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| Invictarx zephyri
McDonald & Wolfe, 2018
Illustration by Kara Kelley
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Abstract
Nodosauridae is a clade of armored dinosaurs with a rich fossil record and long history of study in North America. Nodosaurid fossils have been collected throughout the western United States and Canada. Here, we report three new nodosaurid specimens from the Upper Cretaceous (lower Campanian) Allison Member of the Menefee Formation, San Juan Basin, northwestern New Mexico. The three specimens belong to a new genus and species, Invictarx zephyri, characterized by a unique combination of features pertaining to the morphology of the osteoderms. Among the three specimens there are representative cervical/pectoral and thoracic osteoderms, as well as components of a probable co-ossified pelvic shield. The new tax on is most similar to Glyptodontopelta mimus from the Maastrichtian of New Mexico.
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| Figure 1: Stratigraphic occurrences of Invictarx zephyri and other ankylosaurs from the San Juan Basin. Generalized stratigraphic column of Upper Cretaceous strata in the San Juan Basin, northwestern New Mexico, showing the stratigraphic positions of the nodosaurids I. zephyri and Glyptodontopelta mimus and the ankylosaurids Ahshislepelta minor, Nodocephalosaurus kirtlandensis, and Ziapelta sanjuanensis. Ankylosaur occurrence data are from Sullivan & Lucas (2015).
Nodosaurid silhouette by Scott Hartman (creativecommons.org), and ankylosaurid silhouette by Andrew A. Farke (creativecommons.org), both available from PhyloPic. Stratigraphic column is derived from data in Miller, Carey & Thompson-Rizer (1991), Molenaar et al. (2002), Sullivan & Lucas (2006), and Fowler (2017).
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Illustration by Kara Kelley
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Systematic paleontology
Dinosauria Owen, 1842, sensu Baron, Norman & Barrett, 2017
Ornithischia Seeley, 1888, sensu Sereno, 2005
Thyreophora Nopcsa, 1915, sensu Sereno, 2005
Ankylosauria Osborn, 1923, sensu Sereno, 2005
Nodosauridae Marsh, 1890, sensu Sereno, 2005
Invictarx zephyri gen. et sp. nov.
Holotype: WSC 16505, incomplete postcranial skeleton including fragments of a dorsal rib, six complete or partial identifiable osteoderms (WSC 16505.1–WSC 16505.6), and fragments of additional osteoderms.
Referred specimens: Natural History Museum of Utah (UMNH) VP 28350, incomplete postcranial skeleton including three dorsal vertebrae, fragments of dorsal ribs, distal end of left humerus, distal end of left ulna, proximal ends of left and right radii, incomplete metacarpal, numerous incomplete but identifiable osteoderms, and fragments of additional osteoderms; UMNH VP 28351, incomplete postcranial skeleton including fragments of several dorsal centra, fragments of dorsal ribs, numerous incomplete but identifiable osteoderms, and fragments of additional osteoderms.
Etymology: Invictarx is derived from the Latin words invictus (“invincible, unconquerable”) and arx (“fortress”), in reference to the well-armored nature of ankylosaurian dinosaurs. The specific name, zephyri, is the genitive form of the Latin masculine noun zephyrus, “west wind,” in reference to the blustery conditions that prevail among the outcrops where the specimens were discovered. The full name may be translated as “unconquerable fortress of the western wind.”
Locality: All specimens were collected in San Juan County, New Mexico, on land administered by the U.S. BLM. Precise locality data are on file at WSC, UMNH, and the BLM.
Horizon: All specimens were collected from outcrops of the Juans Lake Beds (Miller, Carey & Thompson-Rizer, 1991) (Fig. 1), upper part of the Allison Member, Menefee Formation; lower Campanian, Upper Cretaceous (Molenaar et al., 2002; Lucas et al., 2005).
Specific diagnosis (as for genus by monotypy): nodosaurid ankylosaur distinguished by the following unique combination of characters: (1) observable on WSC 16505, UMNH VP 28350, and UMNH VP 28351 cervical/pectoral, thoracic, and pelvic osteoderms exhibit overall smooth surface texture, with little or 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, similar to Glyptodontopelta mimus but lacking the dense pattern of dendritic grooves that characterizes that taxon (Burns, 2008; Burns & Currie, 2014); (2) observable on WSC 16505 and UMNH VP 28351 some thoracic osteoderms exhibit a low, rounded keel with a deep groove extending craniocaudally along the apex, also present in the ankylosaurids Anodontosaurus lambei (Fig. 13G in Penkalski (2018)) and Platypelta coombsi (Fig. 13O in Penkalski (2018)) (P. Penkalski, 2018, personal communication), but absent in G. mimus (Burns, 2008); and (3) observable on UMNH VP 28351 probably possessed a co-ossified pelvic shield consisting of polygonal osteoderms of uniform size (Category 3 of Arbour, Burns & Currie (2011)), similar to some other nodosaurids, including Nodosaurus textilis (Lull, 1921), Stegopelta landerensis (Moodie, 1910), G. mimus (Ford, 2000; Burns, 2008), and Europelta carbonensis (Kirkland et al., 2013), as well as the ankylosaurid Aletopelta coombsi (Ford & Kirkland, 2001; Arbour & Currie, 2016).
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Conclusions:
The new nodosaurid I. zephyri provides further insight into the poorly known vertebrate fossil record of the Allison Member of the Menefee Formation. Although the known material is fragmentary, the osteoderms exhibit a unique combination of characters. The occurrence of Invictarx in the early Campanian of southern Laramidia aligns with previous hypotheses that nodosaurids were present in Laramidia throughout the Late Cretaceous, even as ankylosaurids suffered a local extinction and later reinvaded from Asia (Arbour, Zanno & Gates, 2016).
Andrew T. McDonald and Douglas G. Wolfe. 2018. A New Nodosaurid Ankylosaur (Dinosauria: Thyreophora) from the Upper Cretaceous Menefee Formation of New Mexico. PeerJ. 6:e5435. DOI: 10.7717/peerj.5435
