The terrestrial Judith River Formation of northern Montana was deposited over an approximately 4 Myr interval during the Campanian (Late Cretaceous). Despite having been prospected and collected continuously by palaeontologists for over a century, few relatively complete dinosaur skeletons have been recovered from this unit to date. Here we describe a new genus and species of ankylosaurine dinosaur, Zuul crurivastator, from the Coal Ridge Member of the Judith River Formation, based on an exceptionally complete and well-preserved skeleton (ROM 75860). This is the first ankylosaurin skeleton known with a complete skull and tail club, and it is the most complete ankylosaurid ever found in North America. The presence of abundant soft tissue preservation across the skeleton, including in situ osteoderms, skin impressions and dark films that probably represent preserved keratin, make this exceptional skeleton an important reference for understanding the evolution of dermal and epidermal structures in this clade. Phylogenetic analysis recovers Zuul as an ankylosaurin ankylosaurid within a clade of Dyoplosaurus and Scolosaurus, with Euoplocephalus being more distantly related within Ankylosaurini. The occurrence of Z. crurivastator from the upper Judith River Formation fills a gap in the ankylosaurine stratigraphic and geographical record in North America, and further highlights that Campanian ankylosaurines were undergoing rapid evolution and stratigraphic succession of taxa as observed for Laramidian ceratopsids, hadrosaurids, pachycephalosaurids and tyrannosaurids.

Supplementary Figures and Videos of ROM 75860, Zuul crurivastatorSupplementary Figure 1: ROM 75860 skull in right lateral view. Supplementary Figure 2: ROM 75860 skull in left lateral view. Supplementary Figure 3: ROM 75860 skull in anterior view. Supplementary Figure 4: ROM 75860 skull in posterior view. Supplementary Figure 5: ROM 75860 skull in dorsal view. Supplementary Figure 6: ROM 75860 skull in ventral view. Supplementary Figure 7: ROM 75860 left lower jaw in lateral view. Supplementary Figure 8: ROM 75860 left lower jaw in medial view. Supplementary Figure 9: ROM 75860 left lower jaw in dorsal view. Supplementary Figure 10: ROM 75860 left lower jaw in ventral view. Supplementary Figure 11: ROM 75860 right lower jaw in lateral view. Supplementary Figure 12: ROM 75860 right lower jaw in medial view. Supplementary Figure 13: ROM 75860 right lower jaw in dorsal view. Supplementary Figure 14: ROM 75860 right lower jaw in ventral view. Supplementary Figure 15: ROM 75860 right lower jaw in anterior view. Supplementary Figure 16: ROM 75860 right lower jaw in posterior view. Supplementary Figure 17: ROM 75860 tail in dorsal view. Supplementary Figure 18: ROM 75860 tail in dorsal view. Supplementary Video: Photogrammetric reconstruction of the skull and jaws of ROM 75860.Zuul supplementary files.zip