Panoplosaurus is a genus of armoured dinosaur from the Late Cretaceous of Alberta, Canada. Few specimens of the genus are known, all from the middle Campanian of the Dinosaur Park Formation, roughly 76 to 75 million years ago. It was first discovered in 1917, and named in 1919 by Lawrence Lambe, named for its extensive armour, meaning "well-armoured lizard". Panoplosaurus has at times been considered the proper name for material otherwise referred to as Edmontonia, complicating its phylogenetic and ecological interpretations, at one point being considered to have existed across Alberta, New Mexico and Texas, with specimens in institutions from Canada and the United States. The skull and skeleton of Panoplosaurus are similar to its relatives, but have a few significant differences, such as the lumpy form of the skull osteoderms, a completely fused shoulder blade, and regularly shaped plates on its neck and body lacking prominent spines. It was a quadrupedal animal, roughly 5 m (16 ft) long and 1,600 kg (3,500 lb) in weight. The skull has a short snout, with a very domed surface, and bony plates directly covering the cheek. The neck had circular groups of plates arranged around the top surface, both the forelimb and hindlimb were about the same length, and the hand may have only included three fingers. Almost the entire surface of the body was covered in plates, osteoderms and scutes of varying sizes, ranging from large elements along the skull and neck, to smaller, round bones underneath the chin and body, to small ossicles that filled in the spaces between other, larger osteoderms. Panoplosaurus was originally classified as a stegosaur related to the similarly armoured form Ankylosaurus, a group that was later divided with ankylosaurs becoming their own group. It was then considered close to Edmontonia in the subfamily Panoplosaurinae, but then moved into a general placement in Nodosauridae. Edmontonia was for a time considered the same taxon as Panoplosaurus, making it the only nodosaur from the Campanian of North America, but this was quickly disputed and they are now considered separate. Following consistent placements in phylogenetic analyses close to Edmontonia and the American taxon Animantarx, Panoplosaurus was placed into the clade Panoplosaurini, related but not close to Nodosaurus or Struthiosaurus, which it was considered close to around when it was named. Panoplosaurus is from deposits slightly younger than Edmontonia rugosidens, and existed alongside hadrosaurids like Corythosaurus and Lambeosaurus, ceratopsids like Centrosaurus, and the tyrannosaurid Gorgosaurus, as well as other small dinosaurs like Stegoceras, Dromaeosaurus and Ornithomimus, and various fishes, amphibians, crocodiles and pterosaurs.
Discovery
In 1917, Canadian paleontologist Charles M. Sternberg of the Geological Survey of Canada discovered a complete skull and significant amount of the skeleton of an armoured dinosaur in the sandstone layers of Quarry 69 of the Belly River Group, 64 m (210 ft) above sea level. The specimen, designated by the Canadian Museum of Nature accession number CMN 2759, and excavated 4.43 km (2.75 mi) south of the mouth of the Little Sandhill tributary of the Red Deer River in Alberta, includes a nearly complete skull in articulation, most or all of the cervical vertebrae and the front dorsal vertebrae, and armour plates covering them, a majority of the disarticulated forelimb and three articulated fingers, a fragment of the pelvis and partial sacrum, a few bones of the foot, and multiple hundreds of osteoderms and dermal ossicles. This material was being described by Canadian paleontologist Lawrence M. Lambe of the Geological Society, who completed the description of the skull and osteoderms prior to his death in March of 1919. Lambe's work was published posthumously by the Royal Society of Canada, where he named the new material as Panoplosaurus mirus, and a supplemental description of the vertebrae and limb material, which Lambe had not yet gotten to, was published by Sternberg in 1921. The name Panoplosaurus derived from the hoplon of Greece, translating as "well -armoured lizard".
While the beds of discovery of the holotype, CMN 2759, were originally described as the Belly River beds, they are now considered part of the upper level of the Dinosaur Park Formation, pertaining to the late middle Campanian, 75 to 76 million years ago. Many additional specimens have since been referred to the genus, including Royal Ontario Museum (ROM) 1215 and Royal Tyrrell Museum of Paleontology (RTMP) 83.25.2 from the Dinosaur Park Formation of Alberta, and an unnumbered Oklahoma Museum of Natural History (OMNH) scapulocoracoid from the Aguja Formation of Texas. A scapula from the Naashoibito member of the Kirtland Formation in New Mexico was referred to Panoplosaurus in 1981, but as it is from a different age and location from other specimens, may instead represent the ankylosaurid taxon Nodocephalosaurus, although this is uncertain due to lack of overlapping material. 18 other specimens in the ROM, CMN, American Museum of Natural History, and Princeton University were referred to the genus by Walter P. Coombs in 1978, but these were referred on the assumption that Edmontonia was a synonym of Panoplosaurus, which was considered unjustified by Kenneth Carpenter in a 1990 review on Late Cretaceous nodosaurids. The referral of ROM 1215 to Panoplosaurus was questioned by Coombs in 1990, and Roland A. Gangloff referred it and the Alaskan specimen DPMWA 90–25 to Edmontonia in 1995 based on the anatomy of the lack of diagnostic traits, but Victoria Arbour and colleagues retained ROM 1215 in Panoplosaurus in 2009, limiting the genus to it, the holotype CMN 2759, and specimen AMNH 3072, all from the upper Dinosaur Park Formation. Arbour and Philip J. Currie further restricted Panoplosaurus to just the holotype in 2016, as a result of uncertainties surrounding the referrals of various specimens between it, Edmontonia rugosidens and Edmontonia longiceps, all from the Dinosaur Park Formation and similar deposits.
Description
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