Description and Classification

Ankylosaurus was one of the largest known ankylosaurid dinosaurs, often described as a living tank. It was a heavily built, four-legged herbivore that grew to be about 6 to 8 meters (20 to 26 feet) long and may have weighed between 4.8 and 8 metric tons (about 5 to 9 short tons). Its body was low to the ground and very wide, covered from its head to its tail in bony armor plates called osteoderms. These osteoderms were embedded in its skin and varied in size, creating a formidable shield.

The skull of Ankylosaurus was also unique, being broad, triangular, and heavily armored with fused bony plates. It even had bony shutters that could protect its eyes and horn-like projections at the back of its head. Inside its snout were complex air passages, which might have helped with its sense of smell, warming the air it breathed, or even making sounds. Despite its large body, Ankylosaurus had a relatively small brain. Its teeth were small and leaf-shaped, suitable for nipping at plants rather than heavy chewing.

Ankylosaurus belongs to the order Ornithischia, the “bird-hipped” dinosaurs. More specifically, it is part of the Thyreophora group, which includes other armored dinosaurs like Stegosaurus. Within Thyreophora, it is classified in the family Ankylosauridae, of which it is the most famous member and gives the family its name. Ankylosaurids are known for their broad, armored heads and massive tail clubs, distinguishing them from their nodosaurid cousins who generally lacked such clubs. The only recognized species is Ankylosaurus magniventris, with “magniventris” meaning “great belly,” likely referring to its wide body.

Distinguishing Features

Ankylosaurus had several distinct features that set it apart from other dinosaurs:

• A massive, bony club at the end of its tail, formed by several large osteoderms fused to the last few tail vertebrae.
• An extremely wide, heavily armored body covered in bony plates (osteoderms) and knobs for defense.
• A broad, low, triangular skull with fused bones, two horns pointing backward from the rear of the head, and two horns below these that pointed down and backward.
• Short, powerful legs, with the back legs being slightly longer than the front legs.
• Complex nasal passages within its snout, possibly for an enhanced sense of smell or vocalization.
• Small, leaf-shaped teeth designed for cropping vegetation rather than extensive chewing.

Paleoenvironment and Diet

Ankylosaurus lived during the final stage of the Cretaceous Period, known as the Maastrichtian age, in what is now western North America. Its fossils have been found in geological formations such as the Hell Creek Formation (Montana, North Dakota, South Dakota), Lance Formation (Wyoming), and Scollard Formation (Alberta, Canada). During this time, the environment was a mix of coastal plains, forests, and river systems, with a warm and humid climate. Ankylosaurus shared this world with many other well-known dinosaurs, including the fearsome predator Tyrannosaurus rex, the horned Triceratops, and the duck-billed Edmontosaurus.

As an herbivore, Ankylosaurus fed on plants. Its low-slung body meant it likely browsed on low-growing vegetation such as ferns, cycads, and possibly early flowering plants. Its small teeth and weak chewing ability suggest that it didn’t grind its food much in its mouth. Instead, it probably swallowed mouthfuls of plant material that were then broken down in its large digestive system, perhaps through fermentation in its “great belly.”

Significance and Ongoing Research

Ankylosaurus is a significant dinosaur because it was one of the last and largest members of the armored ankylosaurids, existing right up until the mass extinction event that ended the age of dinosaurs. It represents a peak in defensive evolution, showcasing incredible armor and a formidable tail club likely used to defend against large predators like Tyrannosaurus rex. Discoveries of Ankylosaurus fossils, though not as common as some other dinosaurs from its time, provide valuable information about the ecosystems of Late Cretaceous North America.

Ongoing research continues to explore various aspects of Ankylosaurus. Scientists are studying the biomechanics of its tail club to understand how it was swung and the force it could generate, debating whether it was primarily for defense or also for display or fighting rivals. The exact arrangement and function of its body armor are still being investigated with new fossil finds. Paleontologists also work to understand more about its diet, how it moved, and its growth patterns. While famous, complete skeletons of Ankylosaurus are rare, so each new discovery has the potential to reveal more secrets about this fascinating armored dinosaur and its place in the prehistoric world.