Saturday, July 21, 2018

[Paleontology • 2018] Akainacephalus johnsoni • A New southern Laramidian Ankylosaurid from the upper Campanian Kaiparowits Formation of southern Utah, USA

Akainacephalus johnsoni 
Wiersma​ & Irmis, 2018 

 DOI: 10.7717/peerj.5016


A partial ankylosaurid skeleton from the upper Campanian Kaiparowits Formation of southern Utah is recognized as a new taxonAkainacephalus johnsoni, gen. et sp. nov. The new taxon documents the first record of an associated ankylosaurid skull and postcranial skeleton from the Kaiparowits Formation. Preserved material includes a complete skull, much of the vertebral column, including a complete tail club, a nearly complete synsacrum, several fore- and hind limb elements, and a suite of postcranial osteoderms, making Akainacephalus johnsoni the most complete ankylosaurid from the Late Cretaceous of southern Laramidia. Arrangement and morphology of cranial ornamentation in Akainacephalus johnsoni is strikingly similar to Nodocephalosaurus kirtlandensis and some Asian ankylosaurids (e.g., Saichania chulsanensis, Pinacosaurus grangeri, and Minotaurasaurus ramachandrani); the cranium is densely ornamented with symmetrically arranged and distinctly raised ossified caputegulae which are predominantly distributed across the dorsal and dorsolateral regions of the nasals, frontals, and orbitals. Cranial caputegulae display smooth surface textures with minor pitting and possess a distinct conical to pyramidal morphology which terminates in a sharp apex. Character analysis suggests a close phylogenetic relationship with N. kirtlandensis, M. ramachandrani, Tarchia teresae, and S. chulsanensis, rather than with Late Cretaceous northern Laramidian ankylosaurids (e.g., Euoplocephalus tutus, Anodontosaurus lambei, and Ankylosaurus magniventris). These new data are consistent with evidence for distinct northern and southern biogeographic provinces in Laramidia during the late Campanian. The addition of this new ankylosaurid taxon from southern Utah enhances our understanding of ankylosaurid diversity and evolutionary relationships. Potential implications for the geographical distribution of Late Cretaceous ankylosaurid dinosaurs throughout the Western Interior suggest multiple time-transgressive biogeographic dispersal events from Asia into Laramidia.

Life reconstruction: Andrey Atuchin 

Systematic paleontology

Dinosauria Owen, 1842 sensu Padian and May, 1993
Ornithischia Seeley, 1887 sensu Padian and May, 1993
Thyreophora Nopcsa, 1915 sensu Sereno, 1986

Ankylosauria Osborn, 1923 sensu Carpenter, 1997
Ankylosauridae Brown, 1908 sensu Sereno, 1998
Ankylosaurinae Brown, 1908 sensu Sereno, 1986
Ankylosaurini Arbour and Currie, 2016

Akainacephalus, gen. nov.

Akainacephalus johnsoni, sp. nov.

Figure 4: Skull of Akainacephalus johnsoni (UMNH VP 20202).
Photographs of the skull of Akainacephalus johnsoni in (A), dorsal; and (B), ventral views. Line drawings in (C), dorsal; and (D), ventral views highlight major anatomical features.

Study sites: bpt, basipterygoid; bs, basisphenoid; ch, choana; exo, exoccipital; fm, foramen magnum; fca, frontal caputegulum; ins, internarial septum; laca, lacrimal caputegulum; loca, loreal caputegulum; mx, maxilla; mxtr, maxillary tooth row; naca, nasal caputegulum; ns, nuchal shelf; oc, occipital condyle; pal, palatine; prfca, prefrontal caputegulum; pmx, premaxilla; pmxs, interpremaxillry suture with oblong depression; pop, paroccipital process; ptv, pterygoid vacuity; q, quadrate; qj, quadratojugal; qjh, quadratojuga horn; so, supra occipital; snca, supranarial caputegulum; sob, supraorbital boss; sqh, squamosal horn.

Figure 3: Skull of Akainacephalus johnsoni (UMNH VP 20202). Photographs of the skull of Akainacephalus johnsoni in (A), left lateral; and (B), right lateral views. Line drawings in (C), left lateral; and (D), right lateral views highlight major anatomical features.

Study sites: en, external naris; fca, frontal caputegulum; j, jugal; jca, jugal caputegulum; l, lacrimal; laca, lacrimal caputegulum; loca, loreal caputegulum; mx, maxilla; n, nasal; naca, nasal caputegulae; ns, nuchal shelf; orb, orbit; pmx, premaxilla; prfca, prefrontal caputegulum; snca, supranarial caputegulum; sob, supraorbital boss; q, quadrate; qjh, quadratojugal horn; sqh, squamosal horn. 

Holotype: UMNH VP 20202, a partial skeleton comprising a complete skull, both mandibles, predentary, four dorsal, four dorsosacral, three sacral, one caudosacral, and eight caudal vertebrae, dorsal ribs, a complete tail club, both scapulae, left coracoid, right humerus, right ulna, partial left ilium, left femur, left tibia, left fibula, phalanx, two partial cervical osteoderm half rings, and 17 dorsal and lateral osteoderms of various sizes and morphologies.

Type locality: UMNH VP Locality 1109 (“HMG Quarry”), Horse Mountain area, GSENM, Kane County, southern Utah, USA.

Type stratigraphic horizon and age: UMNH VP Locality 1109 is a multitaxic bonebed deposited in a crevasse splay sandstone within the lower portion of the middle unit of the upper Campanian Kaiparowits Formation (Fig. 2A). The stratigraphic position of this site is approximately 190 m from the base of the formation (Roberts et al., 2013: fig. 6.3) and within approximately one meter stratigraphic proximity of the recently dated bentonite ash bed KP-07, which has produced a U-Pb zircon age of 76.26 ± 0.10 Ma (Roberts et al., 2013), providing a precise age constraint for Akainacephalus johnsoni.

