towards the sides and together form an hourglass configuration. Like Nodocephalosaurus, the lacrimals are ornamented with a tetrahedral-shaped lacrimal caputegulum, although the lacrimal in Nodocephalosaurus is well defined in lateral view and has a rugose surface texture. The postorbitals have an ornamented side surface that forms the rear border of the orbit and is continuous with the posterodorsal ornamentation of the jugal. The jugals consist of a broad element that touches the front-most part of the quadratojugal and the rear margin of the maxilla. The jugal forms a shelf that is ornamented towards the sides with a small ridge of rugose co-ossified bone. The quadratojugal is a small wall of bone that projects towards the front and forms the front extension of the quadratojugal horn.
The external nares are ornamented with a supranasal caputegulum that is arranged along the premaxillary beak at the back and the sides. The caputegulum has a smooth surface texture, with some rugosity, and a tetrahedral-shaped lacrimal caputegulum is present near it. Along the dorsal surface of the nasal region are eight caputegulae that are ordered in a symmetrical pattern, but are separated by a row of pyramid-like caputegulae. Large, hexagonal caputegulae form a sagittal midline row which terminate in an apex towards the rear and towards the sides of sagittal midline row is a row of both polygonal and pyramid-shaped caputegulae. The side projecting nasal caputegulum forms a flaring nostril in side view. Positioned on the sides of each prefrontal is a subtriangular caputegulum with a keeled apex, which is anterior to the anterior-most supraorbital boss. A caputegulum that has a hexagonal base and a bulbous shape is surrounded by six smaller caputegulae towards the sides and front, which are similar to the nasal caputegulae but are different to the large, posterior caputegulum. The nuchal shelf has little rugosity and three, poorly preserved nuchal caputegulae, which vary between subrounded to elongate polygonal, are present on the posterior-most portion. The circumorbital complex consists of a supraorbital horn, a lacrimal caputegulum, a jugal osteoderm, and the thickened rim along the posterior margin of the orbital. The postorbital horn is formed by the fused anterior and posterior supraorbital caputegulae. Due to the damaged squamosal horns and more vertically oriented quadratojugal horns, it is unknown if Akainacephalus had squamosal horns and quadratojugal horns that exceeded the width of the postorbital horns. The postorbital horns are triangular towards the back, with a rugose and bulbous surface texture. The lacrimal caputegulum is a small caputegulum with a small surface texture that is arranged towards the underside of the supraorbital boss, while the jugal caputegulum is concave towards the underside and extends towards the sides which creates a small shelf. The quadrates are visible towards the sides. A subtriangular caputegulum with a blunt keel is present on the mandible and is shorter than that of other ankylosaurids.
The quadrates have a shaft that expands to the sides and the middle to touch the middle face of the quadratojugals and the side faces of the pterygoids. Additionally, the quadrates are relatively robust and have a strong inclination towards the front of in side view. The predentary forms the mandibular counterpart to the premaxillary rostrum and articulates with the mandibular symphysis. In addition, the rear end of the predentary has a smooth surface texture. As in other ankylosaurids, the predentary of Akainacephalus is transversely almost straight. The total number of alveoli is unknown as they have been eroded away. The surangular, along with the posterior margin of the dentary, contributes to the tallest portion of the mandible and forms the coronoid process. The prearticular comprises the retroarticular process, along with the surangular.
Postcrania
The posterior cervical vertebra has a spool-shaped centrum and a distinct keel on the ventral side. The dorsal vertebrae centra are spool-shaped and appear oblong towards the front and back as they are compressed towards the sides. However, the dorsal vertebrae of the mid-dorsal series show minor deformation. Unlike other ankylosaurids, the underside of the posterior surface of the centrum possesses a recurved, hook-like projection, which is followed by a longitudinal keel. Some of the dorsal vertebrae preserve portions of the fused ribs. The ribs that are fused to the dorsal vertebrae are T-shaped in cross section. The synsacrum consists of four dorsosacral, three sacral vertebrae and a single caudosacral vertebra, which are all fused along the centra and the neural spines. The sutural contacts are nearly concealed between individual vertebrae, which may be an ontogenetic feature. The dorsosacral vertebrae have short ribs that are fused to the centrum and transverse processes. These ribs, along with the sacral ribs, form broad contact surfaces that articulate with the ilium. The sacral ribs have a much less pronounced T-shaped cross section as the dorsal surfaces are less developed horizontally, unlike the ribs on the dorsosacral vertebrae. The caudal vertebrae have a short but tall boot and centra that have anterior and posterior articular surfaces that are spool-shaped and are slightly amphicoelous. The neural spines of the caudal vertebrae are tall and the centra of all proximal caudal vertebra are ellipsoid in shape, with the exception of the more distal caudal vertebrae in the proximal series as they are more round. The proximal ribs have dorsal surfaces that expand horizontally to form a T- or L-shaped cross-section, with three proximal ribs having an L-shaped cross-section and arch towards the underside. The proximal ribs with a T-shaped cross section have a wide horizontal expansion on the upper side of both sides of the ribs and arch less sharply, unlike the L-shaped ribs, which suggests they were positioned more towards the front of the vertebral column.