Invictarx (meaning "unconquerable fortress") is a monospecific genus of nodosaurid dinosaur from New Mexico that lived during the Late Cretaceous (lower Campanian, 78.5 Ma) in what is now the upper Allison Member of the Menefee Formation. The type and only species, Invictarx zephyri, is known from three isolated, incomplete postcranial skeletons. It was named in 2018 by Andrew T. McDonald and Douglas G. Wolfe. Invictarx shares similarities with Glyptodontopelta from the Naashoibito member of the Ojo Alamo Formation, New Mexico.
Discovery and naming
In May 2011, an incomplete postcranial skeleton of an ankylosaur was discovered from the upper Allison Member of the Menefee Formation, San Juan Basin by Daniel Williamson. A second incomplete postcranial skeleton was discovered in October 2011 by Andrew T. McDonald while a third specimen was discovered in October 2015 by Keith Brockmann. The specimens were subsequently named and described in 2018 by Andrew T. McDonald and Douglas G. Wolfe. The holotype specimen, WSC 16505, consists of fragments of a dorsal rib, complete or partial identifiable osteoderms, and fragments of additional osteoderms. Two other specimens were referred to Invictarx: UMNH VP 28350, dorsal vertebrae, fragments of dorsal ribs, distal end of humerus, distal end of ulna, proximal ends of radii, incomplete metacarpal, numerous incomplete osteoderms, and fragments of additional osteoderms; and UMNH VP 28351, dorsal centra, fragments of dorsal ribs, numerous incomplete osteoderms, and fragments of additional osteoderms. The three specimens are housed at the Western Science Center, California and Natural History Museum of Utah, Salt Lake City, Utah. The generic name, Invictarx, is derived from the Latin words "invictus" (invincible or unconquerable) and "arx" (fortress), in reference to the fact that Invictarx, like all other ankylosaurs, were well-armoured. The specific name, zephyri, is derived from is the genitive form of the Greek noun Zephyrus, meaning (of the) western wind, referring to the "blustery conditions that prevail among the outcrops where the specimens were discovered".
Description
Distinguishing traits McDonald & Wolfe (2018) diagnosed Invictarx based on the cervical or pectoral, thoracic, and pelvic osteoderms exhibiting an overall smooth surface texture, with little to no projecting rugosity, with abundant pits distributed randomly over the entire external surface, and with no neurovascular grooves or a small number of bifurcating and non-bifurcating neurovascular grooves distributed randomly, a feature similar to Glyptodontopelta; thoracic osteoderms exhibit a low, rounded keel with a deep groove extending craniocaudally along the apex, similar to Anodontosaurus and Platypelta; and the presence of possible a co-ossified pelvic shield that consists of polygonal osteoderms of uniform size, similar to Nodosaurus, Stegopelta, Glyptodontopelta, Europelta and Aletopelta.
Postcrania The dorsal vertebrae are incomplete but well-preserved, as one vertebra consists of the centrum while the other two vertebrae preserve the base of the neural arch and partial prezygapophyses. Based on Sauropelta and Europelta, the vertebrae are identified as middle dorsal vertebrae, although there are no indications of ribs fused to the three dorsal vertebrae. The dorsal vertebrae have the cranial and caudal faces only slightly concave. Both the cranial and caudal surfaces are sub circular. The neural arch forms a right angle with the long axis of the centrum as it rises vertically from the craniodorsal margin of the centrum. The vertebrae possess parapophyses that are distinct and are rugose swellings on the sides of the neural arch. The prezygapophyses are joined towards the underside and form a short parapet on the neural arch. As in the nodosaurids Sauropelta, Europelta, Silvisaurus and Struthiosaurus austriacus, the neural canal is ellipse-shaped with its axis oriented towards the upper edge and sides.
All of the appendicular elements are incomplete and poorly preserved. Although the distal end of the left humerus is crushed, the ulnar condyle was larger than the radial condyle. As in other nodosaurids, the radial condyle was sub spherical shaped and matches the circular shape of the proximal end of the radius. The distal end of the ulna is broken and in poor condition. The distal ends of the ulnae resemble those of Sauropelta and Niobrarasaurus. The left radius consist of only a fragment while the right radius is more complete. The fragment of the left radius reveals little about the element's morphology. The proximal articulation surface of the radii are shallowly concave and circular. In cross-section, the shaft of the radii are sub circular. The sides of the shaft of the radius forms an almost flat surface to articulate with the ulna. A metacarpal assigned to one of the referred specimens lacks the proximal and distal ends, and the morphology, orientation, and placement of the metacarpal is unknown.
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