|Preserved elements and skeletal reconstructions|
Wiersma and Irmis, 2018
Wiersma and Irmis, 2018
Akainacephalus is a genus of herbivorous ankylosaurid dinosaur from the Campanian age Kaiparowits Formation of southern Utah. The type and only species is Akainacephalus johnsoni, 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.
Discovery and naming
In 2008, in the Grand Staircase-Escalante National Monument in Kane County, Utah, an ankylosaurian skeleton was discovered at a site first located by Scott Richardson, an employee of the Bureau of Land Management. In 2009, the excavation was completed by a team headed by R. Irmis. The skull of the skeleton was during four years prepared by volunteer Randy Johnson, a retired chemist. In January 2014, the skull was investigated by means of a CAT-scan.
In 2018, the type species Akainacephalus johnsoni was named and described by Dutch paleontologist Jelle P. Wiersma and Randall Benjamin Irmis. The generic name combines a Greek ἄκαινα, akaina, "spike", a reference to the spiky head armour, with κεφαλή, kephalè, "head". The specific name honours Randy Johnson as preparator.
The holotype, UMNH VP 20202, was found in a layer of the Kaiparowits Formation dating from late Campanian. The age of the layer was determined at 75.97 million years by zircon dating. The holotype consists of a partial skeleton with skull. It contains the complete skull and lower jaws, including the predentary; four back vertebrae; eight sacral vertebrae; eight "free" tail vertebrae; eleven tail vertebrae forming the handle of the tail club; the tail club itself; ribs; both shoulder blades; a left coracoid; a right humerus; a right ulna; a left ilium; a left thighbone; a left shinbone; a left calfbone; a toe phalanx; a claw; two cervical halfrings; and fourteen osteoderms of the back and flanks. It represents about 45% of the skeletal elements. It is part of the collection of the Natural History Museum of Utah in Salt Lake City.
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.
The head armour of Akainacephalus is strikingly similar to that of Nodocephalosaurus, a related form also found in southern Laramidia, in New Mexico. The description therefore closely compared the two genera in order to prove that Akainacephalus was a valid taxon. Akainacephalus and Nodocephalosaurus share certain traits such as pyramid-shaped osteoderms on the snout and flaring armour above the nostrils. There are, however, also differences. With Akainacephalus, the osteoderms of the front and rear supraorbitals form a single back-swept high structure. In N. kirtlandensis, these remain separate elements of far smaller size. Akainacephalus has a much smaller squamosal horn. Its cheek horn points straight to below as an enormous triangle, while that of Nodocephalosaurus curves to behind like a smaller curved fin. A comparison is complicated by the fact that the skull of Nodocephalosaurus is only partly known. Besides, with the holotype of Akainacephalus the squamosal horns have broken off, making it impossible to determine their exact shape, and the skull as a whole has been compressed from front to rear, creating a kink from which snout and rear are appending, and narrowing and raising the area around the eye socket. However, Nodocephalosaurus was found in the Kirtland Formation, in layers that are three million years younger, which precludes the taxa from being identical.
The front snout bones, the premaxillae, form a wide, U-shaped, upper beak, wider than long. The sides of the snout are not covered by armour. The number of maxillary teeth, per side, has been estimated at a minimum of sixteen. The bony nostrils are oriented to the side of the snout. The nostril is rather small and not clearly subdivided into smaller openings. The quadrate bone is strongly inclined, causing the jaw joint to be positioned, visible in side view, in front of the cheek horn point, which has not been reported in other ankylosaurids.
Akainacephalus' phylogenetic analysis reveals it to form a clade with fellow Laramidian ankylosaurid Nodocephalosaurus. Additional data from cladistic analysis also shows that Akainacephalus and Nodocephalosaurus are likewise more closely related to Asian ankylosaurid genera such as Saichania and Tarchia than to North American genera such as Euoplocephalus and Ankylosaurus itself. The discovery indicates a strong case for provincialism between dinosaur populations in Northern and Southern Laramidia. Furthermore, the discovery of Akainocephalus also indicates at least two faunal migrations between Asia and North America, created when dropping sea levels allowed migrations between the continents via the Beringian Land Bridge.
During the Late Cretaceous period, the site of the Kaiparowits Formation was located near the western shore of the Western Interior Seaway, a large inland sea that split North America into two landmasses, Laramidia to the west and Appalachia to the east. The plateau where dinosaurs lived was an ancient floodplain dominated by large channels and abundant wetland peat swamps, ponds and lakes, and was bordered by highlands. The climate was wet and humid, and supported an abundant and diverse range of organisms. This formation contains one of the best and most continuous records of Late Cretaceous terrestrial life in the world.
Akainacephalus shared its paleoenvironment with other dinosaurs, such as dromaeosaurid theropods, the troodontid Talos sampsoni, tyrannosaurids like Teratophoneus, the duckbilled hadrosaurs Parasaurolophus cyrtocristatus and Gryposaurus monumentensis, the ceratopsians Utahceratops gettyi, Nasutoceratops titusi and Kosmoceratops richardsoni and the oviraptorosaurian Hagryphus giganteus. Some fossil evidence suggests the presence of the tyrannosaurid Albertosaurus and the ornithomimid Ornithomimus velox, but the existing assessment of the material is not conclusive. Paleofauna present in the Kaiparowits Formation included chondrichthyans (sharks and rays), frogs, salamanders, turtles, lizards and crocodilians. A variety of early mammals were present including multituberculates, marsupials, and insectivorans.
- Jelle P. Wiersma; Randall B. Irmis (2018). "A new southern Laramidian ankylosaurid, Akainacephalus johnsoni gen. et sp. nov., from the upper Campanian Kaiparowits Formation of southern Utah, USA". PeerJ. 6: e5016. doi:10.7717/peerj.5016. PMC 6063217. PMID 30065856.
- Titus, Alan L. and Mark A. Loewen (editors). At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah. 2013. Indiana University Press. Hardbound: 634 pp.
- Clinton, William. "Presidential Proclamation: Establishment of the Grand Staircase-Escalante National Monument". September 18, 1996. Archived from the original on 2013-08-28. Retrieved 9 November 2013.
- Media related to Akainacephalus at Wikimedia Commons