Imagine Dinosaurs's video: Ankylosaurs: Ankylosaurus

Often compared to an army tank or bus, Ankylosaurus was a heavily armored dinosaur with a large club-like protrusion at the end of its tail. Ankylosaurus means "fused lizard" in Greek, and it was given that name because bones in its skull and other parts of its body were fused, making the dinosaur extremely rugged. Ankylosaurus lived in the late Cretaceous Period, about 65.5 million to 66.8 million years ago, and roamed the Western United States and Alberta, Canada. While this herbivorous dinosaur was a massive animal, a re-examination of its fossils in 2004 by armored dinosaur expert Kenneth Carpenter downsized it a bit. The largest Ankylosaurus specimen ever found was 20.5 feet (6.25 meters) long, 5.6 feet (1.7 m) tall at the hips and 4.9 feet (1.5 m) wide, according to the study, published in the Canadian Journal of Earth Sciences. It likely weighed up to 4 tons (3.6 metric tons). The top of the dinosaur was almost completely covered with thick armor consisting of massive knobs and oval plates of bone, known as osteoderms or scutes, which are also common on crocodiles, armadillos and some lizards. "They are bones that form within the skin, just like crocodiles," Carpenter, who's the director of the USU Eastern Prehistoric Museum in Utah, told Live Science. The osteoderms of all ankylosaurs (dinosaurs in Ankylosaurus' suborder, Ankylosauria) were composed of an thin outer cortical or compact bone and an thick inner cancellous bone (spongy, porous bone), according to an analysis Carpenter and his colleagues published in the journal Acta Palaeontologica Polonica in 2010. The osteoderms were probably covered with skin and keratin, the fibrous protein that makes up hair and nails in people.