Panoplosaurus
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 long and 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, above sea level. The specimen, designated by the Canadian Museum of Nature accession number CMN 2759, and excavated 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 1215 and Royal Tyrrell Museum of Paleontology 83.25.2 from the Dinosaur Park Formation of Alberta, and an unnumbered Oklahoma Museum of Natural History 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
Panoplosaurus was a rather large animal at, a comparable size to other ankylosaurs from the same location, and heavier than or approximately equivalent to all predators it coexisted with. Panoplosaurus was about long, and was armoured like most ankylosaurs, but lacked prominent spikes anywhere on its body. The characteristics of its armour are one of the features that distinguishes Panoplosaurus from its relative and contemporary Edmontonia, along with a tapering snout, lumpy cranial armour, a swollen vomer bone in the palate, tall neural arches and neural spines in the vertebrae, a small and round coracoid fused to the scapula, and a hand that may have only had three digits. There is also a prominent armour plate covering the cheek in the type specimen of Panoplosaurus, which may be unique feature of the taxon, or individual, depending on what additional skulls are referred to P. mirus.Skull
The skull of Panoplosaurus is broad and depressed, narrowing towards the end of the blunt snout to form a triangular shape. At a total length of in a straight line, the skull is curved in a way that across the the same length is. Behind the, the skull reaches its maximum width of, giving it a very short and broad appearance. As other nodosaurid specimens from the same location that may be referred to Panoplosaurus lack the distinctively short snout of the holotype, it may be that its unique appearance is due to it being a different age or sex than other individuals. The orbits of Panoplosaurus are very small, and placed far from the snout, which is very squared and bearing laterally open nostrils. Bony plates are present across the entire exterior of the skull, including large osteoderms on all the sides of the skull and mandible, small scutes within the nostrils, and underneath the head in the form of an aggregation of small ossicles covering the surface between the two lower jaws. The, where the skull articulates with the neck, is short and thick, facing nearly directly downwards, which would have meant the head was held with the snout down in life, about 20 degrees below the horizontal. Unlike in Edmontonia, the groove separating cranial osteoderms in Panoplosaurus never disappear, which show that there is a unique narrow scute across the entire rear of the skull.Due to fusion and the covering of osteoderms, the only individual bone of the long mandible than can be identified is the. The rami diverge strongly towards the read of the skull, where they curve inwards at the jaw articulation, and towards the front they bend inwards slightly where the predentary articulates with the, with the thin predentaries meeting at the midline of the jaw. The mandibles are deepest near the rear of the skull, approximately maintaining their depth along the tooth row before narrowing sharply at the front where the predentaries are. Predentaries, which make up the lower portion of the snout, are somewhat horseshoe-shaped, form a sharp beak that fits within the overhang of the in the upper jaw. Both the predentaries and their premaxillary counterparts lack any teeth, a derived feature among nodosaurids where premaxillary teeth are sometimes present. The teeth of the mandible are hidden by armour on the right side, and on the left side of the skull where the cheek plate is not in place, the mandibular teeth are hidden by the teeth of the in the upper jaw. Eight maxillary teeth are preserved, and though it is not certain that was the full tooth count there is not room for many more in the jaw. All the teeth are similar to those of Edmontonia and Palaeoscincus, with a mild expansion of the crown above the root, and formed by prominent ridges on both the front and rear edges of the crown, though there are more on the front than rear edge of the tooth. There are not significant differences between the teeth of Panoplosaurus and those of other armoured dinosaurs.