Ankylosaurus

Lore & Background

Ankylosaurus is a member of the family Ankylosauridae, and its closest relatives appear to be Anodontosaurus and Euoplocephalus. Possibly the largest known ankylosaurid, it is estimated to have been between 6 and 8 m long and to have weighed between 4.8 and 8 MT. It was quadrupedal, with a broad, robust body, a wide, low skull, two horns pointing backward from the back of the head, and two below these that pointed backward and down. Unlike other ankylosaurs, its nostrils faced sideways. The front part of the jaws was covered in a beak, with rows of small, leaf-shaped teeth behind it. It was covered in armor plates (osteoderms), with bony half-rings covering the neck, and had a large club on the end of its tail. Bones in the skull and other parts of the body were fused, increasing their strength. Ankylosaurus is thought to have been a slow-moving animal, able to make quick movements when necessary. Its broad muzzle indicates it was a non-selective browser. Sinuses and nasal chambers in the snout may have been for heat and water balance or may have played a role in vocalization. The tail club is thought to have been used in defense against predators or in intraspecific combat. Specimens have been found in the Hell Creek, Lance, Scollard, Frenchman, and Ferris formations, but it appears to have been rare in its environment. It lived alongside Denversaurus, Tyrannosaurus, Triceratops, and Edmontosaurus.

Reader's Guide

The type specimen (AMNH 5895) was discovered in 1906 in the Hell Creek Formation, Montana, by an American Museum of Natural History expedition led by Barnum Brown. Brown described it in 1908. His skeletal reconstruction restored missing parts in a fashion similar to Stegosaurus, and he likened the result to the extinct armored mammal Glyptodon. The reconstruction showed robust forelimbs, a strongly arched back, a pelvis with prongs projecting forwards from the ilium and pubis, and a short, drooping tail without a tail club, which was unknown at the time. Brown also reconstructed the armor plates in parallel rows running down the back; this arrangement was purely hypothetical. In 1910, another AMNH expedition led by Brown discovered a specimen (AMNH 5214) in the Scollard Formation, Alberta, Canada, which included the first and only tail club known of this genus. In 1947, Charles M. Sternberg and T. Potter Chamney collected a skull and mandible (CMN 8880) near the same area. A section of caudal vertebrae (CCM V03) was discovered in the 1960s in the Powder River drainage, Montana. In 1990, Walter P. Coombs noted differences in teeth among specimens but refrained from naming a new species. In 2004, Kenneth Carpenter accepted all specimens as belonging to A. magniventris. In 2017, Victoria M. Arbour and Jordan Mallon redescribed the genus, concluding that though Ankylosaurus is the best-known member of its group, it was bizarre in comparison to related ankylosaurs.

Did You Know?

• The generic name Ankylosaurus means 'fused lizard', referring to the fusion of bones in the skull and body.
• The specific name magniventris means 'great belly', referring to the great width of the animal's body.
• The first tail club of Ankylosaurus was discovered in 1910 in Alberta, Canada.
• Ankylosaurus is known from far fewer remains than its closest relatives.

Physical Form and Defensive Armament

Ankylosaurus is estimated to have measured between six and eight meters in length and carried a mass of roughly 4.8 to 8 metric tonnes, making it possibly the largest known member of the ankylosaurid family. It moved on four legs atop a broad, heavily built frame. The skull was wide and low, bearing a pair of backward-pointing horns at the rear of the head and a second, lower pair angled backward and downward. A distinctive departure from other ankylosaurs is that its nostrils opened to the sides rather than forward. The front of the jaws was capped by a beak, with rows of small, leaf-shaped teeth set further back. The body was encased in osteoderms—bony armor plates—with half-ring structures shielding the neck, and a massive club anchored the tail's tip. Bones throughout the skull and skeleton were fused together, a trait that both reinforced structural strength and gave the genus its name.

Discovery and the Controversial 1908 Description

Collector Peter Kaisen had located the bones, which included the upper skull, a couple of teeth, shoulder girdle elements, various vertebrae, ribs, and over thirty armor plates. Brown depicted the creature with a strongly arched back, robust forelimbs, and a short drooping tail lacking any club, with armor plates arranged in neat parallel rows. Samuel Wendell Williston attacked the restoration as built on too little evidence and even argued the animal was simply a synonym of Stegopelta. That synonymy claim was never accepted.

Ecology, Diet, and Life in Late Cretaceous North America

Ankylosaurus roamed western North America during the final stretch of the Cretaceous, roughly 70 to 66 million years ago, placing it among the very last non-avian dinosaurs. Fossils have surfaced in the Hell Creek, Lance, Scollard, Frenchman, and Ferris formations, though the animal appears to have been uncommon in its surroundings. Its broad muzzle suggests it was a non-selective browser, likely sweeping through vegetation without much discrimination. Paleontologists infer it was a slow mover capable of short bursts of quick motion when the situation demanded. The sinuses and nasal chambers within its snout may have helped regulate body temperature and water balance, or perhaps played a part in producing sounds. The enormous tail club likely served as a weapon against predators such as Tyrannosaurus or in confrontations with rival individuals. Ankylosaurus shared its world with Triceratops, Edmontosaurus, and the nodosaurid Denversaurus, yet its ecological niche does not seem to have overlapped with that last species, hinting it may have favored upland terrain.

Taxonomy, Naming, and Archetypal Status

Ankylosaurus is a monotypic genus, containing only the species A. magniventris, and sits within the family Ankylosauridae. Its closest known relatives are Anodontosaurus and Euoplocephalus. Despite the fact that other ankylosaurid lineages are represented by far more extensive fossil collections, Ankylosaurus is frequently regarded as the archetypal member of the entire Ankylosauria group. This status persists even though it possesses certain unusual anatomical features, such as its laterally oriented nostrils, and even though only a handful of incomplete specimens have been excavated to date. Early confusion over which osteoderms belonged to Ankylosaurus versus Tyrannosaurus or Euoplocephalus complicated the picture for decades, but modern re-examination has clarified the picture considerably.

Common Misconceptions (Editorial)

- Some people think Ankylosaurus had forward-facing nostrils, but its nostrils actually faced sideways, unlike other ankylosaurs. - Many assume a complete skeleton has been found, but only a handful of specimens have been excavated, and no complete skeleton exists. - It is often believed that all ankylosaurs had tail clubs, but the tail club of Ankylosaurus was unknown when it was first reconstructed, and its earliest skeletal reconstruction actually resembled Stegosaurus.

Why It Matters (Editorial)

Ankylosaurus endures in the popular imagination as a living fortress, a testament to the evolutionary arms race between predator and prey at the end of the age of dinosaurs. Its rarity in the fossil record and unusual features, such as sideways-facing nostrils, remind us that even iconic species often remain mysterious, compelling scientists and the public alike to keep asking questions about life’s ancient past.

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