ARTICLE OSTEOLOGY AND PHYLOGENETIC RELATIONSHIPS OF THE MID-CRETACEOUS NEORNITHISCHIAN DINOSAUR ORYCTODROMEUS CUBICULARIS VARRICCHIO, 2007 L. J. KRUMENACKER, 1,2 * DAVID J. VARRICCHIO, 3 CHRIS ORGAN, 4 JACOB D. GARDNER, 5 BROOKS B. BRITT, 6 and CLINT BOYD 7 1 Department of Geosciences, Idaho State University, Pocatello, Idaho, 83209, krumlaur@isu.edu; 2 Department of Physical Sciences, College of Eastern Idaho, Idaho Falls, Idaho, 83404, lj.krumenacker@cei.edu; 3 Department of Earth Sciences, Montana State University, Bozeman, Montana, 59717, djv@montana.edu; 4 School of Biological Sciences, University of Reading, RG6 6EX, UK, c.l.organ@reading.ac.uk; 5 School of Biological Sciences, University of Reading, RG6 6EX, UK, jacob.gardner@reading.ac.uk; 6 Department of Geological Sciences, Brigham Young University, Provo, Utah, 84602, brooks_britt@byu.edu; 7 North Dakota Geological Survey, Bismarck, North Dakota, 58505, ndgspaleo@nd.gov ABSTRACT—The vertebrate assemblages of the Albian to Cenomanian Wayan Formation of southeastern Idaho and southwestern Montana’s coeval Vaughn Member of the Blackleaf Formation are dominated by the small, burrowing orodromine dinosaur Oryctodromeus cubicularis. Here, we describe in detail the osteology of Oryctodromeus based on new specimens from Idaho and Montana that add substantially to the preliminary description of the types from Montana, and provide a suite of additional diagnostic characters for the taxon: ilium with elongate preacetabular process; elongate cervical vertebra centra with an anteroposterior length 1.6 times the dorsoventral height; elongate dorsal vertebra centra with an anteroposterior length 1.4 times the dorsoventral height; more than 55 elongate caudal vertebrae enveloped in hypaxial and epaxial ossified tendons; and a femoral head on an elongate neck—similar to that of Koreanosaurus— projecting from the greater trochanter at about 35°. The tail, comprising two-thirds of the animal’s roughly 3 meters length, and associated tendon sheaths in the axial column indicate greater flexibility than previously supposed for ossified tendons or, alternatively, suggest that the Oryctodromeus burrows had separate, or multiple entrances and exits. The elongated and angled femoral head likely facilitated digging via a braced splayed-leg posture. Our phylogenetic analysis incorporates new characteristics and supports the monophyly of Orodrominae, a clade of neornithischian dinosaurs from the middle to Late Cretaceous of Asia and western North America that utilized burrowing. SUPPLEMENTARY FILES—Supplementary files are available for this article for free at www.tandfonline.com/UJVP. Citation for this article: Krumenacker, L. J., Varricchio, D. J., Organ, C., Gardner, J. D., Britt, B. B., & Boyd, C. (2024) Osteology and phylogenetic relationships of the mid-Cretaceous neornithischian dinosaur Oryctodromeus cubicularis Varricchio, 2007. Journal of Vertebrate Paleontology. https://doi.org/10.1080/02724634.2024.2330581 Submitted: May 23, 2021 Revisions received: February 11, 2024 Accepted: March 8, 2024 INTRODUCTION The terrestrial vertebrate record for the lower part of the Upper Cretaceous of North America has grown dramatically over the past two decades largely due to discoveries in the Mus- sentuchit Member of the Cedar Mountain Formation of eastern Utah (e.g., Kirkland & Madsen, 2007; McDonald et al., 2012; Zanno & Makovicky, 2013). However, discoveries outside of this area are uncommon and equivalent faunas are poorly under- stood. The Wayan Formation of eastern Idaho and the Vaughn Member of the Blackleaf Formation in southwestern Montana are coeval geological units in a single sedimentary basin (Krume- nacker, 2020). Oryctodromeus cubicularis, from the Wayan For- mation and Vaughn Member, is the most completely known North American Cenomanian dinosaur taxon north of the Mus- sentuchit assemblage (Krumenacker, 2010, 2017; Krumenacker et al., 2017, 2019; Varricchio et al., 2007). Orodromines, as defined by Brown et al. (2013), share characteristics that include a sacropubal articulation, a sharp scapular spine, and a D-shaped cross-section of the fibula. Excluding Oryctodromeus, other orodromine taxa include Zephyrosaurus schaffi from the Aptian–Albian Cloverly Formation of Montana (Sues, 1980), Orodromeus makelai from the Campanian Two Medicine For- mation of Montana (Horner & Weishampel, 1988; Scheetz, 1999), Albertadromeus syntarsus from the Campanian Belly River Group of Alberta (Brown et al., 2013), Koreanosaurus boseongensis from the Santonian to Campanian Seonso Con- glomerate of South Korea (Huh et al., 2010), and an unnamed ‘Kaiparowits orodromine’ from the Campanian Kaiparowits For- mation of southern Utah (Boyd, 2015; Gates et al., 2013). Oryctodromeus was erected based on an adult (MOR 1636a) and two juveniles (MOR 1636b) found in association in the expanded distal chamber of an infilled burrow in the Cenoma- nian Vaughn Member of the Blackleaf Formation in southwes- tern Montana (Varricchio et al., 2007). This discovery provided the first evidence of burrowing and denning behavior among *Corresponding author. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ujvp. Journal of Vertebrate Paleontology e2330581 (28 pages) © by the Society of Vertebrate Paleontology DOI: 10.1080/02724634.2024.2330581 Published online 25 Apr 2024