Ankylosaurus is a genus of armored dinosaur. Fossils of Ankylosaurus have been found in geological formations dating to the very end of the Cretaceous Period, between about 68–66 million years ago, in western North America, making it among the last of the non-avian dinosaurs. It was named by Barnum Brown in 1908, and the only species classified in the genus is A. magniventris. The genus name means “fused lizard” and the specific name means “great belly”. A handful of specimens have been excavated to date, but a complete skeleton has not been discovered. Though other members of Ankylosauria are represented by more extensive fossil material, Ankylosaurus is often considered the archetypal member of its group.
The largest known ankylosaurid, Ankylosaurus measured up to 6.25 m (20.5 feet) in length, 1.7 m (5.6 feet) in height, and weighed 6 tonnes (13,000 lb). It was a quadrupedal animal, with a broad, robust body. It had a wide, low skull, with two horns pointing backwards from the back of the head, and two horns below these that pointed backwards and down. The front part of the jaws were covered in a beak, with rows of small, leaf-shaped teeth further behind it. It was covered in armor plates, or 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, and this feature is the source of the genus name.
Ankylosaurus is a member of the family Ankylosauridae, and its closest relatives appear to be Anodontosaurus and Euoplocephalus. 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 played a role in vocalization. The tail club is thought to have been used in defense against predators or in intraspecific combat. Ankylosaurus has been found in the Hell Creek, Lance, and Scollard formations, but appears to have been rare in its environment. Although it lived alongside a nodosaurid ankylosaur, their ranges and ecological niches do not appear to have overlapped, and Ankylosaurus may have inhabited upland areas. Ankylosaurus also lived alongside dinosaurs such as Tyrannosaurus, Triceratops, and Edmontosaurus.
Ankylosaurus is the largest known ankylosaurid dinosaur, estimated to have been up to 6.25 m (20.5 feet) long, 1.5 m (4.9 feet) wide, and 1.7 m (5.6 feet) tall at the hip. This length has been proposed by American palaeontologist Kenneth Carpenter, and is based on the largest known skull (specimen NMC 8880), which is 64.5 cm (25.4 inches) long and 74.5 cm (29.3 inches) wide. The smallest known skull (specimen AMNH 5214) is 55.5 cm (21.9 inches) long and 64.5 cm (25.4 inches) wide, and this specimen is estimated to have been 5.4 m (17.7 feet) long and around 1.4 m (4.6 feet) tall. Other authors have proposed a body length of 7 m (23 feet), 8–9 m (26.2–29.5 ft), or more than 9 m (29.5 feet). The weight of the animal has been estimated at 6 tonnes (13,000 lb).
The structure of much of the skeleton of Ankylosaurus, including most of the pelvis, tail and feet, is still unknown. It was quadrupedal, and its hind limbs were longer than the forelimbs. The scapula (shoulder blade) and coracoid (a rectangular bone connected to the lower end of the scapula) of specimen AMNH 5895 were fused, and had entheses (connective tissue) for various muscle attachments. The scapula was 61.5 cm (24.2 inches) long. The humerus (upper arm bone) was short and very broad, and about 54 cm (21 inches) long in specimen AMNH 5214. The femur (thigh bone) was very robust, and 67 cm (26 inches) long in AMNH 5214. While the feet of Ankylosaurus are incompletely known, the hindfeet probably had three toes, as is the case in related animals.
The three known Ankylosaurus skulls differ in various details, but this is thought to be the result of taphonomy (changes happening during fossilisation of the remains) and individual variation. The skull was low and triangular in shape, wider than it was long. It had a broad beak on the premaxillae. The orbits (eye sockets) were almost round to slightly oval and did not face directly sideways, because the skull tapered towards the front. Crests above the orbits merged into the upper squamosal horns (their shape has been described as “pyramidal”), which pointed backwards to the sides from the back of the skull. The crest and horn were probably separate elements originally, as seen in the related Pinacosaurus and Euoplocephalus. Below the upper horns, jugal horns were present, which pointed backwards and down. The horns may have originally been osteoderms (armor plates) that fused to the skull. However, the scale pattern on the skull surface was instead the result of remodelling of the skull. This obliterated the sutures between skull elements, which is common for adult ankylosaurs. The scale pattern of the skull was variable between specimens, though some details are shared; it had a diamond-shaped scale (internarial scale) at the font of the snout, two squamosal osteoderms above the orbit, and a ridge of scales at the back of the skull.
Since Ankylosaurus and other Late Cretaceous North American ankylosaurids grouped with Asian genera (in a tribe the authors named Ankylosaurini), Arbour and Currie suggested that earlier North American ankylosaurids had gone extinct by the late Albian or Cenomanian ages of the Middle Cretaceous. Ankylosaurids thereafter recolonised North America from Asia during the Campanian or Turonian ages of the Late Cretaceous, and diversified there again, leading to genera such as Ankylosaurus, Anodontosaurus, and Euoplocephalus. This explains a 30 million year gap in the fossil record of North American ankylosaurids between these ages.
Ankylosaurus existed between 68 and 66 million years ago, in the final, or Maastrichtian, stage of the Late Cretaceous Period. It was among the last dinosaur genera that appeared before the Cretaceous–Paleogene extinction event. The type specimen is from the Hell Creek Formation of Montana, while other specimens have been found in the Lance Formation of Wyoming and the Scollard Formation in Alberta, Canada, all of which date to the end of the Cretaceous. Fossils of Ankylosaurus are rare in these sediments, and the distribution of its remains suggest that it was restricted to the uplands of the formations, rather than the coastal lowlands. Another ankylosaur, an indeterminate nodosaur (formerly referred to as Edmontonia sp.), is also found in the same formations, but the range of the two genera does not seem to have overlapped. Their remains have so far not been found in the same localities, and the nodosaur appears to have inhabited the lowlands. The narrow muzzle of the nodosaur suggests it had a more selective diet than Ankylosaurus, further indicating ecological separation.
The Hell Creek, Lance and Scollard Formations represent different sections of the western shore of the Western Interior Seaway that divided western and eastern North America during the Cretaceous. They represent a broad coastal plain, extending westward from the seaway to the newly formed Rocky Mountains. These formations are composed largely of sandstone and mudstone, which have been attributed to floodplain environments. The regions where Ankylosaurus and other Late Cretaceous ankylosaurs have been found had a warm subtropical/temperate climate, which was monsoonal, had occasional rainfall, tropical storms, and forest fires. In the Hell Creek Formation, many types of plants were supported, primarily angiosperms, with less common conifers, ferns and cycads. An abundance of fossil leaves found at dozens of different sites indicates that the area was largely forested by small trees. Ankylosaurus shared its environment with dinosaurs including the ceratopsids Triceratops and Torosaurus, the hypsilophodont Thescelosaurus, the hadrosaurid Edmontosaurus, an indeterminate nodosaur, the pachycephalosaurian Pachycephalosaurus, and the theropods Struthiomimus, Ornithomimus, Troodon, and Tyrannosaurus.