Ankylosaurus

Ankylosaurus was one of the largest known ankylosaurids. It lived on the western side of North America near the close of the Cretaceous Period, about 68–66 million years ago. A broad, low torso, bony plates within the skin and a specialised club at the tail tip made it a highly distinctive herbivore.

No complete skeleton has been found. The familiar full-body image combines several partial specimens with better-preserved relatives. That distinction matters: the skull, tail weapon and many individual armour pieces are directly known, while the exact arrangement of plates over the body remains a reconstruction.

Quick facts

Name, discovery and type specimen

The genus name refers to stiffness or fusion, while the species name magniventris means “large belly”. Barnum Brown chose it for an animal with a rigid trunk and remarkably broad body.

Collector Peter Kaisen found the type specimen, AMNH 5895, near Gilbert Creek in Montana in 1906 during an American Museum of Natural History expedition led by Brown. Brown named the genus and species in 1908. The specimen includes parts of the skull, two teeth, vertebrae, ribs, a shoulder element and numerous osteoderms.

A second important skeleton, AMNH 5214, was found in Alberta in 1910. It preserves the most complete skull of the genus, both lower jaws, limb bones, armour and the only well-preserved tail club securely referred to Ankylosaurus. CMN 8880, found in 1947, provides the largest known skull. Other assignments based on isolated teeth or armour require more caution.

Classification and species

Ankylosaurus belongs to Thyreophora, the armoured branch of ornithischian dinosaurs. Within it, ankylosaurids differ from nodosaurids partly in their specialised tail clubs. The wider relationships are set out in the guide to dinosaur classification.

Only one species, A. magniventris, is generally recognised. Differences among a few teeth do not provide enough evidence for a second species. Early proposals that the genus was the same as the fragmentary Stegopelta were not accepted, and old museum practice sometimes mixed its material with Euoplocephalus.

What the fossils preserve and omit

The holotype remains the most informative postcranial specimen, with parts of the spine, ribs, shoulder and armour. AMNH 5214 adds a complete skull, jaws, several ribs and tail vertebrae, both upper arm bones, a thigh bone, part of the pelvis and the club. More than seventy osteoderms found with a theropod once called Dynamosaurus also belong in this history.

Complete hands, feet and pelvis, a continuous tail and an articulated covering of body armour are all missing. Individual pieces show what types of osteoderm existed, but not a precise map of every plate. Artistic rows should therefore be read as informed arrangements, not a fossil photograph.

Size and body proportions

Scaling AMNH 5214 gives a broad range near 6–8 metres. Scaling the largest skull has produced figures from roughly 7.6 to 10 metres, but the upper end is probably excessive because the vertebrae are not proportionally gigantic. A working adult range of 6–8 metres is safer.

Mass is less certain still. Estimates near 6 tonnes and around 8 tonnes depend on body volume, specimen choice and scaling. The dinosaur size comparison therefore treats 5–8 tonnes as a range rather than a universal value.

The torso was unusually wide and low, supported by short powerful limbs. A stiffened back helped carry the heavy body. The skull was low, wider than long and ornamented with bony knobs and horn-like projections behind the eyes. About 34–36 tiny teeth lined each upper jaw, extremely small relative to the skull.

Armour was not a continuous shell

Osteoderms are bones that form inside the skin. Large elements were separated by smaller ossicles and flexible tissue, so the covering was not a solid turtle-like shell. Transverse half-rings protected the neck, while flatter plates covered the body. Their living surfaces probably carried keratin and skin.

Old restorations often give Ankylosaurus enormous shoulder spines copied from nodosaurids. Secure evidence for those spikes is absent. It could not roll into a ball, and gaps between armour pieces were a necessary part of a mobile body.

The club and what it could do

The club consisted of a stiff handle built from tightly interlocking tail vertebrae and enlarged terminal osteoderms. On AMNH 5214 the bony knob is about 60 cm long and 49 cm wide. The preserved handle is about 1.21 metres long, and the entire specialised region may have approached two metres.

Biomechanical work on ankylosaurids shows that a large club could deliver forceful lateral blows. Defence against predators is plausible, while display or combat between members of the same species may also have mattered. No fossil shows this individual genus striking a Tyrannosaurus, so the popular broken-leg scene is possible storytelling, not direct evidence.

Feeding and environment

A low head and beaked mouth suited vegetation close to the ground. Ferns, soft shoots and low shrubs are reasonable foods. Small teeth provided limited processing, while microbial fermentation probably occurred in a capacious gut. No stomach contents reveal a precise menu.

Ankylosaurus inhabited floodplains, river margins, wooded areas and open ground represented by the Hell Creek, Lance, Scollard and related rocks. Tyrannosaurus, Triceratops and Edmontosaurus lived within this broad regional record. Sharing a formation does not establish a particular encounter.

The rarity of its fossils might reflect low abundance, preference for habitats poorly represented in sediments, or both. Isolated adult skeletons do not prove that the animals always lived alone. The broader limits of such deductions are explained in how fossils form.

Evidence, inference and reconstruction