Dyoplosaurus (meaning “double-armoured lizard”) is a monospecific genus of ankylosaurid dinosaur from Alberta that lived during the Late Cretaceous (middle Campanian, ~76.5–75 Ma) in what is now the Dinosaur Park Formation. Dyoplosaurus represents a close relative of Scolosaurus and Anodontosaurus, two ankylosaurids known from the Horseshoe Canyon and Dinosaur Park Formation. Discovery and naming The holotype specimen was obtained in 1919 from the bottom ten metres of the Dinosaur Park Formation by Levi Sternberg, near what is now the Red Deer River in Alberta, Canada. The holotype specimen, ROM 784, consists of a partial skull roof, mandible fragments with teeth, osteoderms, skin impressions, articulated post-thoracic vertebrae, partial thoracic ribs, a partial ilium, both ischia, tail club, associated radius, metacarpal, femur, tibia, fibula, and pes. The holotype is currently housed at the Royal Ontario Museum. Two specimens were referred to Dyoplosaurus, ROM 7761 and UA 47273, and both consist of partial tail clubs. The generic name, Dyoplosaurus, is derived from the Greek words “dyo” (double), “hoplon” (weapon, shield, armour) and “sauros” (lizard). The specific name, acutosquameus, is derived from the Latin words “acutus” (sharp) and “squama” (scale). In 1956, Evgeny Maleev named a second species of Dyoplosaurus: D. giganteus. The species was based on the very large specimen PIN 551/29, which consists of a series of caudal vertebrae, metatarsals, phalanges and osteoderms including tail club knob from the Nemegt Formation of Mongolia. The species was diagnosed based on the short anterior caudal vertebrae; large chevrons that were fused to the caudal vertebrae; low, long distal caudal vertebrae; short, wide metatarsals; thick, hoofed-shaped unguals; sharp, thin-walled osteoderms with numerous pits and channels on the external surface. However, a 2014 study by Arbour and colleagues considered that the holotype lacked diagnostic traits, as such traits are present in all ankylosaurines, and considered the species to be a nomen dubium. Description Dyoplosaurus has an estimated body length of and body mass of . All referred specimens represent almost fully mature individuals. Dyoplosaurus can be distinguished from all other ankylosaurids in having sacral ribs that are anterolaterally-directed, triangular unguals in dorsal view and a tail club knob that is longer than wide. It differs from Scolosaurus in having a proportionately shorter postacetabular process of the ilium and triangular osteoderms on the lateral sides of the anterior portion of the tail. Dyoplosaurus also differs from Euoplocephalus in the pelvis as it has anterolaterally projecting, ventrally directed