At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah

18 Review of Late Cretaceous Ankylosaurian Dinosaurs from the Grand Staircase Region, Southern Utah

445 18 Review of Late Cretaceous Ankylosaurian Dinosaurs from the Grand Staircase Region, Southern Utah Mark A. Loewen, Michael E. Burns, Michael A. Getty, James I. Kirkland, and Matthew K. Vickaryous New ankylosaur specimens from Grand Staircase– Escalante National Monument, Utah, provide data on the distribution and diversity of ankylosaurian dinosaurs of southern Laramidia. These materials are from the Dakota, Straight Cliffs, Wahweap, Kaiparowits, and laterally equiva­ lent formations of the Grand Staircase of southern Utah. The earliest record of ankylosaurs from Grand Staircase– Escalante National Monument is based on a tooth from the Dakota Formation. Teeth and postcranial elements of both nodosaurids and ankylosaurids are present in the Straight Cliffs and Wahweap formations, and their lateral equiva­ lents. Specimens from the Kaiparowits Formation include nodosaurid teeth and a distal cervical spine, and several ankylosaurid specimens represented by teeth, cranial frag­ ments, postcrania, osteoderms, and tail clubs. Analysis of the ankylosaurid material from the Kaiparo­ wits Formation indicates that there are two distinct, essen­ tially coeval, ankylosaurids from the lower middle unit of the Kaiparowits Formation. Introduction Ankylosaur dinosaurs are a characteristic component of di­ nosaur faunas from the Upper Cretaceous of western North America (Laramidia). Campanian strata have yielded numer­ ous specimens of nodosaurids and ankylosaurids, mostly in the north, particularly in Alberta, Canada. Previously, our knowledge of ankylosaurs in the southern part of Laramidia has largely been limited to isolated teeth and osteoderms in Utah, New Mexico, and Texas. In Alberta, a number of ankylosaurids including Dyo­ plosaurus acutosquameus (Parks, 1924), Scolosaurus cutleri (Nopsca, 1928), and Anodontosaurus lambei (Sternberg, 1929) were synonymized into Euoplocephalus tutus (Lambe, 1902, 1910) by Coombs (1978). Although this taxonomic lumping of Campanian ankylosaurids has been questioned (Penkalski, 2001), it has until recently remained unaltered. However, as part of the results of an ongoing revision of Euoplocephalus (Arbour, Burns, and Sissons, 2009; Arbour, 2010), Dyoplo­ saurus is now recognized as distinct from Euoplocephalus (Arbour, Burns, and Sissons, 2009). Nodosaurids from north­ ern Laramidia are best represented by three taxa from Alber­ ta, Edmontonia rugosidens and Panoplosaurus mirus from the late Campanian Dinosaur Park Formation and Edmontonia longiceps from the late Campanian to early Maastrichtian Horseshoe Canyon Formation (Vickaryous, Maryańska, and Weishampel, 2004; Ryan and Evans, 2005). Late Cretaceous ankylosaurs from the southern portion of Laramidia are less well known. Nodocephalosaurus kirt­ landensis (Sullivan, 1999; Sullivan and Fowler, 2006) is an ankylosaurid represented by a partial skull from the Kirtland Formation of the San Juan Basin of New Mexico. This speci­ men is geologically younger than any of the specimens from the Grand Staircase of southern Utah (Roberts, Deino, and Chan, 2005; Sullivan and Lucas, 2006; Roberts et al., this volume, Chapter 6). Nodosaurid material has been recov­ ered from the San Juan Basin of New Mexico but is younger than the material reviewed here. This material includes Glyptodontopelta mimus from the Naashoibito Member of the Maastrichtian Ojo Alamo Formation (Ford, 2000; Burns, 2008). Finally, Aletopelta coombsi (Ford and Kirkland, 2001) has been described from the upper Campanian of south­ ern California and is distinguished in having a mosaic of ankylosaurid (basally excavated osteoderms and short distal limb elements) and nodosaurid (large teeth and a mosaic of polygonal osteoderms over the pelvis) characters. Although ankylosaur remains are considered rare in the late Cretaceous of Utah, previous workers have recovered remains of specimens from the Grand Staircase region. Most of these specimens are teeth recovered as a result of intensive screen washing of sediments by R. Cifelli and J. Eaton (Eaton, Cifelli et al., 1999, Eaton, Diem et al., 1999). Isolated ankylosaur teeth and small osteoderms (i.e., ossi­ cles) are rarely encountered in vertebrate microfossil sites, in contrast to their relative abundance in contemporary north­ ern formations of the Western Interior Basin. Macrofossil ankylosaur material recently collected under the Kaiparo­ wits Basin Project, mostly from the Kaiparowits Formation, has greatly increased our knowledge of ankylosaurs from Loewen et al. 446 southern Utah (Getty, Vickaryous, and Loewen, 2009). Asso­ ciated macrofossil ankylosaur remains represent less than 5% of over 32 recorded associated dinosaur localities in the Kai­ parowits Formation (Getty, Vickaryous, and Loewen, 2009), underscoring their rarity. Although ankylosaurs represent a relatively small component of the total Kaiparowits Forma­ tion dinosaur fauna, the new material from the Kai­ parowits and other Campanian formations of southern Utah is diag­ nostic enough to warrant a preliminary comparison with contemporaneous ankylosaurs to the north and south along Laramidia. Materials and Methods Here we reevaluate all available fossil specimens...