Edmontosaurus did not have a single 'defense' — it relied on a combination of speed, herd behavior, and size to avoid becoming prey. This is a fascinating predator-prey dynamic from the Late Cretaceous (around 75 million years ago in what is now Alberta, Canada).
Edmontosaurus was a large hadrosaur (duck-billed dinosaur) that lived in herds — fossil trackways and bone beds show dozens of individuals together. Herds provide safety in numbers: more eyes to spot predators, confusion that makes it harder for a hunter to target a single individual, and collective defense (adults surrounding juveniles). A solitary Edmontosaurus was much more vulnerable than one in a group.
Edmontosaurus was bipedal and quadrupedal — it could rear up on hind legs to run. At roughly 13 meters long and 4 tons, it was not as fast as smaller dinosaurs, but it could outrun Albertosaurus in short bursts, especially in broken terrain or through dense vegetation. Albertosaurus, despite being a 9-meter apex predator, was built for power and bite force, not pure sprint speed over distance.
An adult Edmontosaurus outweighed Albertosaurus by roughly 2 tons. While Edmontosaurus was an herbivore without teeth or claws designed for combat, a full-grown adult could inflict serious injury with tail strikes or trampling — enough to deter an attack. Young or juvenile Edmontosaurus were the real targets; adults were higher-risk prey.
Fossils show that Albertosaurus likely targeted young, sick, or isolated Edmontosaurus individuals rather than engaging healthy adults in herds. This is how modern predators work too — lions target young wildebeest or stragglers, not a buffalo in the middle of the herd. Predator and prey evolved together: Edmontosaurus evolved herding and size, Albertosaurus evolved pack hunting (evidence suggests they may have hunted in small groups) and selective prey choice.
Pro tip: The real 'defense' was evolutionary — populations of Edmontosaurus that lived in herds and had better hearing/smell to detect predators survived and reproduced. Over millions of years, natural selection shaped both the prey's vigilance and the predator's hunting strategy. This arms race is visible in the fossil record.