The Ankylosaurs

The ankylosaurs were the tanks of the dinosaur world. These bird-hipped (ornithischian) dinosaurs were closely related to the stegosaurs and distantly related to the ceratopsians, pachycephalosaurs, and ornithopods. Ankylosaurs belonged to the ornithischian sub-group Thyreophora. This group's name comes from a combination of two Greek words that can be read "shield bearers"; they are distinguished by body armor lined up in longitudinal rows along their bodies. Early thyreophorans had simple body armor, while the later members--such as the ankylosaurs and stegosaurs--developed full-fledged spikes and plates. As a general rule, thyreophorans had a small brain-to-body-size ratio, implying that their intelligence may have been on par with (if not lower than) modern-day cows. Also, thyreophorans had smaller forelimbs than they did hind-limbs.

Before thyreophora split into ankylosauria and stegosauria, early thyreophorans looked nothing like the armored dinosaurs most of us know. Scutellosaurus, a forerunner of the larger thyreophorans, was lightly-built, small (just under four feet long), and could probably walk on its hind-legs. 

note the armor along Scutellosaurus' hide

As the thyreophorans evolved, two different routes were taken: that of the stegosaurs (characterized by spikes or plates) and the ankylosaurs. The ankylosaurs were bulky quadrupeds that had squat, powerful limbs. Because of their weight, ankylosaurs probably couldn't run faster than ten kilometers an hour. Bony osteoderms (a type of body armor) covered much of their skin. The armor was arranged in side-by-side rows along their bodies, and the first two rows of armor had a tendency to form a half-ring around the neck. In nodosaurids, this came from adjacent plates fusing with one another; ankylosaurids usually had the plates fused to the top of a third band of bone. Ankylosaur skulls were fused with armor, including a distinctive armor on the outside-rear of the lower jaw. Ankylosaurs have been discovered on every continent except Africa. The group Ankylosauria subdivides into two categories: the nodosauridae and the ankylosauridae. 

The nodosaurids are the less well-known ankylosaurs. Nodosaurids lack a bony club on the ends of their tails (as is peculiar to the ankylosaurids), and they sport large shoulder armor, such as spikes. Here are a few examples:

The ankylosaurids are the most well-known ankylosaurs. Ankylosaurus belongs in this category. These ankylosaurs are marked by their heavy armor and clubbed tails (though some primitive ankylosauridae didn't have tail clubs). Here are a few examples: