Abstract
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.
Highlights
The Judith River Formation of northern Montana is a terrestrial siliciclastic unit deposited over an approximately 4 Myr interval during a Campanian transgressive-regressive cycle of the Western Interior Seaway [1]
Seven ornithischian dinosaur species that are currently considered valid have been identified from this unit, with most based on highly incomplete remains, and theropod species-level records are largely restricted to isolated teeth [7]
The phylogenetic analysis recovered 10 most parsimonious trees, each with a tree length of 563, a consistency index of 0.403, and a retention index of 0.666 and a best tree-bisection reconnection osteoderm position left or right side anteroposterior length apicobasal length comments only a portion of a horny sheath is visible at present
Summary
The Judith River Formation of northern Montana is a terrestrial siliciclastic unit deposited over an approximately 4 Myr interval during a Campanian transgressive-regressive cycle of the Western Interior Seaway [1]. V. Hayden, when mapping the area for the US Geological Survey [2]. Hayden, when mapping the area for the US Geological Survey [2] This collection is historically significant because it included specimens recognized as the first dinosaurs from North America [3]. Dinosaur biodiversity in the Judith River Formation remains represented primarily by fragmentary specimens that are difficult to identify to low taxonomic levels. Seven ornithischian dinosaur species that are currently considered valid have been identified from this unit, with most based on highly incomplete remains, and theropod species-level records are largely restricted to isolated teeth [7]. Knowledge of other aspects of the fauna, such as non-dinosaurian reptiles, is based on isolated and fragmentary material
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