7-3 New Nodosaurid (Dinosauria: Thyreophora) Armor and Postcrania from the Upper Cretaceous Allison Member, Menefee Formation, New Mexico, USA

Session: The Campanian Crucible: A synthesis of vertebrate paleobiogeography and ecosystem dynamics in Laramidia (Posters)

Poster Booth No.: 16

Presenting Author:

Benjamin Mohler

Authors:

Mohler, Benjamin Franciscus¹, Silverstein, Jessie R.², Brand, Harrison Rory³, Smith, Heather F.⁴, Adrian, Brenton⁵

(1) Department of Geosciences, The University of Arizona, Tucson, Arizona, USA, (2) Southwest Paleontological Society, Mesa, AZ, USA, (3) Southwest Paleontological Society, Mesa, AZ, , (4) Department of Anatomy, Midwestern University, Glendale, Arizona, ; School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, USA, (5) School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, USA,

Abstract:

Fossils recovered from the Menefee Formation are almost ubiquitously fragmentary, so it is no surprise that armored and shelled vertebrates (turtles, crocodyliforms, and thyreophoran dinosaurs) with more durable skeletal elements are the best represented fauna from this unit. Here we report newly collected fossils of nodosaurid dinosaurs, including isolated osteoderms and fragmentary skeletal elements from exposures of the Juans Lake Beds in the Allison Member of the Menefee Formation, outcropping in the San Juan Basin of northwestern New Mexico. Identified among these fossils are likely dorsal or cervical osteoderms, a likely pelvic osteoderm, interstitial ossicles, and associated partial appendicular elements. The osteoderms are referrable to Nodosauridae over Ankylosauridae due to their thickness and lack of deep ventral excavation. The nodosaurid taxon Invictarx zephyri was founded upon fossils from the Juans Lake Beds and established entirely on characteristics of osteoderm morphology. Among these characters, the combination of traits considered consistent and exclusive to Invictarx are smooth surface texture, poorly developed or absent rugosity, abundant, random pitting, and underdeveloped neurovascular grooves, compared to osteoderms of the Maastrichtian nodosaurid Glyptodontopelta from upsection in the San Juan Basin. 

Newly collected Menefee nodosaurid osteoderms conform with these surface traits, though some differ in overall morphology (length, width, and traits of the keel), likely reflecting shield elements not represented by the holotype and previously referred specimens of Invictarx. Among recovered osteoderms is a laterally compressed, scalene form, which has not been previously observed in Invictarx, possibly positioned on the tail or distally along the cervical or dorsal shield. In addition, a distinct isolated specimen that is large, flat, and plate-like, with a weak rounded keel, was found in association with an osteoderm of the more typical Invictarx morphology. The “split keel” trait described for Invictarx is not apparent in these newly reported nodosaurid osteoderms; however, this trait is only present in a minority of osteoderms previously attributed to Invictarx, and is not exclusive to Invictarx among Campanian ankylosaurians, so its absence in this sample is of limited taxonomic significance. Frequent abrasion of the surface texture of the keel can also obscure this feature. The new material expands the hypodigm of Invictarx and helps clarify its morphological distinctness from other Late Cretaceous San Juan Basin ankylosaurians.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

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