NEW ANKYLOSAURID DINOSAURS (ORNITHISCHIA: THYREOPHORA) FROM THE UPPER CAMPANIAN KAIPAROWITS FORMATION OF SOUTHERN UTAH AND IMPLICATIONS FOR INTERCONTINENTAL DISPERSAL PATTERNS OF ANKYLOSAURIDAE

Ankylosaurid dinosaurs from Laramidia (western North America) are a poorly understood group of armored ornithischians that are known from seven published Late Cretaceous taxa. Most of these taxa are known from northern Laramidia in Montana, Wyoming, and Alberta, including Ankylosaurus magniventris, Euoplocephalus tutus, Dyoplosaurus acutosqameus, Oohkotokia horneri, Anodontosaurus lambei, and Scolosaurus cutleri. The only southern Laramidian taxon is Nodocephalosaurus kirtlandensisfrom northern New Mexico. Here we present two new ankylosaurid taxa from the upper Campanian Kaiparowits Formation of southern Utah which significantly increases the taxonomic diversity of the clade, with biogeographic implications for the dispersal of North American ankylosaurids. Taxon A comprises nearly 60% of the skeleton, including a complete cranium, handle and tail club, osteoderms, and various postcranial elements; taxon B consists of an isolated partial skull.

Character analysis suggests a close phylogenetic relationship between taxon A and Nodocephalosaurus from the upper Campanian Kirtland Formation of New Mexico, as well as Asian taxa such as Saichania from the middle and upper Campanian Barun Goyot Formation of Mongolia. The occurrence of Taxon A and Nodocephalosaurus suggest a new clade that is restricted to southern Laramidia. Taxon B shows a close relationship with Late Cretaceous northern Laramidian ankylosaurids, such as Dyoplosaurus and Euoplocephalus, providing evidence that this clade of ankylosaurids extended into southern Laramidia. The divergent phylogenetic relationship between the northern and southern Laramidian clades suggests at least two intercontinental dispersal events between Asia and North America during the Late Cretaceous. The oldest occurrence of northern Laramidian ankylosaurids is recorded from the middle Campanian stage (77 Ma) of the Dinosaur Park Formation, suggesting an earlier dispersal from Asia compared to the younger, southern Laramidian clade, which stratigraphically first appear at 76 Ma in the upper Campanian Kaiparowits Formation. These new data support the hypothesis for distinct northern and southern biogeographic provinces for ankylosaurids during the upper Campanian in Laramidia.