Ankylosaurus

Ankylosaurus magniventris, meaning "fused great-belly lizard," is the type genus of Ankylosauridae and lived until the end of the Cretaceous. Although no complete specimen has been found, researchers have recovered several partial skeletons and skulls. These remains define the family's key traits: a broad, heavily armoured skull with co-ossified osteoderms that hide the underlying bone sutures, large nasal sinuses, and a wide cropping beak. Its tail ended in a large club formed by a fused mass of vertebrae and dermal bone that weighed tens of kilograms.

The animal had a low-slung body that was notably broad across the hips. Its limbs were stout, with the forelimbs slightly shorter than the hindlimbs. Bone armour covered the dorsal surface, ranging from small studs to large keeled plates and lateral spikes. Even the eyelids were reinforced with bone. Ankylosaurus had small, leaf-shaped teeth suited for shearing low vegetation against its horny beak.

Ankylosaurus shared its Late Cretaceous environment with Tyrannosaurus rex, Triceratops, Edmontosaurus, and Pachycephalosaurus. Biomechanical studies by Arbour & Snively (2009) confirm that its tail club could deliver bone-breaking impacts, potentially fracturing the metatarsals or tibia of a T. rex. Whether the club was used mainly for predator defence, combat within the species, or both remains a subject of debate. This dinosaur was a low-bulk feeder that moved slowly across the landscape, likely consuming ferns, low conifer browse, and flowering plants.

Growth rings in the osteoderms and bone histology suggest that juveniles grew rapidly, a pattern seen in other ornithischians. Large adult specimens are relatively rare compared to the juveniles of related taxa like Pinacosaurus. This may indicate that adult ankylosaurs were more solitary than bonebed-forming dinosaurs such as centrosaurines.

• AMNH 5895 — Brown's holotype, American Museum of Natural History; partial skull, vertebrae, ribs, scutes, and a tail club.
• AMNH 5214 — second key specimen, also AMNH; preserves a more complete skull.
• CMN 8880 — Canadian Museum of Nature; partial skull and tail-club specimen.

Ankylosaurus is a relatively well-defined genus with little internal taxonomic dispute (it's monotypic), but key questions persist: tail-club function (defence vs combat); maximum size (mass estimates have ranged from 4 to 8 tonnes); comparison with Asian relatives (whether Ankylosaurus is a North American immigrant from Asia or an in situ Laramidian endemic). The animal's relative rarity compared with Triceratops in the Hell Creek may reflect genuinely lower abundance or sampling bias in habitat preference.

Ankylosaurus is a fixture of children's dinosaur books and toy lines, and appears in Jurassic World (2015) and Prehistoric Planet (2022). Its tail club is consistently the most-emphasised feature.

• ankylosauria
• euoplocephalus
• nodosaurus
• tyrannosaurus-rex

• Carpenter, K. (2004). Redescription of Ankylosaurus magniventris Brown 1908 (Ankylosauridae) from the Upper Cretaceous of the Western Interior of North America. Canadian Journal of Earth Sciences, 41, 961–986.
• Arbour, V. M., & Snively, E. (2009). Finite element analyses of ankylosaurid dinosaur tail club impacts. Anatomical Record, 292, 1412–1426.
• Arbour, V. M., & Currie, P. J. (2016). Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs. Journal of Systematic Palaeontology, 14, 385–444.
• Brown, B. (1908). The Ankylosauridae, a new family of armored dinosaurs from the Upper Cretaceous. Bulletin of the AMNH, 24, 187–201.