Edmontonia rugosidens
Edmontonia rugosidens was a heavily armoured, four-legged plant-eater with a low, broad, barrel-shaped body about six to nearly seven metres long. Rows of bony plates called osteoderms were embedded in the skin across its neck, back and flanks, and a pair of long spines projected forwards over the shoulders.
Edmontonia rugosidens
Edmontonia rugosidens
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Weight
2500–4000 kg
Length
6–6.8 m
Diet
Herbivore
Period
Cretaceous
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Edmontonia rugosidens next to Tyrannosaurus rex and a human, to the same scale.
Edmontonia rugosidens
Tyrannosaurus rex
2500–4000 kg
6–6.8 m
Cretaceous
Quadrupedal
1.4–1.8 m
10 km/h
Herbivore
9 recorded fossil finds in United States and Canada. Most in Wyoming, Alberta, and Montana. First described in 1930 by Gilmore.
Sites and counts from the Paleobiology Database
The timeline shows the whole age of dinosaurs – and how much time has passed since. The glowing part is when Edmontonia rugosidens lived.
million years ago
Lived 76.5–74 million years ago
If the whole age of dinosaurs were a single day, Edmontonia rugosidens lived at 22:48.
Chapter • Meet the dinosaur
Edmontonia rugosidens was a heavily armoured, four-legged plant-eater with a low, broad, barrel-shaped body about six to nearly seven metres long. Rows of bony plates called osteoderms were embedded in the skin across its neck, back and flanks, and a pair of long spines projected forwards over the shoulders. In this species the leading shoulder spines were especially robust and split at the tip, which is one of the clearest features separating it from the closely related Edmontonia longiceps. The skull was long and low, ending in a wide, toothless beak, with small leaf-shaped cheek teeth further back. Unlike the ankylosaurids, Edmontonia had no tail club — all of its heavy defensive gear sat on the front half of the body. As a nodosaurid, Edmontonia moved slowly and close to the ground. With an estimated top speed of around ten kilometres per hour it could not outrun a tyrannosaurid, so palaeontologists interpret the armour and shoulder spines as a combined defence and display system. The most likely strategy was to turn the armoured front end toward an attacker and hold its ground like a living shield. The large forked shoulder spines may also have been used in shoving contests between rivals, in which two animals pushed against each other without inflicting serious injury — behaviour comparable to that of modern hoofed mammals. Edmontonia rugosidens was a dedicated herbivore. Its broad beak was well suited to cropping low-growing vegetation such as ferns, horsetails, cycads and the flowering plants that were spreading rapidly through the Late Cretaceous. The teeth were small and simple, so chewing was limited; instead the animal carried an enormous gut in which fermenting bacteria broke down tough plant fibre. The barrel-shaped torso was therefore as much a product of digestion as of armour. The first bones later assigned to this species were described in 1930 by the American palaeontologist Charles W. Gilmore, who named them Palaeoscincus rugosidens on the basis of a well-preserved specimen from the Two Medicine Formation of Montana. Edmontonia had been erected as a genus by Charles M. Sternberg in 1928 for Edmontonia longiceps, and rugosidens was eventually transferred into it. In 1988 Robert Bakker argued that the species was distinct enough to warrant its own name, Chassternbergia, but most researchers still treat it as a species of Edmontonia. Additional material is known from the Dinosaur Park Formation of Alberta, Canada.
Chapter • Where it lived
Edmontonia rugosidens lived during the Campanian stage of the Late Cretaceous, roughly 76.5 to 74 million years ago. Fossils come from the Two Medicine Formation of Montana, USA, and the Dinosaur Park Formation of Alberta, Canada. The landscape was one of broad river floodplains and coastal lowlands along the western shore of the Western Interior Seaway, with meandering channels, wetlands and stands of conifers, ferns and early flowering plants. The climate was warm and humid with a marked wet season. Edmontonia shared this world with hadrosaurs such as Gryposaurus, horned dinosaurs such as Chasmosaurus, and predators including Gorgosaurus and Daspletosaurus.
Chapter • Wild behavior
Diurnal
The forward shoulder spines of Edmontonia rugosidens were split at the tip — a feature absent in Edmontonia longiceps.
Unlike Ankylosaurus, Edmontonia had no tail club; all of its heavy armament was concentrated on the neck, shoulders and back.
The species name rugosidens means roughly 'wrinkled tooth', referring to the coarse surface of the tooth crowns.
It weighed about as much as a modern rhinoceros, but was far longer and much lower slung.
Its armour plates sat embedded in the skin, like the osteoderms of living crocodiles and armadillos, rather than fused to the skeleton.
With an estimated walking pace of about ten kilometres per hour, Edmontonia could not flee predators and had to stand its ground.
The species was first described in 1930 under an entirely different name, Palaeoscincus rugosidens, and has been shuffled between genera ever since.
The wide, toothless beak worked like a lawnmower for low ferns and horsetails.
Chapter • Scientific place
Cambridge University Press (in Dinosaur Systematics)
Reviewed: 9/4/2026
Peer-reviewed systematic treatment placing Edmontonia rugosidens within Nodosauridae (Ankylosauria, Thyreophora, Ornithischia) alongside Panoplosaurus mirus and Edmontonia longiceps.
Open sourcePaleobiology Database (Fossilworks gateway)
Reviewed: 9/4/2026
Paleobiology Database taxon page listing the type specimen USNM 11868 from the Two Medicine Formation of Montana and Cretaceous occurrences in Alberta (Canada) and Montana/Wyoming (USA).
Open sourceUniversity of Montana (ScholarWorks)
Reviewed: 9/4/2026
University-hosted palaeobotanical study documenting the Campanian floodplain vegetation and depositional palaeoenvironment of the Two Medicine Formation in which Edmontonia rugosidens lived.
Open sourceNatural History Museum, London
Reviewed: 9/4/2026
Museum reference listing Edmontonia among plant-eating (herbivorous) dinosaurs, supporting the low-browsing herbivore diet.
Open sourceGeological Society of America Bulletin
Reviewed: 9/4/2026
Extinct taxon; conservation status NE
Open sourceNatural History Museum, London
Reviewed: 9/4/2026
General authoritative museum overview of Edmontonia, an armoured plant-eating nodosaurid of the Late Cretaceous, used as the reference for the sv, en, da and no description text.
Open sourceNatural History Museum, London
Reviewed: 9/4/2026
General authoritative museum overview of Edmontonia, an armoured plant-eating nodosaurid of the Late Cretaceous, used as the reference for the sv, en, da and no description text.
Open sourceNatural History Museum, London
Reviewed: 9/4/2026
General authoritative museum overview of Edmontonia, an armoured plant-eating nodosaurid of the Late Cretaceous, used as the reference for the sv, en, da and no description text.
Open sourceNatural History Museum, London
Reviewed: 9/4/2026
General authoritative museum overview of Edmontonia, an armoured plant-eating nodosaurid of the Late Cretaceous, used as the reference for the sv, en, da and no description text.
Open source