Ankylosaur systematics: example using Panoplosaurus and Edmontonia (Ankylosauria: Nodosauridae)

Three species of nodosaurid ankylosaurs are present in the Upper Cretaceous of the Western Interior. These are Panoplosaurus mirus Lambe 1919, Edmontonia longiceps Sternberg 1928, and Edmontonia rugosidens (Gilmore 1930). Stratigraphically, P. mirus and E. rugosidens occur in the Middle Campanian Judith River and Two Medicine Formations of Alberta and Montana. E. longiceps occurs in the Campanian–Maastrichtian Horseshoe Canyon Formation. Panoplosaurus sp. is present in the Campanian Aguja Formation of Texas and the Campanian Fruitland or Kirtland Formations of New Mexico. Edmontonia sp. occurs in the Campanian Aguja Formation of Texas, and the Maastrichtian Laramie Formation of Colorado, the Lance Formation of Wyoming, and the Hell Creek Formation of South Dakota. Panoplosaurus is characterized by: skull with tapered snout when viewed dorsally; reniform cranial armor; swollen, grooved vomer; tall neural pedicles; tall, slender neural spines; four co-ossified sacral vertebrae; co-ossified coracoid and scapula; absence of laterally projecting spines; keeled plates longer than wide. Edmontonia is characterized by: skull with parallel sided snout when viewed dorsally; “smooth” cranial scutes; keeled vomer; neural pedicles and neural spines shorter than in Panoplosaurus; three co-ossified sacral vertebrae; coracoid not fused to scapula. Of the two species of Edmontonia , the stratigraphically older E. rugosidens is distinguished from the stratigraphically younger E. longiceps by the presence of postorbital prominences, divergent tooth rows and wide palate, a synsacrum that is longer than wide (hence less robust), and larger lateral body spines.