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.

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,[2] 64 m (210 ft) above sea level.

[1] 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.

[1] 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.

[2] 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.

[10] Panoplosaurus was a rather large animal at 1,600 kg (3,500 lb), a comparable size to other ankylosaurs from the same location, and heavier than or approximately equivalent to all predators it coexisted with.

[2] There is also a prominent armour plate covering the cheek in the type specimen of Panoplosaurus, which may be unique feature of the taxon,[12] or individual, depending on what additional skulls are referred to P.

[1] 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 occipital, 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.

[1] 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.

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 premaxillaries in the upper jaw.

The top margin of the scapula forms a shelf as it approaches the coracoid, terminating in a prominent acromion process that projects outwards from the animal, and directly overhangs a large rugose attachment area for the deltoid muscle.

[2] The humerus is a robust, 430 mm (17 in) long bone, with the shape and projection of the head suggesting the upper arm of Panoplosaurus was held in a flexed position in life.

Three bands of cervical osteoderms were present in both genera, consisting of rounder plates that united on the midline of the animal, and one narrower element on each side with a sharp keel.

[14] Nopcsa's classification of Panoplosaurinae was amended slightly by Charles W. Gilmore in 1930, who moved Palaeoscincus into the subfamily and removed Dyoplosaurus, which was discovered to have the skull of an ankylosaurine.

[17] Disagreeing with the classifications based on the work of Nopcsa, Evgeny Maleev moved Panoplosaurus into the family Ankylosauridae, which he considered to contain all ankylosaurs except Syrmosaurus.

[18] Coombs reviewed and revised the classifications of Ankylosauria in 1978, which he used as the group to encompass all heavily armoured ornithischians in a similar usage to Nodosauridae of Nopcsa.

Panoplosaurus was resolved next to Edmontonia, deep within an unresolved polytomy of all nodosaurids more derived than Animantarx, which included Niobrarasaurus, Nodosaurus, Pawpawsaurus, Sauropelta, Silvisaurus, Stegopelta, and Texasetes.

[23] In 2016, the phylogenetic analysis of Arbour and Currie initially meant to test the relationships of Ankylosauridae was expanded to include many of the nodosaurids known at the time, with Panoplosaurus limited to the holotype due to a lack of consensus about referred specimens.

[34] The reference phylogeny for Panoplosaurini designated by Madzia and colleagues was that of Rivera-Sylva et al. (2018),[34] which is a modified version of the Arbour and Currie analysis expanded to include the Mexican taxon Acantholipan.

While the shape of teeth in Panoplosaurus and other ankylosaurs suggests a simple, soft food diet, their variability implies a less restrictive feeding strategy.

[37] Stomach contents of the closely related taxon Borealopelta were identified amongst gastroliths, showing that at least it had a diet regularly consisting of almost 85% fern material, along with 3.7% cycad matter, trace elements of conifers, and 11.4% undiagnostic plant remains.

[43] The climate of the environment in the Cretaceous was much different than in present day, being warmer and more temperate, with wet and dry seasons allowing for a large variety of flora and fauna.

The forest floor was covered by decaying vegetative matter, small ferns, lycopods and angiosperms, mosses, lichens, and fungi, with plentiful algae where there was standing water.

[45] The constant presence of water in the Dinosaur Park Formation led many forms of freshwater or marine animals to enter the otherwise predominantly terrestrial ecosystem.

[46] In the lower Dinosaur Park Formation, assemblages of crevasse sites show that mollusks were commonly dominated by the freshwater clam Sphaerium, which occurred with abundant gastropods of the genera Goniobasis and Lioplacodes.

The ray Myledaphus is characteristic of the formation and similar deposits, and lived alongside the less common shark Hybodus montanensis, intermediate paddlefish and sturgeons, the long, slender fish Belonostomus, the gar Lepisosteus, bowfins, and an abundance of small teleosts including Paratarpon and Cretophareodus.

[50] Fragmentary material from the choristodere Cteniogenys, and many well-preserved skulls and skeletons of its relative Champsosaurus, is present among the fauna of the Dinosaur Park Formation, representing an extinct group of semi-aquatic animals with crocodilian features that spanned the globe for much of the Mesozoic and Cenozoic.

[54] A rich and diverse vertebrate assemblage is known from the Dinosaur Park Formation, with the lower region, excluding the Lethbridge Coal Zone, being formed by terrestrial and coastal deposits.

[11] Six small lizards are known, representing five different families, with the teiids Socognathus and Glyptogenys, the xenosaurid Exostinus, the helodermatid Labrodioctes, the necrosaurid Parasaniwa, and the varanid Palaeosaniwa.