While this four-legged dino was certainly big – the largest Ankylosaurus ever found was 20.5 feet (6.25 meters) long and weighed around 4 tons (3.6 metric tons) – Ankylosaurus was an herbivore, a plant-eating tank of an animal. And we do mean tank. The Ankylosaurus was covered with thick, bony plates – a defense that came in handy for this hefty herbivore and lent it its name. Ankylosaurus means "fused lizard" in Greek – the bony plates were fused to the dinosaur's skull and other body parts making it extremely sturdy.
Dr. Hans Sues is the senior research geologist and curator of fossil vertebrates in the department of paleobiology at the Smithsonian Institution's National Museum of Natural History in Washington, D.C., and has published widely on many species of dinosaurs, including a 2020 article on the skull of one species from the ankylosaur group in the journal Biological Communications.
"The first line of defense was their outer armor composed of bony plates," Sues says via email. "In fact, ankylosaurs were so heavily armored that even their upper eyelid was protected by a bony plate. One group of ankylosaurs, Ankylosauridae, had massive clubs of bone at the end of their tails, and they could swing their tails to inflict massive damage on the legs of an attacking predator."
The Ankylosaurus' body was thick and squat, its legs sturdy. It had a broad, triangular shaped skull. But it was the armor that made the difference – hard plates covered with bony spikes jutting from its flanks and bony knobs called osteoderms or scutes protruding from its back. Horned protuberances stood out above each eye and on the sides of the face. Not all ankylosaurs had the same armor patterning but all had armor to protect them from predators, most notably Tyrannosaurus, Tarbosaurus and Deinonychus.
And ankylosaurs needed that protection because they spent most of the day eating massive amounts of food – stripping leaves from low-lying plants.
"They have small teeth similar to those of present-day plant-eating lizards," Sues says. "An amazing discovery of a nearly perfect ankylosaur skeleton from Alberta [British Columbia, Canada] preserved gut contents that confirmed that ankylosaurs ate plants." However, Sues shared a startling discovery. "One small-bodied ankylosaur recently discovered in northern China apparently went into lakes and even ate small fish," he says.
Though the first ankylosaur fossil was discovered by Barnum Brown in the Hell Creek Formation in Montana in 1906, Sues says ankylosaurs have been found in many places throughout the world including Europe, Antarctica, Asia, Australia and North and South America.
"In fact, one of the first dinosaurs ever to be discovered was an ankylosaur, Hylaeosaurus from southern England, discovered in 1831," he says.