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The Hell Creek Formation ecosystem was an environment full of dangerous animals. With carnivores like the infamous Tyrannosaurus roaming the woods, many herbivores evolved countermeasures unlike anything seen in nature before or since. Edmontosaurus grew to enormous sizes, rivaling the largest of hadrosaurs. Triceratops developed its characteristic neck shield and brow horns.
Then there were the ankylosaurs.
While their family had been around since the late Jurassic, their largest and most heavily-armored members lived in North America alongside the more fearsome of the carnivores. Its six-ton body was covered with ninety bony plates (called Osteoderms), and the last few vertebrae on its tail had fused into a strong, club-like structure that could be swung around with great speed at a wide angle. Even its eyelids had sheaths of bone for protection. Its shorter limbs may have prevented it from running very quickly, but an adult could easily hold its own in dangerous situations. However, new studies into Ankylosaurus and its relatives have provided a great deal of insight into how this animal would have lived beyond just defense.
First off, it was a very solitary animal. Fossils of ankylosaurids are rare and only found one at a time, suggesting very few times- if any- where they met in any substantial groups. They also had massive paraglossal bones which, in modern birds, corresponds to long, flexible tongues. (Source) They undoubtedly were helpful in browsing fruits, tubers, and other low-lying plants, but there are many possible uses for them. Some species of skink have colorful tongues they display when threatened, and camels have special organs behind them that they inflate outside their mouths during mating season as a way to harmlessly compete with other males. Neither one of these soft tissue aspects preserves well in the fossil record, meaning that such a structure may be plausible in an animal as large as Ankylosaurus.
Another thing to consider is their environment. At the time it lived, western North America was humid and warm, covered in a patchwork of thick rainforest and prairies of ferns and horsetails. The Hell Creek formation especially was rich in swamps and river deltas, meaning that anything that lived here had to swim at least periodically. At first, an armored tank of an animal seems like it wouldn't last long in such an place, let alone thrive, but studies on the thickness of their bones found that they had strong lungs that could hold their breath for long periods of time while their unusually dense limbs kept them down close to the bottoms of the rivers and lakes they needed to swim through. (Source) This isn't entirely dissimilar to modern Hippopotamus, who, while their skeleton may not seem fit for a semiaquatic animal at first glance either, are entirely comfortable in deep water and need it to survive. This may not necessitate a similar lifestyle for Ankylosaurus and its relatives, but the niche was certainly there, and it's not too far a stretch to say an they could have easily filled it.
This image shows a hypothetical situation that fulfills the hypotheses mentioned above, 66 million years ago. A small dromaeosaurid, Achillobator, delicately navigates her way across a river. She does her best to keep her feathers dry and stay out of the mouths of any crocodilians that may be lurking below. Unfortunately for her, she has accidentally stepped into the territory of something even more dangerous. A male Ankylosaur nearby is patrolling his territory, and he has to keep it clear of any unwanted items if he wants to impress a mate, including any potential threats to a nest. His thick skull means his brain is one of the smallest for any large animal, meaning he doesn't care to distinguish between predators and animals just passing by. One look at the intruder is enough to send him into a frenzy, charging at the poor raptor at full force with his tongue unfurled to flash its warning colors. For the little dinosaur, its survival may depend entirely on luck.
