[1] In 2010, Gregory S. Paul considered both main Edmontonia species, E. longiceps and E. rugosidens, to be equally long at six metres and weigh three tonnes.

In 1990, Kenneth Carpenter established some diagnostic traits for the genus as a whole, mainly comparing it with its close relative Panoplosaurus.

The type species, Edmontonia longiceps, is distinguished from E. rugosidens in lacking sideways projecting osteoderms behind the eye sockets; having tooth rows that are less divergent; possessing a more narrow palate; having a sacrum that is wider than long and more robust; and in having shorter spikes at the sides.

The cutting edge of the upper beak continued into the maxillary tooth rows, each containing fourteen to seventeen small teeth.

The top of these was a horizontal oval and represented the bony external nostril, the entrance to the nasal cavity, the normal air passage.

The more rounded second opening below and obliquely in front, was the entrance to a "paranasal" tract, running along the outer side of the nasal cavity, in a somewhat lower position.

A study by Matthew Vickaryous in 2006 proved for the first time the presence of multiple openings in a nodosaurid; such structures had already been well established in ankylosaurids.

The tiles behind the upper eye socket rim in Edmontonia longiceps do not stick out as much as in E. rugosidens, combined with a more narrow, pointed snout in the former.

The neck and shoulder region was protected by three cervical halfrings, each consisting of fused rounded rectangular, asymmetrically keeled, bone plates.

Below each lower end of the second halfring a side spike was present, a separate triangular osteoderm pointing obliquely forward.

In the third halfring over the shoulders, the two pairs of central segments are bordered on each side by a very large forward-pointing spike that is bifurcated, featuring a secondary point above the main one.

Gilmore had trouble believing that the shoulder spikes really pointed to the front as this would have greatly hampered the animal while moving through vegetation.

The front rows have plates oriented along the length of the body, but to the rear the long axis of these osteoderms gradually rotates sideways, their keels ultimately running transversely.

[4] In 1915, the American Museum of Natural History obtained the nearly complete, articulated front half of an armoured dinosaur, found the same year by Barnum Brown in Alberta, Canada.

[17] This taxon was erected by Bakker in 1988 for a skull from the Late Maastrichtian Upper Cretaceous Lance Formation of South Dakota, specimen DMNH 468 found by Philip Reinheimer in 1922.

In 1988 Bakker proposed that the Edmontoniinae with the Panoplosaurinae should be joined into Edmontoniidae, the presumed sister group of the Nodosauridae within Nodosauroidea which he assumed not be ankylosaurians but the last surviving stegosaurians.

[2] Rings in the petrified wood of trees contemporary with Edmontonia show evidence of strong seasonal changes in precipitation and temperature;[1] this may hold an explanation for why so many specimens have been found with their armor plating and spikes in the same position they were in life.

[1] The Edmontonia could have died due to drought, dried up, and then rapidly became covered in sediment when the rainy season began.

[1] Edmontonia rugosidens existed in the upper section of the Dinosaur Park Formation, about 76.5–75 million years ago.

It lived alongside numerous other giant herbivores, such as the hadrosaurids Gryposaurus, Corythosaurus and Parasaurolophus, the ceratopsids Centrosaurus and Chasmosaurus, and ankylosaurids Scolosaurus[11] and Dyoplosaurus[11] Studies of the jaw anatomy and mechanics of these dinosaurs suggests they probably all occupied slightly different ecological niches in order to avoid direct competition for food in such a crowded eco-space.

[25] The only large predators known from the same levels of the formation as Edmontonia are the tyrannosaurids Gorgosaurus libratus and an unnamed species of Daspletosaurus.

[11] Edmontonia longiceps is known from the Horseshoe Canyon Formation, from the middle unit, which was dated to 71.5-71 million years ago in 2009.

The saltwater plesiosaur Leurospondylus has been found in marine sediments in the Horseshoe Canyon, while freshwater environments were populated by turtles, Champsosaurus, and crocodilians like Leidyosuchus and Stangerochampsa.

Along with much rarer ankylosaurians and pachycephalosaurs, all of these animals would have been prey for a diverse array of carnivorous theropods, including troodontids, dromaeosaurids, and caenagnathids.

[26][27] Adult Albertosaurus was the apex predator in this environment, with intermediate niches possibly filled by juvenile albertosaurs.