Etymology: The genus name is derived from the Greek akaina, meaning “thorn” or “spine,” referring to the thorn-like cranial caputegulae of the holotype; and “cephalus,” the Greek meaning for head. The specific epithet honors Randy Johnson, volunteer preparator at the Natural History Museum of Utah, who skillfully prepared the skull and lower jaws of UMNH VP 20202.

Diagnosis: Akainacephalus johnsoni possesses the following autapomorphies: massive supraorbital bosses in lateral view, forming a tall backswept flange extending laterally over the orbits, and enveloping the anterodorsal and posterior margins of the orbit; nearly vertical projecting triangular quadratojugal horns; frontal possesses a large, flat, and centrally positioned hexagonal-shaped caputegulum; a combination of tightly spaced, symmetrically positioned pyramidal and conical-shaped caputegulae across the frontonasal region; a distinct midline row of conical-shaped caputegulae across the nasal region, symmetrically separating caputegulae situated dorsolaterally; basioccipital foramen anterior and dorsally to the occipital condyle. A. johnsoni also possesses a unique combination of character states: shares with Nodocephalosaurus kirtlandensis the presence of a large, laterally oriented supranarial osteoderm forming the postmaxillary/lacrimal ridge dorsal to the external nares; differs from Tsagantegia longicranialis, Talarurus plicatospineus, Pinacosaurus grangeri, all northern Laramidian taxa and Ziapelta sanjuanensis but shares with Nodocephalosaurus kirtlandensis, Minotaurasaurus ramachandrani, Saichania chulsanensis, and Tarchia kielanae the presence of well-pronounced cranial ornamentation located along the nasal and frontal regions of the skull that are characterized by a dense array of well-defined caputegulae with a distinct conical (N. kirtlandensis) and pyramidal (M. ramachandrani, S. chulsanensis, T. kilanae) morphology; shares with Euoplocephalus and Zuul crurivastator a globular surface texture on the tail club knob, which differs from the smoother texture in Ankylosaurus magniventris; differs from ZPAL MgD I/113, cf. Pinacosaurus, Saichania chulsanensis, and Dyoplosaurus acutosquameus, but similar to Anodontosaurus lambei, Euoplocephalus tutus, Zuul crurivastator, and Ankylosaurus magniventris in having a wider than long tail club knob ratio; and shares with ZPAL MgD I/113, cf. Pinacosaurus, D. acutosquameus, and Zuul crurivastator triangular osteoderms along the lateral surfaces on the proximal portion of the tail.

Figure 7: Variation in cranial ornamentation in selected Laramidian and Asian taxa, including Akainacephalus johnsoni.

Comparative line drawings highlighting major areas of cranial ornamentation in Akainacephalus johnsoni and closely related Laramidian and Asian taxa. Akainacephalus johnsoni (UMNH VP 20202) in (A), dorsal; (B), left lateral view compared to Nodocephalosaurus kirtlandensis (SMP VP-900) in (C), dorsal; and (D) left lateral view; Tarchia teresae (PIN 3142/250) in (E) dorsal; (F), left lateral view and Minotaurasaurus ramachandrani (INBR 21004) in (G), dorsal; and (H), left lateral view.

Study sites: acc po, accessory postorbital ossification; asob, anterior supraorbital boss; frca, frontal caputegulum; laca, lacrimal caputegulum; loca, loreal caputegulum; mso, medial supraorbital; mx, maxilla; n, external naris; naca, nasal caputegulae; nuca, nuchal caputegulae; orb, orbital; pmx, premxilla; pnca, postnarial caputegulum; pos postocular ossicles; prfca, prefrontal caputegulum; psob, posterior supraorbital boss; pt, pterygoid; q, quadrate; qjh, quadratojugal horn; snca, supranarial caputegulum; sqh, squamosal horn. 
Color scheme after Arbour & Currie (2013a). Dorsal view of N. kirtlandensis modified after Arbour et al. (2014). T. teresea (=Saichania chulsanensis in Arbour, Currie & Badamgarav, 2014) and M. ramachandrani modified after Arbour, Currie & Badamgarav (2014).

Figure 28: Preserved elements and skeletal reconstructions of Akainacephalus johnsoni.
A composite showing all holotype skeletal material of Akainacephalus johnsoni (UMNH VP 20202) anatomically arranged in dorsal view (A). Cartoon illustrating a full body reconstruction for A. johnsoni in (B), dorsal; and (C), left lateral view. Preserved material in the skeletal reconstructions is highlighted in orange.


Akainacephalus johnsoni (UMNH VP 20202) is a new taxon of ankylosaurid dinosaur from the upper Campanian Kaiparowits Formation of southern Utah, USA. It consists of a complete cranium and significant amount of diagnostic postcranial material that can be distinguished from all other known Late Cretaceous Laramidian ankylosaurids. Akainacephalus johnsoni is closely related to its stratigraphically younger sister taxon, Nodocephalosaurus kirtlandensis from the Kirtland Formation of New Mexico. Both taxa are more closely related to Asian ankylosaurids than they are to other Laramidian ankylosaurids. This suggests multiple ankylosaurid dispersal events from Asia to Laramidia during the Late Cretaceous. Lastly, together with Dyoplosaurus acutossquameus and Scolosaurus cutleri (∼77 Ma) from northern Laramidia, Akainacephalus johnsoni represents one of the older known ankylosaurid dinosaurs (∼76.3 Ma) from the Late Cretaceous of western North America.

Jelle P. Wiersma​ and 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

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